4. Genetic Mapping and Linkage

Trihybrid Cross

4. Genetic Mapping and Linkage

Trihybrid Cross

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Open Question

The following progeny are obtained from a test cross of a trihybrid wild-type plant to a plant with the recessive phenotypes compound leaves (c), intercalary leaflets (i), and green fruits (g). (Traits not listed are wild type.) The test-cross progeny are as follows:

Phenotype Number
Compound leaves 324
Compound leaves, intercalary leaflets 32
Compound leaves, green fruits 5
Compound leaves, intercalary leaflets, green fruits 51
Intercalary leaflets 3
Intercalary leaflets, green fruits 309
Green fruits 42
Wild type 49
815

Calculate the frequencies of recombination between the adjacent genes in the map.

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Open Question

In rabbits, chocolate-colored fur (w⁺) is dominant to white fur (w), straight fur (c⁺) is dominant to curly fur (c), and long ear (s⁺) is dominant to short ear (s). The cross of a trihybrid rabbit with straight, chocolate-colored fur and long ears to a rabbit that has white, curly fur and short ears produces the following results:

Phenotype Number
White, short, straight 13
Chocolate, long, straight 165
Chocolate, long, curly 13
White, long, straight 82
Chocolate, short, straight 436
Chocolate, short, curly 79
White, short, curly 162
White, long, curly 450
1400

Determine the interference value for this cross.

Open Question

The following progeny are obtained from a test cross of a trihybrid wild-type plant to a plant with the recessive phenotypes compound leaves (c), intercalary leaflets (i), and green fruits (g). (Traits not listed are wild type.) The test-cross progeny are as follows:

Phenotype Number
Compound leaves 324
Compound leaves, intercalary leaflets 32
Compound leaves, green fruits 5
Compound leaves, intercalary leaflets, green fruits 51
Intercalary leaflets 3
Intercalary leaflets, green fruits 309
Green fruits 42
Wild type 49
815

How many double-crossover progeny are expected among the test-cross progeny? Calculate the interference for this cross.

Open Question

The following progeny are obtained from a test cross of a trihybrid wild-type plant to a plant with the recessive phenotypes compound leaves (c), intercalary leaflets (i), and green fruits (g). (Traits not listed are wild type.) The test-cross progeny are as follows:

Phenotype Number
Compound leaves 324
Compound leaves, intercalary leaflets 32
Compound leaves, green fruits 5
Compound leaves, intercalary leaflets, green fruits 51
Intercalary leaflets 3
Intercalary leaflets, green fruits 309
Green fruits 42
Wild type 49
815

Determine the order of the three genes, and construct a genetic map that identifies the correct order and the alleles carried on each chromosome in the trihybrid parental plant.

Open Question

In tomatoes, the allele T for tall plant height is dominant to dwarf allele t, the P allele for smooth skin is dominant to the p allele for peach fuzz skin, and the allele R for round fruit is dominant to the recessive r allele for oblong fruit. The genes controlling these traits are linked on chromosome 1 in the tomato genome, and the genes are arranged in the order and with the recombination frequencies shown.

A pure-breeding tall, peach fuzz, round plant is crossed to a pure-breeding plant that is dwarf, smooth, oblong. What are the gamete genotypes produced by each of these plants?

Open Question

Drosophila melanogaster has one pair of sex chromosomes (XX or XY) and three pairs of autosomes, referred to as chromosomes II, III, and IV. A genetics student discovered a male fly with very short (sh) legs. Using this male, the student was able to establish a pure breeding stock of this mutant and found that it was recessive. She then incorporated the mutant into a stock containing the recessive gene black (b, body color located on chromosome II) and the recessive gene pink (p, eye color located on chromosome III). A female from the homozygous black, pink, short stock was then mated to a wild-type male. The F₁ males of this cross were all wild type and were then backcrossed to the homozygous b, p, sh females. The F₂ results appeared as shown in the following table. No other phenotypes were observed.Wild Pink* Black, Black, Pink,Short* ShortFemales 63 58 55 69Males 59 65 51 60*Other trait or traits are wild type.The student repeated the experiment, making the reciprocal cross, F₁ females backcrossed to homozygous b, p, sh males. She observed that 85 percent of the offspring fell into the given classes, but that 15 percent of the offspring were equally divided among b + p, b + +, + sh p, and + sh + phenotypic males and females. How can these results be explained, and what information can be derived from the data?

Open Question

Domestic dogs evolved from ancestral gray wolves. Wolves have coats of short, straight hair and lack 'furnishings,' a growth pattern marked by eyebrows and a mustache found in some domestic dogs. In domestic dogs, coat variation is controlled by allelic variation in three genes. Recessive mutant alleles in the FGF5 gene result in long hair, while dogs carrying the dominant ancestral allele have short hair. Likewise, recessive mutant alleles in the KRT71 gene result in curly hair, whereas dogs with an ancestral dominant allele have straight hair. Dominant mutant alleles in the RSPO2 gene cause the presence of furnishings, while dogs homozygous for the ancestral recessive allele have no furnishings. A pure-breeding curly- and long-haired poodle with furnishings was crossed to a pure-breeding short- and straight-haired border collie lacking furnishings

If dogs of the F₁ generation are interbred, what proportions of genotypes and phenotypes are expected in the F₂?

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