Question

Sex determination in animals such as humans and insects is done using the XX-XO and XX-XY mechanisms. In some organisms ZZ-ZW mechanism of sex determination is seen.
Based on the given statement, answer the question given below.

XX-XO type of sex determination and XX-XY type of sex determination are examples of
[1 mark]

• A. male heterogamety
• B. female heterogamety
• C. male homogamety
• D. haplo-diploid system

Answer 1

The correct option is A male heterogamety

In XX-XO type and XX-XY type sex-determining mechanisms, males can produce two different types of gametes.
(i) XX-XO type - for example in grasshoppers, males have 22A+XO and females have 22A+XX chromosomes. 'A' represents autosomes which are common in all somatic cells. XX and XO are allosomes that determine the sex.
Gametes produced by female’s having AA+XX chromosomes will be of one type: (A+X), i.e, homogametic
Gametes from males having AA+XO chromosomes will be of two types: (A+X) and (A+O), i.e, heterogametic.
(ii) XX-XY type - for example in humans, males have 44A+XY) and females have 44A+XX) chromosomes in their diploid cells.
Gametes from males (44+XY) will be of two types: (22+X) and (22+Y), i.e, heterogametic.
Gametes from females (44+XX) will be of one type: (22+X), i.e, homogametic.

Thus, male gametes are heterogametic as they are
(i) either with or without X - chromosome as seen in the XX-XO type sex determination in grasshoppers
(ii) with X- chromosome or with Y - chromosome as seen in the XX-XY type sex determination in humans. This type of sex determination is an example of male heterogamety.