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. Understand the genetic setup: The two plants are heterozygous for two linked gene pairs (Ab/aB), and the loci are 25 map units (mu) apart. This means there is a 25% chance of recombination between these loci during gamete formation.
2. Determine the parental and recombinant gametes: The parental gametes are 'Ab' and 'aB', while the recombinant gametes are 'AB' and 'ab'. The recombination frequency of 25% means that 25% of the gametes will be recombinant (12.5% 'AB' and 12.5% 'ab'), and 75% will be parental (37.5% 'Ab' and 37.5% 'aB').
3. Set up the Punnett square: Create a 4x4 Punnett square to represent the cross between the gametes of the two heterozygous plants. The rows and columns will include the four types of gametes: 'Ab', 'aB', 'AB', and 'ab', with their respective probabilities.
4. Determine the genotypes of the offspring: Fill in the Punnett square by combining the gametes from each parent. For example, crossing 'Ab' with 'aB' will result in the genotype 'AaBb'. Repeat this for all combinations to determine the genotypes of all offspring.
5. Determine the phenotypic ratio: Since A and B are dominant alleles, any offspring with at least one 'A' and one 'B' allele will display the dominant phenotype. Count the number of offspring with each phenotype (dominant for both traits, dominant for one trait, or recessive for both traits) and calculate the phenotypic ratio based on the probabilities from the Punnett square.

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 linked genes to estimate their distance apart, measured in map units (mu). In this case, the genes are 25 mu apart, indicating a 25% chance of recombination occurring between them.

Heterozygosity and Gamete Formation

Heterozygosity occurs when an organism has two different alleles for a particular gene. In this scenario, both parent plants are heterozygous for two gene pairs (Ab/aB), which means they can produce gametes with different combinations of these alleles. The formation of gametes involves the segregation of alleles and can result in parental types and recombinant types due to crossing over.

Phenotypic Ratios in Dihybrid Crosses

Phenotypic ratios describe the relative frequencies of different phenotypes in the offspring resulting from a genetic cross. In a dihybrid cross involving linked genes, the phenotypic ratio can be influenced by the degree of linkage and the occurrence of crossing over. The expected ratio can be calculated by considering both parental and recombinant gametes produced by the heterozygous parents.