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Ch. 13 - Meiosis and Sexual Life Cycles
Chapter 13, Problem 7

Assume that genes A and B are on the same chromosome and are 50 map units apart. An animal heterozygous at both loci is crossed with one that is homozygous recessive at both loci. What percentage of the offspring will show recombinant phenotypes resulting from crossovers? Without knowing these genes are on the same chromosome, how would you interpret the results of this cross?

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Genetic Linkage

Genetic linkage refers to the tendency of genes located close to each other on the same chromosome to be inherited together during meiosis. When genes are linked, they do not assort independently, which affects the ratios of phenotypes in offspring. In this case, genes A and B are 50 map units apart, indicating they are on the same chromosome but far enough apart to allow for recombination.
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Recombination Frequency

Recombination frequency is the proportion of offspring that exhibit recombinant phenotypes due to crossing over during meiosis. It is measured in map units, where 1 map unit corresponds to a 1% chance of recombination. Since genes A and B are 50 map units apart, the expected recombination frequency is 50%, meaning that half of the offspring will show recombinant phenotypes.
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Mendelian Inheritance

Mendelian inheritance describes the patterns of inheritance for traits controlled by single genes, as established by Gregor Mendel's laws. In the context of this cross, if the genes were not known to be linked, one might expect a 1:1:1:1 ratio of phenotypes in the offspring, reflecting independent assortment. However, the presence of linkage alters this expectation, leading to a different distribution of phenotypes.
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