Complementary Genes Supplementary Genes Polygenic Inheritance Duplicate Genes Modified Duplicate Gene

2. Complementary Genes:
They are those non-allelic genes which independently show a similar effect but produce a new trait when
present together in the dominant form.
Complementary genes were first studied by Bateson and Punnet (1906) in case of flower colour of Sweet Pea
(Lathyrus odoratus).
The latter is also an example of recessive epistasis where the recessive homozygous alleles of one type suppress the effect
of dominant alleles of the other type. Here, the flower colour is purple if dominant alleles of two genes are present
together (С—P—). The colour is white if the double dominant condition is absent (ccP—, С—pp, ccpp).

3. Bateson and Punnet (1906) crossed two white flowered strains (CCpp, ccPP) of Sweet Pea and obtained
purple flowered plants (CcPp) in the F1 generation.
Clearly both the parents have contributed a gene or factor for the synthesis of this purple colour. The
purple flowered plants of F1 generation were then allowed to self-breed.
Both purple and white flowered plants appear in the F2 generation in the ratio of 9: 7.
It is a modification of the di-hybrid ratio of 9: 3: 3: 1. The appearance of purple colour in 9/16 population
shows that the colour is determined by two dominant genes (C and P). When either of the two is absent
(ccPP or CCpp, ccPp or Ccpp), the pigment does not appear

4. Supplementary Genes:
Supplementary genes are a pair of non-allelic genes, one of which produces its effect
independently in the dominant state while the dominant allele of the second gene (supplemen-
tary gene) needs the presence of other gene for its expression.
Supplementary Genes in Lablab:
Lablab has two genes, K and L. In the recessive state the second or supplementary gene (11) has no effect on seed
coat colour.
Dominant К independently produces Khaki colour while its recessive allele gives rise to buff colour irrespective of
the supplementary gene being dominant or recessive. In the dominant state the supplementary gene (L—)
changes the effect of dominant allele of pigment forming gene (K) into chocolate colour. F2 ratio is 9: 3: 4

5. Modified Supplementary Genes/Collaborative Supplementary
Genes/Collaboration:
They are two nonallelic genes which not only are able to produce their own effects independently when
present in the dominant state but can also interact to form a new trait. Comb types in poultry is an example
of collaborative supplementary genes, P and R.
When none of these genes is present in the dominant state (pprr), single comb is formed.
In case P alone is dominant, a pea comb is formed (Pprr, PPrr). If R alone is dominant, a rose comb is
obtained (ppRr, ppRR). A walnut comb is formed when both P and R occur together in dominant state ( P — R
—) to produce supplementary effect.
When pure pea combed and pure rose combed birds are crossed, all the offspring of F, individuals have walnut
comb. On inbreeding the walnut combed birds, the F2 generation comes to have all the four types of combs in
the ratio of 9 walnut: 3 pea: 3 rose: 1 single

7. What is Polygenic Inheritance?
Polygenic inheritance
Many traits and phenotypic characters present in plants and animals such as height, skin pigmentation,
hair and eye colour, milk and egg production are inherited through many alleles present in different loci.
This is known as polygenic inheritance.
If we take an example of height or skin pigmentation in humans, we find many different forms of the two traits.
We can’t categorise people in just two categories like ‘tall’ and ‘short’ for height or ‘dark’ and ‘light’ for the skin
colour.
We find continuous variation for both these traits because these traits are controlled by multiple genes. There
are as many as 400 genes that control the trait of height and are responsible for variation in height present in
the population.
Polygenic Inheritance Definition
Polygenic inheritance is defined as quantitative inheritance, where multiple independent genes have an
additive or similar effect on a single quantitative trait. Polygenic inheritance is also known as multiple gene
inheritance or multiple factor inheritance.

8. Polygenic Inheritance Examples
Polygenic Inheritance in Humans
There are many traits in humans, which show polygenic inheritance, e.g. skin and hair colour, height, eye colour, the
risk for diseases and resistance, intelligence, blood pressure, bipolar disorder, autism, longevity, etc.
Brief description of some of the traits:
1.Skin pigmentation: inheritance of skin pigmentation is polygenic inheritance. Around 60 loci contribute to the
inheritance of a single trait.
1. If we take an example of a pair of alleles of three different and unlinked loci as A and a, B and b, C and c. The
capital letters represent the incompletely dominant allele for dark skin colour.
2.The more capital letters show skin colour towards the darker range and small letters towards the lighter colour of the
skin. Parents having genotype AABBCC and aabbcc will produce offsprings of intermediate colour in the F1 generation,
i.e. AaBbCc genotype.
3. In the F2 generation of two triple heterozygotes (AaBbCc x AaBbCc) mate, they will give rise to varying phenotypes
ranging from very dark to very light in the ratio 1:6:15:20:15:6:1.

9. Punnett square showing F2 generation offsprings continuous variation
From light to dark→
1.Height: There are around 400 genes responsible for the phenotype and environment greatly influences the
expression of genes.
2.Eye colour: The colour of the eye is determined by polygenes. At least 9 colours of eye colour are
recognised in humans. There are two major eye colour genes and 14 more genes that determine the
expression of the phenotype. A different number of alleles contribute to each colour. These are found to be X-
linked.
Skin pigmentation

10. Duplicate Gene
Sometimes a character is controlled by two non-allelic genes whose dominant alleles produce the same
phenotype whether they are alone or together.
In Shepherd’s purse (Capsella bursa-pastoris), the presence of either gene A or gene B or both results in
triangular capsules; when both these genes are in recessive forms, the oval capsules produced

11. Duplicate Gene with Dominance Modification
(11:5):
A character controlled by two gene pairs showing dominance only if two dominant alleles are present. Dominant
phenotype will thus be produced only when two non-allelic dominant alleles or two allelic dominant alleles are
present. Such a case is found in pigment glands of cotton