E5 - E1. If we hypothesize two genes independently...

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E1. If we hypothesize two genes independently assorting, the predicted ratio is 9:3:3:1. There is a total of 427 offspring. The expected numbers of offspring are 9/16 × 427 = 240 purple flowers, long pollen 3/16 × 427 = 80 purple flowers, round pollen 3/16 × 427 = 80 red flowers, long pollen 1/16 × 427 = 27 red flowers, round pollen Plugging these values into our chi square formula, 22 2 2 2 2 (296 240) (19 80) (27 80) (85 27) 240 80 80 27 13.1 46.5 35.1 124.6 219.3 χ −−−− =+ + + =+++ = 2 Looking up this value in the chi square table under 3 degrees of freedom, we find that such a large value is expected by chance less than 1% of the time. Therefore, we reject the hypothesis that the genes assort independently. E2. They could have used a strain with two abnormal chromosomes. In this case, the recombinant chromosomes would either look normal or have abnormalities at both ends. E3. The top of the Conceptual Level column in Figure 5.6 shows the chromosomes of McClintock’s cross. This experiment could be modified to a standard testcross in the following way. In the heterozygous parent, the C (colored) and Wx (starchy) alleles could be on the knobbed, translocation chromosome and the c (colorless) and wx (waxy) alleles on a normal chromosome. The other parent would have two cytologically normal copies of chromosome 9 and be homozygous for the recessive alleles (i.e., cc wxwx ). If the cross were done in this way, nonrecombinant offspring would be colored and starchy, or colorless and waxy; recombinant offspring would be colored and waxy, or colorless and starchy. The recombinant offspring should inherit a chromosome with a knob but no translocation, or a translocation but no knob. E4. A gene on the Y chromosome in mammals would only be transmitted from father to son. It would be difficult to genetically map Y-linked genes because a normal male has only one copy of the Y chromosome, so you do not get any crossing over between two Y chromosomes. Occasionally, abnormal males (XYY) are born with two Y chromosomes. If such males were heterozygous for alleles of Y-linked genes, one could examine the normal male offspring of XYY fathers and determine if crossing over has occurred. E5. The rationale behind a testcross is to determine if recombination has occurred during meiosis in the heterozygous parent. The other parent is usually homozygous recessive, so we cannot tell if crossing over has occurred in the recessive parent. It is easier to interpret the data if a testcross does use a completely homozygous recessive parent. However, in the other parent, it is not necessary for all of the dominant alleles to be on one chromosome and all of the recessive alleles on the other. The parental generation provides us with information concerning the original linkage pattern between the dominant and recessive alleles.
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E5 - E1. If we hypothesize two genes independently...

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