Simple genetics: a recessive symptom is ...

A recessive sign is a symptom that does not manifest itself if the genotype has a dominant allele of the same sign. In order to better understand this definition, let's look at how the coding of attributes takes place at the genetic level.

A bit of theory

Each feature in the human body is encoded by two allelic genes, one from each of the parents. Allelic genes are usually divided into dominant and recessive genes . If the gamete has both a dominant and a recessive allelic gene, then the phenotype will show a dominant feature. This principle illustrates a simple example from a school biology course: if one of the parents has blue eyes and the other has brown eyes, then the child will most likely have brown eyes, since blue is a recessive sign. This rule works if in the genotype of the brown-eyed parent both corresponding alleles are dominant. Let the gene responsible for the brown eyes, A, for the blue - a. Then when crossing, there are several possible variants:

P: AA x aa;

F1: Aa, Aa, Aa, Aa.

All the offspring are heterozygous, and all have a dominant trait - brown eyes.

The second possible option:

P: Aa x aa;

F1: Aa, Aa, aa, aa.

With this crossing, a recessive symptom (these are blue eyes) also appears. The probability that the child will be blue-eyed is 50%.

In a similar way, albinism (a violation of pigmentation), color blindness, hemophilia is inherited. These are recessive signs of a person that manifest themselves only in the absence of a dominant allele.

Features of recessive traits

Many recessive signs arose as a result of gene mutations. Let's recall, for example, the experience of Thomas Morgan with flies-drosophila. The normal eye color for flies is red, and the cause of white flare of some flies was a mutation in the X chromosome. So there was a recessive trait connected to the floor.

Hemophilia A and color blindness also refer to recessive traits linked to the sex.

Consider the crossing of recessive traits on the example of color blindness. Let the gene responsible for the normal perception of colors - X, and the mutant - X ^d . Crossing is as follows:

P: XX x X ^d Y;

F1: XX ^d , XX ^d , XY, XY.

That is, if the father suffered color blindness, and the mother was healthy, then all the children will be healthy, but the girls will be carriers of the color blindness gene, which is likely to occur in 50% of their male children. In women, color blindness is extremely rare, as a healthy X chromosome compensates for the mutant.

Other types of gene interaction

The previous example with eye color is an example of complete dominance, that is, the dominant gene completely drowns the recessive gene. The symptom manifested in the genotype corresponds to the dominant allele. But there are cases when the dominant gene does not suppress recessive completely, and in the offspring there is something middle - a new sign (codomination), or both genes manifest themselves (incomplete dominance).

Co-domination is a rare phenomenon. In the human body, codomination is manifested only by the inheritance of blood groups. Suppose that one of the parents has a second blood group (AA), the second group has a third group (BB). Both signs A and B are dominant. When crossing, we get that all children have a fourth blood group, coded as AB. That is, the phenotype showed both signs.

Also the color of many floral plants is inherited. If you cross a red and white rhododendron, then the result may be red, and white, and a two-color flower. Although red and dominant in this case, it does not suppress a recessive trait. This interaction, in which both signs will manifest in the genotype equally intensively.

Another unusual example is related to codomination. When crossing the red and white cosme, the result may turn pink. Pink color appears as a result of incomplete dominance, when the dominant allele interacts with the recessive allele. Thus, a new, intermediate feature is formed.

Non-allelic interaction

It is worth mentioning that the incomplete dominance is not inherent in the human genotype. The mechanism of incomplete dominance is not applicable to inheritance of skin color. If one of the parents has a dark skin, another has a light, and the child has a swarthy, intermediate variant, then this is not an example of incomplete dominance. In this case, the interaction of non-allelic genes occurs .