Sex-linked Inheritance

Red-green colour blindness is an X-linked recessive trait, meaning the defective allele resides on the X chromosome and is masked by a normal dominant allele. A colour-blind man has the genotype X'Y, where X' carries the recessive allele, while the homozygous normal woman is XX. Sons always inherit their single X chromosome from the mother and the Y chromosome from the father. Since the mother contributes only normal X chromosomes, every son receives a normal X and is therefore normal-visioned. The option stating all sons are colour-blind is wrong because sons do not receive the father's X-linked allele; they get his Y instead. The choice that half the sons are colour-blind would require the mother to be a carrier, which she is not. Males cannot be 'carriers' for an X-linked trait since they have only one X, so that option is biologically invalid. As stated in NCERT Class 12, Chapter 5 (Principles of Inheritance and Variation), X-linked recessive traits typically pass from carrier mothers to sons. As a consistency check, because the trait reaches sons only through the maternal X, a normal mother guarantees normal-visioned sons.