Question at position 3 If two of the offspring produced in the previous question are crossed, what are the expected phenotypes of their offspring, and in what proportions?Full, green pods = 9 Full, yellow pods = 3 Constricted, green pods = 3 Constricted, yellow pods = 1Full, green pods = 16 Full, yellow pods = 0 Constricted, green pods = 0 Constricted, yellow pods = 0Full, green pods = 4 Full, yellow pods = 4 Constricted, green pods = 4 Constricted, yellow pods = 4Full, green pods = 0 Full, yellow pods = 0 Constricted, green pods = 0 Constricted, yellow pods = 16BlackTom题目解析

Question

Question at position 3 If two of the offspring produced in the previous question are crossed, what are the expected phenotypes of their offspring, and in what proportions?Full, green pods = 9 Full, yellow pods = 3 Constricted, green pods = 3 Constricted, yellow pods = 1Full, green pods = 16 Full, yellow pods = 0 Constricted, green pods = 0 Constricted, yellow pods = 0Full, green pods = 4 Full, yellow pods = 4 Constricted, green pods = 4 Constricted, yellow pods = 4Full, green pods = 0 Full, yellow pods = 0 Constricted, green pods = 0 Constricted, yellow pods = 16BlackTom题目解析

BlackTom AI · Accepted Answer

To approach this question, we first identify the genetic scenario described by the cross and then evaluate how each option aligns with expected outcomes.
Option 1: Full, green pods = 9

Full, yellow pods = 3

Constricted, green pods = 3

Constricted, yellow pods = 1

This matches the classic 9:3:3:1 phenotypic ratio produced by a dihybrid cross (two heterozygous parents for two traits with complete dominance). The 9:3:3:1 ratio arises when each parent contributes two alleles (AaBb x AaBb), resulting in 9 dominant-dominant, 3 dominant-recessive for the first trait with the second trait dominant, 3 recessive-dominant, and 1 recessive-recessive combination. This option therefore coherently reflects the expected proportions.

Option 2: Full, green pods = 16

Full, yellow pods = 0

Constricted, green pods = 0

Constricted, yellow pods = 0

This extreme 16:0:0:0 distribution would imply that the cross yields only one phenotypic class with no variation at all. In a typical dihybrid cross with two independently assorted traits showing dominance, we do not expect complete lack of variation across all offspring unless one allele is completely fixed—an unlikely scenario under standard Mendelian two-gene dihybrid inheritance. Thus this option misrepresents the expected outcome.

Option 3: Full, green pods = 4

Full, yellow pods = 4

Constricted, green pods = 4

Constricted, yellow pods = 4

A 4:4:4:4 distribution would imply an exactly even split among all four phenotype classes. However, the genetic mechanics of a two-trait dihybrid cross do not produce an even 1:1:1:1 ratio for phenotypes unless the traits are independent and the parental genotypes produce equal probabilities for each class in a way that sums to equal thirds of a population, which is not the standard dihybrid outcome. This makes this option unlikely for the described cross.

Option 4: Full, green pods = 0

Full, yellow pods = 0

Constricted, green pods = 0

Constricted, yellow pods = 16

This option suggests an inheritance pattern where only one phenotype class is observed (complete dominance for both traits in the same direction) across all offspring, which contradicts the expected variation from a dihybrid cross of two heterozygotes. It also leaves no evidence of the other phenotypes, making it biologically inconsistent with the typical 9:3:3:1 outcome.

Putting it all together, the first option aligns with the well-established 9:3:3:1 phenotypic ratio for a dihybrid cross, while the others fail to capture the expected distribution under standard Mendelian inheritance. The reasoning above highlights why each incorrect option deviates from the expected genetic proportions and why the first option is the correct representation of the ratio.

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