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NCERT Exemplar · Q13

Q.In a certain taxon of insects some have 17 chromosomes and the others have 18 chromosomes. The 17 and 18 chromosome-bearing organisms are:

(a) males and females, respectively
(b) females and males, respectively
(c) all males
(d) all females
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In insects with XO type sex determination, males have one less chromosome than females, meaning 17-chromosome insects are males and 18-chromosome insects are females.

Understanding how sex is determined in different organisms is fundamental to genetics. While humans and many other mammals follow an XY system, many insects, including grasshoppers and some bugs, exhibit an XO type of sex determination. This system is distinct because it lacks a Y chromosome entirely in males.

In the XO system:

  • Females possess two X chromosomes (XX) along with their set of autosomes.
  • Males possess only one X chromosome (XO) along with their set of autosomes. They do not have a second sex chromosome (Y).

This difference in the number of sex chromosomes directly impacts the total chromosome count for each sex. If we consider the autosomes (non-sex chromosomes) to be present in an equal number of pairs in both sexes, then the total chromosome number will differ by one.

Note

Autosomes are all chromosomes other than the sex chromosomes. They are typically present in homologous pairs in both males and females.

Let's say an insect species has 'N' pairs of autosomes.

  • A female (XX) would have $2N$ autosomes plus two X chromosomes, making a total of $2N + 2$ chromosomes.
  • A male (XO) would have $2N$ autosomes plus one X chromosome, making a total of $2N + 1$ chromosomes.

From this, it is clear that males in an XO system will always have one fewer chromosome than females. …

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