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Ch. 13 - Meiosis and Sexual Life Cycles
Campbell - Campbell Biology 12th Edition
Urry12th EditionCampbell BiologyISBN: 9785794169850Non è quello che usi tu?Cambia libro di testo
Tutti i libri di testoUrry 12th EditionCh. 13 - Meiosis and Sexual Life CyclesProblema 8
Capitolo 13, Problema 8

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?

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Identify the genotypes of the parent plants. The homozygous blue oval plant has the genotype BBrr, and the homozygous white round plant has the genotype bbRR.
Determine the genotype of the F1 progeny. Since the genes are linked and the parents are homozygous, the F1 generation will have the genotype BbRr.
Set up the cross for the F1 progeny with the homozygous white oval plants, which have the genotype bbrr. The cross is BbRr x bbrr.
Calculate the expected gamete combinations from the F1 progeny (BbRr). Since the genes are 10 map units apart, the recombination frequency is 10%. This results in 45% non-recombinant gametes (BR and br) and 5% recombinant gametes (Br and bR).
Determine the expected phenotypes and their frequencies among the 1,000 offspring. Non-recombinant offspring (BR x bbrr and br x bbrr) will be 450 blue round (BRbbrr) and 450 white oval (brbbrr). Recombinant offspring (Br x bbrr and bR x bbrr) will be 50 blue oval (Brbbrr) and 50 white round (bRbbrr).

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Genetic Linkage

Genetic linkage refers to the tendency of genes located close to each other on a chromosome to be inherited together during meiosis. The closer the genes are, the less likely they are to be separated by recombination. In this scenario, the two genes controlling petal color and stamen shape are linked, which affects the expected phenotypic ratios in the offspring.
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01:28
Genetic Code

Map Units and Recombination Frequency

Map units, or centimorgans, are a measure of genetic distance based on the frequency of recombination between two genes. One map unit corresponds to a 1% chance of recombination occurring between the genes. In this case, the genes are 10 map units apart, indicating a 10% recombination frequency, which will influence the proportions of phenotypes in the offspring.
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Percorso guidato
06:36
Allele Frequencies

Punnett Square and Phenotypic Ratios

A Punnett square is a tool used to predict the genotypic and phenotypic ratios of offspring from genetic crosses. By setting up a Punnett square for the F1 progeny crossed with homozygous white oval plants, one can calculate the expected ratios of the four phenotypes: blue round, blue oval, white round, and white oval, based on the inheritance patterns of the linked genes.
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01:37
Punnett Squares
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The following diagram shows a cell in meiosis. Identify the stage of meiosis shown.


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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?

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The diagram shows a cell in meiosis. Describe the makeup of a haploid set and a diploid set.

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