Question

What makes the fertilized egg develop either into a boy or a girl?

Answer 1

Fertilization is a process where the male and the female gametes fuse together to form a diploid cell of $46$ chromosomes. The diploid cell is called zygote which undergoes further mitotic cell division to develop into an embryo. In humans, the gender is determined by the ${23^{rd}}$ pair of chromosomes called the allosome. Two types of sex chromosomes are XX and XY.

Complete answer:
Females carries the XX pair of sex chromosomes and males, the XY pair. The gender of the baby is determined by the sperm cell that fertilizes the woman’s egg. The haploid sperm cell carries one sex chromosome, either Y or X. Y gives rise to male and X gives rise to females. The egg cell carries only the X chromosome.
If an egg is fertilized by the X bearing sperm, the resulting embryo will have two X chromosomes i.e., XX and will grow into a girl. If an egg is fertilized by the Y bearing sperm, the embryo will have XY chromosomes which will give rise to a baby boy. Hence gender determination wholly depends on the sperm cell.
The sperm cell also carries half of an embryo’s DNA. The egg carries the other half. When these two cells combine, following sexual intercourse or artificial fertilization in a lab, the resulting embryo will have a full $23$ chromosome pair. The other $22$ pairs of chromosomes determine the phenotypic characteristics of the baby.

Note:
There is a syndrome called testicular feminization when a genetically assigned male (XY) due to various abnormalities in the X chromosome, is resistant to the actions of the androgen hormones which in turn affects the formation of the male genitalia and instead gives female phenotype. This condition is also called androgen insensitivity syndrome. It occurs in $1$ out of $20,000$ births and can be present as either incomplete syndrome or complete syndrome.