home / study / science / biology / biology questions and answers / consider the

ID: 208048 • Letter: H

Question

home / study / science / biology / biology questions and answers / consider the following crosses: cross 1: p: f1 testcross: f2 phenotypes: ssttrr x ssttrr ssttrr ... Question: Consider the following crosses: Cross 1: P: F1 testcross: F2 phenotypes: ssTTRR x SSttrr SsTtRr ... Consider the following crosses: Cross 1: P: F1 testcross: F2 phenotypes: ssTTRR x SSttrr SsTtRr x ssttrr STR 121 str 131 STr 124 stR 127 Str 119 sTR 121 StR 135 sTr 122 Which genes are most likely linked? Draw a combined genetic map based on all the crosses.

Explanation / Answer

One way that recombination frequencies have been used historically is to build linkage maps, chromosomal maps based on recombination frequencies. In fact, studying linkage helped early geneticists establish that chromosomes were in fact linear, and that each gene had its own specific place on a chromosome.

Recombination frequency is not a direct measure of how physically far apart genes are on chromosomes. However, it provides an estimate or approximation of physical distance. So, we can say that a pair of genes with a larger recombination frequency are likely farther apart, while a pair with a smaller recombination frequency are likely closer together together.

Importantly, recombination frequency "maxes out" at 50%50%50, percent (which corresponds to genes being unlinked, or assorting independently). That is, 50%50%50, percent is the largest recombination frequency we'll ever directly measure between genes. So, if we want to figure out the map distance between genes further apart than this, we must do so by adding the recombination frequencies of multiple pairs of genes, "building up" a map that extends between the two distant genes.

Comparison of recombination frequencies can also be used to figure out the order of genes on a chromosome. For example, let's suppose we have three genes, R, S, and T, and we want to know their order on the chromosome (RrSsTt? TTSSTT? rrsstt?) If we look at recombination frequencies among all three possible pairs of genes (TR, RS, TS), we can figure out which genes lie furthest apart, and which other gene lies in the middle. Specifically, the pair of genes with the largest recombination frequency must flank the third gene.

So, according to deta,

F1 testcross:

F2 phenotypes:

Cross 2:

P:
F1 testcross: F2 phenotypes:

Cross 3:
P:
F1 testcross: F2 phenotypes:

According to this :

Now let’s look at recombination between the Rs and the Ts loci. The “input” parental genotypes were Ts TT and so we must calculate the frequency of rS tT Rs and TTTs progeny types (this time, we ignore ). We see that there are 243+233+14+16=506 recombinants; because 506/1008 is very close to an RF of 50 percent, we conclude that the sc and vg loci are not linked and are probably not on the same chromosome. We can summarize the linkagerelationship as follows

Case

Possible Genotype

Frequency of ssAllele

Frequency of tt Allele

Frequency of TtAllele

Frequency of SsAllele

TTSS

TsRR

0.5

RsTT

0.5

sstt

0.25