Pedigree analysis provides a method to record observations on genetics in human families. A number of symbols were reviewed, including square = male, circle = female, filled = affected, etc.. Dominant pedigrees are those in which the trait is found in both sexes, doesn’t skip generations, and about half the offspring of an affected individual are affected. Examples in humans include achondroplasia, polycactyly and brachydactyly.
Recessive pedigrees are those in which the trait is found in both sexes, the parents of affected usually not affected, about 1/4 of children of heterozygous parents are affected, and there is often matings of related individuals (consanguineous mating). Examples in humans include albinism, cystic fibrosis, Tay-Sachs disease and sickle cell anemia.
Alleles don’t always interact in a simple dominant/ recessive manner (unlike what Mendel observed). With incomplete dominance, the heterozygote is distinct (and often intermediate to) the two homzygous types. (Example: red, pink and white snapdragons). With codominance, each of the alleles is independently expressed. (Example: ABO blood group system in humans, three alleles, the IA and IB alleles code for enzymes which put sugars onto a carbohydrate group on the red blood cell surface.
The ABO system was reviewed, refer to text for explanation. This is also an example of multiple (more than two) alleles for a single gene.
These relationships are not necessarily confusing when you consider that the terminology and interpretations depend on what phenotype is being examined. For example, if enzyme activity is the phenotype, one could see incomplete dominance even when the external trait showed a dominant / recessive relationship.
Genes aren’t always expressed in a consistent fashion. This may be due to effects of other genes, or the environment. (This is also unlike what Mendel observed). Sometimes we don’t know what is responsible for this, but seek to describe the phenomenon.
Variable expressivity refers to the severity or expression of a genotype varying among individuals. Examples include varying severity of a genetic disease, and varying age of onset. Incomplete penetrance refers to the percentage of the time that a genotype is expressed. For example, 90% penetrance indicates that a dominant allele results in a phenotype in 9 cases out of 10.
Sometimes, gene influences on other genes are well-understood. Epistasis refers to gene interactions, so that the genotype at one gene influences expression at another gene. This can be expressed as a deviation from a 9:3:3:1 phenotypic ratio in the F2 generation. Examples include a 9:7 ratio and 12:3:1 ratio (see test for explanations).
Another way to define genes is based on a functional test, the complementation test. This involves crossing two recessive mutants to determine if the progeny are wild-type ("normal" in phenotype). Wild type progeny then indicate that the mutations are in different genes ("show complementation"), while mutant progeny indicate that they are in different genes.