Types of gene Action

1. Additive.

• Gene action is additive when the phenotypic value of the heterozygote is halfway between that of the two homozygotes.
• Coat color in Shorthorn cattle is an example.
• Three genotypes and the corresponding phenotypes are:

RR = red, Rr = roan, rr = white.

• Both alleles express themselves in the heterozygote.
• In fact, about half the hairs are red and half are white which gives the roan color; thus, the heterozygote is intermediate to the homozygotes.
• Similarly, genes may act additively for measurable traits. Assume weight is controlled by one pair of genes and the genotypes and corresponding phenotypes are: AA = 10 lbs, Aa = 9 lbs. and aa = 8 lbs.
• The genes act additively. The phenotypic value for the heterozygote is halfway between the values for the two homozygotes.

(AA + aa)/2= Aa

• Any similar genetic models, i.e., BB = 1, Bb = 1.5 and bb = 2 or CC = 100, Cc = 90 and cc = 80, would be examples of additive gene action. The units of measurement depend on the trait and could be lbs., kgs or gms for weight; in., cm for length, etc.

2. Complete Dominance.

• The phenotypic value for the heterozygote is equal to the value for one of the homozygotes.
• The examples given previously for polled vs horned and color in Angus is examples for qualitative traits. Two examples for measurable traits are: DD = 2 units, Dd = 2 units and dd = 1.5 units; or FF = 6 units, Ff = 10 units and ff = 10 units. In the first example D is dominant and d is recessive; the second example has f dominant and F recessive.

3. Over dominance.

• The phenotypic value for the heterozygote is greater or less than the value for either homozygote.
• There aren’t many examples of this type of gene action. Sickle-cell anemia in humans would be one example.
• If untreated, individuals with sickle-cell anemia usually die at a fairly young age.
• Heterozygotes do not experience this condition but seem to have greater resistance to malaria than homozygous normal individuals. Thus, in regions of the world where malaria is present, heterozygotes have a higher fitness value than either homozygote. An example of overdominance for a measurable trait would be:

AA = 2 units, Aa = 3 units and aa = 1 unit.

4. Incomplete Dominance.

• The phenotypic value for the heterozygote is intermediate to, but not exactly halfway between, the two homozygotes.
• Coat color in Shorthorn cattle is sometimes called incomplete dominance. This type of gene action is easier to illustrate with measurable traits.
• An example would be:

AA = 4 units, Aa = 3 units and aa = 1 unit.

5. Epistasis:

• Some traits are controlled by two or more pairs of genes. Then, the possibility exists for interaction among genes at different loci.
• This kind of interaction is called epistasis which means that gene action at one locus is altered by gene action at another locus.
• The F1 progeny all had round seeds and yellow cotyledons.
• When the F2 progeny were mated among themselves the progeny had a ratio of 9 round seeds and yellow cotyledons: 3 round seeds and green cotyledons: 3 wrinkled seeds and yellow cotyledons: 1 wrinkled seeds and green cotyledons.
• This result led to the law of independent assortment. The genes at one locus segregated independently of the genes at the other locus.
• In this experiment, the parental genotypes could be symbolized: RRYY (round seeds, yellow cotyledons) × rryy (wrinkled seeds, green cotyledons). R and Y are dominant alleles and the F1, with genotype RrYy, all had round seeds and yellow cotyledons.
• During meiosis, the genes at the two loci segregated independently and F1 plants produced gametes.