Dihybrid Cross: Building on Mendelian Genetics and Punnett Squares: Two Genes Example
We will build our understanding of Mendelian genetics and Punnett squares by starting to think about two different genes. We’ll consider the pea plant and the genes for pea color and pea shape.
Mendelian Genetics and Punnett Squares
Let’s say we have one parent that is homozygous dominant for both of these genes:
- Gene for pea color: Y (capital Y represents yellow, lowercase y represents green)
- Gene for pea shape: R (capital R represents round, lowercase r represents wrinkled)
The genotype of this parent is capital Y capital Y and capital R capital R. The phenotype, or physical appearance, is yellow and round.
The other parent is homozygous recessive for both genes:
- Gene for pea color: y (lowercase y represents green)
- Gene for pea shape: r (lowercase r represents wrinkled)
The genotype of this parent is lowercase y lowercase y and lowercase r lowercase r. The phenotype is green and wrinkled.
What is the phenotype of the offspring of these parents? The genotype of the offspring will be a combination of the alleles inherited from both parents. In this case, the offspring (F1 generation) will have the genotype capital Y r lowercase y r and the phenotype yellow and round.
Now, let’s consider crossing one of the F1 generation with itself or with another member of the F1 generation (Dihybrid cross). This process is illustrated using a 4×4 Punnett square.
The law of segregation and the law of independent assortment explain how the alleles are inherited from each parent.
- Law of Segregation: An individual receives one copy of each gene from each parent. During formation of gametes, the two alleles separate, so the gamete has a 50% chance of getting either allele.
- Law of Independent Assortment: The alleles of different genes are distributed independently into the gametes. This means that the distribution of one gene’s alleles does not affect the distribution of another gene’s alleles.
The genotype and phenotype of the offspring produced from the cross of two F1 generation members will be varied.
- 9 yellow round plants
- 3 yellow wrinkled plants
- 3 green round plants
- 1 green wrinkled plant
These 16 combinations are expected to produce a 9:3:3:1 ratio for these phenotypes. This is a simplified explanation of the principles behind Mendelian genetics and Punnett squares. The actual outcomes of crosses may have slight variations from this ratio due to randomness and other factors.
Key Points For Dihybrid Cross
- The focus is on two different genes: one for pea color and one for pea shape.
- In the parental generation, one parent is homozygous dominant for both genes (YYRR), resulting in yellow, round peas.
- The other parent is homozygous recessive for both genes (yyrr), resulting in green, wrinkled peas.
- In the F1 generation, all individuals will have the genotype YyRr, meaning they are heterozygous for both genes (dihybrid).
- The phenotype of the F1 generation plants will still have yellow, round peas.
- When F1 generation plants are crossed, the law of segregation and the law of independent assortment apply.
- The law of independent assortment only applies when genes are on different chromosomes, which is true for most genes.
- In the F2 generation, the phenotypic ratio will be 9:3:3:1 for yellow round, yellow wrinkled, green round, and green wrinkled, respectively.
- This ratio will not be exact every time but will be close to it.
