This document summarizes different types of gene interactions: 1) Complete dominance occurs when one allele is always dominant over the other allele for a trait. 2) In incomplete dominance, neither allele is fully dominant and the offspring's phenotype is a blend of the parents'. 3) Codominance means both alleles are fully expressed in the heterozygote, such as blood types. 4) Overdominance or heterosis occurs when heterozygotes have a phenotype superior to either homozygote. 5) Non-allelic gene interactions also occur, such as complementation, epistasis, and polygenic inheritance which are influenced by multiple gene loci.

1. GENE INTERACTION
BY-
DR. ANIRUDDHA RAY

2. Gene interaction
Interaction of allelic genes:
• Complete dominance
• Incomplete dominance
• Codominance
• Superdomonance (overdominance)

3. Complete dominance
In all of Mendel’s experiments, he worked with
traits where a single gene controlled the trait and
where one allele was always dominant to the
other.
Although the rules that Mendel derived from his
experiments explain many inheritance patterns,
the rules do not explain them all. There are in
fact exceptions to Mendel’s rules, and these
exceptions usually have something to do with
the dominant allele

4. Incomplete dominance
• The kind of inheritance of allelic genes where a
cross between organisms with two different
phenotypes (AAxaa) produces offspring with
third phenotype that is blending (Aa) of the
parental traits. Incomplete is a condition when
neither allele is dominant over the other, when
the interaction enzymes are slightly different in
their activity

6. Codominance
All humans and many other primates can be
typed for the ABO blood group. There are four
principal types: A, B, AB, and O. There are two
antigens and two antibodies that are mostly
responsible for the ABO types. The specific
combination of these four components
determines an individual's type in most cases.
The table below shows the possible
permutations of antigens and antibodies with the
corresponding ABO type

9. Overdominance
• The kind of gene interaction in which the
phenotypic expression of the
heterozygous condition exceeds the
phenotype of homozygous dominant
condition.

10. Interaction of non-allelic genes
• Complementation
• Epistasis
• Polygenic inheritance

11. Complementation
Gene interaction where the manifestation
of a character is determined by presence
of two dominant genes of different
allelomorphic pairs simultaneously (A_B_)
(ratio 9:3:4, 9:6:1, 9:7)

13. Epistasis
Epistasis is the term applied when one gene interferes
with the expression of another.
A good example of epistasis is the genetic interactions that produce
coat color in horses and other mammals. In horses, brown coat
color (B) is dominant over tan (b). Gene expression is dependent on
a second gene that controls the deposition of pigment in hair. The
dominant gene (C) codes for the presence of pigment in hair,
whereas the recessive gene (c) codes for the absence of pigment. If
a horse is homozygous recessive for the second gene (cc), it will
have a white coat regardless of the genetically programmed coat
color (B gene) because pigment is not deposited in the hair. The
figure above demonstrates this scenario. Several of the white
horses have genotypes for brown or tan coat color in the first gene,
but are completely white because they are homozygous recessive
for the gene controlling pigment deposition.

15. Polygenic inheritance
• Polygenic inheritance is a pattern responsible for
many features that seem simple on the surface.
Polygenic traits are not expressed as absolute or
discrete characters, as was the case with
Mendel's pea plant traits. Instead, polygenic
traits are recognizable by their expression as a
gradation of small differences (a continuous
variation). The results form a bell shaped curve,
with a mean value and extremes in either
direction.

16. Human polygenic traits include:
•
•
•
•
•
•
Height
Weight
Eye Color
Intelligence
Skin Color
Many forms of behavior

18. Pleiotropy
• So far we have only considered genes that affect a
single phenotypic character. This actually is a rare
situation because it is more common that one gene can
have multiple effects

19. Pleiotropy
• Primary (Marfan`s syndrome, Hartnup
diseases)
• Secondary (sickle-cell anaemia,
phenylketonuria)

20. Marfan`s syndrome

21. Hartnup diseases

25. Variation in Gene Expression
Not all traits are expressed 100% of the time
even though the allele is present.
Penetrance - the frequency of expression of an
allele when it is present in the genotype of the
organism (if 9/10 of individuals carrying an allele
express the trait, the trait is said to be 90%
penetrant)
Expressivity - variation in allelic expression
when the allele is penetrant

26. For example the dominant allele P produces polydactyly in humans, a
trait that is characterized by extra toes and/or fingers. Two normal
appearing adults have been known to mate and produce offspring
that express polydactyly. Thus one parent must carry at least one
dominant allele (P allele) and its genotype is probably Pp. This
parent with the Pp genotype exhibits reduced penetrance for
the P allele.
Not all phenotypes that are expressed are manifested to the same
degree. For polydactyly, an extra digit may occur on one or more
appendages, and the digit can be full size or just a stub. Therefore,
when the P allele is present it expresses variable expressivity.
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