11-3 Exploring Mendelian Genetics 11–3 Other Inheritance Patterns Copyright Pearson Prentice Hall

Beyond Dominant and Recessive Alleles What inheritance patterns exist aside from simple dominance? Not all alleles show simple dominant-recessive patterns. One allele may be not be completely dominant over another. Most genes have more than 2 alleles (more than 2 contrasting forms). Many traits are controlled by more than one gene. Copyright Pearson Prentice Hall

Beyond Dominant and Recessive Alleles Some alleles are neither dominant nor recessive. Copyright Pearson Prentice Hall

Beyond Dominant and Recessive Alleles Incomplete Dominance  When one allele is not completely dominant over another it is called incomplete dominance. In incomplete dominance, the heterozygous phenotype is between the two homozygous phenotypes. The offspring is a “blend” of its parents. Copyright Pearson Prentice Hall

Beyond Dominant and Recessive Alleles RR A cross between red (RR) and white (WW) four o’clock plants produces pink-colored flowers (RW). WW Some alleles are neither dominant nor recessive. In four o’clock plants, for example, the alleles for red and white flowers show incomplete dominance. Heterozygous (RW) plants have pink flowers—a mix of red and white coloring. Copyright Pearson Prentice Hall

Beyond Dominant and Recessive Alleles Codominance  In codominance, both alleles contribute to the phenotype. The traits of both parents are seen. In certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers. Heterozygous chickens are speckled with both black and white feathers. The black and white colors do not blend to form a new color, but appear separately. Copyright Pearson Prentice Hall

Beyond Dominant and Recessive Alleles Multiple Alleles  Many genes exist in several different forms, and are therefore said to have multiple alleles. An individual can’t have more than two alleles in their cells. However, more than two possible alleles for each trait can exist in a population. A rabbit's coat color is determined by a single gene that has at least four different alleles. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall

Beyond Dominant and Recessive Alleles Polygenic Traits   Many traits are produced by the interaction of several genes. Traits controlled by two or more genes are said to be polygenic traits. Skin color in humans is a polygenic trait controlled by more than four different genes. Copyright Pearson Prentice Hall

Genetics and the Environment Characteristics of any organism are not determined just by the genes that are inherited. Genes provide a plan for development, but how that plan unfolds depends also on the environment. Environmental conditions can affect gene expression and influence genetically determined traits. Copyright Pearson Prentice Hall

For example, consider the Western white butterfly For example, consider the Western white butterfly. Western white butterflies that hatch in the summer have different color patterns on their wings than those hatching in the spring. Scientific studies revealed that butterflies hatching in springtime had greater levels of pigment in their wings than those hatching in the summer. In other words, the environment in which the butterflies develop influences the expression of their genes for wing coloration.

In order to fly effectively, the body temperature of the Western white butterfly needs to be 28–40°C. More pigmentation allows a butterfly to reach the warm body temperature faster. Similarly, in the hot summer months, less pigmentation prevents the butterflies from overheating.