Genetics 210 Gene Mapping Consider the following results from a three-point test

Question

Genetics 210 Gene Mapping Consider the following results from a three-point testcross similar to the one you read about your laboratory handout. In this study Fl trihybrids were used in a testcross involving the following three linked genes (S/s, E/e. V/v) List the genotypes of the two parents involved in this cross. Here are the data for the eight different phenotypes that resulted from this testcross E S e s V 430 s 417 29 E sV 25 37 4 Total Progeny 991 Which of the above offspring represent the parental phenotypes in this study? Next to each of these phenotypes write how many there were in the progeny pool Ordering the linked genes. When a 3-point test cross is done it is easy to determine the relative order of the genes First find the two offspring phenotypes with the lowest numbers- these are the double crossovers. Now, considering only these double crossovers you will notice that each of them will display either one dominant trait and two recessive traits OR one recessive trait and two dominant traits. Determine which of the three genes does not match the other two (in terms of dominant vs. recessive) and this will be the one situated in the middle of the three. For example, if the double crossovers were ABc and abC, then the C/c gene would be the one that didn't match with the other two and would therefore be situated between the other two. The order of the three genes would therefore be. A/a --c/c - - B/b on one double crossover and a/a --C/c --b/b on the other What is the order of the linked genes on the chromosome whose data you were given in the table above?

Explanation / Answer

From the data given in the question, we can say that, the most frequent reciprocal pairs of classes (phenotypes) which arise from non-crossover gametes tell us the original parental progeny (ESV – 430 & esv – 417) and less frequent reciprocal pairs of classes (phenotypes) are double cross over progeny (Esv – 4 & eSV -5).

By comparing one wild type (ESV) and one double cross over (eSV) we can found that SV are found in both cases. [Two phenotypes should same in one mode, either dominant or recessive; here both SV are in dominant condition. If one recessive or one dominant or vice versa is same it will never occur]. Besides this if we compare another set, such as one wild type (esv) with another double cross over (Esv) we will see that sv is common.

This indicates that S and V should not be together.

Thus the order of the linked genes on the chromosome is –

S------------------------------E----------------------------------V

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