The dihybrid cross
To understand the dihybrid cross it is necessary to understand the process of meiosis. You must have a clear understanding of the stages of meiosis. In the first meiotic division, the homologous pairs of chromosomes have aligned at random before being separated from each other. The separation of the chromosomes gives rise to the independent assortment of the chromosomes. In the second meiotic division, the four cells that are formed develop into haploid gametes.
All genetic crosses show how characteristics are inherited through the generations:
a monohybrid cross shows the inheritance of one through two generations. From parents to the F1 generation and then from the F1 generation to the F2 generation.
a dihybrid cross shows the inheritance of two through two generations. In a dihybrid cross the genes that determine the two characteristics are located on different pairs of homologous chromosomes. Each of these genes can have two or more alleles.
The table show the alleles for two genes on two different pairs of homologous chromosomes in gamete mother cells from two different pea plants (parents).
Colour and shape of pea plants
Colour Y = allele for yellow seeds y = allele for green seeds
Shape R = allele for round seeds r = allele for wrinkled seeds
Crosses have a recognised format and the structures of chromosomes are not shown. Instead, the letters used to identify the alleles of the genes are used to represent the chromosome. Thus, when showing a cross, YYRR would represent two homologous pairs of chromosomes. On one of the homologous pairs both alleles would be for the dominant form of the gene for seed colour and in the other, both alleles would be for the dominant form of the gene for seed shape.
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