Sex of an individual is determined by different factors in various species. Some animals rely entirely on the environmental cues, while in some other animals the individuals can change their sex during their life time indicating that sex of some species is not genetically determined. However, in human beings, the sex of an individual is largely determined genetically.

(a) In what way are the sex chromosomes 'X' and 'Y' different in size? Name the mismatched pair of sex chromosome in humans.

(b) Write the number of pair/pairs of sex chromosomes present in human beings. In which one of the parent (male / female) perfect pair / pairs of sex chromosomes are present?

(c) Citing two examples, justify the statement "Sex of an individual is not always determined genetically".

OR

Draw a flow chart to show that sex is determined genetically in human beings.

Part (a)

• Difference in size: The '$X$' chromosome is normal-sized, whereas the '$Y$' chromosome is short.
• Mismatched pair: In human males, there is a mismatched pair of sex chromosomes consisting of one normal-sized '$X$' and one short '$Y$' chromosome (represented as $XY$).

Part (b)

• Number of pairs: There is $1$ pair of sex chromosomes in human beings (out of $23$ total pairs).
• Parent with perfect pair: The female (mother) has a perfect pair of sex chromosomes, both being '$X$' chromosomes ($XX$).

Part (c)

Justification that sex is not always genetically determined:

1. Environmental temperature in reptiles: In some reptiles, the temperature at which fertilized eggs are kept determines whether the developing offspring will be male or female.
2. Sex change in snails: In animals like snails, individuals can change their sex during their lifetime, showing that their sex is not fixed genetically.

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OR

Flow Chart of Genetic Sex Determination in Human Beings:

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Parents: Mother (XX) $\times$ Father (XY)

Gametes (Ova/Sperm): All (X) (X) and (Y)

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Fusion of Gametes: (X) + (X) (X) + (Y)

Zygote/Offspring: XX XY

Sex of Child: Girl Boy

(50%) (50%)

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