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Variations on Mendel's laws (overview)

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Extensions, exceptions, and revisions to these laws. Get an overview of variations on Mendel's laws, including multiple alleles, incomplete dominance, co-dominance, pleiotropy, lethal alleles, sex linkage, genetic interactions, polygenic traits, and environmental effects.

Review: Mendel's basic model

The basic principles of Gregor Mendel’s model of inheritance have held up for over a century. They can explain how many different characteristics are inherited, in a wide range of organisms including human beings.
Some of the key elements of Mendel’s original model were:
  1. Heritable traits are determined by heritable factors, now called genes. Genes come in pairs (that is, are present in two copies in an organism).
  2. Genes come in different versions, now called alleles. When an organism has two different alleles of a gene, one (the dominant allele) will hide the presence of the other (the recessive allele) and determine appearance.
  3. During gamete production, each egg or sperm cell receives just one of the two gene copies present in the organism, and the copy allocated to each gamete is random (law of segregation).
  4. Genes for different traits are inherited independently of one another (law of independent assortment).
These rules still form the foundation of our understanding of inheritance—that is, how traits are passed on and how an organism's genotype (set of alleles) determines its phenotype (observable features). However, we now know of some exceptions, extensions, and variations, which must be added to the model in order to fully explain the inheritance patterns we see around us.

Variations involving single genes

Some of the variations on Mendel’s rules involve single genes. These include:
  • Multiple alleles. Mendel studied just two alleles of his pea genes, but real populations often have multiple alleles of a given gene.
  • Incomplete dominance. Two alleles may produce an intermediate phenotype when both are present, rather than one fully determining the phenotype.
  • Codominance. Two alleles may be simultaneously expressed when both are present, rather than one fully determining the phenotype.
  • Pleiotropy. Some genes affect many different characteristics, not just a single characteristic.
  • Lethal alleles. Some genes have alleles that prevent survival when homozygous or heterozygous.
  • Sex linkage. Genes carried on sex chromosomes, such as the X chromosome of humans, show different inheritance patterns than genes on autosomal (non-sex) chromosomes.
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Variations involving multiple genes

Other variations on Mendel’s rules involve interactions between pairs (or, potentially, larger numbers) of genes. Many characteristics are controlled by more than one gene, and when two genes affect the same process, they can interact with each other in a variety of different ways. For example:
  • Complementary genes. Recessive alleles of two different genes may give the same phenotype.
  • Epistasis. The alleles of one gene may mask or conceal the alleles of another gene.
In addition, some gene pairs lie near one another on a chromosome and are genetically linked, meaning that they don’t assort independently.

Polygenic inheritance and environmental effects

Many characteristics important in our everyday lives, such as height, skin color, eye color, and risk of diseases like diabetes, 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.
  • 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.
Genetic background and environment contribute to incomplete penetrance, in which not all individuals with a genotype display a corresponding phenotype, and variable expressivity, in which individuals of a particular genotype may have stronger or weaker versions of a phenotype.
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  • duskpin ultimate style avatar for user Ishaan
    Is people with multicolored eyes (like one eye brown and the other blue) an example of incomplete dominance?
    (4 votes)
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    • male robot donald style avatar for user Tybalt
      No; genetic heterochromia is the result of a mutation on at least one gene that determines eye color (scientists discovered that eye color is controlled by at least eight genes in what is called polygenic inheritance). An individual may have also acquired it because of an injury, meaning that it had nothing to do with genetics at all.

      Did this help?
      (17 votes)
  • blobby green style avatar for user 😊
    what is the relationship between mendels law to genetics
    (1 vote)
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  • blobby green style avatar for user Muhammad Irfan Mohd Isa
    How does epistasis relate to getting a 9:7 ratio? how does this concept work? is there an example we can relate this to?

    also does complementary gene mean the same as principle of complementation? cos from what I understand of complementation, is when two mutant pure breeding parents cross and give rise to a progeny all with wild type phenotype. I don't get the explanation of recessive alleles of two different genes giving rise to the same phenotype.
    (3 votes)
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    • aqualine ultimate style avatar for user Joseph K
      You will get 9:7 fenotipe ratio in Duplicate Recessive Epistasis (DRE) for P: AaBb x AaBb, then the F1's genotipes will be 9:3:3:1 (9[A_B_], 3[A_bb], 3[aaB_], 1[aabb]). In DRE, any recessive homozigote is epistatic and they all give the same fenotipe. So we look for how many of the F1's genotipes that doesn't has any recessive homozigote then compare to how many of those that has recessive homozigote (epistatic gene, it means all of them have same fenotipe), then we got 9:7.

      Example: Snail Shell Color

      Also, Complimentary genes are those genes in DRE. They are complimentary because for a hypostatic fenotipe to occure needs all of them to be homozigotely or heterozigotely dominant (which are also hipostatic to the homozigotely recessive genes).
      (2 votes)
  • blobby green style avatar for user 😊
    how cell cellular and chromosomal basis inheritance relate?
    (3 votes)
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  • starky ultimate style avatar for user Adwaith
    what is lethal alleles?
    (2 votes)
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  • starky seedling style avatar for user Emon Chandra
    I'm still confused. Are the variations (i.e. epistasis) still a part of Mendelian genetics or it rather be separated from Mendelian genetics? Like when you're talking about Mendelian genetics, people already know that you're talking about the single gene inheritance, not the deviation of the Mendelian genetics? (Sorry, it's kinda hard to explain what I'm trying to say)
    (2 votes)
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    • winston default style avatar for user Aiyan Alam
      Aside from Mendelian genetics, there are some exceptions to the laws. All the other laws listed are the exception. There is no reason why this happened, other than further research has been made after Mendel's research was rediscovered. And research is still going on, so there can be changes to the variations in the next 10 years or so!
      (1 vote)
  • blobby green style avatar for user MAHIB
    Is the Incomplete dominant same as the bleeding rule, which was proposed before the Mendel genetics?
    (2 votes)
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    • stelly blue style avatar for user Roop Kiran
      In incomplete dominance, no blending takes place. It happens due to incomplete enzyme or no functional enzyme.
      Basically, it is the enzymes that produce the phenotypic character. In the nucleus, mRNA is produced from the DNA. This mRNA goes into the ribosome, the protein synthesis house, wherein these enzymes are synthesized. Sometimes, the enzyme malfunctions or is produced in insufficient amount. Hence, the phenotypic character is affected.
      This phenomenon is observed in the plant snapdragon or Antirrhinum majus
      (1 vote)
  • aqualine ultimate style avatar for user Lakshmi Sriram
    Can someone clearly explain the difference between epistatis and complimentarity of genes?
    (1 vote)
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    • aqualine ultimate style avatar for user Joseph K
      In Epistasis, the epistatic gene will mask/hide the hypostatic gene's fenotype.
      So in most of the epistasis, there are epistatic gene and hypostatic gene present.
      Complimentary genes are present in an special epistasis, called Duplicate Recessive Epistasis. Complementary genes are recessively epistatic to each other. So, if there is any one of the compliment gene that is recessive homozigote, it is the epistatic gene.
      Complimentary genes need to be both/all Dominant so a fenotype occure. If any one of them is recessive then the fenotype won't occure.
      (3 votes)
  • starky tree style avatar for user Reem
    What exactly is the difference between incomplete dominance and codominance?
    (2 votes)
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  • aqualine ultimate style avatar for user Jung, Haesol
    What is autosomal linkage and how does it deviate from Mendel's conclusions?
    (2 votes)
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