Beyond Mendel

Mendelian Heredity What did Mendel say about Heredity? Alternate versions of genes One allele from each parent Dominant and recessive alleles Most traits are not inherited in the simple patterns identified by Mendel (single allele trait) Not all genes have only 2 alleles Not all traits are controlled by one gene In nature, other patterns exist

Polygenic Inheritance When several genes affect a trait Children may have blended traits of both parents Examples Eye color Skin color Height

Incomplete Dominance A form of intermediate inheritance in which heterozygous alleles are both expressed, resulting in a combined phenotype. Neither allele is completely dominant over another Example: If a snapdragon with red flowers is crossed with a snapdragon with white flowers, the offspring will have pink flowers

Multiple Alleles Traits that can be determined by 3 or more possible alleles Still, only two alleles for a gene can be present in one individual Three or more alleles – or types of genes – can be passed on, but only 2 will be passed on

Multiple Alleles, an Example: A human’s blood type is determined by 2 of three possible alleles IA, IB, & i Different combinations of these three alleles can produce four different blood types A, B, AB & O Genotype Phenotype (Blood Group) IAIA A IAi IBIB B IBi IAIB AB ii O

Codominance A condition in which two traits can appear at the same time, both fully expressed Example: Roan Fur on Cattle If red furred cattle are crossed with white furred cattle, the offspring will have red and white fur DAD MOM OFFSPRING + =

Sex-Linked Traits X* X X*X XX Y X*Y XY When a gene is located on either an X or Y chromosome Most are X linked because the X chromosome is longer than the Y chromosome Example: Color Blindness is linked on the X chromosome