6BP2 Variations of Mendel’s Theories Heredity 6BP2 Variations of Mendel’s Theories

Objectives I will understand how geneticists model inheritance. I will be able to explain: incomplete dominance; codominance; polygenic inheritance and multiple alleles

Modeling Inheritance A Punnett square is a model used to predict possible genotypes and phenotypes of offspring.

Modeling Inheritance A pedigree shows phenotypes of genetically related family members.

Variations in Mendel's Theories Incomplete dominance Codominance Polygenic inheritance Multiple alleles

Variations in Mendel's Theories Sometimes traits appear to be blends of alleles. Alleles show incomplete dominance when the offspring’s phenotype is a blend of the parents’ phenotypes. Codominance occurs when both alleles can be observed in a phenotype.

Incomplete dominance a type of inheritance in which the alleles expressing a particular characteristic are neither dominant or recessive; two traits combine or blend together to produce a different trait (a blend of two traits) (when offspring of two homozygous parents show an intermediate phenotype)

Incomplete dominance example: flower color in snapdragons & four o’clock flowers

Codominance In codominance both traits are expressed; there are no dominant and recessive traits Example: roan cattle

Codominance In incomplete dominance neither allele is fully dominant. This is different from codominance, in which both alleles are fully expressed, resulting in organisms that display the characteristics of both parents.

Codominance In codominance, both alleles are expressed independently and are uniquely recognizable.

Mutiple Alleles Some traits, such as human ABO blood type, are determined by more than two alleles.

Polygenic Inheritance two or more genes producing a single trait occurs when multiple genes determine the phenotype of a trait (examples in humans: hair color, skin color, eye color, height, intelligence, body build, etc.)

Genes and the Environment An organism’s environment can affect its phenotype. Some examples of environmental factors that affect phenotype are soil type that a flower is growing in or time of year that a butterfly develops.

Genes and the Environment Example: Hydrangea For most French hydrangeas (Hydrangea macrophylla), the flower color indicates the pH of the soil. In strongly acid soil (pH below 6), flowers turn blue. In alkaline soil (pH above 7), flowers turn pink or even red. In slightly acid or neutral soil (pH 6 to 7), blooms may be purple or a mix of blue and pink on a single shrub