Chapter 12

A man who is an achondroplastic dwarf with normal vision marries a color-blind woman of normal height. The man's father was 6 feet tall, and both the woman's parents were of average height. Achondroplastic dwarfism is autosomal dominant, and red-green color blindness is X-linked recessive. They have a daughter who is a dwarf with normal color vision. What is the probability that she is heterozygous for both genes?

100%

A space probe discovers a planet inhabited by creatures that reproduce with the same hereditary patterns seen in humans. Three phenotypic characters are: height (T = tall, t = dwarf) head appendages (A = antennae, a = no antennae) nose morphology (S = upturned snout, s = downturned snout) Since the creatures are not "intelligent," Earth scientists are able to do some controlled breeding experiments using various heterozygotes in testcrosses. Part A For tall heterozygotes with antennae, the offspring are: tall-antennae 46 dwarf-antennae 7 dwarf-no antennae 42 tall-no antennae 5 Calculate the recombination frequency between T and A. Enter your answer as a percentage (for example, 10%).

12%

Part C Scientists do a further testcross using a heterozygote for height and nose morphology. The offspring are: tall-upturned snout 40 dwarf-upturned snout 9 dwarf-downturned snout 42 tall-downturned snout 9 Calculate the recombination frequency between T and S. Enter your answer as a percentage (for example, 10%).

18%

A plantlike organism on the planet Pandora can have three recessive genetic traits: bluish leaves, due to an allele (a) of gene A; a feathered stem, due to an allele (b) of gene B; and hollow roots due to an allele (c) of gene C. The three genes are linked and recombine as follows: A geneticist did a testcross with an organism that had been found to be heterozygous for the three recessive traits and she was able to identify progeny of the following phenotypic distribution (+ = wild type): Which of the following are the phenotypes of the parents in this cross?

4 and 8

Of the following human aneuploidies, which is the one that generally has the most severe impact on the health of the individual?

47, +21

Part B For heterozygotes with antennae and an upturned snout, the offspring are: antennae-upturned snout 47 antennae-downturned snout 2 no antennae-downturned snout 48 no antennae-upturned snout 3 Calculate the recombination frequency between A and S. Enter your answer as a percentage (for example, 10%).

5%

Imagine a human disorder that is inherited as a dominant, X-linked trait. How would the frequency of this disorder vary between males and females?

Females would display this disorder with greater frequency than males.

Why does recombination between linked genes continue to occur?

New allele combinations are acted upon by natural selection.

Down syndrome has a frequency in the U.S. population of ~1/700 live births. In which of the following groups would you expect this frequency to be significantly higher?

No groups have such higher frequency.

A couple has a child with Down syndrome. The mother is 39 years old at the time of delivery. Which of the following is the most probable cause of the child's condition?

One member of the couple underwent nondisjunction in gamete production.

If cell X enters meiosis, and nondisjunction of one chromosome occurs in one of its daughter cells during meiosis II, what will be the result at the completion of meiosis?

One-fourth of the gametes descended from cell X will be n + 1, 1/4 will be n - 1, and 1/2 will be n.

Gregor Mendel set up a dihybrid cross with one pea plant from the parental generation (P) producing round yellow peas and the other pea plant producing wrinkled green peas. The F2 generation included 315 plants producing round yellow peas, 108 with round green peas, 101 with wrinkled yellow peas, and 32 with wrinkled green peas. How would these results have differed if pea shape and pea color had been linked genes, located close together on the same chromosome?

The F2 generation would have included a higher percentage of pea plants producing round, yellow peas.

Which of these descriptions of the behavior of chromosomes during meiosis explains Mendel's law of independent assortment?

The arrangement of each pair of homologous chromosomes on the metaphase plate during metaphase I is random with respect to the arrangements of other pairs.

Which of the following statements is true of linkage?

The closer two genes are on a chromosome, the lower the probability that a crossover will occur between them.

What is the reason that closely linked genes are typically inherited together?

The likelihood of a crossover event between these two genes is low.

Select the correct statement(s) about sex determination in animals.

The mechanism of sex determination varies with different animal species.

Which of these descriptions of the behavior of chromosomes during meiosis explains Mendel's law of segregation?

The two alleles for each gene separate as homologous chromosomes move apart during anaphase I.

How would one explain a testcross involving F1 dihybrid flies in which more parental-type offspring than recombinant-type offspring are produced?

The two genes are closely linked on the same chromosome.

Part D What is the correct sequence of the three linked genes?

T−A−S

Duchenne muscular dystrophy is a serious condition caused by a recessive allele of a gene on the human X chromosome. The patients have muscles that weaken over time because they have absent or decreased dystrophin, a muscle protein. They rarely live past their 20s. How likely is it for a woman to have this condition?

Very rarely: it is rare that an affected male would mate with a carrier female.

Normally, only female cats have the tortoiseshell phenotype because

a male inherits only one allele of the X-linked gene controlling hair color.

You design Drosophila crosses to provide recombination data for gene a, which is located on the chromosome shown in the figure below. Gene a has recombination frequencies of 14% with the vestigial-wing locus and 26% with the brown-eye locus. Approximately where is a located along the chromosome?

about one-third of the distance from the vestigial-wing locus to the brown-eye locus

A man who is an achondroplastic dwarf with normal vision marries a color-blind woman of normal height. The man's father was 6 feet tall, and both the woman's parents were of average height. Achondroplastic dwarfism is autosomal dominant, and red-green color blindness is X-linked recessive. What proportion of their sons would be color-blind and of normal height?

half

What is the source of the extra chromosome 21 in an individual with Down syndrome?

nondisjunction or translocation in either parent

A man who is an achondroplastic dwarf with normal vision marries a color-blind woman of normal height. The man's father was 6 feet tall, and both the woman's parents were of average height. Achondroplastic dwarfism is autosomal dominant, and red-green color blindness is X-linked recessive. How many of their daughters might be expected to be color-blind dwarfs?

none

In cats, black fur color is caused by an X-linked allele; the other allele at this locus causes orange color. The heterozygote is tortoiseshell. What kinds of offspring would you expect from the cross of a black female and an orange male?

tortoiseshell females; black males