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Ch. 3 - Mendelian Genetics
Klug - Essentials of Genetics 10th Edition
Klug10th EditionEssentials of GeneticsISBN: 9780135588789Non è quello che usi tu?Cambia libro di testo
Tutti i libri di testoKlug 10th EditionCh. 3 - Mendelian GeneticsProblema 23c
Capitolo 3, Problema 23c

Two true-breeding pea plants are crossed. One parent is round, terminal, violet, constricted, while the other expresses the contrasting phenotypes of wrinkled, axial, white, full. The four pairs of contrasting traits are controlled by four genes, each located on a separate chromosome. In the F1 generation, only round, axial, violet, and full are expressed. In the F2 generation, all possible combinations of these traits are expressed in ratios consistent with Mendelian inheritance.
Which F2 phenotype is expected to occur least frequently? Write a mathematical expression that predicts this frequency.

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Identify the dominant and recessive alleles for each trait based on the F1 phenotype. Since the F1 generation shows round, axial, violet, and full phenotypes, these are the dominant traits, and wrinkled, terminal, white, and constricted are recessive.
Assign symbols for each gene, for example: R (round) dominant over r (wrinkled), A (axial) dominant over a (terminal), V (violet) dominant over v (white), and F (full) dominant over f (constricted).
Determine the genotype of the F1 plants. Since the parents are true-breeding and contrasting, the F1 will be heterozygous for all four genes: Rr Aa Vv Ff.
Recall that in the F2 generation, the genes assort independently, and the phenotypic ratio for each gene follows Mendelian inheritance: 3 dominant : 1 recessive. The least frequent phenotype will be the one that is recessive for all four traits.
Write the mathematical expression for the frequency of the least frequent phenotype (all recessive traits) as the product of the probabilities of each recessive phenotype: \(\left(\frac{1}{4}\right)^4\).

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Mendelian Inheritance and Independent Assortment

Mendelian inheritance describes how traits are passed from parents to offspring through dominant and recessive alleles. Independent assortment states that genes on different chromosomes segregate independently during gamete formation, leading to various combinations of traits in offspring.
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Gamete Genetics and Independent Assortment

Genotype and Phenotype Ratios in Dihybrid and Multihybrid Crosses

In crosses involving multiple gene pairs, phenotype ratios result from combining the probabilities of each trait's expression. For four gene pairs, the F2 generation exhibits a 9:3:3:1-like distribution extended to multiple traits, with dominant and recessive phenotypes appearing in predictable ratios.
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Branch Diagram

Calculating Phenotype Frequencies Using Probability Multiplication

The frequency of a specific phenotype in the F2 generation is found by multiplying the probabilities of each individual trait's phenotype. For traits assorting independently, the overall frequency equals the product of each trait's phenotype probability, allowing prediction of rarest phenotypes.
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Multiple Cross Overs and Interference
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Draw all possible conclusions concerning the mode of inheritance of the trait portrayed in the following limited pedigree.

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To assess Mendel's law of segregation using tomatoes, a true-breeding tall variety (SS) is crossed with a true-breeding short variety (ss). The heterozygous F₁ tall plants (Ss) were crossed to produce two sets of F₂ data, as follows.

Using the X² test, analyze the results for both datasets. Calculate X² values and estimate the p values in both cases.

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