You design Drosophila crosses to provide recombination data for gene a, which is located on the chromosome shown in Figure 15.12. 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?
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What is the physical basis for the independent assortment of alleles into offspring?

a. There are chromosome divisions during gamete production.

b. Homologous chromosome pairs are separated during gamete production.

c. Sperm and eggs are produced by different sexes.

d. Each gene codes for more than one protein.

e. The instruction manual for producing a human is incomplete.

Assume that genes A and B are on the same chromosome and are 50 map units apart. An animal heterozygous at both loci is crossed with one that is homozygous recessive at both loci. What percentage of the offspring will show recombinant phenotypes resulting from crossovers? Without knowing these genes are on the same chromosome, how would you interpret the results of this cross?

Two genes of a flower, one controlling blue (B) versus white (b) petals and the other controlling round (R) versus oval (r) stamens, are linked and are 10 map units apart. You cross a homozygous blue oval plant with a homozygous white round plant. The resulting F1 progeny are crossed with homozygous white oval plants, and 1,000 offspring plants are obtained. How many plants of each of the four phenotypes do you expect?

Triploid (3n) watermelons, which are seedless, are produced by crossing a tetraploid (4n) strain with a diploid (2n) plant. Explain why this mating produces a triploid individual.

Meiosis results in independent assortment of maternal and paternal chromosomes. If 2n=6 for a given organism, and there is no crossing over, what is the chance that a gamete produced by this diploid organism will receive only paternal chromosomes?

a. 0

b. 1/16

c. 1/8

d. 1/3

Why are individuals with an extra chromosome 21, which causes Down syndrome, more numerous than individuals with an extra chromosome 3 or chromosome 16?

a. There are probably more genes on chromosome 21 than on the others.

b. Chromosome 21 is a sex chromosome, and chromosomes 3 and 16 are not.

c. Down syndrome is not more common, just more serious.

d. Extra copies of the other chromosomes are probably fatal.

Select True or False for each statement.

T/FLinked genes are always inherited together.

T/FGenetic map distances measure the number of nucleotides between a pair of genes.

T/FThe farther apart genes are on a chromosome, the more likely there is to be a crossover between these genes during meiosis.

T/FCrossing over occurs between genes on different homologs of a homologous chromosome pair.