63. The human MN blood group is determined by alleles (M and N) for a single gene that code for two specific, codominant markers found on the surface of red blood cells. If two individuals with the MN blood type produce offspring, which phenotypic ratio of offspring would result?
Answer: C
50% MN; 25% NN; 25% MM
When two individuals with the MN blood type (genotype MN) produce offspring, the potential genotypes of the offspring can be determined through a Punnett square. This results in a phenotypic ratio of 50% MN, 25% NN, and 25% MM.
A) 50% MM; 25% MN; 25% NN
This option is incorrect because it misrepresents the genotypic outcomes of the cross between two MN individuals. The expected offspring would not yield 50% MM, as this only accounts for one of the possible genotypes.
B) 100% MN
This option is incorrect as well, as it assumes that all offspring will inherit the MN genotype. However, the cross between two MN individuals also allows for the possibility of producing MM and NN offspring.
C) 50% MN; 25% NN; 25% MM
This option is correct because it accurately reflects the expected phenotypic ratio from a cross between two MN individuals. The Punnett square shows the combinations of alleles resulting in 50% MN, 25% NN, and 25% MM.
D) 50% MM; 50% NN
This option is incorrect since it suggests that the offspring would only exhibit MM and NN phenotypes, completely disregarding the MN phenotype that arises from the codominance of the alleles.
Conclusion
The correct answer, 50% MN; 25% NN; 25% MM, accurately represents the genetic outcomes of a cross between two MN blood type individuals. All other options fail to account for the full range of potential offspring phenotypes resulting from the codominance of the M and N alleles.