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    Genetics Problem Set on Patterns of Inheritance

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    1. G6PD is an enzyme found in human white blood cells. It is made by a gene that has 2
    alleles, A and B. In females, cells either make the A type or the B type, but never both.
    This is very different from most other enzymes, where all cells in a heterozygote make
    both types of enzyme. What is the most reasonable explanation for this unusual
    behavior of G6PD?

    2. In humans, red-green color blindness is recessive and X-linked, while albinism is
    recessive and autosomal. What types of children can be produced as the result of
    marriages between two homozygous parents, a normal-visioned albino woman and a
    color-blind, normally pigmented man? (Give your answer separately for males and

    3. Suppose that a mutation occurred in the SRY gene on the human Y chromosome,
    knocking out its ability to produce the testis-determining factor. Predict the
    phenotype of an individual who carried this mutation and a normal X chromosome?

    4. Sex determination in Gryllus campestris (cricket) is by the XO method. The somatic
    cells of a cricket are analyzed and found to contain 29 chromosomes.
    A. What sex is this individual?
    B. What is the diploid number of the opposite sex?

    5. Ichthyosis hystrix gravior (a greatly thickened horny condition of the skin) is a rare
    human condition, but in the single extensive pedigree which has been studied it occurs
    only in males. All the sons of each affected father have the condition. Females are
    not only unaffected, but never transmit the gene for this defect. Can you suggest a
    possible explanation for this curious and unusual type of inheritance?

    6. Dosage compensation is only found in organisms that have sex chromosomes. What is
    dosage compensation? Briefly describe how dosage compensation is achieved in
    Drosophila and humans.

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    Solution Preview

    Question 1

    An individual cell makes either A or B, but not both; however, the individual female can be AB. Strange, isn't it? Well, what's the solution. We must conclude that since this is a female-specific situation, that we're dealing with sex chromosomes. Females are XX. Let us hypothesize that the G6PD gene is on the X-chromosome. Therefore, a female can have both alleles; however, due to gene dosage and compensation mechanisms (see question 9), each cell inactivates one X chromosome at random. In some cells, the chromosome with the A allele is inactivated while in other cells the X chromosome with the B allele is inactivated. It's a completely random process. Therefore, each cell can only express one of the alleles even though the female-as-a-whole can be heterozygous.


    Question 2

    We are told that both parents are homozygous. Therefore, genotypes of both gene are homozygous. That will simplify the question incredibly.

    In the following problem all capital letters represent dominant alleles and lowercase letters represent recessive alleles.

    C is normal vision; c is color-blind. A is normal skin; a is albino.

    Normal-visioned albino woman must be X^CX^Caa where X^C means X with a superscript C. ...

    Solution Summary

    This problem set contains six genetics questions with carefully worked out solutions that are easy to follow. This is a must for anyone studying and trying to understand sex linkage, patterns of inheritance, somatic mutations, and dosage compensation.