heterosis

(hybrid vigour) The exhibition by a hybrid of more vigorous growth, greater yield, or increased disease resistance than either parent. The effect is thought to be due to an accumulation of dominant alleles, each having additive effects, and masking the effects of deleterious recessive alleles. Thus, if a characteristic is affected by two genes X and Y, and the parents are XXyy and xxYY, the hybrid would be XxYy. Setting or crossing between identical F₁ hybrids would produce an extremely variable F₂ generation with only half the heterozygosity of the F₁. Thus to maintain optimum heterosis and uniformity the seed of some crop plants (e.g. Brussels sprouts) is produced each year by crossing two different parental pure lines. Such crops are termed F₁ hybrids.