NCERT Class 12 Biology: Chapter –5 Principle of Inheritance and Variation Part 9

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Question 4:

Discuss in detail the contributions of Morgan and Sturvant in the area of genetics.

Answer:

The experimental verification of the chromosomal theory of inheritance was done by Thomas Hunt Morgan and his colleagues. Morgan carried out several dihybrid crosses in Drosophila to study genes that were sex-linked. For example Morgan hybridised yellow bodied, white eyed females to brown bodied, red eyed males and intercrossed their F1 progeny. He observed that the two genes did not segregate independently of each other and the F2 ratio deviated very significantly from 9:3:3:1 ratio.

Morgan and his group knew that the genes where located on the sex chromosome and saw that when the two genes in a dihybrid cross were situated in the same chromosome, the proportion of parental gene combinations were much higher than that of the non-parental type. Morgan concluded this due to the physical association of two genes or linkage. The term recombination was to describe the generation of non-parental gene combinations. Morgan also 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.

His student 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.

Question 5:

Define aneuploidy. How is it different from polyploidy? Describe the individuals having following chromosomal abnormalities.

a. Trisomy of 21st Chromosome

b. XXY

c. XO

Answer:

Failure of segregation of chromatids during cell division cycle results in the gain or lose of a chromosome(s), called aneuploidy.

Failure of cytokinesis after telophase stage of cell division results in an increase in a whole set of chromosomes in an organism and this phenomenon is known as polyploidy.

a. Trisomy of 21st Chromosome

The genetic disorder of the presence of an additional copy of the chromosome number 21 is called Down’s syndrome. The affected individual is shot statured with small round head, furrowed tongue and partially open mouth. Palm is broad with characteristic palm crease. Physical, psychomotor and mental development is retarded.

b. XXY

The genetic disorder of the presence of an additional copy of X chromosome resulting into a karyotype of 47, XXY is called Klinefelter’s syndrome. Such an individual has overall masculine development, however the feminine development (development of breast: Gynaecomastia) is also expressed. Such individuals are sterile.

c. XO

The disorder caused due to the absence of one of the X chromosomes, that is 45 with XO is called Turner’s syndrome. Such females are sterile as ovaries are rudimentary besides other features including lack of other secondary sexual characters.

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