Dominant inheritance and recessive inheritance

We all know that many of a child's genes are inherited from their parents. For children, some of their parents' genes are good and some are bad. In junior high school biology, we learned about dominant and recessive genes. So what exactly are dominant and recessive genes? Let me give you a detailed introduction below!

Recessive inheritance

The so-called recessive inheritance means that parents carry a certain gene but do not develop the disease, but the gene is passed on to their offspring, causing them to develop the disease. This is the case with autosomal recessive polycystic kidney disease. The parents carry the gene but do not show any symptoms of polycystic kidney disease throughout their lives. Their offspring inherit the gene and become ill, with very severe symptoms at birth and infancy (some even during the fetal period). Such as red and green color blindness, hemophilia, and albinism.

Dominant inheritance

Dominant genetic disease (AA or Aa) means that whether or not you get the disease is determined by the dominant gene A, that is, as long as there is one A, it is dominant and you will get the disease. AA and Aa both have the disease, but aa has no disease-causing gene and will not get the disease.

Recessive inheritance

Recessive inheritance is divided into sex-linked inheritance and non-sex-linked inheritance.

Non-sex-linked recessive inheritance

A non-sex-linked recessive genetic disease (Aa or aa) means that whether or not you get the disease is determined by a and has nothing to do with chromosome X/Y. As long as A is there, it will be dominant, and the disease is recessive, so you will only get the disease when aa is present.

Example:

For example, dominant genes are capital letters, such as A

Recessive genes are lowercase letters, such as a

Two genes together form a pair of alleles, there are three types: AA, Aa, aa

As long as there is one A, such as AA, Aa, it will be dominant.

X-linked recessive inheritance

Genes related to a trait or genetic disease are located on the X chromosome. The nature of these genes is recessive and is passed on with the behavior of the X chromosome. The inheritance mode is called X-linked recessive inheritance (XR).

When inherited in a recessive manner, since women have two X chromosomes, when the recessive pathogenic gene is in a heterozygous state (XAXa), the trait or genetic disease controlled by the recessive gene does not appear, and such women are phenotypically normal carriers of the pathogenic gene. The disease is only manifested when both X chromosomes are homozygous for the recessive pathogenic gene (XaXa). In male cells, there is only one X chromosome, and the Y chromosome lacks the homologous segment, so as long as there is a recessive disease gene (XaY) on the X chromosome, the disease will occur. In this way, the male's cells only have one gene in the paired alleles, so it is called hemizygote.

Red and green color blindness

Red and green color blindness is a recessive genetic disease on the X chromosome (X-linked recessive inheritance). Color blindness can be divided into achromatopsis and dyschromatopsia of the protan and deutan. The former cannot distinguish any color and is generally considered to be autosomal recessive inheritance; the latter is the most common and manifests as reduced discrimination of red and green colors. It is inherited in an X-linked recessive manner, and the pathogenic gene is located at Xq28. According to reports, the incidence rate in males is 7.0% and in females is 0.5%. A male patient with red-green color blindness (XbY) marries a female with normal color discrimination ability (XBXB). Their daughters should receive an X chromosome from their father and a normal X chromosome from their mother and become heterozygous carriers of the disease gene (XBXb). Their sons must receive an XB from their mother, so their color discrimination abilities are all normal (XBY). When a woman who carries the disease-causing gene (XBXb) marries a man with normal color vision, in the next generation, half of the sons will be normal (XBY) and half will be green-blind (XbY), and half of the daughters will be carriers of the disease-causing gene (XBXb) and half will be completely normal (XBXB). Therefore, the father of a female patient must be a patient and her mother must be a carrier of the disease-causing gene. Here we can see the phenomenon of criss-cross inheritance of "from father to daughter, from mother to son". If a female carrier (XBXb) marries a male patient (XbY), among the offspring, 1/2 of the daughters may develop the disease, 1/2 may be carriers, and among the only sons, 1/2 will be diseased and 1/2 will be normal.