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Chapter 5: Problem 8

What two essential criteria must be met in order to execute a successful mapping cross?

Short Answer

Expert verified
Answer: The two essential criteria for a successful mapping cross are: 1) the organisms being crossed must be pure-breeding or true-breeding for the traits being studied, and 2) the traits being studied must be linked, meaning they are inherited together more often than expected by chance due to their proximity on the same chromosome.

Step by step solution

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1. Pure-breeding organisms

In a successful mapping cross, the organisms being crossed must be pure-breeding, or true-breeding, for the traits being studied. This means that these organisms consistently pass down their traits unchanged to their offspring, ensuring that the genes contributing to those traits are homozygous. This ensures clear and consistent inheritance patterns in F1 and F2 generations, which are crucial for making accurate predictions and conclusions about the inheritance of the traits being studied.
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2. Linked traits

The second essential criterion for a successful mapping cross is that the traits being studied must be linked. This means they are located close to each other on the same chromosome, causing them to be inherited together more often than expected by chance as there is a lower probability of crossing over occurring between them during meiosis. As a result, the recombination frequency between linked traits is less than 50%. By analyzing the recombination frequency, the distance between the linked traits can be determined and the order of the genes on the chromosome can be mapped.

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Most popular questions from this chapter

List some of the differences between a linkage map obtained by analyzing crossovers and a physical map obtained by sequencing the DNA.

Why does more crossing over occur between two distantly linked genes than between two genes that are very close together on the same chromosome?

Assume that investigators crossed a strain of flies carrying the dominant eye mutation Lobe on the second chromosome with a strain homozygous for the second chromosome recessive mutations smooth abdomen and straw body. The \(\mathrm{F}_{1}\) Lobe females were then backcrossed with homozygous smooth abdomen, straw body males, and the following phenotypes were observed: (a) Give the gene order and map units between these three loci. (b) What is the coefficient of coincidence?

In this chapter, we focused on linkage, chromosomal mapping, and many associated phenomena. In the process, we found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, what answers would you propose to the following fundamental questions? (a) How was it established experimentally that the frequency of recombination (crossing over) between two genes is related to the distance between them along the chromosome? (b) How do we know that specific genes are linked on a single chromosome, in contrast to being located on separate chromosomes? (c) How do we know that crossing over results from a physi- cal exchange between chromatids? (d) How do we know that sister chromatids undergo recombination during mitosis? (e) When designed matings cannot be conducted in an organism (for example, in humans), how do we learn that genes are linked, and how do we map them?

In Drosophila, a female fly is heterozygous for three mutations, Bar eyes \((B),\) miniature wings \((m),\) and ebony body \((e)\) Note that Bar is a dominant mutation. The fly is crossed to a male with normal eyes, miniature wings, and ebony body. The results of the cross are as follows. Interpret the results of this cross. If you conclude that linkage is involved between any of the genes, determine the map distance(s) between them.
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