5 Most FAQ’s of Genetics Chapter in Inter 2nd Year Zoology (TS/AP)

8 Marks

LAQ-1 : What are multiple alleles? Describe multiple alleles with the help of ABO blood groups in man.

Introduction

In genetics, the concept of ‘multiple alleles’ refers to the existence of more than two possible alleles for a single gene. These multiple alleles lead to a variety of genotypes and phenotypes. The ABO blood group system in humans exemplifies multiple alleles.

Understanding Multiple Alleles

A gene typically has two forms, or ‘alleles‘: one dominant and one recessive. In the case of multiple alleles, a gene possesses more than two alleles. The number of potential genotypes with multiple alleles can be calculated with the formula n(n+1)/2, where ‘n’ represents the number of alleles. For example, the ABO blood groups in humans, determined by three alleles, result in six possible genotypes.

ABO Blood Groups and Their Antigens

The human blood types, A, B, AB, and O, are defined by the presence or absence of specific antigens on red blood cells (RBCs). These antigens are sugar polymers connected to lipid molecules.

1. Group A: Individuals have antigen ‘A’ on RBCs and anti-B antibodies in plasma.
2. Group B: Individuals have antigen ‘B’ on RBCs and anti-A antibodies in plasma.
3. Group AB: Individuals have both ‘A’ and ‘B’ antigens on RBCs, with no anti-A or anti-B antibodies in plasma.
4. Group O: Individuals lack antigens on RBCs but have both anti-A and anti-B antibodies in plasma.

Genetic Basis of ABO Blood Groups

Proposed by Karl Landsteiner, the ABO system is primarily determined by three alleles: IA, IB, and IO (or i).

1. The IA and IB alleles produce antigen A and antigen B, respectively.
2. The IO allele does not produce any antigen.
3. IA and IB are dominant over IO and codominant to each other, allowing their coexistence.
4. Six possible genotypes result from the combination of alleles inherited from parents.

Genotypes and Corresponding Phenotypes

1. IAIA and IAIO: Blood group A.
2. IBIB and IBIO: Blood group B.
3. IAIB: Blood group AB.
4. IOIO: Blood group O.

Blood Compatibility for Transfusion

Different blood groups exhibit specific compatibility profiles for blood transfusion:

1. Group A: A+ can receive from A+, A-, and O-. A- can receive from A- and O-.
2. Group B: B+ can receive from B+, B-, and O-. B- can receive from B- and O-.
3. Group AB: AB+ can receive from all groups, being the ‘universal recipient.’ AB- can receive from A-, B-, AB-, and O-.
4. Group O: O+ can receive only from O+ and O-. O- can donate to any group, making it the ‘universal donor.’

Summary

Multiple alleles, where a gene has more than two possible forms, play a crucial role in genetics. The ABO blood group system, a prime example of multiple alleles, significantly impacts human health, especially in the context of blood transfusions. Each blood group has its unique compatibility profile, underlining the importance of this genetic phenomenon.

---

LAQ-2 : Describe Chromosomal theory of Sex Determination.

Introduction

The chromosomal theory of sex determination hinges on the idea that sex chromosomes, also known as allosomes, are crucial in determining an organism’s sex. It outlines two principal methods: the male heterogametic method and the female heterogametic method.

Male Heterogametic Method

This method distinguishes between females with two identical X chromosomes and males with dissimilar chromosomes.

XX-XO Type

1. Example: Grasshoppers.
2. Female Karyotype: AAXX.
3. Male Karyotype: AAXO.
4. Sex Determination: Relies on the presence of the X chromosome in the sperm.

XX-XY Type

1. Example: Humans.
2. Female Karyotype: AAXX (homogametic).
3. Male Karyotype: AAXY (heterogametic).
4. Sex Determination: Dictated by the presence of either an X or a Y chromosome in the sperm.

Female Heterogametic Method

In this method, females produce two types of eggs, while males produce only one type of sperm.

ZO-ZZ Type

1. Example: Fumea (moths).
2. Female Karyotype: AAZO (heterogametic).
3. Male Karyotype: AAZZ (homogametic).
4. Sex Determination: Based on the type of ovum fertilized.

ZW-ZZ Type

1. Example: Birds.
2. Female Karyotype: AAZW (heterogametic).
3. Male Karyotype: AAZZ (homogametic).
4. Sex Determination: Depends on the ovum that is fertilized.

Summary

The chromosomal theory of sex determination is foundational in understanding how sex is determined in various species. It emphasizes that specific chromosome patterns, such as XX-XO, XX-XY, ZO-ZZ, or ZW-ZZ, play a pivotal role in determining whether an offspring will be male or female. This concept is vital in the biology of a wide array of species, including humans, insects, and birds.

---

LAQ-3 : What is crisscross inheritance? Explain the inheritance of one sex linked recessive character in human beings.

Introduction

Criss-cross inheritance is a genetic pattern where an X-linked recessive trait is transmitted from parents to offspring in a distinctive manner. First identified by T. H. Morgan in fruit flies, this type of inheritance is notable in conditions like colour blindness and haemophilia.

Understanding Genetics and Gender

To comprehend criss-cross inheritance, it’s essential to understand the basics of genetics and gender. In this context, the phenotypic expression of genes, whether dominant or recessive, is crucial. Males, having one X and one Y chromosome, are more prone to exhibit X-linked recessive traits compared to females, who possess two X chromosomes.

Colour Blindness

Red-green colour blindness serves as a prime example of an X-linked recessive trait. This condition arises due to a recessive gene on the X chromosome affecting cones in the retina, which are vital for color perception.

In the instance of criss-cross inheritance, consider a woman with normal vision and two dominant genes (AAXCXC) married to a colour-blind man (AAXcbY). Their daughters will be carriers of the colour blindness gene but not colour-blind, while their sons will have normal vision.

The next generation sees a continuation of this pattern. If a carrier daughter (from the first example) marries a man with normal vision, approximately half of their sons could be colour blind. This illustrates the trait ‘criss-crossing’ from the maternal grandfather to the grandson.

Other Possible Scenarios of Colour Blindness Inheritance

1. If both parents are colour blind, all offspring will inherit colour blindness.
2. A colour-blind mother and a father with normal vision will produce colour-blind sons and carrier daughters.
3. A carrier mother and a colour-blind father can have around half of their children, regardless of gender, inheriting colour blindness.

Summary

Criss-cross inheritance represents a unique pattern in genetics, predominantly seen in X-linked recessive traits like colour blindness and haemophilia. This inheritance mode highlights how males, with a single X chromosome, are more susceptible to these traits. It demonstrates the intriguing pathways our genes follow across generations, including the potential for a trait to ‘criss-cross’ from grandparents to grandchildren.

---

LAQ-4 : Write an essay on common Genetic Disorders.

Introduction

Genetic disorders, caused by abnormalities in genes or chromosomes, fall into two primary categories: Mendelian and chromosomal disorders.

Part I: Mendelian Disorders

Mendelian disorders, named after Gregor Mendel, arise from a single DNA mutation, making them monogenic diseases. These can be either autosomal or sex-linked.

1. Haemophilia: Known as a bleeder’s disease, haemophilia causes excessive bleeding due to poor blood clotting. It is more common in men as it is an X-linked disorder.
2. Colour Blindness: This sex-linked disorder affects color perception. Red and green colour blindness are X-linked, while blue colour blindness is autosomal.
3. Sickle Cell Anaemia: An autosomal recessive disorder caused by a defective gene on chromosome 11. Symptoms include physical weakness and organ damage.
4. Phenyl Ketonuria: An autosomal metabolic disorder due to a gene on chromosome 12, leading to mental retardation.
5. Thalassemia: An autosomal recessive disorder causing severe anaemia.
6. Cystic Fibrosis: This autosomal recessive disorder leads to the build-up of thick mucus in various organs and can be life-threatening.

Part II: Chromosomal Disorders

Chromosomal disorders occur due to changes in chromosome number or structure.

1. Klinefelter Syndrome: Caused by an extra X chromosome in males, leading to reduced fertility and slight breast enlargement.
2. Turner Syndrome: Results from a missing X chromosome in females, characterized by short stature and a flat chest.
3. Down Syndrome: Caused by an extra chromosome 21, leading to developmental delays.
4. Edwards Syndrome: Due to an extra chromosome 18, more common in females and often results in early death.
5. Patau Syndrome: Caused by an extra chromosome 13, leading to severe abnormalities and often early death.
6. Cri-du-chat Syndrome: Results from a deletion on chromosome 5, characterized by a cat-like cry and other abnormalities.
7. Chronic Myelogenous Leukemia (CML): A cancer type caused by a translocation between chromosomes 9 and 22.

Summary

Understanding common genetic disorders is crucial for insights into how our genes and chromosomes affect health. Despite the challenges posed by these disorders, ongoing research and medical advances are improving diagnosis, management, and treatment options. The study of genetic disorders remains a vital part of medical and health education.

---

LAQ-5 : What is DNA fingerprinting? Mention its applications.

Introduction

DNA fingerprinting, also referred to as DNA typing, is a technique for identifying individuals based on their distinctive DNA structures. Unique sequences in our DNA, known as Variable Number Tandem Repeats (VNTRs), enable this identification. DNA can be sourced from various tissues or fluids, including barred tissues and bones.

Part I: The Process of DNA Fingerprinting

The process of DNA fingerprinting encompasses several key steps:

1. Obtaining DNA: DNA is extracted from sources like blood, saliva, hair roots, or semen and can also be replicated through cloning or DNA amplification.
2. Fragmenting DNA: Using enzymes known as restriction endonucleases, DNA is cut into smaller fragments.
3. Separation of DNA Fragments by Electrophoresis: DNA fragments are placed on an agarose gel plate and separated by applying an electric current, allowing smaller pieces to travel farther.
4. Denaturing DNA: The DNA strands are separated using alkaline chemicals.
5. Blotting: In a technique called Southern blotting, a nylon membrane captures the DNA fragments from the gel.
6. Using Probes to Identify Specific DNA: Radioactive probes bind to sample DNA strands through hydrogen bonding.
7. Hybridisation with Probe: After hybridization, excess probes are washed away, and photographic film is placed over the membrane.
8. Exposure on Film to Make a DNA Fingerprint: The radioactive labels create an image on the film, displaying specific DNA bands and forming the DNA fingerprint.

Part II: Applications of DNA Fingerprinting

DNA fingerprinting has a wide array of applications:

1. Forensic Analysis: Utilized in criminal investigations for suspect identification.
2. Medico-Legal Cases: Establishes paternity or maternity with high precision.
3. Pedigree Analysis: Aids in studying the inheritance of genes across generations.
4. Taxonomy: Helps in understanding phylogeny or evolutionary relationships between organisms.
5. Conservation of Wildlife: Assists in maintaining DNA records for species conservation.
6. Anthropological Studies: Used in tracing human population origins and migration patterns.

Summary

DNA fingerprinting is a crucial tool for identifying individuals by their unique DNA sequences. Its diverse applications span from forensic science to conservation of biodiversity. This technique offers profound insights into our genetic makeup and its significance in various aspects of life.