12-4 Beyond Medelian Genetics

Alleles are the possible “options” for a trait.

Summary of Mendel’s Principles 1. The inheritance of traits is determined by genes which are passed from parent to offspring. 2. When 2 or more alleles for a gene exist, some alleles may be dominant and other alleles may be recessive. (_______________________) 3. Genes are segregated from each other when gametes are formed. (____________________) 4. The alleles for different genes usually segregate independently of one another. (__________________________________)

Exceptions to Mendel’s Principles Not all genes show simple patterns of dominant and recessive alleles. The majority of genes have more than 2 alleles Traits are sometimes controlled by more than one gene.

Exceptions to Mendel’s Principles Incomplete Dominance – one allele is not completely dominant over another. There is an appearance of a 3 rd phenotype. Heterozygous phenotypes show up somewhere in between. Example snapdragon flowers Red FlowersxWhite flowersPink Flowers RRrrRr

Exceptions to Mendel’s Principles Codominance – both alleles contribute to the phenotype of the organism. The offspring have a phenotype that expresses both alleles equally. Examples Chickens: Black feathers x white feathers speckled black and white

Blue roanRed roan

Exceptions to Mendel’s Principles Multiple Alleles – genes that have more than 2 alleles for a specific trait Examples: Rabbit fur color has 4 different alleles C = full color C ch = chinchilla C h = himalayan c = albino – no color

Rabbit Fur Colors

Exceptions to Mendel’s Principles Multiple Alleles continue: Examples: The alleles I A and I B are always expressed – they are codominant. Both I A and I B are dominant to i. GenotypesPhenotypes I A I A or I A iA I B I B or I B iB I A I B AB i O

Exceptions to Mendel’s Principles Polygenic Traits – traits controlled by 2 or more genes. The genes can be on the same chromosome or on different chromosomes. In polygenic inheritance, all heterozygotes are intermediate in phenotype. Examples: Eye color in fruit flies – 3 genes Skin color in humans – there are three or four genes involved in the inheritance of skin color in humans

Linkage Linkage: Some genes that are close on the same chromosomes are less likely to be separated during crossing-over. i.e. they end up getting shuffled together most of the time. Example: red hair and freckles.

Sex Linked Traits that can only be passed to males or females. The gene can be found on the X or Y chromosome. Example: color blindness Can be found on the X chromosome because females have two, males only have one chance for a “good” color vision gene.