Multiple Choice
During which stage of meiosis does crossing over primarily occur?
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4. Genetic Mapping and Linkage
Crossing Over and Recombinants
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After a single crossing-over event between genes A and B on homologous chromosomes with the original gene order ABC and abc, what would be the sequence of genes on each of the four resulting chromatids?
A
ABc, abC, AbC, aBc
B
ABC, abc, Abc, aBC
C
ABC, abc, ABC, abc
D
Abc, aBC, Abc, aBC
Verified step by step guidance1
Identify the original gene order on the homologous chromosomes before crossing over. Here, the chromosomes are ABC and abc, where uppercase and lowercase letters represent different alleles of the same genes.
Understand that a single crossing-over event occurs between genes A and B. This means the crossover point is between the first gene (A) and the second gene (B) in the sequence.
Determine the segments that will be exchanged between the homologous chromosomes. Since the crossover is between A and B, the segment containing genes B and C will be swapped between the two chromatids.
Construct the sequences of the four resulting chromatids after the crossover. Two chromatids remain non-recombinant (unchanged), and two chromatids are recombinant, having exchanged the segments distal to the crossover point.
Write out the gene sequences for each chromatid: the two original parental types and the two recombinant types, reflecting the crossover between A and B.
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