Exceptions to Mendel

Not all genes show simple patterns of dominant and recessive alleles Genetics is more complicated in most organisms

4 Exceptions to Mendel’s Genetics Incomplete dominance Codominance Multiple alleles Polygenic traits

Incomplete Dominance One allele is not completely dominant over another The heterozygous phenotype is somewhere in between the two homozygous phenotypes Examples: four-o’clock flowers, betta fish

RR = red WW = white RW = pink

B1B1 = green B2B2 = steel blue B1B2 = royal blue

Codominance Both alleles are dominant and contribute to the phenotype Example: roan cattle, appaloosa horses, human blood type (AB)

RR = red WW = white RW = roan GG = gray WW = white GW = appaloosa

Type AB is codominant

Multiple Alleles Genes that have more than two alleles No individual can have more than two alleles, but more than two alleles can exist in a population Examples: human blood type = 3 different alleles

Polygenic Traits Poly = many genic = genes Traits are controlled by two or more genes Examples: eye color (3 diff. genes, human skin color (4 diff. genes)

At least 3 different genes determine eye color in humans Order of dominance: brown > green > blue. At least 3 different genes determine eye color in humans

An epistatic gene can interfere with other genes. Example: albinism

Phenotype is a combination of genotype and environment Examples: sea turtles male or female height amount of nutrition hair color effects of sunlight

Pedigrees & Karyotypes 7.4

Pedigrees A pedigree chart is used to trace the phenotypes and genotypes throughout a family. Boxes = males Circles = females Shaded shape = person shows the trait White shape = person doesn’t show trait Half-shaded = person is a carrier

Karyotypes A karyotype is a picture of all the chromosomes in a cell. X Y

Karyotypes can show changes in chromosomes. deletion or loss of chromosome extra chromosomes or duplication of part of a chromosome duplication deletion