If two genes (M and N) were so close that crossing over never occurred between them, and we performed a test cross between MMNN and mmnn parents, what is the expected ratio of the F2 offspring?
The correct answer and explanation is:
Correct Answer: 1:1
Explanation (Approx. 300 Words)
To understand the outcome of this genetic cross, we must examine the scenario step by step using the principles of Mendelian genetics and linkage.
Step 1: Understanding the Parental Genotypes
- Parent 1 (MMNN): Homozygous dominant for both genes M and N.
- Parent 2 (mmnn): Homozygous recessive for both genes.
These two genes are said to be so close on the same chromosome that crossing over never occurs between them. This implies complete linkage, meaning the alleles on the same chromosome are inherited together.
Step 2: F1 Generation (from MMNN × mmnn)
Each F1 offspring receives:
- One chromosome with M and N from MMNN parent.
- One chromosome with m and n from mmnn parent.
So, all F1 offspring have the genotype: Mn/mn (or more precisely, [MN]/[mn] to indicate the allele pairs are linked on their respective chromosomes).
Step 3: Test Cross
The F1 individual (MN/mn) is test-crossed with a double recessive individual (mn/mn).
The possible gametes from each parent are:
- F1 can produce only MN and mn gametes (since no crossing over occurs).
- The test cross parent can produce only mn gametes.
So, the offspring genotypes will be:
- MN/mn – receives MN from F1 and mn from tester.
- mn/mn – receives mn from both parents.
Since both types of gametes are produced equally (50% MN, 50% mn), the resulting offspring ratio is:
1 MN/mn : 1 mn/mn, or 1:1 phenotypic ratio
Conclusion
When two genes are completely linked and a test cross is performed, no recombinants appear, and offspring only reflect the parental types. Therefore, the expected F2 ratio is 1:1.