A recessive trait is a trait that does not manifest itself if there is a dominant allele of the same trait in the genotype. To better understand this definition, we will examine how the coding of characters occurs at the genetic level.
Bit of theory
Each trait 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. If the gamete has both a dominant and a recessive allelic gene, then a dominant trait will appear in the phenotype. This principle is illustrated by 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 gene A be responsible for the brown eyes, and for blue. Then, when crossing, several options are possible:
R: AA x aa;
F1: Aa, Aa, Aa, Aa.
All offspring are heterozygous, and all show a dominant trait - brown eyes.
The second possible option:
R: Aa x aa;
F1: Aa, Aa, aa, aa.
With this crossing, a recessive sign also appears (these are blue eyes). 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 traits of a person that appear only in the absence of a dominant allele.
Features of recessive symptoms
Many recessive traits have arisen as a result of gene mutations. Recall, for example, the experience of Thomas Morgan with Drosophila flies. The normal eye color for the flies is red, and the cause of the white eyes of some flies was a mutation in the X chromosome. So there was a recessive sign linked to the floor.
Hemophilia A and color blindness also relate to recessive symptoms linked to the floor.
Consider the crossbreeding of recessive symptoms by the example of color blindness. Let the gene responsible for the normal perception of colors - X, and the mutant - X d . Crossing occurs as follows:
P: XX x X d U;
F1: XX d , XX d , XY, XY.
That is, if the father suffered from color blindness, and the mother was healthy, then all children will be healthy, but the girls will be carriers of the color blind gene, which with a 50% probability will be manifested in their male children. In women, color blindness is extremely rare, since a healthy X chromosome compensates for the mutant one.
Other types of gene interaction
The previous example with eye color is an example of complete dominance, that is, the dominant gene completely drowns out the recessive gene. The trait manifested in the genotype corresponds to a dominant allele. But there are cases when the dominant gene does not completely suppress the recessive one, and something intermediate appears in the offspring - a new trait (coding), or both genes manifest themselves (incomplete dominance).
Codomination is a rare occurrence. In the human body, coding is manifested only by the inheritance of blood groups. Let one of the parents have a second blood group (AA), the second - the third group (BB). Both traits A and B are dominant. When crossing, we get that all children have a fourth blood group, encoded as AB. That is, both signs appeared in the phenotype.
The color of many flower plants is also inherited. If you cross the red and white rhododendron, then the result can be red, and white, and a two-color flower. Although red is dominant in this case, it does not suppress the recessive trait. This is an interaction in which in the genotype both characters appear equally intensely.
Another unusual example is related to coding. When crossing red and white cosmea, pink may result. The pink color appears as a result of incomplete dominance when the dominant allele interacts with the recessive one. Thus, a new, intermediate attribute is formed.
Non-allelic interaction
It is worth mentioning that the incomplete dominance is not characteristic of the human genotype. The mechanism of incomplete dominance is not applicable to the inheritance of skin color. If one of the parents has dark skin, the other has fair skin, and the child has a dark, intermediate version, then this is not an example of incomplete dominance. In this case, the interaction of non-allelic genes occurs .