Write a short note on Sex Determination from Heredity and evolution
NCERT page no. 146
Answers
Answer:
A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most organisms that create their offspring using sexual reproduction have two sexes. Occasionally, there are hermaphrodites in place of one or both sexes. There are also some species that are only one sex due to parthenogenesis, the act of a female reproducing without fertilization.
Explanation:
(1)Normal human cells have 46 chromosomes or 23 pairs of chromosomes in the the nucleus. Out of 46 chromosomes, particularly one pair is of sex chromosomes. Sex chromosomes are named X and Y. In females, pair of sex chromosomes have both X chromosomes i.e XX pair, whereas, in males pair of sex chromosomes have X and Y chromosomes i.e. XY pair. In males as well as females, the gametes i.e. sperm or ova contains 23 chromosomes i.e half of total 46 chromosomes. More specifically male or female gamete contains single chromosome. Ova in females contains only X chromosomes, whereas, in males half sperms contain X chromosomes, and other half contains Y chromosomes.
(2)At the time of fertilisation of male and female gamete, if sperm containing X chromosome fuse with ova containing X chromosomes, zygote formed after fertilisation contains XX pair of chromosome. Thus, zygote develop into female individual. However, during fertilisation if sperm containing Y chromosomes fuses with ova containing X chromosomes then zygote formed after fertilisation will contain XY pair of chromosomes. Hence, zygote will develop into a male individual
(3)Sex determination of child i.e. whether child is male or female completely depends on male parent rather than the female parent.