Module 6

1. Module 6

1.17. Page 5

Lesson 3

Module 6—Mendelian Genetics: The Transmission of Traits to the Next Generation

Lesson Summary

In this lesson you examined the following focusing questions:

What happens when one allele is not completely dominant over another?
How does having more than two alleles for a gene affect the possible phenotypes for a trait?

In cases where one allele is not completely dominant over another, each allele is somewhat expressed. If the heterozygous phenotype appears to be more of a blend, as with flower colour, this is called incomplete dominance. Co-dominance is when each allele is fully expressed in different parts, like with black and white hairs in roan horses.

When multiple alleles are present for a trait there will be many possible phenotypes. Each individual still has only two alleles, but there may be many possible combinations for these two alleles in the population as a whole. Each allele set can still be analyzed with the patterns we’ve learned. One allele will either be dominant over the other, or it will be incompletely dominant. With many alleles, this may lead to an order of dominance, which you will examine in Lesson 8.

Lesson Glossary

Consult the glossary in the textbook for other definitions that you may need to complete your work.

biodiversity: the variation present in the genes, species, and ecosystems of a biome; at the species level refers to the variety of alleles, genotypes, and phenotypes present for each gene; used as an indicator of the biological health of a species, ecosystem, or biome

co-dominance: similar to incomplete dominance—neither allele is dominant; in the heterozygote, both distinct phenotypes appear—not a mixture (e.g., red and white cow colour alleles produce red and white hairs in the heterozygote; in blood types the AB heterozygote makes both A antigens and B antigens—not a mixture)

incomplete dominance: some genes have two alleles—neither of which are dominant; in the heterozygote the phenotype is a mixture of both alleles (e.g., if the white and black alleles are incompletely dominant, the heterozygote will be grey)

multiple alleles: some genes have more than two alleles; an order of dominance is given to determine phenotypes from genotypes (e.g., in a particular flower, colour alleles are red, amber, yellow, and albino in order of decreasing dominance)

pedigree: a chart outlining the generations and relationships within a family line