Genetic Characteristics that cannot be Explained by Simple Mendelian Genetics

Part 1: Genetics Problem- Human Blood Types

Mendel is the father of modern genetics, but there are some genetic characteristics that cannot be explained by simple Mendelian genetics. Such is the case with the human blood types in which there are 3 alleles for the same gene, A B, and o. A parent can pass allele A, B, or o to the offspring based on the parent's genotype.

From these 3 alleles, there are 4 blood types (phenotypes): A, B, AB, and O, and there are six genotypes: AA, Ao, BB, Bo, AB, or oo. This is an example of codominance in which both A and B alleles are codominant to each other.

Blood types can be used in forensics to determine if blood is from the victim or criminal. Blood types can be used to determine parental source in situation where the father is unknown; however, blood types can only eliminate certain blood types. DNA fingerprinting is a better method that is used often in criminal and parental determination cases.

Punnett squares such as the one shown above are used to determine the probabilities (percentages) for genotypes of offspring given specific genotypes for the parents.

A) In the example above, the Punnett Square represents a cross (mating) between a male (on the left side) with blood type AB, and a female, (top of square), with blood type A, genotype Ao.

Answer the following for the cross represented above.

1) What are the possible blood types for the offspring?
2) What are the ratios or percentages for each possible blood type from this cross?
3) What blood type is not possible from this cross?

B) Fill out two Punnett squares for a cross between a male with blood type B and a female with blood type AB. (Note that we do not know if the father is genotype BB or Bo from the information given. Thus there are two solutions to the possible cross.)

Set up two Punnett squares and answer the following questions about them.

1) What are the possible blood types for the cross between the type B (BB or Bo?) male and AB female?
2) What are the percentages (%) or probabilities for each blood type in the offspring?
3) What blood type(s) would not be possible in a cross between these two parents?

Hint: There are two answers for questions 1 & 2 above and only one for 3.

Part 2: Cell division, mutations and genetic variability.

Eukaryotic cells can divide by mitosis or meiosis. In humans, mitosis produces new cells for growth and repair. And, meiosis produces sex cells (gametes), called sperm and eggs. Changes or mutations in genes in sex cells can be inherited by human offspring. Genetic variation in a population of organisms is good; however, sometimes mutations can be harmful or cause genetic disorders.

Briefly, answer the following questions:

How do meiosis and sexual reproduction (fertilization) produce offspring that differ genetically from the parents?