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Ch. 20 - Quantitative Genetics and Multifactorial Traits
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. 20 - Quantitative Genetics and Multifactorial TraitsProblema 11
Capitolo 20, Problema 11

In a cross between a strain of large guinea pigs and a strain of small guinea pigs, the F₁ are phenotypically uniform, with an average size about intermediate between that of the two parental strains. Among 1014 F₂ individuals, 3 are about the same size as the small parental strain and 5 are about the same size as the large parental strain. How many gene pairs are involved in the inheritance of size in these strains of guinea pigs?

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1
Recognize that the problem describes incomplete dominance or additive gene action, where the F₁ phenotype is intermediate between the two parental phenotypes, and the F₂ shows a range of phenotypes including parental types and intermediates.
Identify that the number of gene pairs (n) involved can be estimated by analyzing the proportion of F₂ individuals that resemble the parental phenotypes, which correspond to the homozygous genotypes at all loci.
Recall that for n gene pairs with additive effects, the proportion of F₂ individuals showing the parental phenotype is given by the formula \(\left(\frac{1}{4}\right)^n\) for each parental type, because each gene pair segregates independently and the parental phenotype requires homozygosity at all loci.
Calculate the observed frequency of each parental phenotype in the F₂ generation by dividing the number of individuals with that phenotype by the total number of F₂ individuals (e.g., for the small phenotype, frequency = \(\frac{3}{1014}\)).
Set up the equation \(\left(\frac{1}{4}\right)^n = \text{observed frequency}\) and solve for n by taking the logarithm of both sides: \(n = \frac{\log(\text{observed frequency})}{\log(\frac{1}{4})}\). This will give the number of gene pairs involved.

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Incomplete Dominance and Intermediate Phenotypes

Incomplete dominance occurs when the heterozygote phenotype is intermediate between the two homozygotes, as seen in the F₁ guinea pigs with sizes between large and small parents. This suggests that neither allele is completely dominant, resulting in blending traits rather than a dominant-recessive pattern.
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04:37
Variations on Dominance

Polygenic Inheritance

Polygenic inheritance involves multiple gene pairs contributing additively to a single trait, such as size. The continuous variation and intermediate phenotypes in the F₂ generation indicate that size is controlled by several genes, each with small effects, rather than a single gene.
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05:13
Organelle Inheritance

Segregation Ratios and Estimating Number of Gene Pairs

The number of gene pairs can be estimated by analyzing the frequency of parental phenotypes in the F₂ generation. The proportion of individuals resembling parental extremes follows a (1/4)^n pattern, where n is the number of gene pairs, allowing calculation of n from observed counts of extreme phenotypes.
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07:56
Sex-Linked Genes
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100          20    

110          60    

120          90    

130         130    

140         180   

 150         120    

160          70    

170         50    

180         40 

Calculate 

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(b) the variance, 

(c) the standard deviation, and 

(d) the standard error of the mean. 

Plot a rough graph of plant height against frequency. Do the values represent a normal distribution? Based on your calculations, how would you assess the variation within this population?

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