Beyond Dominant and Recessive Alleles

Exceptions to Mendel’s Principles

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.

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.

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.

Incomplete DominanceIn the case of incomplete dominance one allele

is not completely dominant over the other. Both show in the offspring but they are

blended.

Incomplete Dominance

Curly Hair (CC)

Wavy Hair (Cc)

Straight Hair (cc)

Codominance

Codominance occurs when both alleles contribute to the phenotype of the organism.

Both alleles can be seen in the offspring.

Red Bull White Cow

Roan Cow

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.

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.

Crossing Over Crossing over can create even more genetic

variation by creating new combinations of alleles.

Environmental Influence on Heredity Article on Fingerprints of Twins See worksheet