A family has a history of colour blindness. During genetic testing, it was found that the mother is a carrier of colour blindness (XCX) and the father has normal vision (XY). What is the probability of their sons being colour-blind?
(a) 25%
(b) 50%
(c) 75%
(d) 0%
(b) 50%
Explanation: A son receives the Y chromosome from his father and one X chromosome from his mother. The carrier mother can pass on either XC or X with equal probability.
| Maternal X chromosome | Son’s genotype | Vision |
|---|---|---|
| XC | XCY | Colour-blind |
| X | XY | Normal |
One of the two possible sons is colour-blind. Probability = 1/2 = 50%.
More ICSE Moderate level questions
Which of the following sets shows examples of genetic disorders? P. Colour blindness and Malaria Q. Albinism and Cholera R. Haemophilia and Colour blindness S. Haemophilia and Albinism (a) P and Q (b) R and S (c) P and R (d) Q and S
Ajay and Vijay are siblings. Ajay can roll his tongue while Vijay cannot. What is the genotype of their parents? (a) Both the parents are homozygous dominant. (b) Both the parents are homozygous recessive. (c) One parent is homozygous dominant, and the other is homozygous recessive. (d) Both the parents are heterozygous dominant.
A sequence of DNA has 300 nitrogenous base pairs, of which 75 are Guanine. What is the number of Thymine in this sequence? (a) 150 (b) 100 (c) 50 (d) 75
Four friends P, Q, R and S were discussing the examples of genetic disorders. The examples they quoted were as follows: P. Colour blindness and Malaria Q. Albinism and Cholera R. Haemophilia and Colour blindness S. Haemophilia and Albinism Who gave the correct examples? (a) P and Q (b) R and S (c) P and R (d) Q and S
State Mendel's 'Law of Segregation'.
Given below is a schematic representation of the inheritance of the shape of seeds of garden pea. Answer the questions that follow: (a) Which is the dominant and recessive allele of the trait? (b) What does the ratio 3 : 1 in the F₂ generation represent? (c) State Mendel’s Law of Dominance.
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A zygote which has an Y chromosome inherited from the father will develop into: (a) It will depend on the chromosome inherited from the mother (b) Girl (c) Either boy or girl (d) Boy
The sex chromosome in a human ovum is: (a) X chromosome (b) Y chromosome (c) Both X and Y chromosomes (d) Either X or Y chromosome
Define the term Allele.
Define the term Mutation.
Name the following: (i) The organelle that forms the aster during cell division. (ii) A genetic disorder in which the blood does not clot. (iii) The permanent stoppage of menstruation in human females around the age of 45 years. (iv) The openings on the barks of trees through which transpiration occurs. (v) A gaseous plant hormone which promotes ripening of fruits.
