IGA 8/e Chapter 2

14 Chapter TwoAs can be seen, the phenotype of the endosperm correlates to the predominantallele present.33. What is Mendel’s first law?Answer: Mendel’s first law states that alleles segregate into gametes duringmeiosis. This discovery came from his monohybrid experimental crosses.34. If you had a fruit fly (Drosophila melanogaster) that was of phenotype A, whattest would you make to determine if the fly’s genotype was A/A or A/a?Answer: Do a test-cross (cross to a/a). If the fly was A/A, all the progeny will bephenotypically A; if the fly was A/a, half the progeny will be A, and half will bea.35. In examining a large sample of yeast colonies on a petri dish, a geneticist findsan abnormal-looking colony that is very small. This small colony was crossedwith wild type, and products of meiosis (ascospores) were spread on a plate toproduce colonies. In total, there were 188 wild-type (normal-size) colonies and180 small ones.a. What can be deduced from these results regarding the inheritance of thesmall-colony phenotype? (Invent genetic symbols.)b. What would an ascus from this cross look like?Answer:a. A diploid meiocyte that is heterozygous for one gene (for example, s + /swhere s is the allele that confers the small colony phenotype) will, afterreplication and segregation, give two meiotic products of genotype s + andtwo of s. If the random spores of many meiocytes are analyzed, you wouldexpect to find about 50 percent normally sized colonies and 50 percent smallcolonies if the abnormal phenotype is the result of a mutation in a singlegene. Thus, the actual results of 188 normally sized and 180 small-sizedcolonies support the hypothesis that the phenotype is the result of a mutationin a single gene.b. The following represents an ascus with four spores. The important detail isthat two of the spores are s and two are s + .ss +s s +