Types of Non-Mendelian Genetics

Austrian scientist Gregor Mendel is known as the father of genetics for his pioneering work with pea plants. However, he only was able to describe simple or complete dominance patterns in individuals based on what he observed with those plants. There are many other ways that genes are inherited other than what Mendel described in his research findings. Since Mendel's time, scientists have learned much more about these patterns and how they influence speciation and evolution.

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Incomplete Dominance

Rabbit fur color is an example of incomplete dominance
Rabbits with different colored fur. Getty/Hans Surfer

Incomplete dominance is the blending of traits expressed by the alleles that combine for any given characteristic. In a characteristic that shows incomplete dominance, the heterozygous individual will have a mix or blend of the two alleles' traits. Incomplete dominance will give a 1:2:1 phenotype ratio with the homozygous genotypes each showing a different feature and the heterozygous showing one more distinct phenotype.

Incomplete dominance can affect evolution when the blending of two traits becomes a desirable characteristic. It is often seen as desirable in artificial selection as well. For example, rabbit coat color can be bred to show a blend of the parents' colors. Natural selection may also work that way for the coloring of rabbits in the wild if it helps camouflage them from predators.

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Codominance

The white and pink petals shows codominance
A rhododendron showing codominance. Darwin Cruz

Codominance is another non-Mendelian inheritance pattern that is seen when neither allele is recessive or masked by the other allele in the pair that code for any given characteristic. Instead of blending to create a new feature, in codominance, both alleles are equally expressed and their features are both seen in the phenotype. Neither allele is recessive or masked in any of the generations of offspring in the case of codominance. For example, a cross between a pink and white rhododendron may result in a flower with a mix of pink and white petals.

Codominance affects evolution by ensuring both alleles are passed down instead of being lost. Since there is no true recessive allele in the case of codominance, it is harder for a trait to be bred out of the population. As in the case of incomplete dominance, new phenotypes are created and can help an individual survive long enough to reproduce and pass down those traits.

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Multiple Alleles

Human blood types are controlled by multiple alleles
Blood Types. Getty/Blend Images/ERproductions Ltd

Multiple allele inheritance occurs when there are more than two alleles that are possible to code for any one characteristic. It increases the diversity of traits that are coded by the gene. Multiple alleles can also encompass incomplete dominance and codominance along with simple or complete dominance for any given characteristic.

The diversity afforded by multiple alleles gives natural selection an extra phenotype, or more, to exploit. This gives species an advantage for survival as there are many different traits within a single population; in such cases, a species is more likely to have a favorable adaptation that will help it survive and reproduce.

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Sex-linked Traits

Colorblindness is controlled on the X chromosome
Color blindness test. Getty/Dorling Kindersley

Sex-linked traits are found on the sex chromosomes of the species and are passed down through reproduction. Most of the time, sex-linked traits are seen in one sex and not the other, although both sexes are physically able to inherit a sex-linked trait. These traits are not as common as other traits because they are found only on one set of chromosomes, the sex chromosomes, instead of the multiple pairs of non-sex chromosomes.

Sex-linked traits are often associated with recessive disorders or diseases. The fact they are rarer and usually found only in one sex makes it difficult for the trait to be selected against by natural selection. That is why such disorders continue to be passed down from generation to generation despite the fact that they are not useful adaptations and can cause severe health issues.