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# Practice problems

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1. You're working on golden rice, a plant that promises to alleviate nutritional deficiencies in some developing countries. You cross two true breeding plants. One parent is has fat grains, wavy leaves, red flowers and is short in stature, while the other parent expresses the contrasting phenotypes of thin grains, straight leaves, white flowers and is tall in stature. The four pairs of contrasting traits are controlled by four genes, each located on a separate chromosome. In the F1, only fat grains, straight leaves, red flowers, and tall stature were expressed. In the F2, all possible combinations of these traits were expressed in ratios consistent with Mendelian inheritance.
(a) What conclusions can you draw about dominance relationships from the F1 results?
(b) In the F2 results, which phenotype appeared most frequently? Calculate the probability of this phenotype occurring.
(c) Which F2 phenotype is expected to occur least frequently? Calculate the probability of this phenotype occurring.
(d) In the F2 generation, what is the probability of getting one or the other parental phenotypes?
(e) If the F1 plants were test crossed, how many different phenotypes would be produced? How does this number compare with the number of different phenotypes in the F2 generation discussed in parts a-d?

2. Red-green color blindness is X-linked recessive. A woman with normal color vision has a father who is red-green color-blind. The woman has four sons, none of whom are color-blind. Assume everyone in the family has a normal complement of chromosomes. Each of the next three statements has an explanation for why none of the sons are color-blind. For each, state if it is possible or not possible, then give the reason for your choice.

i) None of the sons are color-blind because the mother does not carry the color-blindness allele.

ii) None of the sons are color-blind because none of them inherited the color-blindness allele from the mother.

iii) None of the sons are color-blind because the mother inactivated the X chromosome with the recessive color-blindness allele, and that is the one each son inherited.

3. You have discovered a new orange body color gene on the X chromosome of Drosophila similar to the gene that confers orange coat color in cats. You know that X chromosome inactivation in female cats heterozygous for the coat color gene can result in tortoiseshell colored coats. Do you expect to see similar tortoiseshell body color in Drosophila? Explain your answer?

4. You are studying the genetics of hair characteristics in mice. You find two new interesting mutants one with abnormally thick hair and one with abnormally curly hair. You describe these new mutants to three of your colleagues. The first colleague says that she has seen the mutants before and that they can be explained by two codominant alleles of the same gene. The second colleague says he too has seen mutants like these before but tells you that they are due to two incompletely dominant alleles. The third colleague thinks the two mutants can be explained by two alleles of the same gene and that thick hair is dominant to curly hair. Assume you have a true breeding strain of each mutant. Could you determine which of your colleagues is right by looking at F1 progeny from crossing the two strains or would you have to look at F2 progeny? What would the F1 progeny look like if the mutants were allelic and were i)codominant ii) incompletely dominant or iii) completely dominant?