Biology 1201A Lecture Notes - Lecture 15: Gamete, Punnett Square, Mendelian Inheritance

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27 Mar 2016

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Morgan was a scientist and didn"t exactly agree with what mendel was saying. He did his own research with fruit flies- drosophila. Wild type have red eyes and symmetrical wings. Crossed it with individual with purple eye mutation and vestigial wings. F1 all has red eyes and symmetrical wings. He did a test cross and took an individual where you do not necessarily know it"s genotype and you cross it with a homozygous recessive individual. The offspring were a mix- some were completely wild type, some were completely mutant, some had wild type eyes and mutant wings, some had mutant eyes and symmetrical wings. We expect half the offspring to get half the offspring to get mutant wings. Based on mendel, we should expect equal amounts of each type of offspring. But that"s not what morgan found- we didn"t get the equal frequencies. Maybe the eye colour and wing genes are on the same chromosome- physically associated.

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Below are data from a three-point linkage experiment in Drosophila melanogaster. These data represent the offspring from a testcross between flies expressing the recessive mutant phenotypes for black body, vestigial wings and brown eyes and flies that express normal wild type phenotypes but are heterozygous for all three traits. All the heterozygous wild type parental flies in this cross start out with mutant alleles in the cis configuration. Answer the following questions based on these data and show your work for question 1 (a-d).

Body	Wing	Eye	Phenotype	Count
b⁺	vg⁺	bw⁺	Gray body, normal wings, red eyes	103
b	vg	bw	Black body, vestigial wings, brown eyes	95
b⁺	vg	bw	Gray body, vestigial wings, brown eyes	32
b	vg⁺	bw⁺	Black body, normal wings, red eyes	29
b⁺	vg⁺	bw	Gray body, normal wings, brown eyes	71
b	vg	bw⁺	Black body, vestigial wings, red eyes	68
b⁺	vg	bw+	Gray body, vestigial wings, red eyes	10
b	vg⁺	bw	Black body, normal wings, brown eyes	13

1a)Which combinations of phenotypes are only possible in those offspring derived from gametes resulting from double crossover events?

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1b)What is the distance between the body color (gray vs. black) and wing loci (normal vs. vestigial)?

1c)What is the distance between the body color (gray vs. black) and eye color loci (red vs. brown eyes)?

1d)What is the coefficient of coincidence for double crossovers?

In this study you will learn about a modification of Mendelian inheritance which is also an important source of genetic variation.

You are about to perform what looks like a classic dihybrid testcross ("A" is the gene for eye color and "B" is the gene for wing color.)

A allele for red eyes

a allele for white eyes

B allele for clear wings

b allele for spotted wings

(AaBb) X (aabb) Note the genotypes and phenotypes.

Question #1: Predict the expected phenotypic ratio of this P X P cross.

Observed results:

500 red eyes clear wings

500 white eyes spotted wings

Question #2: How do the expected results compare with the observed results?

Question #3: Considering Mendel's second law (regarding dihybrid crosses), suggest an explanation for the discrepancy between the observed results and those predicted.

Genes are located at specific positions on a chromosome. The distance between genes is measured in terms of "mapping units". In this exercise the gene for wing phenotype âBâ will be moved from its original position of zero to different positions on the chromosome. You will observe the effect of this gene movement.

First, gene "B" (wing color) is moved from position zero to position 10 on the same chromosome. Gene "A" (eye color) stays at position 0.

Observed Results: (position 10)

450 red eyes clear wings

50 red eyes spotted wings

50 white eyes clear wings

450 white eyes spotted wings

Question #4: How do these results differ from those predicted by Mendel's second law?

Question #5: From those in the first cross?

Question #6: Can you suggest a hypothesis to explain these new results?

Gene "B" (wing color) is now moved to position 20 on the same chromosome. Gene "A" (eye color) stays at position 0.

Observed Results: (position 20)

400 red eyes clear wings

100 red eyes spotted wings

100 white eyes clear wings

400 white eyes spotted wings

Question #7: How do the proportions of the phenotypic classes in this cross differ from the proportions you obtained in the earlier crosses?

The wing color gene "B" is now moved farther away (positions 30, 40, 50, and 60).

Observed Results: (position 30)

350 red eyes clear wings

150 red eyes spotted wings

150 white eyes clear wings

350 white eyes spotted wings

Observed Results: (position 40)

292 red eyes clear wings

220 red eyes spotted wings

195 white eyes clear wings

293 white eyes spotted wings

Observed Results: (position 50)

250 red eyes clear wings

250 red eyes spotted wings

250 white eyes clear wings

250 white eyes spotted wings

Observed Results: (position 60)

250 red eyes clear wings

250 red eyes spotted wings

250 white eyes clear wings

250 white eyes spotted wings

Question #8: Prepare a line graph plotting the % of recombinant offspring (Y-axis) vs. difference in position between the two genes (X-axis). (Hint: your graph should be one line with labeled points at map positions 0, 10, 20, 30, 40, 50, & 60).

Question #9: A genetic definition of "1 map unit" is the distance between 2 genes that gives 1% recombinants out of the total number of progeny. Using this definition, how many map units separate the "A" and "B" genes when the eye color gene "A" is at position 0 and the wing color gene "B" is at position 40? (Show how you determined the results.)

Question #10: How does the distance between two genes on the same chromosome affect the proportion of recombinants? Why?

Question #11: Maximum crossover frequency can be defined as the point at which no more recombination can be detected no matter how far apart the genes are located. Do your results indicate that you are approaching the maximum crossover frequency?

Question #12: Show (using a Punnett Square) the expected phenotypic rations if the two genes were located on different chromosomes.

Question #13: Which of your simulated cross or crosses do the data from the Punnett Square most closely fit? Why?

Question #14: A wild-type fly (heterozygous for gray body color and normal wings - b⁺ vg⁺/b vg) was mated to a black fly with vestigial wings (b vg/b vg). The F1 had the following phenotypic distribution: wild type, 850; black body - vestigial wings, 899; black body - normal wings, 79; gray body - vestigial wings, 85. What is the recombination frequency (RF) between these genes for body color and wing type? (show work)

Question #15: A cross produced 915 offspring with normal pigment and 310 with albinism. Conclusion?

a. One of the parents was homozygous for albinism.

b. Both parents were heterozygous.

c. One parent was homozygous for normal pigmentation.

d. Both parents were albinos. e. 605 albino zygotes must have failed to develop.

NOTE: Please show all work to support your answers.

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Biology 1201A Lecture Notes - Lecture 15: Gamete, Punnett Square, Mendelian Inheritance

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