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How to solve incomplete and codominance problems


Incomplete dominance

Sometimes, when you inherit some traits, the result phenotype of the heterozygous is different, then phenotype of homozygous by dominant trait and homozygous by recessive trait, and has the intermediate value between them. It was found that in all these cases is occures genetic interactions between alleles of the same gene. If in the heterozygous state, none of alleles is not dominant over the other, that mean, it is not completly dominant or recessive, then such a genetic interaction is called incomplete dominance. The phenotypic ratio in incomplete dominance is equal to the genotypic ratio (1:2:1). Thus, incomplete dominance is an exception from the rules of inheritance for monohybrid crosses, which are described by Mendel. Fortunately, Mendel chose for his experiment traits, that do not have an incomplete dominance, since otherwise it was be very complicate of his first study. Of course, when we talk about the interaction of genes, we mean the interaction of the products of these genes.

Coloration of flowers in plants Mirabilis jalapa (the four o'clock flower or marvel of Peru plant ).

Incomplete dominance can be observed both in plants and animals. A classic example of incomplete dominance is the inheritance of flower color in plants Mirabilis jalapa (The four o'clock flower or marvel of Peru). When crossing a plant with white flowers (recessive homozygous "aa") with a plant whose flowers are red (homozygous dominant "AA"), in the first generation we have hybrid offspring ("Aa"), which will have pink flowers. And when we cross these hybrids we get the offspring with phenotypes ratio 1 red : 2 pink : 1 white. Using the rules of making genetic traits files we can create our traits file:

A a:pink

color:A a

Genetic calculation: You can open this traits file ( Incomplete dominance 1.txt ) and calculate results for Traits phenotypes. As result of crossing the hybrids of first generation we get the offspring with phenotypes ratio: 1 red : 2 pink : 1 white. From the version 3.3, on the tab "Find" you can choose each phenotype and see from what kind of genotypes it is consist.

Plumage coloration in Andalusian chickens.

Another example of incomplete dominance can be Andalusian chickens (Andalusian chickens), which are obtained from crosses of pure line of black hens and "splashed white" hens. The black color of hens determined by the allele, which is responsible for the synthesis of melanin pigment ("B"). In "splashed white" hens, this allele is lack ("bb"). And in heterozygotes ("Bb") melanin is synthesized in small quantities and only gives a bluish sheen in the plumage. Genetic traits file for this case should be:

B b:blue andalusian
b:splashed white

color:A a

Genetic calculation: You can open this traits file( Incomplete dominance 2.txt ) and calculate results for Traits phenotypes. As result of crossing the hybrids of first generation we get the offspring with phenotypes ratio: 1 black : 2 blue andalusian : 1 splashed white. On the tab "Find" you can choose each phenotype and see from what kind of genotypes it is consist.


Sometimes the heterozygous individual can exceeded by its properties of his homozygous recessive and homozygous dominant parents. We can say that heterozygotes have some selective advantage over homozygous individuals. This advantage may be invisible and appears only under certain environmental conditions. This phenomenon was be called overdominance. But it should be noted that overdominance is a special case of incomplete dominance. Let's look at the example of resistance to the malaria, which have the heterozygous carriers of the gene of sickle-cell anemia.

Inheritance of sickle-cell anemia.

It often happens that the heterozygous individual is similar to a homozygous dominant, and the differences are observed only under certain conditions. Such heterozygous are called the carriers of the trait. This is typical for some hereditary diseases in humans, when it is necessary to determine if a person is a carrier of the disease. The example of it, can be the inheritance of sickle-cell anemia. The main function of erythrocytes is the delivery of oxygen from the lungs to the tissues and carbon dioxide from the tissues to the lungs. This transfer is possible due to the fact that red blood cells have a special respiratory pigment - hemoglobin. Hemoglobin in people with sickle cell anemia is different from the normal only in that as a result of a single mutation, the glutamic acid at position 6 in the B-chain is replaced by valine. Such a small difference but has a significant influence on the properties and function of hemoglobin. If normally the red blood cells have a biconcave disk shape, then in the people with the disease, they get sickle-cell form and become more brittle and can decay more quickly. Reducing the number of platelets in humans causes symptoms of anemia. In present time is known a quite a number of abnormal forms of hemoglobin. Inheritance of each of them is determined by a particular gene. Let's mark the recessive allele of a gene, that causes sickle-cell anemia as "<Hbs>", as well as the dominant allele "<HbA>". People, who are homozygous by the recessive allele ("<Hbs> <Hbs>") will have sickle cell anemia, and people with genotype "<HbA> <HbA>" will be completely healthy. First, this severe form of anemia was found and described in the West Indies in 1910. Most people with sickle cell anemia died at an early age until will become available the qualified medical aid. Children with this disease are born from two heterozygous parents with the genotype "<HbA><Hbs>", each of them is a carrier of gene of sickle-cell anemia. Red blood cells of heterozygous carriers have normal shape and contain a mix of normal or abnormal hemoglobins. Heterozygous individuals are healthy, but under certain conditions at them may appear symptoms of the disease. If the oxygenation level of the blood in these people is going down, their red blood cells is turning in to the sickle-cell form and become brittle. This can happen for example in the mountains, where as you know there is a reduced concentration of oxygen. The attack of the disease can also cause the intense physical activity, when the body loses the water. Carriers of the gene "<Hbs>" are most common in areas where malaria is prevalent. Stirrer of malaria, which is often fatal, is Plasmodium falciparum. It was found, that patients with sickle-cell anemia have an hereditary increased resistance ( but not absolute ) to infection by this parasite. The parasite simply cannot survive in the red blood cells with sickle-cell forms, of such individuals. Heterozygous carriers of the gene that do not suffer from sickle cell anemia, also have increased resistance to malaria. They are much less suffer from malaria, and if get sick, then recover faster. Such advantage of heterozygotes is probably explains the fact, that the gene of sickle-cell anemia is still prevalent in African populations of humans. Thus, this case is a perfect illustration of overdominance. Now let's write the genetic traits file:

<HbA> <Hbs>:sickle-cell anemia carrier
<HbA>:normal hemoglobin
<Hbs>:sickle-cell hemoglobin

hemoglobin:<HbA> <Hbs>

Genetic calculation: You can open this traits file ( Overdominance.txt ) and calculate results for Traits phenotypes. As result of crossing the hybrids "<HbA><Hbs>" we get the offspring with phenotypes ratio: 1 normal hemoglobin : 2 sickle-cell anemia carrier : 1 sickle-cell hemoglobin. On the tab "Find" you can choose each phenotype and see from what kind of genotypes it is consist.

Multiple alleles

In all the examples, that described above, a trait was controlled by a single gene, which was represented by one of two alleles. However, we know quite a lot of cases, where a trait is expressed in several different variants and controlled by three or more alleles of one gene. In the corresponding loci of homologous chromosomes can be located any of the two alleles of the group. In such cases is talking about multiple alleles. For example, for a locus "white" of drosophila, which determines the color of the eyes, known more than 20 alleles. The wild allele "w +" gives a brick-red color of the wild type. The opposite allele "w" - completely suppresses the formation of pigment and gives white eyes. Other alleles of this group, such as "wa" (apricot), "we '(eosinic)," wi "(ivory), give us the intermediate color between brick-red and completely white.

Coloration of rabbits.

Another interesting example can be a series of multiple alleles of the gene in the rabbit, which control the color of the fur. These alleles can be marked as C<+>; C<Ch>; C<H> and C<a>. Wild-type allele C <+> is dominate over other alleles. Rabbits with this allele in the genotype have a wild-type coloration - gray (agouti), that is typical for these animals. Color of individuals, which homozygous by the allele C<Ch>, is lighter, than the wild type, an it's looks like a chinchilla. Heterozygous rabbits with genotypes C<Ch>C<H> and C<Ch>C<a> have a light gray fur - intermediate between schinchilla and white. Rabbits with genotypes C<H>C<H> and C<H>C<a> have himalayan phenotype. They have white fur everywhere except the legs, tail, ears and nose. And rabbits homozygous by allele C<a> - is albino. They have white fur and pink eyes. Let's write the traits file for this case:

C<+>:agouti rabbit
C<Ch> C<H>:light gray rabbit
C<Ch> C<a>:light gray rabbit
C<Ch>:chinchilla rabbit
C<H>:himalayan rabbit
C<a>:albino rabbit

color:C<+> C<Ch> C<H> C<a>

Genetic calculation: You can open this traits file ( Multiple alleles.txt ) and calculate results for Traits phenotypes. For a parents you can choose such heterozygotes with genotypes C<Ch>C<H>, C<Ch><a> or C<Ch>C<a> - in any combination and see what ratio was be obtained in the Traits phenotypes. On the tab "Find" you can choose each phenotype and see from what kind of genotypes it is consist.

Such traits like a color of mice, eye color in mice and human blood groups are controlled by multiple alleles. In human populations the multiple alleles are widespread. For example in humans is now known at least 51 independent loci of blood groups with more than 70 alleles. Consider this in more detail on the example of the inheritance of blood system of ABO.


But before we proceed to the consideration of this example is necessary to define some concepts. Because on the example of inheritance of the ABO blood we will also consider the codominance, which also refers to a variants of allelic interaction. There is a difference between incomplete dominance and codominance. This difference lies in the fact that the formation of phenotype of heterozygotes in codominance is due to the presence of products of the both interacting genes. We can say that the alleles have an additive effect on the phenotype.

ABO blood type inheritance.

Blood type is controlled by three alleles of one gene - I<o>, I<A>, I<B> (such marking of allelic genes can be used in the writing of genetic traits files and solving the problems with multiple alleles). These genes are responsible for the formation or lack of isoagglutinogens, which is belong to the mucopolysaccharides, and are found in erythrocyte membranes. Red blood cells of homozygotes I<A>I<A> (blood group A) have on their surface antigen "A", and homozygotes I<B>I<B> (blood group B) - antigen "B". Red blood cells of homozygotes I<o>I<o> have a lack of both these antigens. Red blood cells of heterozygotes I<A>I<o> and I<B>I<o> on the surface have only antigens "A" or only antigens "B" - accordingly. Accordingly, the blood in the first case will be "A", and in the second "B". It is already known case of complete dominance. If a individual has a heterozygous genotype I<A>I<B>, then his red blood cells will have both antigens - "A" and "B" on the surface (blood group AB). In this case, it is codominance. Alleles I<A> and I<B> work in the heterozygote, as though independently of each other. Knowledge of the genetic control of blood groups is essential. The fact that people with blood group "o" in the blood plasma contains haemagglutinin "a" and "b", with the group "A" - hemagglutinin ''b", and with group "B"- hemagglutinin "a". But in people with blood group "AB" do not have these agglutinins. Agglutinin "a" is able to bind and precipitate the red blood cells with antigen "A", and the agglutinin "b" - the red blood cells with antigen "B". On these interactions is based the system of blood transfusion. Blood of group "o" can be transfused to all the people, blood of group "A" to people with groups "A" and "o", blood of group "B" - to people with groups "B" and "o", and the blood of group "AB" only to people with the same group. Since agglutinins of blood plasma are able to bind red blood cells, the violation of these rules may lead to hemorrhagic shock. hese patterns are also used in criminalistics to identify the spots of blood and to determination of paternity. Now let's write the genetic traits file:

I<A> I<B>:blood group AB
I<A>:blood group A
I<B>:blood group B
I<o>:blood group o

blood groups:I<A> I<B> I<o>

Genetic calculation: You can open this traits file ( Codominance.txt ) and calculate results for Traits phenotypes. As an illustration, take the following example. Heterozygous mother has blood "A", and the heterozygous father the blood "B". Then their childrens can get any of the four blood groups with equal probability. On the tab "Find" you can choose each phenotype of these childrens and see from what kind of genotypes it is consist.

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