Inheritance of Two Genes: According to this law, the alleles of two pairs of trait separate independently of each other during gamete formation, and get randomly rearranged in the offspring at the time of fertilization, producing both parental and new combinations of traits.
Mendel first studied the inheritance of one gene in the plant through monohybrid cross. He considered only a single character (plant height) on pairs of pea plants with one contrasting trait. Later, he studied the inheritance of two genes in the plant through dihybrid cross.
Mendel studied the following seven characters with contrasting traits:
- Stem height: Tall/dwarf
- Seed shape: Round/wrinkled
- Seed colour: Yellow/green
- Pod colour: Green/yellow
- Pod shape: Inflated/constricted
- Flower colour: Violet/white
- Flower position: Axial/terminal
Inheritance of two genes is defined as the study of the inheritance of two genes controlling two different characters. A dihybrid cross is a cross between two individuals who differ in two traits controlled by two separate genes. If both parents are homozygous for both genes, the offspring of the F1 generation will be uniformly heterozygous for both genes and will have the dominant phenotype for both characteristics.
When homozygous parents with two pairs of contrasting characteristics cross, one pair’s inheritance is independent of the others. In other words, when a dihybrid produces gametes, the assortment (distribution) of alleles or traits is unaffected by the parents’ initial pairings. The dihybrid cross or dihybrid ratio can be used to explain this law.
Inheritance of Two Genes or Dihybrid Cross
A dihybrid cross is a breeding experiment between two organisms which are identical hybrids for two traits. In other words, a dihybrid cross is a cross between two organisms, with both being heterozygous for two different traits. The individuals in this type of trait are homozygous for a specific trait. These traits are determined by DNA segments called genes.
In a dihybrid cross, the parents carry different pair of alleles for each trait. One parent carries homozygous dominant allele, while the other one carries homozygous recessive allele. The offsprings produced after the crosses in the F1 generation are all heterozygous for specific traits.
Mendel crossed a pair of contradictory features at a time, such as seed colour and shape. He took the wrinkled-green seed and crossed it with the round-yellow seed. In the F1 generation, he only got round-yellow seeds. This showed that seeds with a round shape and a yellow colour are dominant.
Seeds’ wrinkled shape and green colour, on the other hand, are recessive traits. The F1 offspring were then self-hybridised. In the F2 generation, this resulted in four possible seed combinations. In the phenotypic ratio of 9:3:3:1, there were round-yellow, round-green, wrinkled-yellow, and wrinkled-green seeds.
Seed colour and seed shape are the two traits being studied in this dihybrid cross. The dominant characteristics of yellow seed colour (YY) and round seed shape (RR) are homozygous in one plant (YYRR). In contrast, the recessive traits of green seed colour and wrinkled seed shape are homozygous in the other (yyrr).
Consider the letters “Y” for yellow seeds and “y” for green seeds, “R” for round seeds and “r” for wrinkled seeds. As a result, the parental genotype will be “YYRR” (yellow-round seeds) and “yyrr” (yellow-round seeds) (green-wrinkled seeds).
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