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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 3rd phenotype. Heterozygous phenotypes show up somewhere in between. Example snapdragon flowers Red Flowers RR x White flowers rr Pink Flowers Rr 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 roan Red 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 Cch = chinchilla Ch = himalayan c = albino – no color Rabbit Fur Colors Exceptions to Mendel’s Principles Multiple Alleles continue: Examples: The alleles IA and IB are always expressed – they are codominant. Both IA and IB are dominant to i. Genotypes Phenotypes IA IA or IA i A IB IB or IB i B IA IB AB ii 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.