Read the following passage carefully and answer the given questions.
Mendelian Disorders
Mendelian disorders are mainly determined by alteration or mutation in the single gene. These disorders are transmitted to the offspring on the same lines as we have studied in the principle of inheritance. The pattern of inheritance of such Mendelian disorders can be traced in a family by the pedigree analysis. Most common and prevalent Mendelian disorders are Haemophilia, Cystic fibrosis, Sickle cell anaemia, Colour blindness, Phenylketonuria, Thalassemia, etc. It is important to mention here that such Mendelian disorders may be dominant or recessive. By pedigree analysis one can easily understand whether the trait in question is dominant or recessive. Similarly, the trait may also be linked to the sex chromosome as in case of haemophilia. It is evident that this X-linked recessive trait shows transmission from carrier female to male progeny.
The son of a woman who carries the gene of colour blindness has __________ chance of being colour blind.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 50 percent
Colour blindness is a sex-linked recessive disorder. The gene is on the X chromosome. A woman who carries the gene (a carrier) has one normal X (X) and one X with the gene (Xc). She is not colour blind herself, because the normal dominant gene hides the recessive one. Her husband is taken as normal (XY).
Mother: X Xc Father: X Y
| X (father) | Y (father) | |
|---|---|---|
| X (mother) | XX — normal daughter | XY — normal son |
| Xc (mother) | XXc — carrier daughter | XcY — colour blind son |
A son always gets his Y from the father and his X from the mother. The mother gives X or Xc with equal chance. So, of her sons, one in two (50 per cent) is colour blind. A son has only one X, so the recessive gene shows in him.
Note: This passage is taken from NCERT as it is. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.