Q: Q15. What is point mutation? Give one example.

### Point Mutation\n\n**Definition:** A point mutation is a type of gene mutation that involves a change in a single base pair of DNA. This change can be a substitution (one base replaced by another), an insertion (addition of a base), or a deletion (removal of a base) within a gene sequence.\n\n**Example: Sickle Cell Anemia**\nSickle cell anemia is a classic example of a disorder caused by a point mutation. In this genetic disorder, a single base substitution occurs in the beta-globin chain of hemoglobin.\n* Specifically, at the sixth codon of the beta-globin gene, the normal DNA sequence GAG (which codes for glutamic acid) is mutated to GUG (which codes for valine).\n* This single amino acid substitution (glutamic acid to valine) alters the structure and function of the hemoglobin molecule (HbS), leading to the characteristic 'sickle' shape of red blood cells under low oxygen tension. These sickle-shaped cells are rigid, can block blood vessels, and have a reduced oxygen-carrying capacity, causing various symptoms of the disease.

Question 1

Q1. Mention the advantages of selecting pea plant for experiment by Mendel.

Accepted Answer

Mendel selected the garden pea plant (Pisum sativum) for his experiments due to several distinct advantages that made it an ideal model organism for studying inheritance patterns. These advantages contributed significantly to the success and clarity of his findings:

### Advantages of Pea Plant for Mendel's Experiments:

1.  **Easily Cultivated and Short Life Cycle:** Pea plants are annuals, meaning they complete their life cycle within a single growing season. This allowed Mendel to study multiple generations in a relatively short period, accelerating his research and enabling him to collect extensive data.

2.  **Clearly Observable Contrasting Traits:** Pea plants exhibit several well-defined, distinct, and contrasting characters (e.g., tall/dwarf stem height, round/wrinkled seed shape, yellow/green seed colour, violet/white flower colour, etc.). These clear-cut differences made it easy to categorize and track the inheritance of specific traits without ambiguity.

3.  **Bisexual Flowers and Self-Pollination:** Pea flowers are bisexual, containing both male (stamens) and female (pistil) reproductive organs. They naturally undergo self-pollination, leading to true-breeding lines (homozygous for specific traits). This was crucial for establishing pure parental stocks, ensuring that the starting generations were genetically uniform for the traits being studied.

4.  **Easy Cross-Pollination (Hybridization):** Despite natural self-pollination, pea plants can be easily cross-pollinated manually. Mendel could remove the anthers (emasculation) from a flower before it matured and then dust its stigma with pollen from another desired plant. This allowed him to create specific crosses between different varieties and study the inheritance patterns between contrasting traits.

5.  **Large Number of Offspring:** Pea plants produce a large number of seeds (offspring) per cross. A large sample size increases the statistical reliability of the experimental results, making it easier to observe and quantify the ratios of different phenotypes and genotypes, which were crucial for formulating his laws.

6.  **Inexpensive and Readily Available:** Pea plants are common, inexpensive to grow, and readily available, making them practical for large-scale experimental setups without significant resource constraints.

Question 2

Q2. Distinguish between the following – (a) Dominance and Recessive (b) Homozygous and Heterozygous (c) Monohybrid and Dihybrid.

Accepted Answer

Here are the distinctions between the given genetic terms:

**(a) Dominance and Recessive**
| Feature         | Dominant Trait                                     | Recessive Trait                                    |
|-----------------|----------------------------------------------------|----------------------------------------------------|
| **Expression**  | Expressed in both homozygous and heterozygous conditions. | Expressed only in the homozygous condition.        |
| **Presence**    | Requires at least one dominant allele for expression. | Requires two recessive alleles for expression.     |
| **Masking**     | Masks the expression of the recessive allele.      | Its expression is masked by the dominant allele.   |
| **Notation**    | Represented by an uppercase letter (e.g., 'T' for tall). | Represented by a lowercase letter (e.g., 't' for dwarf). |
| **Example**     | Tallness in pea plants (TT or Tt).                 | Dwarfness in pea plants (tt).                      |

**(b) Homozygous and Heterozygous**
| Feature         | Homozygous                                         | Heterozygous                                       |
|-----------------|----------------------------------------------------|----------------------------------------------------|\ | **Alleles**     | Possesses two identical alleles for a particular gene. | Possesses two different alleles for a particular gene. |\ | **Genotype**    | Can be homozygous dominant (e.g., TT) or homozygous recessive (e.g., tt). | Always consists of one dominant and one recessive allele (e.g., Tt). |\ | **Gametes**     | Produces only one type of gamete with respect to that gene. | Produces two different types of gametes in equal proportions (e.g., T and t). |\ | **Phenotype**   | Expresses the dominant phenotype if homozygous dominant, or the recessive phenotype if homozygous recessive. | Expresses the dominant phenotype (as the dominant allele masks the recessive one). |\ | **Example**     | Pure tall (TT) or pure dwarf (tt) pea plants.      | Hybrid tall (Tt) pea plants.                       |

**(c) Monohybrid and Dihybrid**
| Feature         | Monohybrid                                         | Dihybrid                                           |\ |-----------------|----------------------------------------------------|----------------------------------------------------|\ | **Definition**  | Involves the study of inheritance of a single pair of contrasting characters. | Involves the study of inheritance of two pairs of contrasting characters simultaneously. |\ | **Cross**       | A cross between two parents differing in one character. | A cross between two parents differing in two characters. |\ | **F2 Ratio**    | Phenotypic ratio in F2 generation is 3:1. Genotypic ratio is 1:2:1. | Phenotypic ratio in F2 generation is 9:3:3:1. Genotypic ratio is more complex. |\ | **Law**         | Explains the Law of Segregation.                   | Explains the Law of Independent Assortment.        |\ | **Example**     | Cross between tall and dwarf pea plants (e.g., TT x tt). | Cross between pea plants with round yellow seeds and wrinkled green seeds (e.g., RRYY x rryy). |

Question 3

Q3. Write a brief account of the contribution of T.H. Morgan in genetics.

Accepted Answer

Thomas Hunt Morgan, an American evolutionary biologist, geneticist, and embryologist, made significant contributions to the field of genetics, particularly through his work with the fruit fly, *Drosophila melanogaster*. His research established the chromosome theory of inheritance and laid the groundwork for modern genetics. He was awarded the Nobel Prize in Physiology or Medicine in 1933 for his discoveries concerning the role played by the chromosome in heredity.

### Key Contributions of T.H. Morgan:

1.  **Choice of *Drosophila melanogaster* as a Model Organism:**
    *   Morgan chose the fruit fly, *Drosophila melanogaster*, for his genetic experiments. This proved to be an excellent choice due to several advantages:
        *   They can be grown on simple synthetic media in the laboratory.
        *   They complete their life cycle in about two weeks.
        *   A single mating can produce a large number of progeny flies.
        *   There is a clear differentiation of sexes (male and female).
        *   They have many types of hereditary variations that can be seen with low power microscopes.

2.  **Discovery of Linkage and Recombination:**
    *   Morgan conducted several dihybrid crosses in *Drosophila* to study genes that were located on the same chromosome. He observed that when two genes were situated on the same chromosome, the proportion of parental gene combinations was much higher than the non-parental (recombinant) types.
    *   This observation led him to coin the term **linkage** to describe the physical association of genes on a chromosome and the term **recombination** to describe the generation of non-parental gene combinations.
    *   He found that genes located on the same chromosome do not always segregate independently, contrary to Mendel's Law of Independent Assortment, especially if they are closely linked.

3.  **Concept of Gene Mapping and Chromosome Maps:**
    *   Morgan's student, Alfred Sturtevant, used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes and 'mapped' their positions on the chromosome.
    *   The higher the frequency of recombination, the farther apart the genes were on the chromosome, and vice versa. This concept led to the construction of **genetic maps** or **chromosome maps**, which show the linear arrangement of genes on a chromosome.
    *   These maps are crucial for understanding genome organization and for genetic engineering.

4.  **Elucidation of Sex-Linked Inheritance:**
    *   Morgan's experiments with *Drosophila* also provided definitive evidence for the chromosomal basis of sex determination and sex-linked inheritance. He studied the inheritance of the white-eye trait in *Drosophila* and showed that the gene for eye color was located on the X chromosome.
    *   He demonstrated that the inheritance pattern of this trait was different in males and females, confirming that certain traits are linked to sex chromosomes.

5.  **Confirmation of the Chromosomal Theory of Inheritance:**
    *   Through his extensive work, Morgan provided experimental verification for the chromosomal theory of inheritance, which states that chromosomes are the carriers of genetic material. His findings solidified the idea that genes are located on chromosomes and that these chromosomes undergo segregation and independent assortment during meiosis, explaining Mendelian inheritance patterns.

In summary, T.H. Morgan's meticulous experiments with *Drosophila* revolutionized genetics by providing concrete evidence for linkage, recombination, gene mapping, and sex-linked inheritance, thereby firmly establishing the chromosomal theory of inheritance.

Question 4

Q4. What is pedigree analysis? Suggest how analysis of the following traits might be useful: (a) Sickle cell anaemia (b) Phenylketonuria (c) Thalassemia.

Accepted Answer

### What is Pedigree Analysis?
Pedigree analysis is a strong tool in human genetics that involves the study of the inheritance pattern of a particular genetic trait across generations in a family. It is represented by a diagram called a pedigree chart, which uses standardized symbols to depict family relationships and the presence or absence of a trait in each individual. Since controlled crosses are not possible in humans, pedigree analysis helps geneticists to understand the mode of inheritance (e.g., autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, mitochondrial inheritance) of genetic disorders or traits, predict the probability of a trait appearing in future generations, and provide genetic counseling.

**Key features of a pedigree chart:**
*   Squares represent males, circles represent females.
*   Shaded symbols indicate affected individuals.
*   Unshaded symbols indicate unaffected individuals.
*   A horizontal line between two individuals represents a mating.
*   A vertical line extending downwards from a mating line connects to their offspring.
*   Offspring are shown in birth order from left to right.
*   Roman numerals (I, II, III...) denote generations.
*   Arabic numerals (1, 2, 3...) denote individuals within a generation.

### Usefulness of Pedigree Analysis for Specific Traits:

Pedigree analysis is invaluable for understanding and managing genetic disorders. Here's how it's useful for the mentioned traits:

**(a) Sickle Cell Anaemia:**
*   **Nature of the trait:** Autosomal recessive disorder. Individuals homozygous for the recessive allele (HbS HbS) suffer from the disease, while heterozygotes (HbA HbS) are carriers and usually asymptomatic (sickle cell trait), but show resistance to malaria.
*   **Usefulness of Pedigree Analysis:**
    *   **Identifying Carriers:** Pedigree analysis can help identify asymptomatic carriers (heterozygotes) within a family, especially if affected individuals are present in previous generations or among siblings. This is crucial for genetic counseling.
    *   **Risk Assessment:** It allows for the calculation of the probability of future offspring inheriting the disease. For example, if two carriers (HbA HbS x HbA HbS) plan to have children, pedigree analysis helps them understand the 25% chance of having an affected child, 50% chance of having a carrier child, and 25% chance of having an unaffected non-carrier child.
    *   **Early Diagnosis and Management:** By identifying families at risk, early diagnostic measures (like prenatal diagnosis) can be offered, allowing for timely intervention and management strategies.
    *   **Population Screening:** Understanding the prevalence and inheritance pattern through pedigree studies can inform population-level screening programs in high-risk communities.

**(b) Phenylketonuria (PKU):**
*   **Nature of the trait:** Autosomal recessive metabolic disorder. It results from a deficiency of the enzyme phenylalanine hydroxylase, leading to the accumulation of phenylalanine and its derivatives, causing intellectual disability if untreated.
*   **Usefulness of Pedigree Analysis:**
    *   **Carrier Detection:** Similar to sickle cell anaemia, pedigree analysis can pinpoint carriers (heterozygotes) who do not show symptoms but can pass on the defective gene. This is vital for reproductive planning.
    *   **Genetic Counseling:** It provides families with accurate information about the risk of recurrence in future children. If both parents are carriers, there's a 25% chance of their child being affected.
    *   **Preventive Measures:** Early identification of at-risk families through pedigree analysis can lead to newborn screening programs. If PKU is detected early, a special diet low in phenylalanine can prevent severe intellectual disability.
    *   **Understanding Disease Etiology:** Pedigree charts can reveal patterns of inheritance that confirm the autosomal recessive nature of PKU, helping researchers understand the genetic basis of the disorder.

**(c) Thalassemia:**
*   **Nature of the trait:** Autosomal recessive blood disorder characterized by abnormal hemoglobin production. It leads to severe anemia. There are two main types: alpha-thalassemia and beta-thalassemia, depending on which globin chain is affected.
*   **Usefulness of Pedigree Analysis:**
    *   **Carrier Identification and Risk Stratification:** Pedigree analysis is crucial for identifying carriers (heterozygotes) of thalassemia, who may have mild or no symptoms. This is particularly important in populations where thalassemia is prevalent.
    *   **Prenatal Diagnosis and Family Planning:** For couples identified as carriers, pedigree analysis helps in assessing the risk of having a severely affected child. This information is critical for genetic counseling and decisions regarding prenatal diagnosis (e.g., chorionic villus sampling or amniocentesis) to detect the condition in the fetus.
    *   **Preventive Strategies:** By understanding the inheritance pattern within a family, healthcare providers can advise on family planning options, including preimplantation genetic diagnosis (PGD) or adoption, to prevent the birth of affected children.
    *   **Differential Diagnosis:** In cases of unexplained anemia, a detailed family history through pedigree analysis can suggest thalassemia as a possible diagnosis, guiding further genetic testing.

In essence, pedigree analysis serves as a fundamental tool for geneticists and clinicians to unravel complex inheritance patterns, provide accurate risk assessments, and offer informed genetic counseling for a wide range of inherited disorders, including these specific examples.

Question 5

Q5. How is sex determined in human beings?

Accepted Answer

Sex determination in human beings is based on the chromosomal mechanism, specifically involving the sex chromosomes. Humans have a total of 46 chromosomes (23 pairs) in their somatic cells. Out of these, 22 pairs are autosomes (non-sex chromosomes), and one pair consists of sex chromosomes, which determine the sex of an individual.

### Chromosomal Basis of Sex Determination:

1.  **Sex Chromosomes:**
    *   There are two types of sex chromosomes: X and Y.
    *   **Females** are homogametic, possessing two X chromosomes (XX).
    *   **Males** are heterogametic, possessing one X chromosome and one Y chromosome (XY).

2.  **Gamete Formation:**
    *   **Females (XX):** During meiosis, females produce only one type of gamete (ova), each carrying a single X chromosome.
    *   **Males (XY):** During meiosis, males produce two types of gametes (sperms) in equal proportions: 50% carry an X chromosome and 50% carry a Y chromosome.

3.  **Fertilization and Sex Determination:**
    *   The sex of the offspring is determined at the time of fertilization, depending on which type of sperm fertilizes the egg.
    *   If an **X-carrying sperm** fertilizes an X-carrying egg, the resulting zygote will be XX, developing into a **female**.
    *   If a **Y-carrying sperm** fertilizes an X-carrying egg, the resulting zygote will be XY, developing into a **male**.

4.  **Role of the Y Chromosome (SRY Gene):**
    *   The presence or absence of the Y chromosome is the critical factor in human sex determination. Specifically, a gene located on the Y chromosome, called the **SRY gene** (Sex-determining Region Y), plays a pivotal role.
    *   The SRY gene codes for a transcription factor called Testis-Determining Factor (TDF). TDF initiates the development of testes in the embryo. Once testes develop, they produce male hormones (androgens) that lead to the development of other male secondary sexual characteristics.
    *   In the absence of the Y chromosome (and thus the SRY gene), the embryonic gonads develop into ovaries, leading to female development.

### Punnett Square Representation:

Let's illustrate this with a Punnett square:

From the Punnett square, it is clear that there is an equal probability (50%) of having a male child and a 50% probability of having a female child in each pregnancy.

### Conclusion:
In humans, the male is heterogametic (XY) and determines the sex of the offspring. The presence of the Y chromosome, specifically the SRY gene, is responsible for initiating male development. The female is homogametic (XX) and contributes only X chromosomes to the offspring.

Question 6

Q6. Define and design a test-cross.

Accepted Answer

### Definition of Test Cross
A test cross is a cross used to determine the genotype of an individual with a dominant phenotype. The individual showing the dominant phenotype (e.g., tall pea plant) could be either homozygous dominant (TT) or heterozygous (Tt). To ascertain its genotype, it is crossed with a homozygous recessive individual (tt). The phenotypic ratio of the offspring from this cross reveals the genotype of the unknown dominant parent.

### Design of a Test Cross
Let's consider a monohybrid cross involving the trait of stem height in pea plants, where 'T' represents the dominant allele for tallness and 't' represents the recessive allele for dwarfness.

**Objective:** To determine the genotype of a tall pea plant (which could be TT or Tt).

**Procedure:**
1.  **Identify the unknown parent:** A pea plant exhibiting the dominant phenotype (tall). Its genotype is either TT or Tt.
2.  **Choose the known parent:** A pea plant exhibiting the recessive phenotype (dwarf). Its genotype must be homozygous recessive (tt).
3.  **Perform the cross:** Cross the unknown tall plant with the known dwarf plant.

**Possible Outcomes and Inferences:**

**Case 1: If the unknown tall plant is Homozygous Dominant (TT)**
*   **Parents:** Tall (TT) x Dwarf (tt)
*   **Gametes:** T (from tall parent) ; t (from dwarf parent)
*   **Offspring (F1 generation):** All Tt (Tall)
*   **Phenotypic Ratio:** All offspring are tall.
*   **Inference:** If all offspring are tall, the unknown parent's genotype is homozygous dominant (TT).

**Case 2: If the unknown tall plant is Heterozygous (Tt)**
*   **Parents:** Tall (Tt) x Dwarf (tt)
*   **Gametes:** T, t (from tall parent) ; t (from dwarf parent)
*   **Offspring (F1 generation):** 1/2 Tt (Tall) : 1/2 tt (Dwarf)
*   **Phenotypic Ratio:** 1 Tall : 1 Dwarf (or 50% tall, 50% dwarf).
*   **Inference:** If the offspring show a 1:1 ratio of tall to dwarf plants, the unknown parent's genotype is heterozygous (Tt).

**Summary:**
By observing the phenotypes of the offspring from a test cross, one can definitively determine whether an individual showing a dominant trait is homozygous dominant or heterozygous.

Question 7

Q7. Using a Punnett Square, work out the distribution of phenotypic features in the first filial generation after a cross between a homozygous female and a heterozygous male for a sing

Accepted Answer

Let's consider a single trait, for example, flower color in a hypothetical plant, where 'R' represents the dominant allele for red flowers and 'r' represents the recessive allele for white flowers.

**Given:**
*   Homozygous female: This means her genotype is either homozygous dominant (RR) or homozygous recessive (rr). Since the question doesn't specify dominant or recessive, we will assume homozygous dominant for the purpose of demonstrating the cross clearly. Let's denote her genotype as **RR**.
*   Heterozygous male: This means his genotype is **Rr**.

**Parents:**
*   Female: Homozygous Dominant (RR)
*   Male: Heterozygous (Rr)

**Gametes produced by parents:**
*   Female (RR) will produce only one type of gamete: **R**
*   Male (Rr) will produce two types of gametes: **R** and **r** (in equal proportions)

**Punnett Square for the cross (RR x Rr):**

| Gametes from Male (Rr) | R | r |
| :--------------------- | :- | :- |
| **Gametes from Female (RR)** | | |
| **R** | RR | Rr |

**Distribution of Genotypes in F1 Generation:**
From the Punnett Square, the possible genotypes in the F1 generation are:
*   **RR:** 1 out of 2 squares (or 50%)
*   **Rr:** 1 out of 2 squares (or 50%)

**Genotypic Ratio:** 1 RR : 1 Rr

**Distribution of Phenotypic Features in F1 Generation:**
*   **RR:** Red flowers (since R is dominant)
*   **Rr:** Red flowers (since R is dominant)

Therefore, all offspring (100%) will have red flowers.

**Phenotypic Ratio:** All Red flowers (or 1:0 Red:White)

**Conclusion:**
When a homozygous dominant female (RR) is crossed with a heterozygous male (Rr), all the offspring in the first filial generation (F1) will exhibit the dominant phenotype (red flowers in this example). Their genotypes will be 50% homozygous dominant (RR) and 50% heterozygous (Rr).

*Note: If the homozygous female was assumed to be homozygous recessive (rr), the cross would be rr x Rr. In this case, the Punnett square would show 50% Rr (Red) and 50% rr (White) offspring, leading to a 1:1 phenotypic ratio of Red:White flowers.*

Question 8

Q8. When a cross is made between plants having violet flowers and plants having white flowers, all the offspring plants have violet flowers.
(a) Which flower colour is dominant?
(b)

Accepted Answer

Let's denote the alleles for flower colour:
*   Let 'V' represent the allele for violet flowers.
*   Let 'v' represent the allele for white flowers.

### (a) Which flower colour is dominant?
**Answer:** Violet flower colour is dominant.

**Explanation:** According to Mendel's Law of Dominance, when two contrasting traits are crossed, only one trait (the dominant one) appears in the F1 generation. Since all the offspring plants in the F1 generation have violet flowers, it indicates that the allele for violet colour (V) is dominant over the allele for white colour (v).

**Parental Cross (P generation):**
*   Violet flowers (unknown genotype, but must contribute 'V') x White flowers (must be homozygous recessive 'vv')
*   Since all F1 are violet, the violet parent must be homozygous dominant (VV).
*   So, P: VV (Violet) x vv (White)
*   Gametes: V (from violet parent) ; v (from white parent)
*   **F1 Generation:** All Vv (Violet)

### (b) If F1 plants are self-pollinated, what will be the phenotypic ratio in the F2 generation?
**Answer:** The phenotypic ratio in the F2 generation will be **3 Violet : 1 White**.

**Explanation:**
*   **F1 Generation Genotype:** All F1 plants are heterozygous (Vv) for violet flowers.
*   **Self-pollination of F1:** Vv (Violet) x Vv (Violet)
*   **Gametes from F1:** Each F1 plant will produce two types of gametes: V and v (in equal proportions).

**Punnett Square for F1 x F1 cross:**

| Gametes from F1 Male (Vv) | V | v |
| :------------------------ | :- | :- |
| **Gametes from F1 Female (Vv)** | | |
| **V** | VV | Vv |
| **v** | Vv | vv |

**Genotypes in F2 Generation:**
*   VV: 1 (Homozygous dominant, Violet)
*   Vv: 2 (Heterozygous, Violet)
*   vv: 1 (Homozygous recessive, White)

**Genotypic Ratio:** 1 VV : 2 Vv : 1 vv

**Phenotypes in F2 Generation:**
*   Violet flowers: VV and Vv (1 + 2 = 3 parts)
*   White flowers: vv (1 part)

**Phenotypic Ratio:** 3 Violet : 1 White

### (c) State the Law that is illustrated in this cross.
**Answer:** The Law of Dominance and the Law of Segregation are illustrated in this cross.

**Explanation:**
1.  **Law of Dominance:** This law is illustrated by the appearance of only one parental trait (violet flowers) in the F1 generation, even though both contrasting traits (violet and white) were present in the parents. The violet allele (V) completely masks the expression of the white allele (v) in the heterozygous condition.
2.  **Law of Segregation (or Law of Purity of Gametes):** This law is illustrated when the F1 plants (Vv) are self-pollinated, and both violet and white flowers reappear in the F2 generation in a 3:1 ratio. This demonstrates that the two alleles (V and v) for a trait, which remain together in the F1 heterozygote, do not blend but separate or segregate from each other during gamete formation. Each gamete receives only one of the two alleles. Upon fertilization, these alleles combine randomly, leading to the reappearance of the recessive trait in F2.

Question 9

Q9. Explain the Law of Segregation with a suitable example.

Accepted Answer

### Law of Segregation (Mendel's First Law)

**Definition:** The Law of Segregation states that during the formation of gametes, the two alleles for a heritable character (trait) separate or segregate from each other such that each gamete receives only one allele. These alleles do not blend or contaminate each other and remain distinct. When fertilization occurs, the alleles combine randomly.

**Key Points:**
*   **Allele Separation:** Each individual carries two alleles for a given trait, but only one of these alleles is passed on to each gamete it produces.
*   **Purity of Gametes:** Gametes are 'pure' in the sense that they carry only one allele for each trait, not both.
*   **No Blending:** The alleles do not mix or blend; they maintain their individuality and can reappear unchanged in subsequent generations.
*   **Random Fertilization:** The union of gametes during fertilization is a random event, restoring the diploid condition.

### Suitable Example: Monohybrid Cross in Pea Plants for Stem Height
Let's consider the trait of stem height in pea plants, where:
*   'T' represents the dominant allele for tallness.
*   't' represents the recessive allele for dwarfness.

**1. Parental Generation (P):**
Mendel crossed true-breeding tall pea plants with true-breeding dwarf pea plants.
*   **Parents:** Homozygous Tall (TT) x Homozygous Dwarf (tt)
*   **Gametes:** The tall parent (TT) produces only 'T' gametes. The dwarf parent (tt) produces only 't' gametes.

**2. First Filial Generation (F1):**
When the 'T' gamete from the tall parent fuses with the 't' gamete from the dwarf parent, all the offspring in the F1 generation are heterozygous (Tt).
*   **F1 Genotype:** All Tt
*   **F1 Phenotype:** All Tall (due to the dominance of 'T' over 't')

**Illustration of Segregation (during F1 gamete formation):**
At this stage, the Law of Segregation comes into play. Each F1 plant (genotype Tt) has two alleles for height. During gamete formation (meiosis), these two alleles (T and t) segregate from each other. Consequently, 50% of the gametes produced by an F1 plant will carry the 'T' allele, and the other 50% will carry the 't' allele. Each gamete is 'pure' for either tallness or dwarfness, but not both.

**3. Second Filial Generation (F2):**
Mendel allowed the F1 plants to self-pollinate (Tt x Tt).

**Punnett Square for F1 x F1 cross:**

| Gametes from F1 Male (Tt) | T | t |
| :------------------------ | :- | :- |
| **Gametes from F1 Female (Tt)** | | |
| **T** | TT | Tt |
| **t** | Tt | tt |

**Results in F2 Generation:**
*   **Genotypes:** 1 TT : 2 Tt : 1 tt
*   **Phenotypes:** 3 Tall : 1 Dwarf

**How the Law of Segregation is illustrated:**
*   The reappearance of the recessive dwarf phenotype (tt) in the F2 generation, which was absent in the F1 generation, is a direct consequence of the segregation of alleles. If the alleles had blended in F1, the dwarf trait would not have reappeared distinctly.
*   The 3:1 phenotypic ratio in F2 (and 1:2:1 genotypic ratio) can only be explained if the alleles 'T' and 't' in the F1 heterozygote (Tt) separated cleanly during gamete formation, with each gamete receiving only one allele. These segregated alleles then combined randomly during fertilization to produce the observed F2 genotypes and phenotypes.

This example clearly demonstrates that alleles do not blend but segregate during gamete formation, ensuring that each gamete carries only one allele for a given trait.

Question 10

Q10. A child has blood group O. If the father has blood group A and mother blood group B, work out the genotypes of the parents and the possible genotypes of the other offsprings.

Accepted Answer

This problem involves the inheritance of ABO blood groups in humans, which is an example of multiple alleles and co-dominance. **1. Deducing Parental Genotypes:** * The child has blood group O. The only possible genotype for blood group O is **i/i**. * A child inherits one allele from each parent. Therefore, the child must have inherited one 'i' allele from the father and one 'i' allele from the mother. * The father has blood group A. His genotype could be I^AI^A or I^Ai. Since he passed the 'i' allele to his child, his genotype must be **I^Ai**. * The mother has blood group B. Her genotype could be I^BI^B or I^Bi. Since she passed the 'i' allele to her child, her genotype must be **I^Bi**. **2. Determining Possible Genotypes of Other Offspring:** To find the possible genotypes of other children, we perform a cross between the parents (I^Ai × I^Bi). **Cross:** * Parental Genotypes: I^Ai (Father) × I^Bi (Mother) * Gametes from Father: I^A and i * Gametes from Mother: I^B and i Using a Punnett square: | | I^B | i | |---|---|---| | **I^A** | I^AI^B | I^Ai | | **i** | I^Bi | ii | From the cross, the possible genotypes of the offspring are: * **I^AI^B** (Blood Group AB) * **I^Ai** (Blood Group A) * **I^Bi** (Blood Group B) * **ii** (Blood Group O) - This is the genotype of the first child. Therefore, the possible genotypes of the *other* offsprings are **I^AI^B, I^Ai, and I^Bi**.

Question 11

Q11. Explain the following terms with example
(a) Co-dominance
(b) Incomplete dominance

Accepted Answer

Both co-dominance and incomplete dominance are types of non-Mendelian inheritance where the dominant-recessive relationship between alleles is not observed. **(a) Co-dominance** **Definition:** Co-dominance is a phenomenon in which both alleles of a gene pair in a heterozygote are fully and independently expressed. Neither allele is dominant or recessive to the other, and the F1 generation resembles both parents. **Mechanism:** In a heterozygous individual (e.g., A1A2), both allele A1 and allele A2 produce their respective proteins or products, and both products are observed phenotypically. **Example: ABO Blood Group in Humans** * The gene 'I' controls the ABO blood group and has three alleles: I^A, I^B, and i. * Alleles I^A and I^B produce slightly different forms of a sugar antigen on the surface of red blood cells, while allele 'i' does not produce any antigen. * I^A and I^B are both dominant over 'i'. * When I^A and I^B are present together in an individual (genotype I^AI^B), both alleles express themselves completely. Such an individual has both A-type and B-type sugar antigens on their RBCs, resulting in the AB blood group. * This is a classic example of co-dominance because both parental phenotypes (A and B antigens) are expressed together in the offspring. **(b) Incomplete Dominance** **Definition:** Incomplete dominance is a phenomenon in which the phenotype of the F1 hybrid is intermediate between the phenotypes of the two parents. It results in a blending of parental traits. **Mechanism:** In a heterozygous individual (e.g., Rr), the dominant allele (R) is not completely dominant over the recessive allele (r). The resulting phenotype is a blend or an intermediate form. The phenotypic and genotypic ratios in the F2 generation are the same (1:2:1). **Example: Flower Colour in Snapdragon (Antirrhinum sp.)** * A cross is made between a true-breeding red-flowered plant (genotype RR) and a true-breeding white-flowered plant (genotype rr). * **F1 Generation:** All offspring have the genotype Rr and produce pink flowers. The pink colour is an intermediate phenotype between the parental red and white colours. This demonstrates that the R allele (for red) is not completely dominant over the r allele (for white). * **F2 Generation:** When the F1 pink-flowered plants (Rr) are self-pollinated, the offspring appear in the following ratio: * **Phenotypic Ratio:** 1 Red : 2 Pink : 1 White * **Genotypic Ratio:** 1 RR : 2 Rr : 1 rr * Here, the phenotypic ratio is identical to the genotypic ratio, which is a characteristic feature of incomplete dominance.

Question 12

Q12. What is pedigree analysis? Suggest how such an analysis, can be useful.

Accepted Answer

**Pedigree Analysis**

Pedigree analysis is a method of studying the inheritance of a particular genetic trait or disorder in human beings over several generations. It involves creating a family tree or chart, called a pedigree chart, that uses standardized symbols to represent family members, their relationships, and their status with respect to the trait in question (affected or unaffected).

**Standard Symbols in a Pedigree Chart:**
*   **Square:** Represents a male.
*   **Circle:** Represents a female.
*   **Diamond:** Represents sex unspecified.
*   **Shaded Symbol (Square or Circle):** Represents an affected individual.
*   **Unshaded Symbol:** Represents an unaffected individual.
*   **Half-shaded Symbol or a dot inside:** Represents a carrier (heterozygous for a recessive trait).
*   **Horizontal Line between a square and a circle:** Represents mating (marriage).
*   **Double Horizontal Line:** Represents consanguineous mating (mating between relatives).
*   **Vertical Line descending from the mating line:** Connects parents to their offspring.
*   **Siblings:** Connected by a horizontal line (sibship line) below the parents.

**Usefulness of Pedigree Analysis**

Pedigree analysis is a powerful tool in human genetics because controlled crosses, as performed in pea plants, are not possible in humans. Its usefulness includes:

1.  **Determining the Mode of Inheritance:** It helps to determine whether a trait is dominant or recessive. For example, if a trait appears in every generation, it is likely dominant. If it skips generations, it is likely recessive.

2.  **Identifying Autosomal or Sex-linked Traits:** It helps to trace whether the gene for the trait is located on an autosome or a sex chromosome (X or Y). For instance, X-linked recessive traits are more common in males than females.

3.  **Genetic Counselling:** Pedigree analysis is crucial for genetic counsellors to advise couples who have a family history of a genetic disorder. It helps predict the risk of the disorder appearing in their future children, allowing them to make informed decisions.

4.  **Tracing the Origin of a Trait:** It can be used to trace the inheritance of a specific trait, abnormality, or disease through generations and identify its origin in the family history.

5.  **Understanding Gene Expression:** It provides valuable information about how a gene is expressed and how it might be influenced by other genes or environmental factors.

In summary, pedigree analysis serves as a strong tool for geneticists to study human genetic diseases and for predicting the pattern of inheritance, which is vital for medical and genetic counselling.

Question 13

Q13. How is sex determined in human beings?

Accepted Answer

Sex determination in human beings is based on the chromosomal mechanism, specifically the XX-XY type.

**1. Chromosomal Basis:**
*   Humans have a total of 46 chromosomes (23 pairs) in each somatic cell.
*   Out of these, 22 pairs are autosomes, which are the same in both males and females.
*   The 23rd pair consists of sex chromosomes, which differ between males and females.
    *   **Females** have a pair of identical sex chromosomes, designated as **XX**.
    *   **Males** have a pair of distinct sex chromosomes, designated as **XY**. The Y chromosome is generally shorter than the X chromosome.

**2. Role of Gametes:**
*   During gamete formation (meiosis), the chromosome number is halved.
*   **Females (XX)** are **homogametic**. They produce only one type of ovum (egg), and each ovum contains 22 autosomes and one X chromosome.
*   **Males (XY)** are **heterogametic**. They produce two types of sperm in approximately equal numbers:
    *   50% of the sperm contain 22 autosomes and an **X chromosome**.
    *   50% of the sperm contain 22 autosomes and a **Y chromosome**.

**3. Determination at Fertilization:**
*   The sex of the child is determined at the moment of fertilization and depends entirely on the type of sperm that fertilizes the ovum.
*   If a sperm carrying the **X chromosome** fertilizes the ovum (which always carries an X chromosome), the resulting zygote will have the genotype **XX**. This zygote will develop into a **female** child.
*   If a sperm carrying the **Y chromosome** fertilizes the ovum, the resulting zygote will have the genotype **XY**. This zygote will develop into a **male** child.

**Conclusion:**
There is a 50% probability of a male child and a 50% probability of a female child being born. Since the mother can only contribute an X chromosome, the sex of the child is determined by the father's sperm. The presence or absence of the Y chromosome is the deciding factor; the SRY (Sex-determining Region Y) gene on the Y chromosome is responsible for initiating male development.

**Chart showing Sex Determination in Humans:**

| Parents | Father | Mother |
|---|---|---|
| **Genotype** | 44+XY | 44+XX |
| **Gametes** | 22+X and 22+Y | 22+X |

**Punnett Square:**

| | Ovum (22+X) |
|---|---|
| **Sperm (22+X)** | Zygote (44+XX) - Female |
| **Sperm (22+Y)** | Zygote (44+XY) - Male |

Question 14

Q14. Explain the following terms with example: (a) Co-dominance (b) Incomplete dominance

Accepted Answer

### (a) Co-dominance

**Definition:** Co-dominance is a phenomenon in which both alleles of a gene are expressed equally and simultaneously in the heterozygote. Neither allele is dominant or recessive to the other; instead, both contribute to the phenotype, resulting in a distinct phenotype that shows characteristics of both parental alleles.

**Mechanism:** In co-dominance, the gene product of both alleles is functional and detectable. This means that the heterozygote expresses both traits associated with the individual alleles, rather than an intermediate or one masking the other.

**Example: ABO Blood Group System in Humans**
The ABO blood group system is a classic example of co-dominance (along with multiple allelism). The gene for blood type (I) has three alleles: I^A, I^B, and i.
*   Alleles I^A and I^B are co-dominant to each other.
*   Allele i is recessive to both I^A and I^B.

When an individual inherits both I^A and I^B alleles (genotype I^A I^B), their red blood cells express both A and B antigens on their surface. This results in blood group AB, which exhibits characteristics of both A and B blood types simultaneously. This is a clear demonstration of co-dominance, as neither A nor B antigen expression is suppressed.

### (b) Incomplete Dominance

**Definition:** Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely expressed over the other allele. This results in a heterozygous phenotype that is intermediate between the two homozygous parental phenotypes.

**Mechanism:** In incomplete dominance, the dominant allele does not completely mask the effect of the recessive allele. Often, the dominant allele produces a certain amount of gene product, but one copy is not enough to produce the full effect seen in the homozygous dominant individual. The heterozygote produces an intermediate amount of the gene product, leading to an intermediate phenotype.

**Example: Flower Colour in Snapdragon (*Antirrhinum majus*) or Four O'clock Plant (*Mirabilis jalapa*)**
Consider the flower colour in Snapdragon:
*   When a homozygous red-flowered plant (RR) is crossed with a homozygous white-flowered plant (rr), the F1 generation produces all pink-flowered plants (Rr).
*   Here, neither the red allele (R) nor the white allele (r) is completely dominant. The pink colour is an intermediate phenotype, a blend of red and white.
*   If the F1 pink-flowered plants (Rr) are self-pollinated, the F2 generation will show a phenotypic ratio of 1 Red (RR) : 2 Pink (Rr) : 1 White (rr). The genotypic ratio is also 1:2:1, which is identical to the phenotypic ratio, a characteristic feature of incomplete dominance.

Question 15

Q15. What is point mutation? Give one example.

Accepted Answer

### Point Mutation

**Definition:** A point mutation is a type of gene mutation that involves a change in a single base pair of DNA. This change can be a substitution (one base replaced by another), an insertion (addition of a base), or a deletion (removal of a base) within a gene sequence.

**Example: Sickle Cell Anemia**
Sickle cell anemia is a classic example of a disorder caused by a point mutation. In this genetic disorder, a single base substitution occurs in the beta-globin chain of hemoglobin.
*   Specifically, at the sixth codon of the beta-globin gene, the normal DNA sequence GAG (which codes for glutamic acid) is mutated to GUG (which codes for valine).
*   This single amino acid substitution (glutamic acid to valine) alters the structure and function of the hemoglobin molecule (HbS), leading to the characteristic 'sickle' shape of red blood cells under low oxygen tension. These sickle-shaped cells are rigid, can block blood vessels, and have a reduced oxygen-carrying capacity, causing various symptoms of the disease.

Question 16

Q16. Who had proposed the chromosomal theory of inheritance?

Accepted Answer

The chromosomal theory of inheritance was proposed by **Walter Sutton** and **Theodor Boveri** independently in 1902.

Question 17

Q17. Briefly describe any two disorders caused by point mutation and chromosomal aberrations, respectively.

Accepted Answer

Genetic disorders can arise from various types of mutations. Here, we describe one disorder caused by a point mutation and another by a chromosomal aberration.

### 1. Disorder Caused by Point Mutation: Sickle Cell Anemia

**Type of Mutation:** Point mutation (specifically, a single base substitution).

**Description:** Sickle cell anemia is an autosomal recessive genetic disorder that affects hemoglobin, the oxygen-carrying protein in red blood cells. It is caused by a single base pair change in the gene encoding the beta-globin chain of hemoglobin.

**Genetic Basis:**
*   The mutation occurs at the sixth codon of the beta-globin gene on chromosome 11.
*   The normal DNA triplet GAG (which codes for the amino acid glutamic acid) is replaced by GUG (which codes for valine).
*   This single amino acid substitution (Glu → Val) results in an abnormal hemoglobin molecule called Hemoglobin S (HbS).

**Phenotypic Effects:**
*   Under low oxygen tension, HbS polymerizes, causing red blood cells to deform into a characteristic 'sickle' shape instead of their normal biconcave disc shape.
*   These sickle cells are rigid, fragile, and can block small blood vessels, leading to tissue damage, pain crises, and organ failure.
*   They also have a shorter lifespan than normal red blood cells, resulting in chronic anemia.

### 2. Disorder Caused by Chromosomal Aberration: Down's Syndrome

**Type of Mutation:** Chromosomal aberration (specifically, aneuploidy – trisomy).

**Description:** Down's syndrome is a genetic disorder caused by the presence of an extra copy of chromosome 21. It is the most common autosomal chromosomal abnormality in humans.

**Genetic Basis:**
*   The primary cause is **Trisomy 21**, meaning an individual has three copies of chromosome 21 instead of the usual two.
*   This usually results from non-disjunction during meiosis (failure of homologous chromosomes to separate during meiosis I or sister chromatids to separate during meiosis II) in either the egg or sperm cell, leading to a gamete with an extra chromosome 21.
*   Less commonly, it can be caused by translocation (a part of chromosome 21 attaches to another chromosome) or mosaicism (some cells have trisomy 21, others are normal).

**Phenotypic Effects:**
*   Individuals with Down's syndrome exhibit a characteristic set of physical features, including a short stature, small round head, furrowed tongue, partially open mouth, broad palm with a characteristic palm crease (simian crease), and flattened facial profile.
*   They typically have moderate to severe intellectual disability.
*   They are also prone to congenital heart disease, gastrointestinal abnormalities, and increased susceptibility to infections and certain cancers.

Question 18

Q18. Briefly describe any two disorders caused by point mutation.

Accepted Answer

A point mutation is a gene mutation that involves a change in a single base pair in the DNA sequence. This change can be a substitution, insertion, or deletion of a single nucleotide. Such a small alteration can have significant consequences, leading to various genetic disorders. Here are two disorders caused by point mutation:

### 1. Sickle Cell Anemia

**Cause:**
Sickle cell anemia is an autosomal recessive genetic disorder. It is caused by a specific point mutation in the gene encoding the beta-globin chain of hemoglobin (HBB gene located on chromosome 11). The mutation involves a single base substitution at the sixth codon of the beta-globin gene. Normally, the sixth codon is GAG, which codes for the amino acid Glutamic acid. In individuals with sickle cell anemia, this GAG codon is replaced by GUG, which codes for Valine. This single base change (adenine replaced by thymine in the DNA sequence) leads to the synthesis of an abnormal hemoglobin molecule called Hemoglobin S (HbS) instead of the normal Hemoglobin A (HbA).

**Effects/Symptoms:**
Under low oxygen tension (e.g., during strenuous exercise, high altitude), the abnormal HbS molecules polymerize and aggregate, forming long, rigid fibers within the red blood cells (RBCs). This causes the RBCs to lose their normal biconcave disc shape and become sickle-shaped (crescent moon shape). These sickle cells are rigid, fragile, and have a reduced lifespan (10-20 days compared to 120 days for normal RBCs), leading to chronic anemia. The sickle cells also tend to block small blood vessels, impairing blood flow to various organs and tissues. This can result in:
*   **Anemia:** Due to rapid destruction of sickle cells.
*   **Pain crises:** Severe pain in bones, chest, and abdomen due to blocked blood vessels.
*   **Organ damage:** Damage to spleen, kidneys, lungs, and brain over time.
*   **Increased susceptibility to infections:** Especially bacterial infections.
*   **Fatigue, shortness of breath, delayed growth.**

### 2. Phenylketonuria (PKU)

**Cause:**
Phenylketonuria (PKU) is an autosomal recessive metabolic disorder. It is caused by a point mutation in the gene (PAH gene located on chromosome 12) that codes for the enzyme phenylalanine hydroxylase (PAH). This enzyme is crucial for the metabolism of the amino acid phenylalanine, converting it into tyrosine. The mutation leads to a deficiency or complete absence of functional phenylalanine hydroxylase.

**Effects/Symptoms:**
Due to the deficiency of phenylalanine hydroxylase, phenylalanine cannot be converted to tyrosine and thus accumulates in the blood and other tissues. Excess phenylalanine is then converted into phenylpyruvic acid and other derivatives (e.g., phenylacetate, phenyllactate). These accumulated substances are toxic to the central nervous system, especially the developing brain. The major symptoms include:
*   **Mental retardation:** Severe intellectual disability if left untreated.
*   **Seizures and behavioral problems.**
*   **Microcephaly (small head size).**
*   **Eczema and skin rashes.**
*   **Reduced hair and skin pigmentation:** Due to insufficient tyrosine, which is a precursor for melanin synthesis.
*   **Musty odor:** In urine and sweat due to the excretion of phenylketones.

Early diagnosis through newborn screening and strict adherence to a phenylalanine-restricted diet from infancy can effectively prevent the severe neurological damage associated with PKU.

Chapter 5: Principles of Inheritance and Variation