Problem

Solutions For An Introduction to Genetic Analysis Chapter 3 Problem 31P

Step-by-Step Solution

Solution 1

7177-3-31P AID: 2975 | 19/03/2013

RID: 2498 | 07/05/2013

The fruit fly Drosophila is usually used for experiments in biology. The normal eye color is red while strains with brown eyes are also available. Usually they have long wings, but short-winged flies are also available.

A female with brown eyes and short wings is crossed to a male with red eyes and long wings. Both parents are from pure lines. The F1 consists of females with red eyes and long wings and short-winged males.

When the F1 are intercrossed, the F2 produced give the following results:

• 3/8 red eyes, long wings

• 3/8 red eyes, short wings

• 1/8 brown eyes, long wings

• 1/8 brown eyes, short wings

1. The word “normal” means wild type flies, which have red color and long wings.

2. The word “line” is used to indicate pure breeds while the word “strain” is also used to indicate the same idea. These two words can be used interchangeably.

3. The diagram showing the parents showing eyes, wings and sexual characteristics is shown:

Picture 1

4. The number of characters given in the problem is three:

• Eye color

• Wing length

• Sex

5. There are 6 phenotypes given in the problem. They are:

• Eye color: Red, Brown

• Wing length: Long, short

• Sex : Male, female

6. The full phenotype of the F1 females designated as normal is - red eyes and long wings.

7. The full phenotype of the F1 males designated as “short-winged” is – red eyes and short wings.

8. The F2 phenotypic ratios for each character in the question are:

• 3/8 red eyes, long wings

• 3/8 red eyes, short wings

• 1/8 brown eyes, long wings

• 1/8 brown eyes, short wings

9. The F2 ratio clearly shows that the 9:3:3:1 ratio is missing. The reasons for this strange ratio could be:

• Sex linkage

• Epistasis

• Two genes on the same chromosome

• Environmental factors

• Reduced penetrance

• Lack of dominance in either or both the genes

10. In sex-linked inheritance it is either X-linked or Y linked. If the mother is a sufferer, her male progeny also suffer since they receive the X chromosome from their mother. Her female progeny become carriers unless the father is also a sufferer. In the case of Y-linked inheritance, the male progeny suffer since they receive the Y chromosome from their father.

In autosomal inheritance, both male and female progeny have equal chances of inheriting the defective allele.

11. The F2 data given does not show such distinguishing sex-linked inheritance.

12. The F1 data shows sex-linked inheritance since all males are like their mother in having short wings while all females are like their father in having long wings and red eyes.

13. The F1 data suggests that long wings are dominant to short wings and red eyes are dominant to brown eyes. In F2 the red to brown eyes ratio is 3:1. This proves the dominance of red eyes. But the ratio of long to short wings is 1:1. This usually results when a testcross is done. Since the results of F2 are inconclusive about wing length we cannot determine the dominant allele without further experiments.

14. The rules of wild-type symbolism, which we can use are of two types:

Mendelian type: the dominant alleles are shown as “R”, “L” and the recessive allele as “r” and “l”.

Drosophila type: the dominant alleles are shown as b+, s+ and the mutants as b and s.

15. The meaning of “deduce the inheritance of these phenotypes” will be to find out the genotypes of the crosses and see that they match with the progeny ratios given.

The details of the crosses are:

Parents:

• Brown eyes, short winged female

• Red eyes, long winged male

The F1 progeny:

• Red eyes long winged females

• Red eyes short winged males

By observing the above ratio we notice that the allele for red eyes is dominant over the allele for brown eyes. Since both the males and females in F1 have red eyes, we can say that it is an autosomally dominant trait.

Regarding the gene for wing length, we can say that it is sex-linked since the allele in the female is seen in the male progeny.

For rewriting the crosses we take B as symbol for red eyes and b as symbol for brown eyes, S for long wings and s for short wings.

Parents:

b/b, ; s/s

B/B, ; S/Y

The F1 progeny: 1:1

B/b ; S/s

B/b ; s/Y

The F2 progeny: 1:1:2:2:1:1:1:1:2:2:1:1

1 – B/B; S/s red eyes, long winged female

1 – B/B; s/s red eyes, short winged female

2 – B/b; S/s red eyes, long winged female

2 – B/b; s/s red eyes, short winged female

1 – b/b; S/s brown eyes, long winged female

1 – b/b; s/s brown eyes, short winged female

1 – B/B; S/Y red eyes, long winged male

1 – B/B; s/Y red eyes, short winged male

2 – B/b; S/Y red eyes, long winged males

2 – B/b; s/Y red eyes, short winged males

1 – b/b; S/Y brown eyes, long winged males

1 – b/b; s/Y brown eyes, short winged males

Add your Solution
Textbook Solutions and Answers Search