The LM and LN alleles at the MN blood group locus exhibit codominance. Give the expected genotypes and phenotypes and their ratios in progeny resulting from the following crosses. a. LMLM Ž LMLN d. LMLN Ž LNLN b. LNLN Ž LNLN e. LMLM Ž LNLN c. LMLN Ž LMLN
Answer
Codominance in the MN Blood Group System: Genotype and Phenotype Ratios
The MN blood group system in humans is controlled by a single gene with two codominant alleles: LM and LN. This genetic system is an excellent example of codominance, where both alleles in a heterozygote are fully expressed, resulting in a distinct phenotype.
- LMLM → M blood group
- LNLN → N blood group
- LMLN → MN blood group
🔬 Genotype & Phenotype Results from Different Crosses
Let’s analyze the five different crosses and calculate the possible genotypes, phenotypes, and their expected ratios in the offspring.
a. Cross: LMLM × LMLN
| Parent 1 Gametes | Parent 2 Gametes | Offspring Genotypes | Phenotypes | Genotypic Ratio | Phenotypic Ratio |
|---|---|---|---|---|---|
| LM | LM, LN | LMLM, LMLN | M, MN | 1 : 1 | 1 : 1 |
Half of the offspring will be homozygous for M (M phenotype), and the other half will be heterozygous showing both M and N antigens (MN phenotype).
b. Cross: LNLN × LNLN
| Parent Gametes | Offspring Genotype | Phenotype | Genotypic Ratio | Phenotypic Ratio |
|---|---|---|---|---|
| LN × LN | LNLN | N | 1 | 100% N |
All offspring will be homozygous for N and will express only the N blood group.
c. Cross: LMLN × LMLN
| Gametes from each parent | Offspring Genotypes | Phenotypes | Genotypic Ratio | Phenotypic Ratio |
|---|---|---|---|---|
| LM, LN | LMLM, LMLN, LNLN | M, MN, N | 1 : 2 : 1 | 1 : 2 : 1 |
This classic codominance cross results in three different genotypes and three observable phenotypes:
- M blood group: 25%
- MN blood group: 50%
- N blood group: 25%
d. Cross: LMLN × LNLN
| Gametes | Offspring Genotypes | Phenotypes | Genotypic Ratio | Phenotypic Ratio |
|---|---|---|---|---|
| LM, LN × LN | LMLN, LNLN | MN, N | 1 : 1 | 1 : 1 |
Half the offspring show the N blood group, while the other half are MN heterozygotes.
e. Cross: LMLM × LNLN
| Gametes | Offspring Genotypes | Phenotypes | Genotypic Ratio | Phenotypic Ratio |
|---|---|---|---|---|
| LM × LN | LMLN | MN | 100% | 100% MN |
This is a simple cross resulting in 100% heterozygous offspring all displaying the MN blood group.
🧬 Summary Table
| Cross | Genotypic Ratio | Phenotypic Ratio |
|---|---|---|
| LMLM × LMLN | 1 LMLM : 1 LMLN | 1 M : 1 MN |
| LNLN × LNLN | 100% LNLN | 100% N |
| LMLN × LMLN | 1 LMLM : 2 LMLN : 1 LNLN | 1 M : 2 MN : 1 N |
| LMLN × LNLN | 1 LMLN : 1 LNLN | 1 MN : 1 N |
| LMLM × LNLN | 100% LMLN | 100% MN |
📚 Final Thoughts on Codominance in MN Blood Groups
The MN blood group is an ideal example to teach and understand the principle of codominance, as both alleles contribute equally and visibly to the phenotype. By mastering these kinds of Punnett square crosses, students can gain strong skills in predicting outcomes and interpreting inheritance patterns.