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Module 6 Mendelian Genetics

Lesson 3.6.6

3.6.6 page 3

Reflect and Connect
By recognizing that genes are located on chromosomes, you can now go far beyond what Mendel was able to do by crossing peas and analyzing outcomes. You should now be able to explain X-linked inheritance, since you know that females have two X chromosomes, and that males have only one. You should also be able to understand how non-disjunction will have dramatic results on offspring by adding or removing thousands of alleles. It has been a few lessons since you’ve looked at your genetic study dictionary. Bring that out again, and add any terms we’ve been using in this lesson that are not in there yet. You may also wish to add entries or flashcards that show patterns of inheritance on one side, with the explanation on the other. To test your understanding of this lesson’s connections to the aspects of genetics that you have already learned about, complete the following questions. If you are having difficulty, consult with your instructor.

 

Diploma Connection

Answer the following questions from a previous Biology 30 Diploma Exam.

In humans, the allele for normal blood clotting (H) is dominant to the allele for hemophilia heart. The trait is X-linked.

  1. A female hemophiliac marries a man who is not a hemophiliac. The row that indicates the probability of this couple having a child that is a hemophiliac, and indicates the sex that the child would be is

    Row

    Probability

    Sex of Affected Child
    A. 0.25 male
    B. 0.25 either female or male
    C. 0.50 male
    D. 0.50 either male or female

    Answer: ___________
  2. A woman who is not a hemophiliac has a father who is a hemophiliac. If this woman marries a man who is a hemophiliac, what is the probability of them having a hemophiliac son?

    Answer: ___________

Use the following information to answer the next question.

A recessive allele causes Drosophila to have white eyes instead of wild-type eyes. This eye colour gene is known to be X-linked. In a cross between homozygous wild-type females and white-eyed males, all F1 progeny have wild-type eyes.

  1. What ratio of wild-type to white-eyed progeny can be expected in each sex if F1 females are crossed to males of the same genotype as their father?

    1. Males – 1:0; females – 1:0
    2. Males – 1:1; females – 1:0
    3. Males – 0:1; females – 1:1
    4. Males – 1:1; females – 1:1

Answer: ___________

 

Use the following information to answer the next question.

  1. To determine whether this is an X chromosome or an autosome, a researcher would have to determine whether these traits are

    1. recessive
    2. dominant
    3. passed from male parents to their male offspring
    4. passed from female parents to their male offspring

    Answer: ___________

Use the following information to answer the next question.


Scientists have identified a genetic condition that apparently makes some men prone to impulsive, violent behaviour. A pedigree was drawn highlighting the violent members of a particular family. It appeared, from the pedigree, that men who displayed this violent behaviour inherited this condition from their mothers, not their fathers. Further evidence showed that this was the mode of inheritance.

—from Richardson, 1993

  1. The inheritance pattern described indicates that this condition is

    1. X-linked
    2. Y-linked
    3. autosomal
    4. codominant

    Answer: ___________
Self-Check

For a very good self-check, go back to the Arizona biology project website and work on the Sex-linked Inheritance Problem set (1–10). As always, each problem has a tutorial to help you if you run into any problems with the questions. Also, the correct answer is explained when you reach it.

 

Lesson Summary

During this lesson you were to examine the following focusing questions:

  • Why do some traits appear more frequently in one gender rather than the other?
  • How did Thomas Hunt Morgan’s work create experimental support for the chromosomal theory of inheritance?

Genes occur on chromosomes. This is true for all chromosomes, including the X chromosome. However, the Y chromosome has few genes. Since males receive only one X chromosome, they will have a different rate of inheritance than females for all traits whose alleles are located on the X chromosome. Most genetic diseases are recessive traits. So, for any recessive traits or diseases appearing on the X chromosome, males will have a higher incidence of that trait or disease than will females. This is because males always express whichever allele they receive on the X chromosome. Therefore, males have a fifty : fifty chance of expressing the trait. Females often have at least one normal, dominant allele to cover up the disease, as in the homozygous dominant and in the heterozygous condition. To express the recessive X-linked trait, a female must be homozygous recessive.

 

Thomas Hunt Morgan and his team worked with fruit flies to try to disprove the chromosomal theory of inheritance. However, his work proved to generate experimental evidence that actually supported the theory. This evidence indicated that genes were in fact located on chromosomes. For this work he received the Nobel Prize in 1933.