Ch 5. Principles of Inheritance and Variations (Stack 3)

Question 1

Explain sex determination in humans

Answer 1

It has already been mentioned that the sex-determining mechanism in the case of humans is XY type. Out of 23 pairs of chromosomes present, 22 pairs are exactly the same in both males and females; these are the autosomes.

A pair of X-chromosomes are present in the female, whereas the presence of an X and Y chromosome are determinants of the male characteristic.

During spermatogenesis among males, two types of gametes are produced . 50 percent of the total sperm produced carry the X-chromosome and the rest 50 percent has Y-chromosome besides the autosomes. Females, however, produce only one type of ovum with an X-chromosome. There is an equal probability of fertilization of the ovum with the sperm carrying either X or Y chromosome. In case the ovum fertilizes with a sperm carrying X-chromosome the zygote develops into a female (XX) and the fertilization of ovum with Y-chromosome carrying sperm results in a male offspring

Question 2

Explain sex determination in Drosophila melanogaster.

Answer 2

Mechanism of sex detem1ination in Drosophila melanogaster is XY type where both male and female have a same number of chromosomes. Among the males an X-chromosome is present but its counterpart is distinctly smaller and called the Y-chromosome.

Females, however, have a pair of X-chromosomes. Both males and females bear a same number of autosomes. Hence, the males have autosomes plus XY, while females have autosomes plus XX. In Drosophila the males have one X and one Y chromosome, whereas females have a pair of X-chromosomes besides autosomes.

Question 3

Explain sex determination in birds

Answer 3

In this case of Birds, the total number of chromosomes is the same in both males and females. But two different types of gametes in terms of the sex chromosomes, are produced by females, i.e., female heterogamety.

ln order to have a distinction with the mechanism of sex detemination described earlier, the two different sex chromosomes of a female bird have been designated to be the Z and W chromosomes. In these organisms, the females have one Z and one W chromosome, whereas males have a pair of Z-chromosomes besides the autosomes.

Question 4

Explain sex determination mechanism in grasshopper

Answer 4

Grasshopper is an example of XO type of sex determination. Where all eggs bear an additional X-chromosome besides the other chromosomes (autosomes). On the other hand, some of the sperms bear the X-chromosome whereas some do not.

Eggs fertilised by sperm having an X-Chromosome become females and, those fertilised by sperm that do not have an X-chromosome become males. In Grasshopper the males have only one X-chromosome besides the autosomes, whereas females have a pair of X-chromosomes

Question 5

Explain sex determination mechanism in honey bees

Answer 5

The sex determination in honey bees is based on the number of sets of chromosomes an individual receives. An offspring formed from the union of a sperm and an egg develops as a female (queen or worker), and an unfertilised egg develops as a male (drone) by means of parthenogenesis.

This means that the males have half the number of chromosomes than that of a female. The females are diploid having 32chromosomes and males are haploid, i.e. , having 16 chromosomes. This is called a haplodiploid sex-determination system and has special characteristic features such as the males produce sperms by mitosis

Question 6

Define muation.

Answer 6

Any alteration in the DNA sequence which can be inherited and may be expressed as a change in the phenotype of an organism is called a mutation.

Question 7

What is deletion mutation

Answer 7

A gene mutation which involves deletion of one or more base pairs of DNA has deleted.

Question 8

What is insertion mutation?

Answer 8

A gene mutation involves the addition of one or more base pairs in DNA.

Question 9

What is point mutation?

Answer 9

A gene mutation which involves change in a single base pair of a DNA is called point mutation

Question 10

Mention an example for point mutation

Answer 10

Sickle cell anaemia

Question 11

What is frame shift mutation?

Answer 11

Insertions and deletion of a base pair or base pairs in a DNA are referred as frame shift mutations.

Question 12

What are mutagens?

Answer 12

Mutation causing agents called as mutagens

Question 13

What is pedigree analysis?

Answer 13

Analysis of traits in a several of generations of a family is called the pedigree analysis

Question 14

Write the representative pedigree analysis (pedigree chart) of myotonic dystrophy as an example for autosomal dominant trait.

Answer 14

Question 15

Write the representative pedigree analysis (pedigree chart) of sickle cell anemia as an example for the autosomal recessive trait

Answer 15

Question 16

What is thalassemia?

Answer 16

Thalassemia is an autosomal recessive blood disorder in which the body makes an abnormal form of hemoglobin due to a mutation in the genes coding for its alpha or beta chain.

Question 17

What are Mendelian disorders

Answer 17

The disorders which arise by alteration or mutation in a single gene are called Mendelian Disorders.

eg : Haemophilia, Cystic fibrosis.

Question 18

Mention an example for a mendelian disorder

Answer 18

Haemophilia.

Question 19

Give an example of sex-linked recessive disease in humans

Answer 19

Haemophilia

Question 20

Name an autosomal recessive Mende lia n disorder in humans

Answer 20

Sickle-cell anaemia.

Question 21

Mention an example of an inborn error of metabolism caused due to a recessive allele on the autosome

Answer 21

Phenylketonuria

Question 22

Name the enzyme that is not produced due to gene mutation which causes phenylketonuria

Answer 22

phenylalanine hydroxylase.

Question 23

Phenyl pyruvic acid and its derivatives are excreted through urine in phenylketonuria. Why?

Answer 23

Phenylketonuria is due to a deficiency of phenylalanine hydroxylase enzyme, which converts phenylalanine into tyrosine.

As a result, phenylalanine accumulates in the circulation which is converted to phenylpyruvate and is excreted in the urine because of its poor absorption by the kidney.

Question 24

Why persons with phenylketonuria normally suffer from mental retardation

Answer 24

Persons with PKU suffer from mental retardation as accumulation of phenylalanine and its derivatives in the blood causes brain damage, seizures, etc.

Question 25

What is haemophilia?

Answer 25

Haemophilia is sex linked recessive disease in which a single protein that is a part of the cascade of proteins involved in the clotting of blood is affected.

Question 26

Why the possibility of a female becoming a haemophilic is extremely rare?

Answer 26

The possibility of a female becoming a haemophilic is extremely rare because the mother of such a female has to be at least carrier and the father should be haemophilic (unviable in the later stage of life).

Question 27

In a haemophilic patient, why even a simple cut results in non-stop bleeding?

Answer 27

In an affected individual a single protein that is a part of the cascade of proteins involved in the clotting of blood is affected. Due to this, a simple cut will result in non-stop bleeding.

Question 28

What is colour blindness

Answer 28

It is a sex-linked recessive disorder due to defect in either red or green cone of the eye resulting in failure to discriminate between red and green colour.

Question 29

What is sickle cell anaemia?

Answer 29

Sickle cell anaemia is an autosome-linked recessive where a mutant haemoglobin molecule undergoes polymerisation under low oxygen tension causing the change in the shape of the RBC from biconcave disc to elongated sickle like structure

Question 30

RBCs become sickle-shaped in persons suffering from sickle cell anaemia. Why?

Answer 30

RBCs become sickle-shaped in persons suffering from sickle cell anemia, because mutant haemoglobin molecule undergoes polymerization under low oxygen tension causing the change in the shape of the RBC.

Question 31

Name the amino acid which substitutes glutamic acid in the beta globin chain of haemoglobin in sickle cell anaemia

Answer 31

Valine

Question 32

Name the amino acid in the beta-globin chain of haemoglobin that is substituted by valine in sickle cell anaemia

Answer 32

Glutamic acid

Question 33

Mention the sixth codon of beta globin chain in normal haemoglobin

Answer 33

GAG

Question 34

. Write the sixth codon of beta-globin chain in haemoglobin of sickle celled anaemic patients

Answer 34

GUG

Question 35

Which genetic disease is characterized by the reduced synthesis of haemoglobin?

Answer 35

Thalassemia