The Comparison between Characters

Fig.1:Gene expression of human dominance/recessiveness
Fig.1:Gene expression of human dominance/recessiveness

Source: Made by the project

【Comparison between Characters】
Genes in human cells are usually inherited in pairs—one from the mother and one from the father, respectively. How do cells express the paired genes? There are so many genes in cells and there are a variety of ways in determining how to produce proteins to express the characters, such as dominance/recessiveness, incomplete dominance, codominace and polygenic inheritance, etc. In addition to a variety of ways to express genes, some gene expressions are affected by other genes or the living environment, e.g. under different growth environments and nutrient conditions, persons with an identical height genotype will have different phenotypes. Hence, under the delicate regulation and expression by many genes, even if all siblings inherit each half of the genes from parents, it is very hard to find persons with identical appearances, except for twins.

Fig.2:Dominance/Recessiveness expression
Fig.2:Dominance/Recessiveness expression

Source: Made by the project

【Dominance/Recessiveness Expression】
The most common way to express genes is dominance/recessiveness. Genotypes are usually formed by two alleles, and when two different kinds of alleles are paired, the allele which suppresses the expression of the other allele is called a dominant allele, and the suppressed allele is called a recessive allele. Hence, among the two alleles from parents, if both are dominant or one of which is dominant, the dominant character will be expressed; only if both ones are recessive, the recessive character will be expressed. For example: In double-folded eyelid or single-edged eyelid inheritance, if both parental genotypes are shown as one dominant allele( shown as A)plus one recessive allele(shown as a), the probability at which their filial generations to show two dominant alleles(AA)is 1/4 and the probability to show one dominant allele plus one recessive allele(Aa)is 2/4, i.e. the probability for children to have double-folded eyelids is 3/4 and the probability for single-edged eyelids(aa)is only 1/4.

Fig 3. ABO blood type
Fig.3:ABO blood type

Source: Made by the project

【ABO Blood Type】
Some genes have three alleles, such as there are two dominant alleles(IA and IB)and one recessive allele(i)in the human ABO blood type, of which each one’s blood type is determined by the combination of two inherited alleles. If the genotype is IAIA  or IAi, one’s blood type is shown as type A; if the genotype is IBIB or IBi, one’s blood type is shown as type B; if the genotype is ii, one’s blood type is shown as type O; if the genotype is IAIB, of which both alleles are dominant, one’s blood type will be shown as type AB. So, if one of the parents has a IAi genotype, and the other one has a IBi genotype, they are likely to bear a baby with blood type O!

【Polygenic Inheritance】
Some characters which is determined by at least one pair of genes, such as skin color, height, etc., are called polygenic inheritance; human beings have more than three pairs of genes liable for inheritance of skin color, as the more the dominant alleles, the darker the skin color. If we document individual heights at the same age and sex and draw a statistical chart, a bell-shaped curve of normal distribution can be obtained. And most individual’s genotypes are shown as medium-sized height, yet the extremely high or short ones are fewer in number.

Reviewed by:Zhang, Yong-Ta professor

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