Read the given paragraph and answer the question:
Humans have 23 pairs of chromosomes. 22 pairs are exactly same in both males and females; these are the autosomes. A pair of X-chromosome are present in the female, whereas the presence of an X and Y chromosome are determinant of the male characteristic.
A number of disorders in human beings have been found to be associated with the inheritance of changed or altered genes or chromosomes. Broadly, genetic disorder may be grouped into two categories – Mendelian disorders and Chromosomal disorders.
What is the probability of having a daughter to a couple expecting their first child?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 50%
In humans the mother is XX and makes only one kind of egg (X). The father is XY and makes two kinds of sperm in equal numbers: X and Y. So the sex of the child is decided by the father's sperm.
| Egg \ Sperm | X | Y |
|---|---|---|
| X | XX (daughter) | XY (son) |
Daughters : sons = 1 : 1, so the chance of a daughter is 1/2 = 50% at every birth.
Option (1) Nil — Incorrect. Half the sperms carry X, so a daughter is always possible.
Option (2) 25% — Incorrect. There are only two equally likely kinds of sperm, not four.
Option (3) 50% — Correct. The mother gives only X eggs; the father gives X and Y sperms in equal numbers. So each child has an equal chance of being XX (a daughter) or XY (a son): the probability of a daughter is 50%.
Option (4) 100% — Incorrect. Half the sperms carry Y and give sons.
Note: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.