Extensions to Mendelian Genetics

Relevant Resources

• Campbell Biology: Chapters 14 and 15

Big Picture

• DNA is our genetic blueprint and controls our phenotype.

• Variation in DNA includes mutations (affect phenotype) and polymorphisms (variation in the genome).

• Mutations in DNA can impact phenotype, including genetic conditions and other traits like coat color in tortoiseshell cats.

• Inheritance within a family can be studied using pedigrees to determine inheritance patterns.

Degrees of Dominance

• Incomplete dominance: One gene controls a trait, but there are three phenotypes.

  • Example: Snapdragon flower color, where the C gene has two alleles, $C^R$ and $C^W$.

  • Red true-breeding snapdragon ($C^RC^R$) crossed with white true-breeding snapdragon ($C^WC^W$) produces pink offspring ($C^RC^W$).

  • Self-crossing the F1 generation ($C^RC^W$ x $C^RC^W$) results in a 1:2:1 ratio of red to pink to white phenotypes.

  • The $C^R$ allele isn't completely dominant over the $C^W$ allele, resulting in the pink phenotype.

Codominance

• Codominance: Both alleles are expressed in the phenotype.

  • Example: Blood type, where one gene has three alleles: $I^A$, $I^B$, and i.

  • $I^A$ and $I^B$ alleles are codominant, while the i allele is recessive.

  • Six possible genotypes: $I^AI^A$, $I^Ai$, $I^BI^B$, $I^Bi$, $I^AI^B$, ii.

  • Four possible phenotypes (blood groups): A, B, AB, O.

  • Blood type is based on the presence or absence of antigens on red blood cells.

Reese Factor

• Refers to the presence or absence of the RhD protein (antigen) on the surface of red blood cells.

  • If positive, the individual has the D antigen.

  • If negative, the individual does not have the D antigen.

  • The Reese factor is also inherited with one allele from each parent.