Recombination and Linked Genes

Fruit flies (Drosophila melanogaster) with a wild-type phenotype have gray bodies and red eyes. Certain mutations can cause changes to these traits. Mutant flies may have a black body and/or cinnabar eyes.

To study the genetics of these traits, a researcher crossed a true-breeding wild-type male fly (with gray body and red eyes) with a true-breeding female fly with a black body and cinnabar eyes. All of the F1 progeny displayed a wild-type phenotype.

Female flies from the F1 generation were crossed with true-breeding male flies with black bodies and cinnabar eyes. The table below represents the predicted outcome and the data obtained from the cross. Explain the difference between the expected data and the actual numbers observed.

F2 Generation Phenotypes Body Color Eye Color Number Predicted Number Observed Gray Red 244 455 Black Cinnabar 432 42 47

Why was it expected that all F2 phenotypes would have the same ratios? Parent cross: GGRR x ggrr F1 hybrids: all GgRr….dominant phenotype: grey bodies red eyes. F2 hybrids: GgRr x ggrr What will the Punnett Square look like for this dihybrid cross?

We assume that the two traits assort independently: they are on different chromosomes The hybrid F1 generation (GgRr)can produce four genotypes among its gametes: GR, Gr, gR, gr The black bodied cinnibar males are all ggrr; all four gametes are gr. Set up a dihybrid Punnett Square…

Four phenotypes, 1:1:1:1 ratio gr GR GgRr Gr Ggrr gR ggRr ggrr Four phenotypes, 1:1:1:1 ratio

Observed does not match predicted… F2 Generation Phenotypes Body Color Eye Color Number Predicted Number Observed Gray Red 244 455 Black Cinnabar 432 42 47

A chi-square test would show a highly significant result X2 = 652 with 3 df!!

These genes appear to be linked – they are on the same chromosome F2 Generation Phenotypes Body Color Eye Color Number Predicted Number Observed Gray Red 244 455 Black Cinnabar 432 42 47

The closer genes are together, the more “linked” they are The last two phenotypes were produced by crossing over. Crossing over does not appear to happen with great frequency here Crossing over frequencies were used by Thomas Hunt Morgan to estimate positions of genes on chromosomes: low frequency, close together

What is the crossing over frequency here? (42 + 47)/(976) = 9.1% F2 Generation Phenotypes Body Color Eye Color Number Predicted Number Observed Gray Red 244 455 Black Cinnabar 432 42 47

College Board Rubric 4 points maximum. Student explanations include the following: Prediction of a 1:1:1:1 ratio with correct phenotypes based on independent assortment. Support for prediction with a diagram of the cross of BbEe x bbee. Correct application of chi-square analysis to show that observed results do not conform to expected Mendelian frequencies. Identification of body color and eye color as linked genes/loci. Use of ratios to show linkage and independent assortment of wing type versus linked traits. Identification of the bottom two phenotypes as products of crossing over (recombinant chromosome). Mentioning that crossover rate is approximately 9–10 percent.

Additional Problems of Dihybrid Inheritance What is the probability that the genotype rrss will be produced by a cross in which the genotypes of the parents are both RrSs?

A female fruit fly hybrid for both gray body (Gg) and normal wings (Nn) is crossed with a male with black body (gg) and vestigial wings (nn): GgNn x ggnn The data for the F1 cross are shown in the chart: Gray normal Black vestigial Gray vestigial Black normal 969 941 190 184

Gray normal Black vestigial Gray vestigial Black normal 969 941 190 184 GgNn x ggnn Assuming that the genes assort independently, what are the frequencies of the expected phenotypes? Draw a Punnett Square to illustrate.

Gray normal Black vestigial Gray vestigial Black normal 969 941 190 184 Given the data above, calculate the rate of cross over that produced the gray vestigial and black normal offspring.