Answer to Question #349185 in Genetics for dav

Question

Explain monohybrid and dihybrid crosses including co-dominance, sex linkage and inherited genetic conditions

Answer 1

As examples of co-dominance, we give the formation of flower variegation in monohybrid cross, as well as the formation of different blood types with the different Rh factors in dihybrid cross.

• Let the gene that is responsible for the purple color of the petals be A, and the gene that is responsible for the white color of the petals be a. Consider this cross:

AA (purple) × aa (white)

↓

4 Aa (variegated)

In this case, genes A and a are expressed in the same way in the heterozygous state, and the phenotype will be expressed as white dots on a purple background or vice versa.

• Let the gene that is responsible for the formation of the second blood type be I^A, and the gene that is responsible for the formation of the third blood type be I^B. The I^O gene in the homozygous state will be responsible for the formation of the first blood type. The I^A and I^B genes will show dominant epistasis in relation to the I^O gene, and the genotypes I^AI^O and I^BI^O will show the same phenotype as the genotypes I^AI^A and I^BI^B, respectively. At the same time, the R gene is responsible for the positive Rh factor, and the r gene is responsible for the negative Rh factor. Consider the following cross:

I^AI^ORr (second blood type, positive Rh factor) × I^BI^Orr (third blood type, negative Rh factor)

↓

In this case, with the I^AI^B genotype, the genes I^A and I^B will be expressed in the same way with the formation of the fourth blood type. That is, the I^A and I^B genes are codominant, but the R gene is completely dominant.

As an example of sex linkage, let's take the development of different states of color vision in different sexes. In this case, only a monohybrid cross is known.

• Let the gene responsible for normal color vision be A, and the gene responsible for the development of color blindness be a. These genes are linked to the X chromosome, and if a woman has the a gene on one of the X chromosomes, she will be a carrier, and a man who has the a gene on the only X chromosome will be colorblind. Among women who have the gene a on two X chromosomes at the same time, there are also colorblind people, but very rarely. Consider several types of cross:

X^AX^A (normal) × X^aY (colorblind)

↓

In this case, all women will be carriers, and men will be owners of normal vision.

X^AX^a (carrier) × X^aY (colorblind)

↓

In this case, women will be both carriers and colorblind, and men will be both normal and colorblind.

X^AX^a (carrier) × X^AY (normal)

↓

In this case, women will be both normal and carriers, and men will be owners of normal vision and colorblind.

As for inherited genetic conditions, let's give an example of the inheritance of such an autosomal recessive disorder as thalassemia. In this case, only a monohybrid cross is also known.

• Let the gene that is responsible for the normal state be A, and the gene that is responsible for the development of thalassemia be a. Let's look at this cross:

Aa (carrier) × Aa (carrier)

↓

In this case, among the offsprings in 1/4 of the cases there will be a normal individual, in another 1/4 of the cases there will be a patient with thalassemia, and in 1/2 of the cases - a carrier.