Essential Knowledge 3.A.4

The inheritance pattern of many traits cannot be explained by simple Mendelian genetics.

Many traits are the product of multiple genes and/or physiological processes. Patterns of inheritance of many traits do not follow ratios predicted by Mendel’s laws and can be identified by quantitative analysis, where observed phenotypic ratios statistically differ from the predicted ratios. Some traits are determined by genes on sex chromosomes. To foster student understanding of this concept, instructors can choose an illustrative

example such as:

• Sex-linked genes reside on sex chromosomes (X in humans).
• In mammals and flies, the Y chromosome is very small and carries few genes.
• In mammals and flies, females are XX and males are XY; as such, X-linked recessive traits are always expressed in males.
• Some traits are sex limited, and expression depends on the sex of the individual, such as milk production in female mammals and pattern baldness in males.

Some traits result from nonnuclear inheritance. Chloroplasts and mitochondria are randomly assorted to gametes and daughter cells; thus, traits determined by chloroplast and mitochondrial DNA do not follow simple Mendelian rules. In animals, mitochondrial DNA is transmitted by the egg and not by sperm; as

such, mitochondrial-determined traits are maternally inherited. NOTE: Epistasis and pleiotropy are beyond the scope of the course and the AP Exam.

Students should be able to:

LO 3.15 Explain deviations from Mendel’s model of the inheritance of traits.

LO 3.16 Explain how the inheritance patterns of many traits cannot be accounted for by Mendelian genetics.

LO 3.17 Describe representations of an appropriate example of inheritance patterns that cannot be explained by Mendel’s model of the inheritance of traits.