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Non-Mendelian inheritance review

Key terms

Incomplete dominancePattern of heredity in which one allele is not completely dominant over another
CodominancePattern of heredity in which both alleles are simultaneously expressed in the heterozygote
Multiple allelesA gene that is controlled by more than two alleles
PleiotropyWhen one gene affects multiple characteristics
Lethal alleleAllele that results in the death of an individual
Polygenic traitTraits that are controlled by multiple genes

Variations involving single genes

Some of the variations on Mendel’s rules involve single genes.
  • Incomplete dominance. Two alleles may produce an intermediate phenotype when both are present, rather than one fully determining the phenotype.
    Diagram of a cross between CWCW (red) and CRCR (white) snapdragon plants. The F1 plants are pink and of genotype CRCW.
    An example of this is the snapdragon plant. A cross between a homozygous white-flowered plant (CWCW) and a homozygous red-flowered plant (CRCR) will produce offspring with pink flowers (CRCW).
  • Codominance. Two alleles may be simultaneously expressed when both are present, rather than one fully determining the phenotype.
Erminette (black and white) chicken resulting from codominance.
Codominance in erminette chicken. Image from Wikimedia, CC BY 2.0.
In some varieties of chickens, the allele for black feathers is codominant with the allele for white feathers. A cross between a black chicken and a white chicken will result in chicken with both black and white feathers.
  • Multiple alleles. Mendel studied just two alleles of his pea genes, but real populations often have multiple alleles of a given gene.
    Allelic series of the color gene C in rabbits.
    • A CC rabbit has black fur.
    • A cchcch rabbit has chinchilla coloration (grayish fur).
    • A chch rabbit has Himalayan (color-point) patterning, with a white body and dark extremities.
    • A cc rabbit is albino, with a pure white coat.
    Image from OpenStax, CC BY 3.0
    An example of this is the gene for coat color in rabbits (the C gene) which comes in four common alleles: C, cch, ch, and c.
  • Pleiotropy. Some genes affect many different characteristics, not just a single characteristic.
    Simple schematic illustrating pleiotropy.
    In pleiotropy, one gene affects multiple features (feature 1, feature 2, feature 3.
    Caption: One gene affects multiple characteristics.
    Based on similar diagram by Ingrid Lobo1.
    An example of this is Marfan syndrome, which results in several symptoms (unusually tall height, thin fingers and toes, lens dislocation, and heart problems). These symptoms don’t seem directly related, but as it turns out, they can all be traced back to the mutation of a single gene.
  • Lethal alleles. Some genes have alleles that prevent survival when homozygous or heterozygous.
    Two yellow mice (AYA genotype) are crossed to one another. The Punnett square for the cross is:
    AYAYAY (dies as embryo)AYA (yellow)
    AAYA (yellow)AA (agouti/brown)
    There is a phenotypic ratio of 2:1 yellow:brown among the mice that survive to birth.
    A classic example of an allele that affects survival is the lethal yellow allele, a spontaneous mutation in mice that makes their coats yellow. Mice that are homozygous (AYAY) genotype die early in development. Although this particular allele is dominant, lethal alleles can be dominant or recessive, and can be expressed in homozygous or heterozygous conditions.

Polygenic inheritance and environmental effects

Many characteristics, such as height, skin color, eye color, and risk of diseases, are controlled by many factors. These factors may be genetic, environmental, or both.
  • Polygenic inheritance. Some characteristics are polygenic, meaning that they’re controlled by a number of different genes. In polygenic inheritance, traits often form a phenotypic spectrum rather than falling into clear-cut categories.
    Human skin color chart showing spectrum of skin pigmentation
    Human skin color chart. Image from Wikimedia, CC BY-SA 4.0,
    An example of this is skin pigmentation in humans, which is controlled by several different genes.
  • Environmental effects. Most real-world characteristics are determined not just by genotype, but also by environmental factors that influence how genotype is translated into phenotype.
    Blue and pink hydrangea flowers
    Blue and pink hydrangea due to variance in soil pH. Image by Lynn Greyling, Public domain
    An example of this is the hydrangea flower. Hydrangea of the same genetic variety may vary in color from blue to pink depending on the pH of the soil they are in.

Common mistakes and misconceptions

  • Some people confuse pleiotropy and polygenic inheritance. The major difference between the two is that pleiotropy is when one gene affects multiple characteristics (e.g. Marfan syndrome) and polygenic inheritance is when one trait is controlled by multiple genes (e.g. skin pigmentation).
  • Codominance and incomplete dominance are not the same. In codominance, neither allele is dominant over the other, so both will be expressed equally in the heterozygote. In incomplete dominance, there is an intermediate heterozygote (such as a pink flower when the parents' phenotypes are red and white).

Want to join the conversation?

  • mr pants teal style avatar for user Allison Kubas
    If codominance is possible, are corecessive traits possible as well?
    (12 votes)
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    • duskpin tree style avatar for user Natrium Chloride
      Good question, there seems to be a lot of discussion about this. The answer is no, there's nothing called co-recessive. In reality, being dominant or recessive is relative. An allele may be recessive to one allele but dominant to another. Which means if one particular locus can hold three alleles, say K k1 and k2, where K is dominant to both k1 and k2. Suppose a person has alleles k1 and k2, they can show co-dominance, incomplete dominance or complete dominance. It depends on the extent of dominance against each other. As this person only has k1 and k2, we are comparing which one is more dominant, rather that comparing them against K.
      (7 votes)
  • leaf red style avatar for user layaz7717
    Will recessive traits eventually disappear from the population, or at least become less common?
    (6 votes)
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    • starky tree style avatar for user Austin
      I think it would depend on whether or not the recessive traits jeopardized an organism's safety. For example, if a recessive trait for a rabbit was bright white fur compared to a muted brown, and the rabbit's habitat was in a jungle or swamp, than the rabbit's with the recessive traits would be easier to target by predators, and therefore the recessive trait would be less likely to be passed on through each generation. The recessive trait would be very hard to get out of a large population compared to a smaller one, and it depends on if the recessive trait effects the animal's ability to adapt and avoid danger. For this reason, most recessive traits in humans will not eventually disappear, because even though the recessive trait is harder to pass down; there are so many humans (around 7.8 billion) that humans with common recessive traits will never be fully extinct. And since humans are the ultimate (apex) predator, people with common recessive traits face no real danger because of the recessive trait they may have. An only exception is if the recessive trait causes some kind of cancer, or other highly dangerous medical condition. It is possible for recessive traits to become less common, but with the human population growing so much, it is hard to predict the trend that recessive traits will take. However; like I noted before, most animals face a different playing field in their natural habitat than humans. So it is possible that recessive traits would become less common to the point of extinction in smaller animal populations.
      (10 votes)
  • leaf red style avatar for user layaz7717
    In this article, I noticed that certain alleles were expressed with a superscript. When is a superscript used? Is it when there are multiple alleles?
    (4 votes)
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  • piceratops sapling style avatar for user ALEXANDERN
    Why are some things albino looking , and what type of mutation is present at the time of the growing in the mother .
    (5 votes)
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  • blobby green style avatar for user rowzeej
    Is it normal to look like you have Marfan Syndrome but not actually have it?
    (4 votes)
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  • duskpin ultimate style avatar for user dyhmondwallace
    So basically pigmentation really depends on the parents? Just how flowers or other living things rely on their developers as well?
    (3 votes)
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  • winston default style avatar for user Tran Phan Minh Kien
    By the way, how does a chicken become white and black like that? I don't understand.
    (3 votes)
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  • ohnoes default style avatar for user sudhanshu singh
    in the example of snapdragon you hae used [ WW-for white ]
    [ RR-for red ], will this not csuse ambiguity,

    in the begning you have convinced us that allele of same gene can't get diffrent alphabatical letter
    (3 votes)
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  • leafers ultimate style avatar for user lilgaben17
    the whole rabbit thing does not make sense to me like what does Cch mean?
    (2 votes)
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  • female robot ada style avatar for user lucija.falamic00
    How is it possible that there could be a codominant relationship between alleles for pigments, for example in cattle, if white allele doesn't produce functional polypeptide?
    (2 votes)
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