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.
A colour blind man marries a carrier woman. Choose the possible progeny outcome.
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 50% of daughters and 50% of sons born will be colour blind.
Colour blindness is X-linked recessive. Write the faulty allele as Xc. The colour blind man is XcY; the carrier woman is XcX.
| Egg \ Sperm | Xc | Y |
|---|---|---|
| Xc | XcXc (colour blind daughter) | XcY (colour blind son) |
| X | XcX (carrier daughter) | XY (normal son) |
Daughters: half colour blind, half carriers with normal vision. Sons: half colour blind, half normal.
Option (1) All sons are born colourblind, while 50% the daughter born will be normal. — Incorrect. A son gets his X from the mother, and half her eggs carry the normal X, so only half the sons are colour blind.
Option (2) All children (sons and daughters) are born colour blind. — Incorrect. For all children to be colour blind, every child would need Xc from the mother too. The carrier mother passes her normal X to half the children, so half the sons are normal and half the daughters are carriers with normal vision.
Option (3) All daughters are normal whereas 50% of the sons born will be colour blind. — Incorrect. This would be true for a normal father. Here every daughter gets her father's Xc, so those who also get Xc from the mother are colour blind.
Option (4) 50% of daughters and 50% of sons born will be colour blind. — Correct. The colour blind man (XcY) gives Xc to daughters and Y to sons; the carrier woman (XcX) gives Xc or X. Daughters are XcXc or XcX, and sons XcY or XY, so half the daughters and half the sons are colour blind.
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.