Question

In this inquiry, you will investigate a variation of the assumption that there are only two alleles per gene. In this exercise, you will cross a purebred horizontally-striped fly with a pure bred vertically-striped fly. F1 results are: F1 Observed Results: 1000 black F1 flies Question #1: Describe the genotypes of the parents? Question #2: Describe the genotype of the F1 offspring. Question #3: What pattern of inheritance seems to be operating here? Now you will cross 2 flies, one with vertical stripes and one with horizontal stripes (derived from other sources of parents), to yield the following F1 results: F1 Observed Results: 25 vertically-striped flies 25 black flies 25 horizontally-striped flies 25 white flies Describe the genotypes of the F1 offspring. How do the results of the first cross differ from the results of the second cross? Try to formulate a hypothesis to account for these differences. One strategy is first to determine which of the four FI phenotypes derived from the second cross represents a "pure line" (i.e. is homozygous). Which of the four phenotypes is homozygous? Why? What is the ratio of this homozygous phenotype (i.e. in those crosses in which it appears along with other phenotypes)? Do these ratios suggest that the phenotype of these homozygotes is determined by a dominant or recessive allele? Why? Formulate a hypothesis to explain the mode of inheritance of the four phenotypes of the abdomen color. You may want to conduct test crosses of individuals displaying the various phenotypes to the homozygous recessive. What is the mode of inheritance in this example? Why? (Mint: make sure your answer includes a specific name for this variation from the typical dominant-recessive Mendelian mode of inheritance.)

Answer 1

1). Assume that the gene coding for horizontal stripes is, H and the genes coding for vertical stripes is, h.

The genotype of pure breed of the fly with the horizontal line is, HH

The genotype of pure breed of the fly with the vertical line is, hh

2). Cross between the pure breeds of flies with the horizontal (HH) and vertical lines (hh) will have the offspring with the following genotypes.

HH* hh ---à Hh (100% black flies) ----à F1

3). The phenotype of the F1 is not similar to either of its parents. Means, the genes are showing incomplete dominance and resulted in an intermediate [phenotype. Thus the inheritance pattern is “incompletely dominant inheritance.”