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Q.Describe chromosomal theory of sex determination.

Telangana TsbieTelangana Board of Intermediate Education 2023Subjective· 8mImportance★★★★★
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Sex is determined by sex chromosomes: in humans (XX-XY) females are XX and males XY, and the sperm (X or Y) decides the sex of the child; other types are XX-XO (grasshopper) and ZZ-ZW (birds).

The chromosomal theory of sex determination explains how the sex of an organism is fixed by chromosomes. Besides the ordinary chromosomes (autosomes), there is a special pair called the sex chromosomes (allosomes) that determine sex.

Different types of chromosomal sex determination:

  1. XX-XY type (as in humans and Drosophila):
  • Females have two similar sex chromosomes, XX; they are homogametic and produce only one kind of egg, carrying an X chromosome.

  • Males have two dissimilar sex chromosomes, XY; they are heterogametic and produce two kinds of sperms - 50% carrying X and 50% carrying Y.

  • At fertilisation: if an X-bearing sperm fertilises the egg (X), the offspring is XX (female); if a Y-bearing sperm fertilises the egg, the offspring is XY (male).

  • Thus the sex of the child is determined by the father's sperm, and there is a 50:50 chance of a boy or a girl.

  1. XX-XO type (as in grasshoppers and some insects):
  • Females are XX (homogametic).

  • Males have only one X chromosome and no Y (XO); they are heterogametic and produce sperms with or without the X chromosome. The presence of XX gives females and XO gives males.

  1. ZZ-ZW type (as in birds and some reptiles): …

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