Question

a. In humans, males are heterogametic and females are homogametic.
Explain. Are there any examples where males are homogametic and females heterogametic?

b. Also describe as to, who determines the sex of an unborn child?
Mention whether temperature has a role in sex determination.

Answer 1

In humans, males are heterogametic and females are homogametic

XX-YY type of sex determination:

• Male heterogamety is the phenomenon in which males produce two different types of gametes. Thus, sperm determines the sex of offspring.
• Human beings have male heterogamety. There are 23 pairs of chromosomes in humans out of which 22 pairs are autosomes and one pair of sex chromosomes or allosomes.
• Females are homogametic. They have XX chromosomes as their sex chromosome (44+XX). They produce ova which contain only X chromosome (22+X).
• Males are heterogametic. They have XY chromosomes as their sex chromosome (44+YY). They produce two different types of sperms, 22+X and 22+Y.

Example with heterogametic females and homogametic males

Unlime humans, many birds follow female heterogamety, also known as ZW-ZZ type of sex determination. Example include hen.

• Female birds have ZW as their sex chromosomes while males have ZZ chromosome as their sex chromosomes.
• Here, the male is homogametic and produce sperms with same sex chromosome (22+Z).
• The female ZW is heterogametic, as they produce two different eggs, one with Z(22+Z) and other one with W chromosomes (22+W).

Sex determination of the unborn child

Sex determination of the unborn child:

• Sex of the child is determined by the father.
• Males have XY chromosomes whereas females have XX chromosomes as their sex chromosomes.
• A sperm can have either X or Y chromosome, but all eggs have an X chromosome.
• When a sperm carrying X chromosome (22+X) fertilises an ovum (22+X), the zygote will result in the birth of a girl child (44+XX).
• When a sperm carrying the Y chromosome (22+Y) fertilises an ovum (22+X), the zygote develops into a male offspring (44+XY).

Hence, the sex is determined by the sperm that fuses with egg at the time of fertilisation.


Role of temperature in sex determination

Temperture has a role in sex determination:

• In turtles, when eggs are incubated at temperature below ${28}^{\circ }$C, male offspring are formed predominantly while incubation at temperature above ${31}^{\circ }$C leads to formation of female offspring. When the temperature is between ${28}^{\circ }$C and ${31}^{\circ }$C, equal numbers of both sexes are fomed.
• In American alligators, exposure to less than ${30}^{\circ }$C produces females while greater than ${34}^{\circ }$C produces males.