What Traits Are Controlled By A Single Gene With Two Alleles

16.5: Not-Mendelian Inheritance

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Family Portrait

This photo of a Due south African family shows some of the variations that exist in human being peel color. The color of human being skin tin can range from very light to very dark with every possible gradation in between. As you might expect, the skin color trait has a more complex genetic basis than just i factor with two alleles, which is the type of simple trait that Mendel studied in pea plants. Like peel colour, many other homo traits accept more complicated modes of inheritance than Mendelian traits. Such modes of inheritance are called non-Mendelian inheritance, and they include inheritance of multiple allele traits, traits with codominance or incomplete potency, and polygenic traits, amidst others, all of which are described below.

a family — Effigy \(\PageIndex{1}\): Family unit

Multiple Allele Traits

The majority of human genes are thought to have more 2 normal versions or alleles. Traits controlled by a single gene with more than than two alleles are chosen multiple allele traits. An example is ABO claret type. Your claret type refers to which of sure proteins called antigens are institute on your red blood cells. There are 3 common alleles for this trait, which are represented by the letters I^A, I^B, and i.

Table \(\PageIndex{1}\): ABO Blood Group
Genotype	Phenotype (blood type)
I^AI^A	A
I^Ai	A
I^BI^B	B
I^Bi	B
ii	O
I^AI^B	AB

Equally shown in the table below, there are half dozen possible ABO genotypes considering the three alleles, taken two at a time, result in half-dozen possible combinations. The IA and IB alleles are ascendant to the i allele. As a result, both I^AI_A and I^Ai genotypes take the same phenotype, with the A antigen in their blood (blazon A blood). Similarly, both I^BI^B and I^Bi genotypes have the same phenotype, with the B antigen in their blood (type B blood). No antigen is associated with the i allele, so people with the 2 genotype have no antigens for ABO blood type in their blood (blazon O blood).

Codominance

Look at the genotype I^AI^B in the ABO blood group table. Alleles I^A and I^B for ABO blood blazon are neither dominant nor recessive to one some other. Instead, they are codominant to each other. Codominance occurs when 2 alleles for a gene are expressed equally in the phenotype of heterozygotes. In the instance of ABO claret blazon, I^AI^B heterozygotes have a unique phenotype, with both A and B antigens in their claret (type AB blood).

Incomplete Dominance

Some other relationship that may occur betwixt alleles for the same cistron is incomplete authorisation. This occurs when the dominant allele is non completely ascendant, so an intermediate phenotype results in heterozygotes who inherit both alleles. By and large, this happens when the two alleles for a given gene both produce proteins but ane protein is not functional. As a outcome, the heterozygote individual produces only half the corporeality of normal protein as is produced by an individual who is homozygous for the normal allele.

An example of incomplete dominance in humans is Tay Sachs disease. The normal allele for the factor, in this case, produces an enzyme that is responsible for breaking down lipids. A defective allele for the cistron results in the production of a nonfunctional enzyme. Heterozygotes who take one normal and one defective allele produce half as much functional enzyme equally the normal homozygote, and this is enough for normal development. However, homozygotes who take only defective alleles produce only the nonfunctional enzyme. This leads to the accumulation of lipids in the brain showtime in utero , which causes pregnant brain damage. Almost individuals with Tay Sachs illness dice at a young historic period, typically past the age of five years.

Polygenic Traits

Many human traits are controlled by more than than one gene. These traits are called polygenic traits. The alleles of each gene have a pocket-size additive effect on the phenotype. There are many possible combinations of alleles, especially if each gene has multiple alleles. Therefore, a whole continuum of phenotypes is possible.

height bell graph polygenic character comparing male and females — Figure \(\PageIndex{2}\). Human Adult Height. Like many other polygenic traits, adult meridian has a bong-shaped distribution.

An case of a human polygenic trait is adult top. Several genes, each with more than than one allele, contribute to this trait, and so at that place are many possible adult heights. For example, one adult'south height might exist ane.655 m (v.430 feet), and some other developed's meridian might be 1.656 g (5.433 anxiety). Adult height ranges from less than 5 feet to more than 6 feet, with males being somewhat taller than females on average. The bulk of people fall virtually the centre of the range of heights for their sex, every bit shown in the graph in Figure \(\PageIndex{two}\).

Environmental Effects on Phenotype

Many traits are affected by the environs every bit well as by genes. This may be especially true for polygenic traits. For example, adult top might be negatively impacted by poor diet or disease during childhood. Skin colour is some other polygenic trait. There is a wide range of skin colors in people worldwide. In add-on to differences in skin colour genes, differences in exposure to ultraviolet (UV) calorie-free cause some of the variations. Equally shown in Figure \(\PageIndex{3}\), exposure to UV light darkens the skin.

tan arm — Figure \(\PageIndex{3}\): Skin on the lower part of the arm is much darker in colour than protected skin near the pinnacle due to the effects of UV radiations.

Pleiotropy

Some genes impact more i phenotypic trait. This is called pleiotropy. At that place are numerous examples of pleiotropy in humans. They generally involve important proteins that are needed for the normal development or functioning of more than ane organ organisation. An example of pleiotropy in humans occurs with the gene that codes for the chief protein in collagen, a substance that helps form bones. This protein is also of import in the ears and eyes. Mutations in the gene result in problems not only in bones but as well in these sensory organs, which is how the cistron's pleiotropic effects were discovered.

Another example of pleiotropy occurs with sickle cell anemia. This recessive genetic disorder occurs when there is a mutation in the gene that ordinarily encodes the red claret cell protein chosen hemoglobin. People with the disorder have two alleles for sickle-cell hemoglobin, so named for the sickle shape (Effigy \(\PageIndex{4}\)) that their red blood cells take on under certain weather condition such as physical exertion. The sickle-shaped red blood cells clog modest blood vessels, causing multiple phenotypic effects, including stunting of physical growth, certain os deformities, kidney failure, and strokes.

Sickle cell Anemia RBC — Figure \(\PageIndex{4}\): The sickle-shaped red claret cell on the left is shown next to several normal red claret cells for comparing.

Epistasis

Some genes impact the expression of other genes. This is called epistasis. Epistasis is similar to dominance, except that it occurs between unlike genes rather than between unlike alleles for the same gene.

Albinism is an example of epistasis. A person with albinism has virtually no paint in the peel. The condition occurs due to an entirely dissimilar gene than the genes that encode skin color. Albinism occurs because a protein called tyrosinase, which is needed for the production of normal pare pigment, is not produced due to a gene mutation. If an individual has albinism mutation, he or she will not have whatsoever skin paint, regardless of the skin color genes that were inherited.

Feature: My Human Torso

Do y'all know your ABO blood blazon? In an emergency, knowing this valuable slice of data could maybe save your life. If you lot ever need a blood transfusion, it is vital that you receive blood that matches your own blood type. Why? If the blood transfused into your body contains an antigen that your ain blood does not contain, antibodies in your blood plasma (the liquid part of your blood) will recognize the antigen every bit foreign to your trunk and cause a reaction called agglutination. In this reaction, the transfused red claret cells volition dodder together, equally shown in the image below. The agglutination reaction is serious and potentially fatal.

Type A blood typing via agglutination — Figure \(\PageIndex{5}\): 2 samples of the same blood are shown here. The sample on the left is mixed with anti-B antibodies; the sample on the right is mixed with anti-A antibodies. Agglutination by the anti-A antibodies on the right shows that the sample is type A blood.

Knowing the antigens and antibodies nowadays in each of the ABO blood types volition aid you sympathize which type(s) of blood you tin safely receive if you e'er need a transfusion. This data is shown in the table below for all of the ABO blood types. For example, if you have blood type A, this ways that your cerise blood cells have the A antigen and that your blood plasma contains anti-B antibodies. If you were to receive a transfusion of type B or type AB blood, both of which accept the B antigen, your anti-B antibodies would assail the transfused red blood cells, causing agglutination.

Table \(\PageIndex{2}\): Antigens and antibodies in ABO claret types
Characteristics	Type A	Type B	Type AB	Blazon O
Cherry-red Claret Cell
Antibodies in Plasma	Anti-B	Anti-A	None	Anti-A and Anti-B
Antigens in Scarlet Blood Cells	A antigen	B antigens	A and B antigens	None

You may have heard that people with blood type O are chosen universal donors and that people with claret type AB are chosen universal recipients. People with type O blood accept neither A nor B antigens in their blood, and so if their blood is transfused into someone with a different ABO claret type, it causes no immune reaction. In other words, they can donate blood to anyone. On the other hand, people with type AB claret take no anti-A or anti-B antibodies in their claret, so they tin can receive a transfusion of claret from anyone. Which blood type(southward) can safely receive a transfusion of type AB blood, and which blood type(s) can be safely received by those with type O blood?

Review

What is not-Mendelian inheritance?
Explain why the homo ABO blood group is an example of a multiple allele trait with codominance.
What is incomplete dominance? Give an example of this type of non-Mendelian inheritance in humans.
Explain the genetic basis of human skin color.
How may the human trait of adult height be influenced past the environment?
Ascertain pleiotropy, and give a human case.
What is the difference between pleiotropy and epistasis?
Which of the following terms all-time matches each trait description? Choose simply the one term that all-time fits each trait. (codominance; multiple allele trait; Mendelian trait; polygenic trait)
1. A trait controlled past four genes.
2. A trait where each allele of a heterozygote makes an equal contribution to the phenotype.
3. A trait controlled by a unmarried gene that has three different versions.
4. A trait controlled past a single gene where one allele is fully dominant to the merely other allele.
People with type AB blood accept:
1. anti-O antibodies
2. anti-A and anti-B antibodies
3. A and B antigens
True or False . People with type O blood cannot receive a blood transfusion from anyone besides others with type O blood.
Truthful or Fake . People with blazon O blood can be heterozygous for this trait.

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