BIOLOGY 1A03 Chapter Notes - Chapter 13: Nettie Stevens, Dihybrid Cross, Blending Inheritance

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BIOLOGY 1A03 Full Course Notes

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Pure lines: when crossed the offspring will have the same genetic material as the parent. Sex of the parents did not affect the expression of a trait. Genes were known as hereditary elements during mendel"s time. Came up with nomenclature: dominant allele labelled with a capital letter, recessive allele labelled with a lower case letter. He showed how genetics are passed from parents to offspring: this formed the basis for chromosome theory of inheritance. Majority of the fruit flies he studied had red eyes however one fly had white eyes. He mated female red eyed flies with male white eyed flies and noticed that red eyed offspring were produced. Over many crosses he eventually created white eyed flies. A reciprocal cross showed that there is a relationship of sex and genetic expression. Found the sex chromosomes in karyotypes of insects. Transmission pattern of x in males and females explained morgan"s reciprocal cross results. X-linked: being carried on the x chromosome.

Related Questions

Please assist with the following. Please no hand-written work, but typed work and thorough explanations please including Punnett Squares. Please Ignore Sex, as these are not sex-linked traits

Dihybrid Testcrosses

In the following crosses, the wild trait is dominant and the brown trait for eye color and ebony trait for body color are both recessive. You will compare the offspring of homozygous wild flies to the offspring of heterozygous wild flies, when each is crossed with a recessive brown-eyed fly with an ebony body.

7. Perform a monohybrid cross between a wild-type female fly and a male fly with brown eyes and ebony body.
Show results for the F₁ offspring. (NOTE: You will use one of these F₁ flies in the next cross.)
ENTER DATA HERE:

PUNNETT SQUARE:

List the genotypes of the flies in this cross:

List the gametes produced by each fly in this cross:

Show the Punnett Square for the above cross.

Question: By examining the offspring, determine whether the wild-type female parent was homozygous or heterozygous for eye color and body color. How did you arrive at this conclusion?

Question: Are the F₁ offspring homozygous or heterozygous for eye color and body color? How did you arrive at this conclusion?

8. Now perform a cross between one wild-type F₁ offspring from the previous cross and a male fly with brown eyes and ebony body.
ENTER DATA HERE:

PUNNETT SQUARE:

List the genotypes of the flies in this cross:

List the gametes produced by each fly in this cross:

Show the Punnett Square for the above cross.

Question: Even though you already knew the genotype of the wild-type fly used in this cross, how can you tell by examining the offspring of this cross whether the wild-type fly in this cross was homozygous or heterozygous for eye color?

Question: What was the phenotypic ratio for the offspring resulting from this testcross?

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.