NCERT Class 12 Biology: Chapter –5 Principle of Inheritance and Variation Part 6

Get top class preparation for IEO right from your home: Get full length tests using official NTA interface: all topics with exact weightage, real exam experience, detailed analytics, comparison and rankings, & questions with full solutions.

Download PDF of This Page (Size: 105K)

Question 7:

In peas, tallness is dominant over dwarfness, and red colour of flowers is dominant over the white colour. When a tall plant bearing red flowers was pollinated with a dwarf plant bearing white flowers, the different phenotypic groups were obtained in the progeny in numbers mentioned against them:

Tall, Red = 138

Tall, White = 132

Dwarf, Red = 136

Dwarf, White = 128

Mention the genotypes of the two parents and of the four offspring types.

Answer:

Genotypes of parents: TtRr and ttrr

Offspring: Tall, Red: TtRr

Offspring: Tall, White: Ttrr

Offspring: Dwarf, red: ttRr

Offspring: Dwarf, White: ttrr

Question 8:

Why the frequency of red-green colour blindness is is many times higher in males than that in the females?

Answer:

Colour blindness is common hereditary condition which means it is passed down from parents. Red/green colour blindness is passed from mother to son on the 23rd chromosome, which is known as the sex chromosome. The faulty gene for colour blindness is found only on the X chromosome. When the father is colour blind, he can only pass an X chromosome to his daughter. Thus she acquires the gene and become the carrier of the disease. While a mother who can pass the X chromosome to both son and daughter. For a son to be colour blind, it is enough for his mother to be a carrier. But for a daughter to be colour blind her father should be colour blind and mother who is a carrier of colour blind gene. For a daughter to be a carrier it is enough to get the faulty gene from carrier mother or colour blind father.

Thus males have higher frequency to be colour blind than females.

Question 9:

If a father and son are both defective in red-green colour vision, is it likely that the son inherited the trait from his father? Comment.

Answer:

Colour blindness is common hereditary condition which means it is passed down from parents. Red/green colour blindness is passed from mother to son on the 23rd chromosome, which is known as the sex chromosome. The faulty gene for colour blindness is found only on the X chromosome.

When the father is colouring blind, he can only pass an X chromosome to his daughter. Hence a colour blind boy can’t receive a colour blind gene from his father, even if his father is colour blind. A father can only pass Y gene to his son.

Question 10:

Discuss why Drosophila has been used extensively for genetical studies.

Answer:

Some reasons why Drosophila was found suitable for studies are:

They could be grown on simple synthetic medium in the laboratory

They complete their life cycle in about 2 weeks

A single mating could produce a large number of progeny flies

Clear differentiation of sexes- the male and females are easily distinguishable

Many type of hereditary variations that can be seen under low power of microscope.

The above mentioned reasons help in easy growth of flies in easily available medium, short life span helps to carry out the research in short time period and fastly, availability of more progenies per mating helps to reduce the effort in developing new progeny flies each time, males are small and differently coloured than females which help the scientist to distinguish them more easily.

Question 11:

How do genes and chromosomes share similarity from the point of view of genetical studies?

Answer:

From the studies of behaviour of chromosomes during mitosis and meiosis it was concluded that the chromosomes and genes occur in pairs and they segregate at the gamete formation and only one of each pair is transmitted to a gamete. The two alleles of a gene-pair are located on homologous sites of homologous chromosomes. The genes and chromosomes segregate independently of another pair and each other respectively.

Question 12:

What is recombination? Discuss the applications of recombination from the point of view of genetic engineering.

Answer:

The term recombination was to describe the generation of non-parental gene combinations. Morgan found that even when the genes are on the same chromosomes, some genes were tightly linked (showed low recombination) while others were loosely linked (showed high recombination). For example he found that the genes white and yellow were tightly linked and showed only 1.3% recombination while white and miniature wing showed 37.2% recombination.

Alfred Sturtevant used the frequency of recombination between gene pairs on the same chromosome as a measure of distance between the genes and mapped their positions on the chromosome (that is the arrangement of genes on the chromosome based on the frequency of recombination).

Today genetic maps are extensively used as a starting point in sequencing of whole genomes as done in Human Genome Sequencing Project.

Developed by: