Chapter 18 | Mendel’s Experiments and Heredity
- Figure 18.6 In pea plants, round peas (R) are dominant to wrinkled peas (r). You do a test cross between a pea plant with wrinkled peas (genotype rr) and a plant of unknown genotype that has round peas. You end up with three plants, all which have round peas. From this data, can you tell if the round pea parent plant is homozygous dominant or heterozygous? If the round pea parent plant is heterozygous, what is the probability that a random sample of 3 progeny peas will all be round?
Figure 18.6 A test cross can be performed to determine whether an organism expressing a dominant trait is a homozygote or a heterozygote. - Figure 18.7 What are the genotypes of the individuals labeled 1, 2 and 3?
Figure 18.7 Alkaptonuria is a recessive genetic disorder in which two amino acids, phenylalanine and tyrosine, are not properly metabolized. Affected individuals may have darkened skin and brown urine, and may suffer joint damage and other complications. In this pedigree, individuals with the disorder are indicated in blue and have the genotype aa. Unaffected individuals are indicated in yellow and have the genotype AA or Aa. Note that it is often possible to determine a person’s genotype from the genotype of their offspring. For example, if neither parent has the disorder but their child does, they must be heterozygous. Two individuals on the pedigree have an unaffected phenotype but unknown genotype. Because they do not have the disorder, they must have at least one normal allele, so their genotype gets the “A?” designation. - Figure 18.13 What ratio of offspring would result from a cross between a white-eyed male and a female that is heterozygous for red eye color?
Figure 18.13 Punnett square analysis is used to determine the ratio of offspring from a cross between a red-eyed male fruit fly and a white-eyed female fruit fly. - Figure 18.17 In pea plants, purple flowers (P) are dominant to white flowers (p) and yellow peas (Y) are dominant to green peas (y). What are the possible genotypes and phenotypes for a cross between PpYY and ppYy pea plants? How many squares do you need to do a Punnett square analysis of this cross?
Figure 18.17 This dihybrid cross of pea plants involves the genes for seed color and texture. - Figure 18.23 In a test cross for two characteristics such as the one shown here, can the predicted frequency of recombinant offspring be 60 percent? Why or why not?
Figure 18.23 Inheritance patterns of unlinked and linked genes are shown. In (a), two genes are located on different chromosomes so independent assortment occurs during meiosis. The offspring have an equal chance of being the parental type (inheriting the same combination of traits as the parents) or a nonparental type (inheriting a different combination of traits than the parents). In (b), two genes are very close together on the same chromosome so that no crossing over occurs between them. The genes are therefore always inherited together and all of the offspring are the parental type. In (c), two genes are far apart on the chromosome such that crossing over occurs during every meiotic event. The recombination frequency will be the same as if the genes were on separate chromosomes. (d) The actual recombination frequency of fruit fly wing length and body color that Thomas Morgan observed in 1912 was 17 percent. A crossover frequency between 0 percent and 50 percent indicates that the genes are on the same chromosome and crossover occurs some of the time. - Figure 18.24 Which of the following statements is true?
- Recombination of the body color and red/ cinnabar eye alleles will occur more frequently than recombination of the alleles for wing length and aristae length.
- Recombination of the body color and aristae length alleles will occur more frequently than recombination of red/brown eye alleles and the aristae length alleles.
- Recombination of the gray/black body color and long/short aristae alleles will not occur.
- Recombination of the red/brown eye and long/ short aristae alleles will occur more frequently than recombination of the alleles for wing length and body color.
Figure 18.24 This genetic map orders Drosophila genes on the basis of recombination frequency.
- Figure 18.26 Which of the following statements about nondisjunction is true?
- Nondisjunction only results in gametes with n+1 or n–1 chromosomes.
- Nondisjunction occurring during meiosis II results in 50 percent normal gametes.
- Nondisjunction during meiosis I results in 50 percent normal gametes.
- Nondisjunction always results in four different kinds of gametes.
Figure 18.26 Nondisjunction occurs when homologous chromosomes or sister chromatids fail to separate during meiosis, resulting in an abnormal chromosome number. Nondisjunction may occur during meiosis I or meiosis II.