Module 6 Mendelian Genetics

Reflect and Connect

By adding knowledge of multiple alleles and incomplete dominance to what you have already learned about genetics, you have greatly expanded your ability to interpret phenotypes and predict inheritance. All this and you are still working only with one gene! When each chromosome has hundreds of genes, and humans have 23 chromosome pairs, think of all the possible variations that exist. Now would be a good time to review your genetics study dictionary and add the new terms from this lesson.

Self-Check

For an excellent self-check, go back to the Arizona biology project website and finish the questions on monohybrid crosses (9–13). Each problem has a tutorial to help, if you have any questions, and also explains the correct answer when given.

Reflect on the Big Picture

Understanding blood types and their inheritance pattern can go a long way when working with family histories, developing pedigrees (charts which trace the inheritance of genetic characteristics through generations), and determining paternity. In biology 20, you learned about the different types of blood and what can happen if a person receives incompatible blood. In this lesson, you will examine the three alleles for blood type and how they account for the four phenotypes possible. Using this information, you can predict possible blood types of future children, study family pedigrees to determine lineage, and even determine if there was a mix up in the hospital!

Lesson Summary

During this lesson you were to focus on the following essential 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 intermediate inheritance. 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, as seen in rabbits.