Exceptions to Mendelian Genetics Incomplete Dominance Codominance Blending of traits: A homozygous white flower traits are expressed simultaneously: A A Both crossed with a homozygous red flower for example homozygous white flower crossed with a will create a pink heterozygote. homozygous red flower would create a red and white flower. Epistasis Pleiotropy genes controlling one phenotype or the gene controlling many phenotypes Many One gene at one locus controlling the expression of a (essentially the opposite of epistasis): The agouti gene at another locus: Consider the biochemical gene in mice is linked to coat color, obesity, and cascade ABCD. If a single mutation occurs certain tumors. in A,B, or C, the product D will be effected. Polygenic Inheritance Multiple Alleles genes controlling one phenotype with an than 2 alleles controlling one trait and Many More additive effect: Laborador Retreiver colors are therefore more than a simple dominant/recessive polygenic relationship between alleles: Blood types are the AABBCC = Black Lab most common example of multiple alleles. Both A and B are dominant while O is treated as recessive. AaBbCc = Lighter Aabbcc= Lightest Type A = IAIA or IAi Type B = IB IB or IBi *Comparatively, epistasis is more like a moving assembly line where each gene is dependent on the Type AB = IA IB next. Type O = ii Sex linked traits Environmental Effects genes of interest are located on the sex cues give to rise to modifications The Environmental chromosomes. Sex linked traits are carried on the in phenotype: The coat of the arctic fox changes X chromosome as no genes shared by both male from white to earthy brown as the temperature rises and female can be carried on the Y chromosome. due to a color related enzyme. The fox benefits Hemophilia is common example on the AP exam. from this well timed camouflage.
Exceptions to Mendelian Genetics Incomplete Dominance A Blending of traits: A homozygous white flower crossed with a homozygous red flower for example will create a pink heterozygote. Codominance Both traits are expressed simultaneously: A homozygous white flower crossed with a homozygous red flower would create a red and white flower. Epistasis Many genes controlling one phenotype or the gene at one locus controlling the expression of a gene at another locus: Consider the biochemical cascade ABCD. If a single mutation occurs in A,B, or C, the product D will be effected. Pleiotropy One gene controlling many phenotypes (essentially the opposite of epistasis): The agouti gene in mice is linked to coat color, obesity, and certain tumors. Polygenic Inheritance Many genes controlling one phenotype with an additive effect: Laborador Retreiver colors are polygenic AABBCC = Black Lab AaBbCc = Lighter Aabbcc= Lightest *Comparatively, epistasis is more like a moving assembly line where each gene is dependent on the next. Multiple Alleles More than 2 alleles controlling one trait and therefore more than a simple dominant/recessive relationship between alleles: Blood types are the most common example of multiple alleles. Both A and B are dominant while O is treated as recessive. Type A = IAIA or IAi Type B = IB IB or IBi Type AB = IA IB Type O = ii Sex linked traits The genes of interest are located on the sex chromosomes. Sex linked traits are carried on the X chromosome as no genes shared by both male and female can be carried on the Y chromosome. Hemophilia is common example on the AP exam. Environmental Effects Environmental cues give to rise to modifications in phenotype: The coat of the arctic fox changes from white to earthy brown as the temperature rises due to a color related enzyme. The fox benefits from this well timed camouflage.
practice_10.1_and_10.2.docx