Two plants in a cross were each heterozygous for two gene pairs (Ab/aB) whose loci are linked and 25 mu apart. Assuming that crossing over occurs during the formation of both male and female gametes and that the A and B alleles are dominant, determine the phenotypic ratio of their offspring.

Verified step by step guidance

1. Identify the genotype of the parent plants: Both are heterozygous for two linked gene pairs, represented as Ab/aB.
2. Understand the concept of linkage and recombination: The loci are 25 map units apart, indicating a 25% recombination frequency.
3. Determine the possible gametes: Due to linkage, the parental gametes (Ab and aB) will be more frequent than the recombinant gametes (AB and ab).
4. Calculate the expected frequency of each type of gamete: Parental gametes (Ab and aB) will each occur 37.5% of the time, while recombinant gametes (AB and ab) will each occur 12.5% of the time.
5. Use a Punnett square to determine the phenotypic ratio of the offspring: Combine the gametes from each parent to predict the offspring's genotypes and phenotypes, considering the dominance of A and B alleles.

Key Concepts

Linkage and Genetic Mapping

Linkage refers to the tendency of genes located close to each other on a chromosome to be inherited together. Genetic mapping uses the frequency of recombination between genes to determine their relative positions on a chromosome. The distance between genes is measured in map units (mu), where 1 mu corresponds to a 1% chance of recombination occurring. In this case, the genes are 25 mu apart, indicating a 25% chance of crossing over.

Heterozygosity and Gamete Formation

Heterozygosity occurs when an individual has two different alleles for a particular gene, such as Ab/aB in this scenario. During gamete formation, particularly in meiosis, these alleles can assort independently or recombine through crossing over. The resulting gametes can be parental (Ab and aB) or recombinant (AB and ab), affecting the genetic makeup of the offspring. Understanding the gamete combinations is crucial for predicting phenotypic ratios.

Phenotypic Ratios in Dihybrid Crosses

Phenotypic ratios represent the relative frequencies of different phenotypes in the offspring resulting from a genetic cross. In a dihybrid cross involving two heterozygous parents, the expected phenotypic ratio can be calculated based on the combinations of alleles. For linked genes, the ratio may deviate from the typical 9:3:3:1 ratio seen in independent assortment, depending on the degree of linkage and crossing over.