Homework 2 sex linked genes

Virtual Lab: Sex-Linked Traits

Worksheet

Please make sure you have read through all of the information in the “Questions” and “Information” areas. If you come upon terms that are unfamiliar to you, please refer to your textbook for further explanation or search the word here: http://encarta.msn.com/encnet/features/dictionary/dictionaryhome.aspx

Next, complete the Punnett square activity by clicking on the laboratory notebook. Please be sure to note the possible genotypes of the various flies:

Female, red eyes

Female, red eyes

Female, white eyes

Male, red eyes

Male, white eyes

XRXR

XrXR

XrXr

XRY

XrY

When you have completed the Punnett square activity, return to the laboratory scene to begin the actual laboratory activity.

In this exercise, you will perform a Drosophila mating in order to observe sex-linked trait transmission. Please click on the shelf in the laboratory. Here you will find vials of fruit flies. On the TOP shelf, please click on one of the female vials (on the left side) and then drag it to the empty vial on the shelf below. Please repeat this step using one of the male vials (on the right side). These flies will be used as the parental (P) generation. You may switch your parent choices at any time by dragging out old selections and dragging in new flies. Use the Punnett square below to predict the genotypes/phenotypes of the offspring (Note: refer to the genotype table you created above if needed):

XR

XR

XR

Genotype: XRXR

Phenotype:Red eye, female

Genotype:XRXR

Phenotype: Red eye, female

Y

Genotype:XRY

Phenotype: Red eye, male

Genotype:XRY

Phenotype: Red eye, male

__50_% Female, red eye _0__% Female, white eye _50__% Male, red eye _0__% Male, white eye

When you are finished, click “Mate and Sort”.

You will now see information appear in the vials sitting on the next shelf below. These are the offspring of the parent flies you selected above, and they represent the first filial (F1) generation. In your “Data Table” on the bottom of the page and/or on Table I found at the end of this Worksheet, please input the numbers of each sex and phenotype combination for the F1 generation. These numbers will be placed into the first row marked “P generation Cross”.

You will next need to select one of the F1 female flies and one of the F1 male flies to create the second filial (F2) generation. Drag your selections down to the empty vial on the next shelf below and fill in the Punnett square below to predict the offspring:

XR

XR

XR

Genotype: XRXR

Phenotype:Red eye, female

Genotype:XRXR

Phenotype: Red eye, female

Y

Genotype:XRY

Phenotype: Red eye, male

Genotype:XRY

Phenotype: Red eye, male

__50_% Female, red eye _0__% Female, white eye _50__% Male, red eye _0__% Male, white eye

After clicking “Mate and Sort”, you will now have information on their offspring (the F2 generation) to input into your “Data Table” or Worksheet below. This information will be placed into the second row marked “F1 generation Cross”.

NOTE: there are additional lines remaining to use if your instructor requires the analysis of additional crosses.

Please finish this exercise by opening the “Journal” link at the bottom of the page and answering the questions.

Table I:

Cross Type

Phenotype of Male Parent

Phenotype of Female Parent

Number of Red eye, Male Offspring

Number of White eye, Male Offspring

Number of Red eye, Female Offspring

Number of White eye, Female Offspring

P Generation Cross

Red

Red

50

0

50

0

F1 Generation Cross

Red

Red

50

0

50

0

P Generation Cross

White

Red

47

0

53

0

F1 Generation Cross

Red

Red

22

25

53

0

P Generation Cross

Red

White

0

49

51

0

F1 Generation Cross

White

Red

27

28

20

25

P Generation Cross

White

White

0

51

0

49

F1 Generation Cross

White

White

0

51

0

49

Post-laboratory Questions:

Through fruit fly studies, geneticists have discovered a segment of DNA called the homeobox which appears to control:

Sex development in the flies

Life span in the flies

Final body plan development in the flies

The genotype of a red-eyed male fruit fly would be:

XRXR

XRXr

XrXr

A or B

None of the above

Sex-linked traits:

Can be carried on the Y chromosome

Affect males and females equally

Can be carried on chromosome 20

A and B

None of the above -2

A monohybrid cross analyzes:

One trait, such as eye color

Two traits, such as eye color and wing shape

The offspring of one parent

A female with the genotype “XRXr”:

Is homozygous for the eye color gene

Is heterozygous for the eye color gene

Is considered a carrier for the eye color gene

A and B

B and C

In T.H. Morgan’s experiments:

He concluded that the gene for fruit fly eye color is carried on the X chromosome

He found that his F1 generation results always mirrored those predicted by Mendelian Laws of Inheritance

He found that his F2 generation results always mirrored those predicted by Mendelian Laws of Inheritance

A and B

All of the above

In this laboratory exercise:

The Punnett square will allow you to predict the traits of the offspring created in your crosses

XR will represent the recessive allele for eye color, which is white

Xr will represent the dominant allele for eye color, which is red

All of the above

In a cross between a homozygous red-eyed female fruit fly and a white-eyed male, what percentage of the female offspring is expected to be carriers?

0%

25%

50%

75%

100%

In a cross between a white-eyed female and a red-eyed male:

All males will have red eyes

50% of males will have white eyes

All females will have red eyes

50% of females will have white eyes

In human diseases that are X-linked dominant, one dominant allele causes the disease. If an affected father has a child with an unaffected mother:

All males are unaffected

Some but not all males are affected

All females are unaffected

Some but not all females are affected

Journal Questions:

1. In a mating between a red-eyed male fruit fly and a red-eyed heterozygous female, what percentage of the female offspring is expected to be carriers? How did you determine the percentage?

2. In a mating between a red-eyed male fruit fly and a white-eyed female fruit fly, what percentage of the male offspring will have white eyes? Describe how you determined the percentage.

3. Hemophilia, a blood disorder in humans, results from a sex-linked recessive allele. Suppose that a daughter of a mother without the allele and a father with the allele marries a man with hemophilia. What is the probability that the daughter's children will develop the disease? Describe how you determined the probability.

4. Colorblindness results from a sex-linked recessive allele. Determine the genotypes of the offspring that result from a cross between a color-blind male and a homozygous female who has normal vision. Describe how you determined the genotypes of the offspring.

5. Explain why sex-linked traits appear more often in males than in females.

6. In humans, hemophilia is a sex-linked recessive trait. It is located on the X chromosome. Remember that the human female genotype is XX and the male genotype is XY. Suppose that a daughter of a mother without the allele and a father with the allele marries a man with hemophilia. What is the probability that the daughter's children will develop the disease? Describe how you determined the probability.

7. Colorblindness also results from a sex-linked recessive allele on the X chromosome in humans. Determine the genotypes of the offspring that result from a cross between a color-blind male and a homozygous female who has normal vision. Describe how you determined the genotypes of the offspring.

8. Based on the traits explained in questions 6 and 7, explain why sex-linked traits in humans appear more often in males than in females.