A female of genotype a b c + + + produces 100 meiotic tetrads. Of these, 68 show no crossover events. Of the remaining 32, 20 show a crossover between a and b, 10 show a crossover between b and c, and 2 show a double crossover between a and b and between b and c. Of the 400 gametes produced, how many of each of the 8 different genotypes will be produced? Assuming the order a–b–c and the allele arrangement previously shown, what is the map distance between these loci?

Verified step by step guidance

1. Identify the parental and recombinant genotypes based on crossover events: Parental (no crossover) = 68 tetrads, Single crossover between a and b = 20 tetrads, Single crossover between b and c = 10 tetrads, Double crossover = 2 tetrads.
2. Calculate the number of gametes for each genotype: Each tetrad produces 4 gametes, so multiply the number of tetrads by 4 to get the number of gametes for each crossover type.
3. Determine the genotypes for each crossover type: Parental = a+b+c+ and abc, Single crossover a-b = a+bc and ab+c+, Single crossover b-c = a+bc+ and ab+c, Double crossover = a+bc and ab+c+.
4. Calculate the map distances: Map distance between a and b = (Number of single crossovers between a and b + Number of double crossovers) / Total number of tetrads * 100, Map distance between b and c = (Number of single crossovers between b and c + Number of double crossovers) / Total number of tetrads * 100.
5. Sum the map distances to find the total map distance between a and c, considering the double crossovers are counted twice.

Key Concepts

Meiotic Tetrads

Meiotic tetrads are structures formed during meiosis, consisting of four chromatids from two homologous chromosomes. They are crucial for understanding genetic recombination, as they allow for the exchange of genetic material between homologous chromosomes during crossover events. The number of tetrads and the types of crossover events that occur can significantly influence the genetic variation in the resulting gametes.

Crossover Events

Crossover events occur during meiosis when homologous chromosomes exchange segments of genetic material. This process increases genetic diversity by producing new allele combinations in gametes. The question specifies different types of crossover events, including single and double crossovers, which affect the frequency of various genotypes in the offspring and are essential for calculating genetic map distances.

Genetic Map Distance

Genetic map distance is a measure of the relative distance between genes on a chromosome, typically expressed in centimorgans (cM). It is calculated based on the frequency of recombination events between genes; a higher frequency indicates a greater distance. Understanding how to calculate map distances is vital for interpreting genetic linkage and predicting the inheritance patterns of traits.