Following gene instructions

The 23 chromosomes you inherited from your father and the 23 chromosomes you inherited from your mother contain two versions of the same set of 24,000 genes. Sometimes, different alleles of a gene give rise to different characteristics. The effects of one allele may mask those of the other. A characteristic that can be masked is called recessive; the characteristic that masks it is called dominant.

Heather and Marcus have dark hair but their children Lucy and Adam have red hair. The gene for red hair is masked in Heather and Marcus by the gene for brown hair. Marcus Pembrey