what is the ratio of genotypic and phenotypic ratio in trihybrid cross
The Correct Answer and Explanation is:
In a trihybrid cross, three traits are being studied, each with two alleles, typically with one being dominant and the other recessive. The most common way to illustrate a trihybrid cross is using the Punnett square.
For example, let’s consider three traits with the following alleles:
- Trait 1: A (dominant) and a (recessive)
- Trait 2: B (dominant) and b (recessive)
- Trait 3: C (dominant) and c (recessive)
The F1 generation of a trihybrid cross will have the following genotypes from the parents: AaBbCc x AaBbCc.
Phenotypic Ratio:
To determine the phenotypic ratio, we consider the probability of each dominant allele being expressed in the offspring. Since dominant alleles mask the effect of recessive ones, the phenotype depends on whether at least one dominant allele is present for each trait.
For each individual trait (A, B, and C), the genotype can be:
- AA or Aa will result in the dominant phenotype.
- aa will result in the recessive phenotype.
The probability of having the dominant phenotype for a single trait is 3/4 (AA, Aa), and the probability of having the recessive phenotype is 1/4 (aa).
For a trihybrid cross, the phenotypic ratio of offspring would be calculated by multiplying the probabilities for each trait:
- Dominant for all three traits: 3/4×3/4×3/4=27/643/4 \times 3/4 \times 3/4 = 27/643/4×3/4×3/4=27/64
- Two dominant traits, one recessive: 3/4×3/4×1/4=9/643/4 \times 3/4 \times 1/4 = 9/643/4×3/4×1/4=9/64 (This happens in several combinations, and there are 3 such combinations for different pairings of the recessive traits)
- One dominant trait, two recessive: 3/4×1/4×1/4=3/643/4 \times 1/4 \times 1/4 = 3/643/4×1/4×1/4=3/64
- Recessive for all three traits: 1/4×1/4×1/4=1/641/4 \times 1/4 \times 1/4 = 1/641/4×1/4×1/4=1/64
Thus, the phenotypic ratio is:
- 27: 27: 9: 9: 3: 3: 1: 1 (Dominant traits in all combinations).
Genotypic Ratio:
The genotypic ratio, on the other hand, is the ratio of the different genotype combinations produced from the trihybrid cross. It is determined by calculating the possible combinations of alleles for each gene.
For each gene (A, B, and C), the possible genotypes are:
- AA, Aa, aa for gene A
- BB, Bb, bb for gene B
- CC, Cc, cc for gene C
Therefore, the total number of genotype combinations in a trihybrid cross would be 27, and their distribution would be as follows:
- AA BB CC: 1/64
- AA BB Cc: 2/64
- AA Bb CC: 2/64
- AA Bb Cc: 4/64
- Aa BB CC: 2/64
- Aa BB Cc: 4/64
- Aa Bb CC: 4/64
- Aa Bb Cc: 8/64
- aa BB CC: 1/64
- aa BB Cc: 2/64
- aa Bb CC: 2/64
- aa Bb Cc: 4/64
Thus, the genotypic ratio is a more detailed breakdown of how the alleles are distributed across the offspring, and it can be derived by expanding the Punnett square for the three genes involved.
Summary:
In a trihybrid cross, the phenotypic ratio is 27:27:9:9:3:3:1:1, while the genotypic ratio is much more complex, with 64 possible genotype combinations. These ratios help in understanding how genetic traits are inherited across generations.
