Question

1. Identical twins are derived from a single egg and fertilized by a single sperm, creating a zygote that later divides in two. Because identical twins originate from a single zygote, they are genetically identical.

Examine the following case study: Caroline Loat and her colleagues examined nine measures of social, behavioral, and cognitive ability in 1000 pairs of identical male twins and 1000 pairs of identical female twins. They found that for three of the measures (prosocial behavior, peer problems, and verbal ability), the two male twins of a pair tended to be more alike in their scores than were then female twins of a pair.

Propose a possible explanation for this observation. Give a complete explanation of the Biological mechanism that influences these observations.

What might this observation mean about the location of the genes that influence the three measures above?

Answer 1

The members of monozygotic twin pairs are identical in genome sequence, they may differ in patterns of gene expression. One early and irreversible process affecting gene expression, which can create differences within pairs of female monozygotic twins, is X inactivation - one twin can express mainly paternally-received genes on the X chromosome while the other twin expresses mainly maternally-received genes. It follows that non-identical X chromosome expression may cause female monozygotic twins to correlate less strongly than male monozygotic twins on complex behavioural traits affected by X-linked loci.