9.8.2 Crossing Over and Recombination: A Tool for Mapping Genes

Question 1

crossing over and recombination

Answer 1

• The inheritance pattern of genes from a heterozygote and a homozygous recessive in a test cross should be four offspring phenotypes, if the genes were independently assorting.
• In performing his experiments, Thomas Hunt Morganfound that crosses with known linked genes yielded four phenotypes in the offspring. The two recombinant phenotypes occurred at a lower frequency than the parental phenotypes.
• Morgan’s student, Alfred Sturtevant, uncovered the reason recombinant phenotypes occur in crosses involving linked genes. He found that crossing over between homologous chromosomes occurs during meiosis, which results in the recombination of alleles.

Question 2

note

Answer 2

• Experimental heterozygous fruit flies have a BbVv genotype. A test cross should help determine whether or not the two genes are on the same chromosome.
• Review: A test cross is a cross with a homozygous recessive individual to determine the unknown genotype of another individual.
• Test-cross organisms can produce only one kind of gamete. In this example, the test cross organism has the bbvv genotype and can only produce bv gametes.

Question 3

Expected outcome if the genes are on

different chromosomes:

Answer 3

• The parental (P) heterozygote should produce four kinds of gametes: BV, Bv, vB, and bv. The four gametes should combine with bv gametes to produce four genotypes in the F1: BbVv, Bbvv, Bbvv, and bbvv.
  Because the four gametes are produced in equal proportions, the genotypes should occur in a 1:1:1:1 ratio.

Question 4

Expected outcome if the genes are

on the same chromosome:

Answer 4

• The parental (P) heterozygote should produce two kinds of gametes. Alleles inherited from each parent determine the two types produced. For example,
  if one parent had the BBVV genotype and the other had a bbvv genotype, the parental heterozygotes will produce BV and bv gametes.
• The experimental results revealed four phenotypes in the F1. At first, this result seems to demonstrate that the genes are on different chromosomes, but the phenotypes do not occur in a 1:1:1:1 ratio. Two phenotype classes comprised 80% of the total. The other two classes each accounted for 10% of the total offspring.
• Question: If the genes are on the same chromosome, why do recombinants appear? Sturtevant discovered that chromosomes break and recombine, resulting in recombinant gametes.

Question 5

Assuming crossing over takes place in fruit flies, which of the following best describes the results obtained from a cross between two fruit flies that are both heterozygous gray-bodied and heterozygous normal-winged with their genes arranged on chromosomes in the following way: BV and bv?

Answer 5

most gray, normal
some black, vestigial
very few gray, vestigial
very few black, normal

Question 6

A researcher notices that a pair of genes sometimes behave as if they are linked on the same chromosome and also behave as if they are on different chromosomes?

Answer 6

• Sometimes genes on homologous chromosomes are switched with each other.

Question 7

Which of the following statements is not a reason that fruit flies are a good model for genetics studies?

Answer 7

• Fruit flies have many chromosomes

Question 8

What surprised Thomas Morgan about his results from the dihybrid test cross that he performed?

Answer 8

• He found more variety in the offspring than he expected.

Question 9

In fruit flies, normal body color (E) is dominant to ebony body color (e) and normal wings (C) are dominant to curly wings (c). A heterozygous female is testcrossed with a male, resulting in the following offspring: 200 ebony body, normal wing 10 normal body, normal wing 5 ebony body, curly wing 150 normal body, curly wing Based on this information, what conclusion can you make?

Answer 9

• The genes are linked. In the female, the alleles for normal body color and curly wings are on the same chromosome.

Question 10

In Thomas Morgan’s dihybrid test cross, 80% of the offspring belonged to one of two phenotypes, and the other 20% of the offspring were evenly split between two other different phenotypes. What conclusion about crossing over can be drawn from these results?

Answer 10

• Crossing over sometimes occurs

Question 11

You cross a black-bodied (a body-color mutant) male with a vestigial-winged female fruitfly. These two genes are linked. You carry out a test cross using females from the F1. What offspring do you expect to be in the smallest numbers?

Answer 11

• two of these choices

Question 12

Useful tools for mapping genes of the human genome include

Answer 12

• pedigree charts.

Question 13

Which one of the following is not true about crossing over?

Answer 13

• It creates more total offspring.

Question 14

Refer to the illustration and label the parental and recombinant gametes correctly.

Answer 14

n                n
   aB              Ab
          AaBb
        n-------AB recombinant
        n--------aB parental
        n--------Ab parental
        n--------aB recombinant

Question 15

Which one of the following is the best description of a test cross?

Answer 15

• A homozygous recessive individual is crossed with an individual with the dominant phenotype.

Question 16

In the fictitious sponge Poreira, sex is determined by genes on the same chromosome, rather than by heteromorphic chromosomes. Sexual phenotypes are determined as follows: P results in male fertility; p causes male sterility; S causes female sterility; s results in female fertility. Populations generally have equal proportions of males and females, but occasionally hermaphrodites are found. Which of the following statements are correct?

Answer 16

• The genotype of a female is ppss. Hermaphrodites result from recombination.

Question 17

Which one of the following best describes the process of crossing over?

Answer 17

• Genes from a chromosome are switched with genes for the same traits on homologous chromosomes.

Question 18

According to Mendel’s second law, the alleles of two or more genes segregate independently from one another during gamete formation. Morgan test crossed a female fly heterozygous for body color and wing type. Only two phenotypes appeared in the offspring. How do Morgan’s results relate to Mendel’s second law?

Answer 18

• The genes controlling the traits must be on the same chromosome.

Question 19

Which one of the following best describes a vestigial structure?

Answer 19

• any biological structure that has no known function