A cross between two true breeding lines one with dark-blue flowers and one with bright-white flowers produces F1 offspring that are light-blue. When the F1 progeny are selfed a 1:2:1 ratio of dark-blue to light-blue to bright-white flowers is observed. What genetic phenomenon is consistent with these results? a. incomplete dominance b. random mating c. codominance d. epistasis e. inbreeding depression
The correct answer and explanation is:
The correct answer is a. incomplete dominance.
In this scenario, when two true-breeding lines (one with dark-blue flowers and one with bright-white flowers) are crossed, the F1 offspring exhibit light-blue flowers. This suggests that neither the dark-blue nor the bright-white allele is completely dominant over the other. Instead, they interact to produce an intermediate phenotype (light-blue flowers). This pattern is characteristic of incomplete dominance, where the heterozygous phenotype is a blend of the two homozygous phenotypes.
When the F1 progeny are selfed, a 1:2:1 ratio of dark-blue to light-blue to bright-white flowers appears in the F2 generation. This is consistent with Mendelian inheritance for a single gene with incomplete dominance. The alleles involved in incomplete dominance are typically represented as follows:
- D for dark-blue (dominant)
- W for bright-white (recessive)
In the F1 generation, the offspring inherit one D allele from one parent and one W allele from the other parent, resulting in a DW genotype, which produces the light-blue color. When these F1 individuals are self-crossed (DW x DW), the possible genotypes in the F2 generation are:
- DD (dark-blue flowers)
- DW (light-blue flowers)
- WW (bright-white flowers)
This results in a 1:2:1 phenotypic ratio (1 dark-blue: 2 light-blue: 1 bright-white), which is a typical outcome of incomplete dominance. The key feature of incomplete dominance is that the heterozygous offspring display an intermediate phenotype, not resembling either of the parental phenotypes exactly.