Human genetics MCQ - Practice Questions with Answers

Thalassemia is mainly caused by mutations in globin genes located on autosomes: the alpha-globin gene cluster is on chromosome 16 and the beta-globin gene is on chromosome 11. Therefore the gene is not carried on the sex chromosomes of the 23rd pair. XX and XY describe female and male sex-chromosome combinations, not the usual chromosomal location of thalassemia genes. Mitochondrial DNA is inherited maternally and is not where the globin genes responsible for thalassemia are located.

Colour blindness is commonly an X-linked recessive trait, so the relevant genes are located on X-chromosomes. Autosomes carry many non-sex-linked traits, but they are not the usual location for the classic red-green colour blindness genes. Y-chromosomes mainly carry genes related to male sex determination and do not carry the standard colour-blindness genes. Mitochondrial DNA is inherited maternally and is associated with mitochondrial traits, not the common X-linked pattern of colour blindness.

Human blood groups, especially the ABO system, show co-dominance because the IA and IB alleles are both expressed together in the AB blood group. Neither allele masks the other. Incomplete dominance would produce an intermediate phenotype, which is not how A and B antigens behave. Pleiotropy means one gene affects several traits, while aneuploidy is an abnormal chromosome number. The blood-group example is therefore classically used to show co-dominance in human genetics.