An illustration of dominance and overdominance. (a) Crossing P1 and P2 allowed their offspring F1 to acquire the dominant alleles needed to negate the effect of the recessive alleles P1 and P2 possessed. (b) P1 was homozygous for a deleterious allele (B*) but, a cross with P2 allowed F1 the advantage of becoming a heterozygote (B*B).

Heterosis occurs when a hybrid performs better than either of its parents (1). In other words, it is the phenomenon of the offspring of a cross being superior to its parents in terms of development, resistance to disease, repoduction etc (23). There are a number of factors that are thought to influence heterosis. They include:
  • Dominance
Aquiring dominant alleles that negate the deliterious effect of recessive alleles accumulated through inbreeding (2).
  • Overdominance
Alleles that when found in a heterozygote are advantageous but when found in a homozygote are harmful (2).
  • Epistasis
Interactions between genes which lead to changes in phenotype. For example, the presence or absence of one gene may affect the expression of another gene (2).


1. Wikipedia (2013), Heterosis

2. Lippman ZB & Zamir D (2007) Heterosis: revisiting the magic. Trends in Genetics 23(2):60-66.

3. Birchler JA, Yao H, Chudalayandi S, Vaiman D, & Veitia RA (2010) Heterosis. The Plant cell 22(7):2105- 2112.