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    4. In yeast, wild type colonies are white. Adenine-requiring auxotrophs (ad) are red when
    grown on medium containing limiting amounts of adenine. This is due to the accumulation
    of a red precursor in adenine biosynthesis. If cells from an ad strain are plated on minimal
    medium most cells don't produce colonies but two rare types of "revertant" colonies are
    produced, one type white in colour, and the other type with a "rose" colour. Strains grown
    from such colonies were crossed to wild type of opposite mating type are yielded the
    following results:
    white "revertant" X wild type (also white) Æ all white auxotrophic progeny
    rose "revertant" X wild type (white) Æ ½ white, ¼ rose, ¼ red
    (a) What is the difference between the genetic mechanisms that produced the white and
    rose colonies?
    (b) Show the genotypes of the parents and progeny in the above crosses.

    5. A lab population of Drosophila was started with a small number of males and females
    from the standard pure line called Oregon R. After several months the population had
    increased to hundreds of thousands of flies. Careful screening of individual flies under
    the microscope revealed many different mutant phenotypes. Three examples were:
    1. A female with forked bristles (wild type is unforked)
    2. A female with apricot eyes (wild type is red)
    3. A male with curled wings (wild type is straight)

    Crosses were made with the following results:
    Forked female X wild type male
    F1: all wild type
    F2: ¾ wild type (males and females)
    ¼ forked (males and females)
    Apricot female X wild type male
    F1: males all apricot; females all wild type
    F2: ½ wild type (males and females)
    ½ apricot (males and females)
    Curled male X wild type female
    F1: ½ curled (males and females)
    ½ wild type (males and females)

    (a) Explain what the results tell us about the nature of these three mutations (include
    genotypes for all individuals involved in the crosses).
    (b) Speculate on whether the mutations could have occurred in the individuals chosen, or
    in their immediate or distant ancestors.

    9. The human genetic disease tuberous sclerosis (TS) is inherited as an autosomal
    dominant. Polycistic kidney disease (PKD) is also inherited as an autosomal dominant.
    Both diseases are dominant because of haplo-insufficiency; in other words in
    heterozygotes the one wild-type allele is insufficient for normal cell function. The genes
    concerned are closely linked on the short arm of chromosome 16. Rare cases have been
    reported of people expressing the symptoms of both TS and PKD. In these cases the two
    diseases are inherited together as one unit through the generations.
    (a) Propose a possible mechanism for the origin of the people who have both diseases,
    and explain how they are inherited together.
    (b) How would you test your idea?

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    Solution Preview

    4. In yeast, wild type colonies are white. Adenine-requiring auxotrophs (ad) are red when
    grown on medium containing limiting amounts of adenine. This is due to the accumulation
    of a red precursor in adenine biosynthesis. If cells from an ad strain are plated on minimal
    medium most cells don't produce colonies but two rare types of "revertant" colonies are
    produced, one type white in colour, and the other type with a "rose" colour. Strains grown
    from such colonies were crossed to wild type of opposite mating type an yielded the
    following results:
    white "revertant" X wild type (also white) Æ all white auxotrophic progeny
    rose "revertant" X wild type (white) Æ ½ white, ¼ rose, ¼ red
    (a) What is the difference between the genetic mechanisms that produced the white and
    rose colonies?
    (b) Show the genotypes of the parents and progeny in the above crosses.

    From what I understand of yeast genetics, normal yeast cells are haploid. Two haploid cells of opposite mating types will combine into a diploid zygote, which will undergo meiosis (after a few mitosis divisions) to produce more haploid yeast cells.

    So, for the first type of revertant colony (white), the mutant gene probably reverted back to the wild type:
    ad -> + (white mutation)

    When this is crossed to a wild type yeast you simply have:
    + X +

    Which will simply give you all white progeny. However, these progeny should be able to synthesize their own adenine, yet the question says that they're auxotrophic. I don't really ...

    Solution Summary

    Yeast cells are assessed. Adenine-requiring auxotrophs on different growth medians are given. The type of opposite mating types which have yielded different results are discussed.

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