Beyond Mendelian Genetics: Complex Patterns of Inheritance

Beyond Mendelian Genetics: Complex Patterns of Inheritance

Articles, Blog , , , , , , , , , , , 15 Comments

Professor Dave again, let’s revisit Mendel. When we looked at Mendelian genetics, we learned
all about genotypes and phenotypes, alleles and Punnett squares. But even then we already began to see that
the basic model of dominant and recessive alleles does not explain certain phenomena. We looked at incomplete dominance, such as
when true-breeding red and and true-breeding white snapdragons are hybridized to yield
an entirely pink F1 generation. Thus a third intermediate phenotype is involved,
totally unlike complete dominance, where only the dominant phenotype is expressed. We also looked at codominance, where two phenotypes
can be expressed simultaneously, as was the case with the cows with both white and brown coloring. Building on what we already know, let’s
continue to extend this understanding into still more interesting and complex patterns
of inheritance. First, it is important to understand that
in cases of dominant and recessive alleles, one allele is dominant simply because it is
visible in the phenotype, and not because it somehow subdues expression of the recessive
allele. With heterozygous individuals, both the dominant
and recessive alleles are expressed, it is simply that the product of the expression
of the dominant allele produces the relevant phenotype. For example, when Mendel examined seed shape,
he noticed that seeds could be round or wrinkled, with round being dominant. We now know that this is because the dominant
allele codes for an enzyme that helps convert unbranched starch into a branched form, while
the recessive allele codes for a defective form of that enzyme, thereby allowing unbranched
starch to accumulate, which causes water to enter the seed by osmosis, and when the seed
dries, it wrinkles. So if at least one dominant allele is present,
in the form of the heterozygous or homozygous dominant genotypes, the correct enzyme will
be produced, and the seed will be round. With the heterozygous organism, both the effective
enzyme and the defective enzyme are produced, but there is enough of the effective version
to get the job done. Only if homozygous recessive will there be
no effective enzyme present at all, in which case the seed will wrinkle. Now with the understanding that all alleles
will be expressed, we can better understand situations involving multiple alleles, meaning
more than simply one dominant and one recessive. An example of this is blood type in humans. With this, there are three alleles that are
possible for a single gene. These are IA, IB, and i, in lowercase, where
the i stands for immunoglobulin. The letters A and B refer to carbohydrates
that can be found on the surface of red blood cells, whereby expression of IA yields the
A carbohydrate, expression of IB yields the B carbohydrate, and expression of i does not
result in any carbohydrate. Therefore, different combinations of these
alleles can yield blood of type A, B, AB, or O, if neither of these carbohydrates are found. We discussed the compatibility of blood types
for transfusion in the anatomy and physiology series, so head over there for more information
in that context. Now let’s step things up a notch. Until now, even in situations with multiple
alleles, we have only seen cases where a particular gene influences only one phenotypic character. Often times, however, a single gene can have
multiple phenotypic effects. This is a situation called pleiotropy. In humans, pleiotropic alleles produce multiple
symptoms associated with hereditary diseases like cystic fibrosis and sickle-cell disease,
whereas with pea plants, a single gene determines both flower color as well as the color of
the outer surface of the seed. In contrast, there are other situations where
multiple genes are involved in the expression of a single phenotype. When this happens because of the interaction
of the products of gene expression, it is called epistasis. For example, in Labrador retrievers, the dominant
coloring is black, while recessive is brown, represented by a capital B and lower case
B respectively. This allele determines the pigment that produces
the coloring of the dog. However, there is another gene that determines
whether or not the pigment will be deposited onto the fur, with a dominant allele resulting
in the pigment being deposited. So if the dog is homozygous recessive for
this trait, then the dog will end up yellow, no matter which alleles are present for the
pigment, as the pigment will not be deposited onto the fur. Only if the dog is heterozygous or homozygous
dominant will the phenotype for color be expressed. So the gene for pigment deposition is said
to be epistatic to the gene that codes for black or brown pigment. Mating two dogs that are heterozygous for
both traits to get a dihybrid cross clearly shows the law of independent assortment in
action, when examining the distribution of dogs with different coloring. An even more complex situation regarding multiple
genes contributing to a single phenotype is called polygenic inheritance. In these cases, traits are not as simple as
existing as one out of two or three discrete options, instead they exist as gradations
along a continuum. An example of this is skin color in humans. Most people have skin of one particular tone,
and this is determined by a variety of genes, but this tone can be very dark, very light,
or any of a number of intermediate shades. This is because all the genes working in tandem
can have dominant or recessive alleles in any combination, and thus produce darkness
in an additive manner, and through a variety of genotypes, given the incredible number
of combinations possible when three or more genes have alleles assorted independently. And finally, there are even phenotypes that
are subject to environmental factors, such as flowers that can display different colors
depending on the acidity of the soil, or other such stimuli. Such characteristics are referred to as multifactorial,
which means that many factors, both genetic and environmental, contribute to the presence
of a particular phenotype. So beyond our previous introduction to Mendelian
genetics, as well as the degrees of dominance we had already examined, we have now broadened
our understanding of methods of inheritance to include phenomena like pleiotropy, epistasis,
polygenic inheritance, and environmental influence. With all of these additional complications,
it is incredible that Mendel was able to do the work that he did. But more importantly, we are now able to elevate
our comprehension of inheritance such that we may examine a variety of genetic disorders,
and come up with strategies for treatment, thanks to our modern understanding of molecular biology.

15 thoughts on “Beyond Mendelian Genetics: Complex Patterns of Inheritance

  • Ayad Ali Post author


  • Asma kh Xasro Post author


  • Ghost Post author

    is there gonna be a video debate between you and (non) dr kent hovind, like when aron ra did it?

  • Hanu Chopra Post author


  • dipty Tandi Post author

    Sir why did you cut off your hair … they were nice

  • Yaseen Nothinyet Post author

    Can you do a video about the moment of interia
    And can you please explain why it's measured by m to the power of 4
    I know m measures distance
    M squared measures area
    M cubed measures volume
    But….. I just don't get why iss gotta why is the moment of interia I'd measured by m to the power of 4

  • Brandon Hamer Post author

    @3:30 not exactly true that the 'i' produces nothing. It produces a smaller saccharide. This allele is also referred to as 'H' in the bombay blood type system, with 'h' producing an even smaller saccharide than 'H'.

  • Samuel Díez Post author

    Very informative video Dave. What about one regarding inheritance dependent on X and Y Chromosomes that can't be explained with Mendelian genetics?

  • S Tman Post author

    Can we touch a little on the
    XYY Turner's syndrome or XXY Klinefelter syndrome..

    XYY Klinefelter syndrome is a genetic condition in which a male has an extra Y chromosome. There are usually few symptoms. These may include being taller than average, acne, and an increased risk of learning problems. The person is generally otherwise normal, including normal fertility.

    I know that this could get a little PC & touchy! Now if you feel that society can't handle the truth about these Syndromes then I'll understand!
    Lgbtq? Explanation

  • vegatronld Post author

    How neat!

  • - Post author

    Indoctrinated Dave explains.

  • Asad Nayyer Post author

    Great timing I’m in Unit 5 Ap Biology: Heredity

  • Hrushikesh Hasabnis Post author

    In the dog example i am bit confused.
    1. Pigment Production & Pigment deposition are two different cases?
    2. The black coat color (BB/Bb) going to be homozygous dominant and Heterrozygous dominant? so what is with bb?
    Sorry i m being.. nerd

  • sandman369 Post author

    700k subs thousands of views on all non flat earth content 🤘⚡👌

  • siva ramaraju siv Post author

    Hiii professor………… looking new without long hair

Leave a Reply

Your email address will not be published. Required fields are marked *