Chapter 14

The genes for the traits that Mendel worked with are either all located on different chromosomes or behave as if they were. How did this help Mendel recognize the principle of independent assortment? a. Otherwise, his dihybrid crosses would not have produced a 9: 3: 3: 1 ratio of \(\mathrm{F}_{2}\) phenotypes. b. The occurrence of individuals with unexpected phenotypes led him to the discovery of recombination. c. It led him to the realization that the behavior of chromosomes during meiosis explained his results. d. It meant that the alleles involved were either dominant or recessive, which gave 3: 1 ratios in the \(\mathrm{F}_{1}\) generation.

The alleles found in haploid organisms cannot be dominant or recessive. Why? a. Dominance and recessiveness describe which of two possible phenotypes are exhibited when two different alleles occur in the same individual. b. Because only one allele is present, alleles in haploid organisms are always dominant. c. Alleles in haploid individuals are transmitted like mitochondrial DNA or chloroplast DNA. d. Most haploid individuals are bacteria, and bacterial genetics is completely different from eukaryotic genetics.

Mendel's rules do not correctly predict patterns of inheritance for tightly linked genes or the inheritance of alleles that show incomplete dominance. Are his hypotheses incorrect? a. Yes, because they are relevant to only a small number of organisms and traits. b. Yes, because not all data support his hypotheses. c. No, because he was not aware of meiosis or the chromosome theory of inheritance. d. No, it just means that the predictions of his hypotheses are limited to certain conditions.

When Sutton and Boveri published the chromosome theory of inheritance, research on meiosis had not yet established that paternal and maternal homologs of different chromosomes assort independently. Then, in 1913 , Elinor Carothers published a paper about a grasshopper species with an unusual karyotype: One chromosome had no homolog (meaning no pairing partner at meiosis \(\mathrm{I}\); another chromosome had homologs that could be distinguished under the light microscope. If chromosomes assort independently, how often should Carothers have observed each of the four products of meiosis shown in the following figure? a. Only the gametes with one of each type of chromosome would occur. b. The four types of gametes should be observed to occur at equal frequencies. c. The chromosome with no pairing partner would disintegrate, so only gametes with one copy of the other chromosome would be observed. d. Gametes with one of each type of chromosome would occur twice as often as gametes with just one chromosome.

Which of the following is the strongest evidence that a trait might be influenced by polygenic inheritance? a. \(\mathrm{F}_{1}\) offspring of parents with different phenotypes have an intermediate phenotype. b. \(\mathrm{F}_{1}\) offspring of parents with different phenotypes have the dominant phenotype. c. The trait shows qualitative (discrete) variation. d. 'The trait shows quantitative variation.

Two black female mice are crossed with a brown male. In several litters, female I produced 9 black offspring and 7 brown; female II produced 57 black offspring. What deductions can you make about the inheritance of black and brown coat color in mice? What are the genotypes of the parents in this case?

A plant with orange, spotted flowers was grown in the greenhouse from a seed collected in the wild. The plant was self-pollinated and gave rise to the following progeny: 88 orange with spots, 34 yellow with spots, 32 orange with no spots, and 8 yellow with no spots. What can you conclude about the dominance relationships of the alleles responsible for the spotted and unspotted phenotypes? What can you conclude about the genotype of the original plant that had orange, spotted flowers?

In flies, small wings are recessive to normal wings. If a cross between two flies produces 8 small wing offspring and 28 normal wing offspring, what are the most likely genotypes of the parents? (Use \(S\) to represent the normal wing allele and \(s\) to represent the short wing allele.)

In garden peas, yellow seeds ( \(Y\) ) are dominant to green seeds \((y),\) and inflated pods (I) are dominant to constricted pods (i). Suppose you have crossed \(Y Y I I\) oarents with vvii parent .Draw the \(\mathrm{F}_{1}\) Punnett square and predict the expected \(\mathrm{F}_{1}\) phenotype(s). .List the genotype(s) of gametes produced by \(\mathrm{F}_{1}\) individuals. .Draw the \(\mathrm{F}_{2}\) Punnett square. Based on this Punnett square, predict the expected phenotype(s) in the \(\mathrm{F}_{2}\) generation and the expected frequency of each phenotype.

In parakeets, two autosomal genes that are located on different chromosomes control the production of feather pigment. Gene \(B\) codes for an enzyme that is required for the synthesis of a blue pigment, and gene \(Y\) codes for an enzyme required for the synthesis of a yellow pigment. Green results from a mixture of yellow and blue pigments, and recessive mutations that prevent production of either pigment are known for both genes. Suppose that a breeder has two green parakeets and mates them. The offspring are green, blue, yellow, and albino (unpigmented). Based on this observation, what are the genotypes of the green parents? What genotypes give each color in the offspring? What fraction of the total progeny should exhibit each type of color?

The smooth feathers on the back of the neck in pigeons can be reversed by a mutation to produce a "crested" appearance in which feathers form a distinctive spike at the back of the head. A pigeon breeder examined offspring produced by a single pair of non-crested birds and recorded the following: 22 non-crested and 7 crested. She then made a series of crosses using offspring from the first cross. When she crossed two of the crested birds, all 20 of the offspring were crested. When she crossed a non-crested bird with a crested bird, 7 offspring were non-crested and 6 were crested. \(\cdot\)For these three crosses, provide genotypes for parents and offspring that are consistent with these results. \(\cdot\)Which allele is dominant?

Suppose you are heterozygous for two genes that are located on different chromosomes. You carry alleles \(A\) and \(a\) for one gene and alleles \(B\) and \(b\) for the other. Draw a diagram illustrating what happens to these genes and alleles when meiosis occurs in your reproductive tissues. Label the stages of meiosis, the homologous chromosomes, sister chromatids, nonhomologous chromosomes, genes, and alleles. Be sure to list all the genetically different gametes that could form and indicate how frequently each type should be observed. On the diagram, identify the events responsible for the principle of segregation and the principle of independent assortment.

The blending-inheritance hypothesis proposed that the genetic material from parents is mixed in the offspring. As a result, traits of offspring and later descendants should lie between the phenotypes of parents. Mendel, in contrast, proposed that genes are discrete and that their integrity is maintained in the offspring and in subsequent generations. Suppose the year is \(1890 .\) You are a horse breeder and have just read Mendel's paper. You don't believe his results, however, because you often work with cremello (very light-colored) and chestnut (reddish-brown) horscs. You know that when you breed a cremcllo individual from a pure-breeding line with a chestnut individual from a pure- breeding line, the offspring are palomino-meaning they have an intermediate (golden-yellow) body color. What additional cross would you do to test whether Mendel's model is valid in the case of genes for horse color? According to his model, what offspring phenotype frequencies would you get from your experimental cross? Explain why your cross would provide a test of Mendel's model versus blending inheritance.

ALD is caused by mutations in one gene. Given the symptoms of ALD, which of the following terms describes the diseaseassociated allele? a. pleiotropic b. dominant c. recessive d. polygenic

Imagine that a woman is heterozygous for a color blindness allele. At a site on the chromosome with the color blindness allele, a new mutation occurs that causes ALD, creating one chromosome with an allele for color blindness and an allele for ALD. A son of this woman is color-blind but does not have ALD. Assuming that no new mutations have occurred, what could account for this color-blind son without ALD?