What is complementary gene action

Surname:

Svea H. 2017-01

Polygenic inheritance / polygeny

Definition: Polygeny / Polygene inheritance is a term in genetics that describes the participation of many genes in the expression of a trait (phenotype).

- This special form of inheritance cannot be explained with the help of Mendel's rules, because these consider clearly distinguishable characteristics, such as the seed shape of the pea (this is simply / monogenically inherited).

- Polgygenically inherited traits are difficult to study because some / many genes make up the same trait, each gene has a small effect on the phenotype and the effects of the different genes add up.

- In polygeny there is a mixture of the gene effect in the characteristic expression, so that no clear Mendelian inheritance can be traced.

If, conversely, several characteristics of the phenotype are determined by a single gene, one speaks of polyphenia.

There are two types of polygenic inheritance:

1. Complementary polygeny

- The genes involved each determine a partial characteristic and in the end complement each other in their effect, so they depend on each other (2 each).

- In the case of a missing gene pair, the overall characteristic is therefore not expressed.


Examples:

- blood clotting factors

- Expression of the comb in chickens

2. Additive polygeny

- Different genes work together in the development of a characteristic and add up in their effect.

- In the absence of a reinforced gene, the characteristic is still pronounced, but weaker.

- The genes can be on different chromosomes and are therefore inherited independently of one another (no coupling).

- Distinguishing features: Graduated differences and smooth transitions in the phenotype (= quantitative features).


Examples:

- size, height

- Grain color of wheat

- Color intensity of the skin color

  • The skin color of humans is produced by the skin pigment melanin, which is deposited in the skin cells (the higher the concentration of the pigment, the darker the skin color).
  • Numerous genes are responsible for skin pigmentation, which are inherited independently of one another (very wide gradation between very dark and very light skin color).
  • The phenotypic expression of the skin color results from the interaction of all genes.
  • The more dominant genes there are, the darker the skin color becomes: AABBCC = dark skin color, aabbcc = light skin color
  • Genotype AaBbCc therefore results in a skin color similar to that of genotype AABBCC.
  • Parents with light and dark skin color can therefore also have children with white skin, provided that both parents pass on alleles for the development of white skin to the child.
  • In 2008, dizygoti twins with phenotypic differences were born in Berlin: the parents had a very dark skin color and a very light skin color, one twin was born with a relatively dark skin color and the other with a relatively light skin color.
  • Such results are very rare: the probability is 1 in 1 million.