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Complex Patterns of Inheritance INCOMPLETE DOMINANCE, CODOMINANCE, MULTIPLE ALLELES, EPISTASIS AND POLYGENIC INHERITANCE Rules and Rulebreakers Mendel’s experiments led to the following conclusions about inheritance Traits are determined by a pair of alleles If the alleles are different, one is expressed (“dominant”) and the other is not (“recessive”) The alleles segregate with equal probability during gamete formation While many traits are inherited by these simple rules, some are not Some traits are controlled not by one pair of alleles, but my several pairs (“polygenic”) Some alleles are not clearly dominant or recessive. Both dominant = “codominance Neither dominant = “incomplete dominance” Sometimes more than 2 alternative alleles exist (“multiple alleles”) Insight into the nature of Alleles Our modern understanding of genetics includes some things Mendel never knew Genes are located on Chromosomes Genes are instructions for the production of proteins There is a distinct protein code that consists of variations in the sequence of nucleotides in the DNA molecules Occasionally mutations occur which create variations in the instructions that a particular gene carries. These may result in the production of a new protein, unlike the original, which may function in a very different way The result may also make only minor variations, which manifest themselves more subtly They may even result in the inability of a cell to produce the protein at all Multiple Alleles As the name suggests, multiple alleles describes a trait for which more than 2 possible alleles exist for a particular gene For example, Mendel’s peas expressed 2 possible seed colors, yellow and green. Yellow was dominant and green was recessive. Simple. If alleles existed for yellow, green and red – That would be a multiple alleles scenario Several more possible combinations would exist for allele pairs If yellow was dominant over green, would it also be dominant over red? If green was recessive to yellow, would red be dominant to green as well? Multiple Alleles – ABO Blood Type ABO blood type is a classic multiple alleles situation. Three alleles exist for blood type at the same “locus” (locus = position on a chromosome) The three alleles are for type A, type B, and type O The A allele is dominant. The B allele is dominant. The O allele is recessive The term for a genetic system with 2 dominant alleles is “codominance” The A allele and the B allele code for the production of proteins on the surface of red blood cells. The type A protein and the type B protein are both “antigens” – proteins used to allow the immune system to distinguish between cells that belong and cells that do not, a necessary part of identifying and fighting infections ABO Blood type Genotypes and Phenotypes Because there are 2 dominant alleles, we use a neutral letter to represent dominance and a notation to identify the type For blood type, we use the letter I Ia is the dominant A allele Ib is the dominant B allele i is the recessive type O allele ABO Blood type Genotypes and Phenotypes More alleles results in more genotypes More genotypes results in more phenotypes Note that there are now 3 different homozygous conditions and 2 different heterozygous conditions Note also that the dominant blood types can be homozygous or heterozygous, but that type O must be homozygous recessive Sample blood type problems Given the following parents, predict the probabilities of each blood type in the offspring: heterozygous type A x heterozygous type B type O x homozygous type B type O x heterozygous type B type O x type O homozygous type A x homozygous type B Type AB x type O More blood type problems For each of these families determine the genotypes of the parents and the children A child with type O blood has a type A father and a type B mother. A man with type AB blood marries a woman with type B blood. Their daughter has type A blood. Father = type A, Mother = type B, 2 kids: type B and type A Same child family: Predict the probability of possible blood types for the next A man with type AB blood marries a woman with type B blood. Their child has type O blood. How angry should the man be? Codominance Problems A roan horse has a combination of chestnut colored hairs and white hairs No individual hair has both chestnut and white Predict the offspring White x White Chestnut x White Roan x White Roan x Roan Incomplete Dominance In these flowers there are alleles for red and for white, neither is dominant. The trait is controlled by a single pair of alleles. Heterozygous flowers are pink. Predict the probable outcomes from these crosses: Red x White White x Pink Pink x Pink Pink x Red
