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chapter 14 (Degrees of Dominance (Most genes have multiple phenotypic…
chapter 14
Degrees of Dominance
Most genes have multiple phenotypic effects, a property called pleiotropy
For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases
In epistasis, expression of a gene at one locus alters the phenotypic expression of a gene at a second locus
In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways
Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype
In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
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quantitative approach
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Each variant for a character, such as purple or white color for flowers, is called a trait
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mating two contrasting, true-breeding varieties, a process called hybridization
test crossing
The F1 offspring produced in this cross were monohybrids, heterozygous for one character
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Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters
A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently
An organism with two different alleles for a gene is a heterozygote and is said to be heterozygous for the gene controlling that character
Therefore, we distinguish between an organism’s phenotype, or physical appearance, and its genotype, or genetic makeup
Fetal Testing
In amniocentesis, the liquid that bathes the fetus is removed and tested
In chorionic villus sampling (CVS), a sample of the placenta is removed and tested
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