As you have already studied, we humans have 46 chromosomes in the form of 23 homologous pair (Fig 17.29). When we compare karyotype of a human male with female, we notice that 22 pairs are common in both. These chromosomes are autosomes. But the 23rd pair is very different in males and female: A woman has two similar chromosomes in her 23rd pair. These are called X – chromosomes. She is XX.
The chromosomes of 23rd pair of a man are morphologically different from each other. One is X – chromosomes while the other much shorter is Y – chromosomes. He is XY.
X and Y chromosomes are called sex chromosomes because they have genes for sex determination. ‘SRY’ is the male determining gene. It is located at the tip of short arm of Y – chromosomes. Its name SRY stands for “sex determining regions of Y” (Fig 17.30).
“Autosomes do not carry any sex – determining gene. All chromosomes are called autosomes. The gene on autosomes determines autosomal traits. “
When chromosomes segregate at meiosis, each homologue of the 23rd pair also moves in one gamete. A woman being XX produces eggs, each with in X – chromosomes. As all her eggs are alike she is homogametic. A man is XY. He produces two types of sperms, half carrying X chromosome, the other half carrying Y. He is heterogametic. Chances for both types of sperm are equal.
Sex of the baby is determined by the type of sperm. If an X – carrying sperm fertilizes the egg, the zygote will be XX, and a baby girl is born. If a Y – carrying sperm fertilizes the egg, the zygote will be XY; SRY gene expresses itself and a baby boy will be born (Fig 17.31).
Sex ratio indicates chances of the sex of the baby. Are chances for a son or a daughter not equal?
“A son receives his only X – chromosome from his mother and Y from his father. A daughter receives one X from her father and the other X from mother.”
Who is more responsible for determination of sex of a newborn baby, father or mother?