Beyond Dominant and Recessive Alleles

1. Beyond Dominant and Recessive Alleles Exceptions to Mendel’s Principles

2. Mendel’s Principles 1. The inheritance of traits is determined by individual units known as genes. In organisms that reproduce sexually, genes are passed from parents to their offspring (children). 2. In cases in which two or more forms of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive.

3. Mendel’s Principles 3. In most sexually reproducing organisms, each adult has two alleles for each gene – one from each parent. These alleles are segregated from each other when gametes are formed. One allele goes into each gamete cell.

4. But… Genetics is more complicated than this!! There are exceptions to Mendel’s principles… Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes.

5. Incomplete Dominance In the case of incomplete dominance one allele is not completely dominant over the other. Both show in the offspring but they are blended.

6. Incomplete Dominance Curly Hair (CC) Wavy Hair (Cc) Straight Hair (cc)

7. Codominance Codominance occurs when both alleles contribute to the phenotype of the organism. Both alleles can be seen in the offspring. White Cow Red Bull Roan Cow

8. Polygenic Traits Many traits are produced by the interaction of several genes. These are called polygenic traits, which means having many genes. • Height • Skin Color • Weight This allows for a lot of variation in a population.

9. Gene-Chromosome Theory • States that genes are located in specific spots on specific chromosomes. • If genes are on the same chromosome they are said to be “linked” and passed onto the next generation together.

10. Crossing Over • Crossing over can create even more genetic variation by creating new combinations of alleles.

11. Environmental Influence on Heredity • Article on Fingerprints of Twins • See worksheet