Background Information:

How can a sex-linked trait be identified?

Sex-linked traits are characteristics carried on genes of the sex chromosomes. Thomas Hunt Morgan was one of the first scientists known to observe sex linkage. In his experiments with the common fruit fly, Drosophila melanogaster, he analyzed the results of the following Drosophila mating:

1. A white-eyed male crossed with a red-eyed female produced all red-eyed F1 offspring, as expected according to Mendelian laws of inheritance. An F1 red-eyed male crossed with an F1 red-eyed female would result in 3 red-eyed F2 offspring (male or female): 1 white-eyed F2 offspring (male or female) according to Mendelian laws of inheritance. But Morgan found that all of the F2 female offspring and half of the F2 male offspring had red eyes, which is not expected according to Mendelian laws of inheritance.

A white-eyed female crossed with a red-eyed male resulted in all of the F1 female offspring having red eyes and all of the F1 male offspring having white eyes, which is expected according to Mendelian laws of inheritance. An F1 red-eyed female crossed with an F1 white-eyed male would result in 3 red-eyed F2 offspring (male or female): 1 white-eyed F2 offspring (male or female) according to Mendelian laws of inheritance. But Morgan found that half of the F1 offspring of each sex had red eyes and the other half had white eyes, which is not expected according to Mendelian laws of inheritance.

2. Morgan's results proved that more complex laws of inheritance exist. The alleles for Drosophila eye-color are not simply dominant or recessive. Because of the ratios that resulted from his Drosophila crosses, Morgan concluded that the gene for eye color is carried by, or linked to, the X chromosome. He also found that the Y chromosome does not carry a gene for this characteristic.

Fruit flies are often used to study genetics because of their short life cycle (about 2 weeks), their small size (several hundred can be housed in a small jar), and their fecundity (a single pair may produce more than a hundred offspring).

English biologist Reginald Punnett developed a simple method for finding the expected proportions of genotypes and phenotypes in the offspring of a genetic cross. It is called a Punnett square. Punnett squares can be used to predict the traits of offspring in genetic crosses. When using Punnett squares to analyze the results of crosses involving eye-color in Drosophila, XR represents the dominant allele for red-eye color and Xr represents the recessive allele for white-eye color.

Fisher

Lab Procedure:

Virtual Lab: Sex-Linked Traits

Genetics Chapter 7

Purpose: Explore sex-linked gene for eye color in Drosophila flies.

Objective: Construct and Analyze Punnett square for monohybrid genetic crosses involving sex-linked traits.

When completing post lab questions please refer to your textbook for further explanation. Textbook pages 201-203 McDougal

Instructions:

  • Go to my website
  • Go to Biology
  • Go to Virtual Labs
  • Go to McGraw-Hill Virtual Labs Link
  • Go to Sex-linked Traits
  • Go to Laboratory Exercise

Procedure:

  1. Click on the T.V. screen found in your virtual lab. Watch the short video.
  2. Next, complete the Punnett square activity by clicking on the laboratory notebook.

Complete the table below:

Female, red eyes / Female, red eyes / Female, white eyes / Male, red eyes / Male, white eyes

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

  1. In this exercise, you will perform a Drosophila mating in order to observe sex-linked trait transmission. Click on the top shelf. There are four types of fruit flies that can be used as parents for your initial mating. Each vial is labeled with the sex and phenotype of the flies it contains. Click and drag a male and a female parent to the empty vial on the second shelf. Use the Punnett square below to predict the genotypes/phenotypes of the offspring (Note: refer to the genotype table you created above if needed):

Genotype: Parental generation
Phenotype: / Genotype:Parental generation

Phenotype:
Offspring: / Offspring:
Genotype:
Phenotype: / Genotype:
Phenotype:

____Female, red eye _____ Female, white eye _____Male, red eye _____ Male, white eye

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

  1. You will now see information appear in the vials sitting on the next shelf below; shelf three. These are the offspring of the parent flies you selected above, and they represent the first filial (F1) generation. In your “Data Table” 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”.
  2. You will next need to select one of the F1 female flies and one of the F1 male flies from shelf three and drag them to the empty vial on shelf four to create the second filial (F2) generation. After you have dragged your selections down to the empty vial on shelf four fill in the Punnett square below to predict the offspring:

Genotype:
Phenotype: / Genotype:
Phenotype:
Genotype:
Phenotype: / Genotype:
Phenotype:

____Female, red eye ______Female, white eye ____Male, red eye ____ 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.” This information will be placed into the second row marked “F1 generation Cross” under Trial #1.

NOTE: there are additional lines remaining to use if your instructor requires the analysis of additional crosses. Click reset to begin the activity with a new P generation, follow the above instructions to complete a second trial.

  1. When you have completed this exercise you may begin answering the post lab questions.

When completing post lab questions please refer to your textbook and/or notes for further explanations. Textbook pages 201-203 McDougal

Data/Observations:

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 / /
F1 Generation Cross / /
P Generation Cross
F1 Generation Cross
  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?

X Y

  1. 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.

X Y

  1. 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.

X Y

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

Post-laboratory Questions:

  1. Through fruit fly studies, geneticists have discovered a segment of DNA called the homeobox which appears to control:
  1. Sex development in the flies
  2. Life span in the flies
  3. Final body plan development in the flies
  1. The genotype of a red-eyed male fruit fly would be:
  1. XRXR
  2. XRXr
  3. XrXr
  4. A or B
  5. None of the above
  1. Sex-linked traits:
  1. Can be carried on the Y chromosome
  2. Affect males more than females
  3. Can be carried on chromosome 20
  4. A and B
  5. None of the above
  1. A monohybrid cross analyzes:
  1. One trait, such as eye color
  2. Two traits, such as eye color and wing shape
  3. The offspring of one parent
  1. A female with the genotype “XRXr”:
  1. Is homozygous for the eye color gene
  2. Is heterozygous for the eye color gene
  3. Is considered infected
  4. A and B
  5. B and C
  1. In T.H. Morgan’s experiments:
  1. He concluded that the gene for fruit fly eye color is carried on the X chromosome
  2. He found that his F1 generation results always mirrored those predicted by Mendelian Laws of Inheritance
  3. He found that his F2 generation results always mirrored those predicted by Mendelian Laws of Inheritance
  4. A and B
  5. All of the above
  1. In this laboratory exercise:
  1. The Punnett square will allow you to predict the traits of the offspring created in your crosses
  2. XR will represent the recessive allele for eye color, which is white
  3. Xr will represent the dominant allele for eye color, which is red
  4. All of the above
  1. 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?
  1. 0%
  2. 25%
  3. 50%
  4. 75%
  5. 100%
  1. In a cross between a white-eyed female and a red-eyed male:
  1. All males will have white eyes
  2. 50% of males will have white eyes
  3. All females will have red eyes
  4. 50% of females will have white eyes
  1. 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:
  1. All male offspring are unaffected
  2. Some but not all males are affected
  3. All females are unaffected
  4. Some but not all females are affected