- 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.

Expected outcome if the genes are on different chromosomes:

- 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.

Expected outcome if the genes are on the same chromosome:

- 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.

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?

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

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?

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

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

- Fruit flies have many chromosomes

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

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

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?

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

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?

- Crossing over sometimes occurs

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

- It creates more total offspring.

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

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

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

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

9.8.2 Crossing Over and Recombination: A Tool for Mapping Genes Flashcards by Irina Soloshenko

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9.8.2 Crossing Over and Recombination: A Tool for Mapping Genes Flashcards

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