With the help of a flow chart explain in brief how the sex of a newborn is genetically determined in human beings. Which of the two parents, the mother or the father, is responsible for determination of sex of a child?

In human beings, the sex of a newborn is genetically determined by the sex chromosomes.

Flow chart:

$$\begin{array}{c} \text{Mother (XX)} \\ \hline \text{Each ovum carries: } \mathbf{X} \\ \end{array} \quad \times \quad \begin{array}{c} \text{Father (XY)} \\ \hline 50\% \text{ sperms carry } \mathbf{X} \\ 50\% \text{ sperms carry } \mathbf{Y} \\ \end{array}$$

$$\Downarrow$$

$$\begin{array}{ll} \text{If sperm carries } X: & X + X \;\Rightarrow\; \text{XX} \;\Rightarrow\; \textbf{Girl (female)} \\ [2mm] \text{If sperm carries } Y: & X + Y \;\Rightarrow\; \text{XY} \;\Rightarrow\; \textbf{Boy (male)} \\ \end{array}$$

$$\text{Probability: } 50\% \text{ boys} \quad : \quad 50\% \text{ girls}$$

Which parent is responsible?

The father is responsible for the sex determination of the child. This is because the mother has only X chromosomes (XX) and therefore can give only an X to the child, whereas the father has both X and Y (XY) and can pass either an X (resulting in a girl) or a Y (resulting in a boy). The sex of the baby therefore depends entirely on whether the sperm that fertilises the ovum carries an X or a Y chromosome.