Question

The chromosome number is fixed for all normal organisms leading to species specification whereas any abnormality in the chromosome number of an organism results into abnormal individuals. For example, in humans, 46 is the fixed number of chromosomes both male and female. In males it is '44 + XY' and in females, it is '44 +XX'. Thus the human male is heterogametic, in other words produces two different types of gametes one with '22 + X' chromosomes and the other with '22 + Y' chromosomes respectively. Human female, on the other hand, is homo gametic i.e. produce only one type of gamete with '22 + X' chromosomes only. Sometimes an error may occur during the meiosis of the cell cycle, where the sister chromatids fail to segregate called nondisjunction, leading to the production of abnormal gametes with altered chromosome numbers. On fertilisation, such gametes develop into abnormal individuals.

(a) State what is aneuploidy. (1)

(b) If during spermatogenesis, the chromatids of sex chromosomes fail to segregate during meiosis, write only the different types of gametes with altered chromosome numbers that could possibly be produced. (1)

(c) A normal human sperm (22 + Y) fertilises an ovum with karyotype '22 +XX'. Name the disorder the offspring thus produced would suffer from and write any two symptoms of the disorder. (2)

OR

(c) Name the best-known and most common autosomal aneuploid abnormality in humans and write any two symptoms. (2)

Answer 1

(a) Aneuploidy is defined as the condition of the abnormal number of chromosomes in a cell. It may occur due to an extra or less number of chromosomes.

(b) 22 + 0, 22 + XY

(c) Klinefelter syndrome: Klinefelter syndrome (sometimes called Klinefelter's, KS or XXY) is where males are born with an extra X chromosome.

Symptoms:

1. Males with feminine characteristics, decreased facial and body hair.

2. In babies and toddlers - weak muscles and very flexible (hypermobile) joints, learning to sit up, crawl, walk and talk later than usual, being quieter and more passive than usual, having undescended testicles, only one testicle, or smaller penis. 

OR

(c) Aneuploidy is a genetic disorder that develops when a cell has an unusually high number of chromosomes. Trisomy of chromosome 21 where three copies of chromosome 21 are present in the patient is the most well-known aneuploidy in the autosomal chromosome. Down's syndrome is a trisomy of the 21-chromosome disorder. Short size, a protruding tongue, a flattened face, and upward-slanting eyelids are the four symptoms of Down's syndrome.