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, let's look at how traits are encoded at the genetic level.
A bit of theory
Each trait in the human body is encoded by two allelic genes, one from each parent. 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 is likely to have brown eyes, since blue is a recessive trait. This rule works if both corresponding alleles are dominant in the genotype of the brown-eyed parent. Let gene A be responsible for brown eyes, and a for blue eyes. 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.
Second possible option:
R: Aa x aa;
F1: Ah, Ah, ah, ah.
With such a crossing, a recessive trait is also manifested (these are blue eyes). There is a 50% chance that a child will be blue-eyed.
Albinism (disturbance of pigmentation), color blindness, hemophilia are inherited in a similar way. These are recessive human traits that appear only in the absence of a dominant allele.
Features of recessive traits
Many recessive traits are the result of gene mutations. Consider, for example, Thomas Morgan's experience with fruit flies. The normal eye color for flies is red, and the cause of white-eyed flies was a mutation on the X chromosome. This is how the sex-linked recessive trait appeared.
Hemophilia A and color blindness are also sex-linked recessive traits.
Let's consider the crossing of recessive traits using the example of color blindness. Let the gene responsible for the normal perception of colors be X, and the mutant gene Xd. Crossing happens like this:
P: XX x XdY;
F1: XXd, XXd, XY, XY.
That is, if the father suffered from color blindness, and the mother was he althy, then all the children will be he althy, but the girls will be carriers of the gene for color blindness, which will manifest itself in their male children with a 50% probability. In women, color blindness is extremely rare, as a he althy X chromosome compensates for a mutant one.
Other types of gene interactions
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 that appears in the genotype corresponds to the dominant allele. But there are cases when the dominant gene does not completely suppress the recessive one, and something in between appears in the offspring - a new trait (codominance), or both genes manifest themselves (incomplete dominance).
Co-dominance is rare. In the human body, coding is manifested only by the inheritance of blood groups. Let one of the parents have the 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 the fourth blood group, coded as AB. That is, both traits appeared in the phenotype.
The color of many flower plants is also inherited. If you cross a red and white rhododendron, then the result can be red, and white, and a two-color flower. Although the red color is dominant in this case, it does not suppress the recessive trait. This is an interaction in which both traits will appear equally intense in the genotype.
Another unusual example has to do with co-dominance. When crossing red and white cosmos, the result can be pink. Pink color appears as a result of incomplete dominance, when the dominant allele interacts with the recessive one. Thus, a new, intermediate sign is formed.
Non-allelic interaction
Worth itmake a reservation that incomplete dominance is not characteristic of the human genotype. The mechanism of incomplete dominance does not apply to the inheritance of skin color. If one of the parents has dark skin, the other is light, and the child has dark skin, an intermediate option, then this is not an example of incomplete dominance. In this case, it is precisely the interaction of non-allelic genes that occurs.
