This tutorial explored the more complex expression patterns of alleles. These patterns of expression do not contradict the ideas and conclusions of Mendel, but show that genes and their products can interact and/or be expressed in more complex ways. In all cases, these genes are still transmitted from generation to generation as distinct alleles on chromosomes that segregate independently during meiosis. The differences lie in how the gene product behaves within the cell, and the number of such products that contribute to a given character.
Some alleles can show incomplete dominance. In the snapdragon flower color we saw that three phenotypes could be traced to two alleles. In other words, two separate homozygous phenotypes resulted (as is usually seen with characters transmitted in a Mendelian manner) and a third phenotype associated with a heterozygous genotype (clearly different than the case observed with Mendelian traits, where the heterozygous phenotype is the same as the homozygous dominant genotype). How can two alleles yield three phenotypes? Consider the snapdragon. When a plant receives two alleles (a double dose) of the red allele, the flower is very red. These alleles encode for enzymes involved in the production of red pigment. The alternative alleles produce an enzyme that is nonfunctional and cannot participate in pigment synthesis. When a plant receives two copies of this nonfunctional alternative allele, pigment is not produced and the flower is white. When the plant receives one copy (a single dose) of the functional red allele, it can produce some pigment, but not as much as with two fully functional alleles, and so, the color is less red (pink).
In the example above, note that the reason for the phenotypic pattern was that one allele was nonfunctional and the other functional allele resulted in a phenotype that was dependent on there being one or two copies of the functional alleles. There is another situation, however, termed codominance, in which both alleles are functional and expressed. The M and N blood groups are examples of codominance. These alleles encode for proteins that are located on the surface of red blood cells. They are similar but not identical; they differ in four amino acids. If the individual is homozygous, then the phenotype is either M or N. If heterozygous, however, then both proteins are expressed on the surface of red blood cells.
So far we have considered fairly simple cases, where the number of alleles is limited to two. In fact, many genes have multiple alleles. This may seem impossible because in the diploid state there is only room for two alleles (one on the maternal chromosome and one on the paternal chromosome) of a gene. At the population level, however, many more allele forms are possible (although in any given diploid individual, only two occur at any given time). The ABO blood type is one example.
In some cases, genes and their alleles may be expressed in complex ways. That is, no single trait can be attributed to a given allele. Pleiotropy describes this situation, and includes the examples of pigmentation and crossed eyes in the case of albinism.
In the case where one gene product is used by (or dependent on) another product, epistasis can occur. This is fairly common because gene products do not function in isolation. In the example of fur color in mice, one can see that pigment synthesis and pigment deposition are two processes that must occur in order for a specific phenotype (i.e., color) to be observed.
We also considered the case of polygenic inheritance, whereby many genes and their alleles are involved in the expression of a given phenotype. If you think about it, you will likely recognize that polygenic inheritance and epistasis are related. In fact, many traits are determined by multiple genes, which makes the analysis of expression patterns complex.
Geneticists often study the expression of particular traits in family lineages, or pedigrees, in order to gain insight into the mode of expression for a given character trait. Not only can pedigree analyses provide insight into the mode of transmission, but importantly, they can be used to predict the genotype of particular individuals.
This tutorial examined some human genetic diseases, including cystic fibrosis, PKU, and sickle-cell disease. These diseases are found in individuals that are homozygous for the recessive allele, but as you learned, the heterozygous state has a well-documented advantage in the case of the sickle-cell disease and a probable advantage in the case of cystic fibrosis. Moreover, you learned that the phenotype can be affected by the environment; in the case of PKU, a person that is homozygous for the allele can escape the disease with a proper diet.
Not all genetic diseases that behave in a Mendelian fashion behave recessively. Huntington's disease is a degenerative disease of the nervous system. Individuals with either homozygous or heterozygous genotypes develop this disease, ensuring that the defective allele is expressed in all generations as a dominant character trait. Individuals carrying this dominant allele do not begin to show symptoms until late in life (after their child-bearing years), and so, natural selection cannot act directly to affect the frequency of this allele in the population. In other words, through their reproductive years, individuals with this detrimental allele are just as fit (likely to reproduce) as normal individuals.