How many pairs of chromosomes are present in human beings? Out of these how many are sex chromosomes? How many types of sex chromosomes are found in human beings?
The sex of a newborn child is a matter of chance and none of the parents may be considered responsible for it. Draw a flow chart showing the determination of sex of a newborn to justify this statement.
Twenty-three pairs of chromosomes are present in human beings. Out of these, one pair is the sex chromosomes.
Two types of sex chromosomes are found in human beings: X and Y. Males contain one X chromosome and one Y chromosome (XY), while females contain two copies of X chromosomes (XX).
The sperm has either X or Y chromosome, while the egg has only X chromosome. So, if the sperm carrying Y chromosome fuses with the egg, it results in the formation of a male child; and if the sperm carrying X chromosome fuses with the egg, it results in the formation of a female child.
Male Female
Sex Chromosomes XY × XX
↓ ↓
Gamete produced X and Y X and X
Fusion of gamete XX Female or XY Male
Thus, there is an equal chance of fusion of either X or Y chromosome with the egg. Therefore, we can say that the sex of a newborn child is a matter of chance and none of the parents is responsible for it.
Define evolution. How does it occur? Describe how fossils provide us evidences in support of evolution.
21.How do Mendel’s experiments show that the
(a) traits may be dominant or recessive
(b) traits are inherited independently
(a) Draw a sectional view of human female reproductive system and label the part where
(i) eggs develop.
(ii) fertilisation takes place.
(iii) fertilised egg gets implanted.
(b) Describe, in brief, the changes the uterus undergoes
(i) to receive the zygote.
(ii) if zygote is not formed.
What is meant by speciation? List four factors that could lead to speciation. Which of them cannot be a major factor in the speciation of a self pollinating plant species. Give reason to justify your answer.
