Genetics (SAQs)

Zoology-2 | Unit-6: Genetics – SAQs:
Welcome to SAQs in Unit-6: Genetics. This page includes the most important FAQs from previous exams. Each answer is provided in simple English, followed by a Telugu explanation, and then presented in the exam format. This method ensures you’re well-prepared to achieve top marks in your final exams.

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SAQ-1 : How is sex determined in Human Beings?

Introduction

Sex determination in humans is a process governed by chromosomal mechanisms that dictate the development of an individual into either male or female. This process primarily involves the sex chromosomes present in a fertilized egg.

1. Chromosomes and Sex Determination: Humans possess 23 pairs of chromosomes, totaling 46. Among these, 22 pairs, known as autosomes, are common to both sexes. The 23rd pair, the sex chromosomes, varies between males and females and is crucial for determining an individual’s sex.
2. Female Chromosomes: Females have identical sex chromosomes, denoted as XX. Therefore, a typical female chromosomal configuration is 44 autosomes plus XX (44XX).
3. Male Chromosomes: Males have differing sex chromosomes, represented as XY. Thus, a male’s chromosomal makeup is 44 autosomes plus XY (44XY).
4. Formation of Gametes:
   • Females produce only one type of egg, each carrying the X chromosome and 22 autosomes (22X).
   • Males produce two types of sperm: one carrying the X chromosome (22X) and the other carrying the Y chromosome (22Y).
5. Fertilization and Sex Determination: The sex of the offspring is determined during fertilization:
   • If a 22X sperm fertilizes a 22X egg, the result is a female child with 44XX chromosomes.
   • If a 22Y sperm fertilizes a 22X egg, the result is a male child with 44XY chromosomes.

Consequently, the male’s sperm is responsible for determining the sex of the child, with a 50 percent probability for each type of sperm, leading to an equal chance of conceiving a male or female child.

Summary

Sex determination in humans hinges on the 23rd pair of sex chromosomes. Females have a 44XX chromosomal pattern, while males have a 44XY pattern. During reproduction, the contribution of the male’s X or Y chromosome to the female’s egg dictates the offspring’s sex. This results in a 50 percent probability for having either a male or female child.

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SAQ-2 : Describe Erythroblastosis Foetalis.

Introduction

Erythroblastosis fetalis, also known as Haemolytic disease, is a medical condition that occurs when there’s a blood type incompatibility between an Rh-negative mother and her Rh-positive baby. The Rh factor is a key protein on red blood cells, determining whether blood is Rh positive or negative.

1. Understanding Rh Incompatibility: The issue arises from Rh incompatibility between the mother’s and the baby’s blood types. If an Rh-negative mother conceives a child with an Rh-positive father, the baby may inherit the Rh-positive trait.
2. First Pregnancy and Sensitization: During a mother’s first pregnancy with an Rh-positive baby, the baby’s Rh-positive cells might enter the mother’s bloodstream during delivery. This event can trigger the mother’s immune system to produce antibodies against these cells, a process known as sensitization. Generally, the first baby remains unaffected, as antibody production usually extends beyond the pregnancy duration.
3. Risk in Subsequent Pregnancies: In later pregnancies with an Rh-positive baby, the antibodies previously produced by the mother can cross the placenta and attack the baby’s red blood cells. This leads to Haemolytic Disease of the Newborn (HDN).
4. Effects on the Baby: As a reaction to red blood cell destruction, the baby starts producing immature red blood cells called erythroblasts. These cells are less efficient in oxygen transportation, leading to the condition’s name: Erythroblastosis fetalis.
5. Prevention: Modern medical practices prevent this condition by administering anti-D antibodies to Rh-negative mothers during pregnancy. This approach prevents sensitization of the mother’s immune system, thereby protecting the baby from developing harmful antibodies.

Summary

Erythroblastosis fetalis represents a significant risk in cases of Rh incompatibility between an Rh-negative mother and an Rh-positive baby, particularly in subsequent pregnancies. With the advent of anti-D antibody injections, it’s now possible to effectively prevent this condition, safeguarding the health of both mother and child.

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SAQ-3 : Describe the genetic basis of ABO blood grouping.

Introduction

The ABO blood grouping system, introduced by Karl Landsteiner, is predicated on the presence of antigens on the surface of red blood cells. The three antigen types in this system are A, B, and O (indicating the absence of an antigen).

1. ABO Blood Group Genes: The determination of ABO blood type hinges on a specific gene on chromosome 9. This gene has three allele variants: IA, IB, and IO (or i).
2. Understanding Alleles: The alleles IA, IB, and IO are crucial in dictating the type of antigens present on red blood cells. IA produces type A antigens, IB leads to type B antigens, and IO results in no antigen production.
3. How Alleles Interact: In this genetic structure, IA and IB alleles are dominant over IO. Consequently, inheriting an IO allele from one parent and either IA or IB from the other results in blood types A or B, respectively. IA and IB are co-dominant when inherited together, producing blood type AB with both A and B antigens.
4. Possible Genotypes and Phenotypes: The combination of alleles inherited from parents leads to six possible genotypes and four blood types:
   • IAIA or IAIO: Results in Group A blood.
   • IBIB or IBIO: Leads to Group B blood.
   • IAIB: Creates Group AB blood.
   • IOIO: Results in Group O blood.

Summary

The ABO blood grouping system’s foundation lies in the specific antigens on red blood cells, determined by three alleles of a single gene. An individual’s blood type, or phenotype, is defined by the genotype – the combination of alleles inherited from the parents. Understanding these genetic bases is critical for ensuring safe blood transfusions and medical compatibility.

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SAQ-4 : Describe the Genic Balance Theory of sex determination.

Introduction

The Genic Balance Theory, proposed by Calvin B. Bridges, describes sex determination as a function of the ratio between X chromosomes and autosomal sets in an organism. This theory was primarily studied in fruit flies (Drosophila).

‘Y’ Chromosome Not Essential

Bridges’ findings revealed that in Drosophila, the ‘Y’ chromosome is not crucial for male sex determination. Instead, the sex is determined by the ratio of X chromosomes to autosomal sets.

Understanding the Ratio

The X/A ratio, where ‘X’ represents the number of X chromosomes and ‘A’ the number of autosomal sets, is key in this theory. For instance, an organism with a 2X/2A composition has an X/A ratio of 1.0.

Observations and Outcomes

Bridges’ experiments with various karyotypes in Drosophila led to diverse X/A ratios and corresponding sexual phenotypes:

1. AAXX (2X/2A): Ratio of 1.0, resulting in a female.
2. AAXY: Ratio of 0.5, resulting in a male.
3. AAXO: Ratio of 0.5, also producing a male.
4. AAXXY: Ratio of 1.0, leading to a female.
5. AAXXX: Ratio of 1.5, leading to a metafemale (overly female).
6. AAXXXY: Ratio of 1.5, resulting in a metafemale.
7. AAAXX: Ratio of 0.67, producing an intersex (characteristics of both sexes).
8. AAAXO: Ratio of 0.33, leading to a metamale (overly male).
9. AAAXXXX: Ratio of 1.33, resulting in a metafemale.

Summary

The Genic Balance Theory emphasizes the pivotal role of the X/A ratio in determining the sexual phenotype of an organism. Contrary to traditional views, the presence of the Y chromosome is not the sole determinant of maleness in this system, but rather the balance between X chromosomes and autosomal sets.