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 MN blood type (genotypes MN) produce offspring, the expected phenotypic ratio among the offspring is 50% MN, 25% NN, and 25% MM due to the codominance of the alleles.
A) 50% MM; 25% MN; 25% NN
This option incorrectly suggests that there would be 50% MM offspring. In a cross between two MN individuals, the possible genotypes are MM, MN, and NN, leading to different proportions than those stated here.
B) 100% MN
This option is incorrect as it assumes that all offspring would inherit the MN genotype. Since both parents can also contribute the N allele, there is a possibility of producing NN offspring as well.
C) 50% MN; 25% NN; 25% MM
This option correctly reflects the expected outcomes from a cross between two MN individuals. The Punnett square would show that the offspring ratios would indeed be 50% with the MN genotype, 25% with the NN genotype, and 25% with the MM genotype.
D) 50% MM; 50% NN
This option is incorrect as it suggests that there would be no MN offspring. The cross between two MN individuals would not yield an equal number of MM and NN offspring, as the MN combination is also a viable outcome.
Conclusion
The correct answer is C, as it accurately represents the genetic outcomes expected from the cross of two MN blood type individuals. The ratios provided in the other options fail to account for the codominance of the alleles and the resultant combinations that can occur, demonstrating a fundamental understanding of Mendelian genetics.