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Chapter 15: Q15-12TYU (page 313)

Assume you are mapping genes A, B, C, and D in Drosophila. You know that these genes are linked on the same chromosome, and you determine the recombination frequencies between each pair of genes to be as follows: A-B, 8%; A-C, 28%; A-D, 25%; B-C, 20%; B-D, 33%.

  1. Describe how you determined the recombination frequency for each pair of genes.

  2. Draw a chromosome map based on your data.

Short Answer

Expert verified

(a). An F1 dihybrid offspring for each pair of genes is generated to obtain recombinant frequencies. The F1 hybrid is then test crossed with a homozygous recessive fly. The parental and recombinant offspring are observed and counted to determine the recombination frequency between the two genes.

(b).

Step by step solution

01

Recombination frequency

The number of recombinants produced concerning the number of offspring in a cross is referred to as recombinant frequency.The recombination frequency between gene pairs helps to calculate the distance between the genes on a chromosome.

1% recombinant frequency refers to one map unit. The distance between the genes on the chromosome is referred to as map units.

02

Calculation of recombinant frequency between the pair of genes

Calculate the recombinant frequency for geneAand geneB. First, a homozygous dominant parent (AABB) is crossed with a recessive parent (aabb). The offspring produced would be F1dihybrid with genotypeAbBb.

The F1dihybrid is then test crossed with a homozygous recessive fly (aabb).The offspring produced from this cross is observed and segregated as parental and recombinants based on parents' genotype.

The recombination frequency between the gene pairs is calculated using the formula:

RecombinantFrequency=NumberofrecombinantsTotalnumberofoffspring脳100

03

Chromosome map

According to the given data, the distance between geneAand geneBis 8 map units, betweenAandCis 28 map units, betweenAandDis 25 map units, betweenBandCis 20 map units, and betweenAandDgene is 33 map units.

The data suggest that gene A and B is located between gene D and C. Therefore, the sequence of genes on the chromosome is DABC.

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Most popular questions from this chapter

The results in the data table are from a simulated F1 dihybrid test cross. The hypothesis that the two genes are unlinked predicts that the offspring phenotypic ratio will be 1:1:1:1. Using this ratio, calculate the expected number of each phenotype out of the 900 total offspring, and enter the values in that data table.

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 crossover? Without knowing these genes are on the same chromosome, how would you interpret the results of this cross?

A planet is 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), and nose morphology (S = upturned snout, s = downturned snout). Since the creatures are not 鈥渋ntelligent,鈥� Earth scientists are able to do some controlled breeding experiments using various heterozygotes in testcrosses. For tall heterozygotes with antennae, the offspring are tall antennae, 46; dwarf antennae, 7; dwarf no antennae, 42; tall no antennae, 5. 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 frequencies for both experiments.

The ABO blood type locus has been mapped on chromosome 9. A father with type AB blood and a mother who has type O blood have a child with trisomy nine and type A blood. Using this information, can you tell in which parent the non-disjunction occurred? Explain your answer. (See Figures 14.11 and 15.13).

Pseudohypertrophic muscular dystrophy is an inherited disorder that causes gradual deterioration of the muscles. It is seen almost exclusively in boys born to apparently unaffected parents and usually results in death in the early teens. Is this disorder caused by a dominant or a recessive allele? Is its inheritance sex-linked or autosomal? How do you know? Explain why this disorder is almost never seen in girls.
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