Ch, 15 - The Chromosomal Basis of Inheritance

Problem 5

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Chapter 15, Problem 5

Using the information from problem 4, 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 from these data, and then use your answer from problem 4 to determine the correct order of the three linked genes.

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Step 1: Calculate the total number of offspring from the testcross. Add up all the offspring counts: 40 (tall upturned snout) + 9 (dwarf upturned snout) + 42 (dwarf downturned snout) + 9 (tall downturned snout) = 100.

Step 2: Identify the recombinant offspring. Recombinant offspring are those that have a combination of traits not found in either parent used in the testcross. In this case, the recombinants are dwarf upturned snout (9) and tall downturned snout (9).

Step 3: Calculate the recombination frequency. Add the number of recombinant offspring and divide by the total number of offspring, then multiply by 100 to get the percentage. Recombination frequency = (9 + 9) / 100 * 100% = 18%.

Step 4: Use the recombination frequency to determine the correct order of the genes. From problem 4, if you have the map distances between the genes, you can use the recombination frequency to infer the order. Typically, the gene with the middle frequency value is in the middle.

Step 5: Confirm the gene order by checking if the calculated map distances from the recombination frequencies fit the observed data. If the order is correct, the sum of the distances between the genes should match the recombination frequencies observed.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Recombination Frequency

Recombination frequency is a measure of the likelihood that two genes will be separated during meiosis due to crossing over. It is calculated by dividing the number of recombinant offspring by the total number of offspring, then multiplying by 100 to express it as a percentage. This frequency helps in determining the genetic distance between linked genes on a chromosome.

Testcross

A testcross is a genetic cross between an individual with an unknown genotype and a homozygous recessive individual. This method is used to determine the genotype of the unknown individual based on the phenotypes of the offspring. In this context, the testcross helps reveal the linkage and arrangement of genes affecting traits like height and nose morphology.

Linked Genes

Linked genes are genes that are located close to each other on the same chromosome and tend to be inherited together during meiosis. The closer the genes are, the lower the chance of recombination occurring between them. Understanding the arrangement of linked genes is crucial for predicting inheritance patterns and calculating recombination frequencies.

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Sex-Linked Inheritance

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Textbook Question

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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Textbook Question

A wild-type fruit fly (heterozygous for gray body color and normal wings) is mated with a black fly with vestigial wings. The offspring have the following phenotypic distribution: wild-type, 778; black vestigial, 785; black normal, 158; gray vestigial, 162. What is the recombination frequency between these genes for body color and wing size? Is this consistent with the results of the experiment in Figure 15.9?

A planet is inhabited by creatures that reproduce with the same hereditary patterns seen in humans. Three phenotypic characters are height (T=tall, t=dwart), head appendages (A=antennae, a=no antennae), and 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. 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.

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Textbook Question

A wild-type fruit fly (heterozygous for gray body color and red eyes) is mated with a black fruit fly with purple eyes. The offspring are wild-type, 721; black purple, 751; gray purple, 49; black red, 45. What is the recombination frequency between these genes for body color and eye color? Using information from problem 3, what fruit flies (genotypes and phenotypes) would you mate to determine the order of the body color, wing size, and eye color genes on the chromosome?

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