Genes do not always follow the patterns Mendel observed.
Patterns of inheritance that follow Mendel's laws are said to follow Mendelian inheritance. These are patterns of inheritance that follow Mendel’s laws of dominance and recessiveness, showing that some genes appear more often than others, and independent assortment, which suggests that genes are inherited randomly. However, scientists now know that many other patterns influence both genotype and phenotype. Patterns of inheritance that that do not follow Mendelian patterns are said to follow non-Mendelian inheritance. These traits have four basic underlying causes:
alleles for a trait are neither completely dominant nor completely recessive
more than two alleles exist for a particular gene
one gene produces multiple phenotypes
multiple genes code for a single phenotype
Additionally, many physical, behavioral, and physiological traits are influenced by the organism's environment, either during development or throughout the organism's life, or both. For example, a person's maximum height is determined by genes, but nutrition, injury, and other environmental factors can cause the person to be shorter than specified by their genes.
Incomplete Dominance and Codominance
Incomplete dominance results in a blending of alleles, while codominance results in expression of both alleles simultaneously.
When alleles for a trait are not completely dominant or recessive, incomplete dominance occurs, or another variation called codominance can occur. Incomplete dominance results when two alleles are blended in the organism's phenotype, while codominance results when two alleles are both expressed in the organism's phenotype. For example, roses show incomplete dominance in petal color. Alleles code for either red or white petals, but heterozygous roses are pink. By convention, because neither trait is dominant or recessive, a single capital letter (in this case, C for color) is used to denote the trait, with a superscript to denote the specific color (R for red and W for white): CR and CW, respectively. A Punnett square of a cross between homozygous roses shows all pink heterozygous roses in the first generation.
In contrast, when alleles are codominant, both are expressed equally in heterozygous individuals. For example, chickens have codominant alleles for feather color. Alleles code for either white (W) or black (B) feathers. Heterozygous chickens are speckled, having feathers that are both white and black, not gray (which would indicate incomplete dominance instead).
Codominance in Chickens
More Than Two Alleles for One Gene
Some genes have more than two alleles.
Mendelian inheritance assumes that each gene has two alleles, a dominant one and a recessive one. Sometimes, however, even more alleles exist, and the ratio of dominant to recessive is variable. For example, humans have three alleles for red blood cells: IA, IB, and i. IA and IB are codominant, while i is recessive. Alleles IA and IB code for different enzymes that add different sugars to the carbohydrates found in the membrane of the red blood cells, while allele i codes for no carbohydrate. The combinations of alleles give four distinct blood types: A, B, AB, and O. The different blood types are phenotypes of blood. Individuals with blood type AB must be heterozygous (have two different alleles for a gene), and individuals with blood type O must be homozygous recessive (have two identical recessive alleles for a gene). Blood types A and B can result either from homozygous dominant (have two identical dominant alleles for a gene) or heterozygous combinations. Knowing a person's blood type is very important when they need a transfusion. Giving the incorrect type of blood can cause clotting within the arteries, which can lead to death. Blood type O is the universal donor because it lacks all markers. Therefore, everyone, regardless of their blood type can receive it.
Human Blood Type
Pleiotropy happens when one gene codes for more than one phenotype.
Mendelian inheritance assumes that one gene codes for one trait, such as flower color or pea shape. When multiple phenotypes arise from a single gene it is called pleiotropy. One example is phenylketonuria in humans, which is a disorder arising from a single genetic mutation that causes the body to improperly metabolize the amino acid phenylalanine, resulting in phenotypic effects ranging from cognitive developmental delays to eczema to lessened skin pigmentation. In fact, a range of human genetic disorders result from pleiotropy, including Crohn's disease, sickle cell anemia, and congenital deafness. Even Mendel's pea plants were subject to pleiotropy: The gene that determines flower color also determines the color of the outer surface coating of the seed, which can be either gray or white.
Polygenic Inheritance and Epistasis
Some traits are the result of multiple genes. Traits can be coded for by more than one gene, known as polygenic inheritance, or one gene can activate another gene, known as epistasis.
Often, many more than two phenotypes exist for some traits. Consider hair color in humans. Even ignoring the various shades of each color, human hair can be black, brown, red, or blond. Polygenic inheritance is when more than one gene codes for a single trait. In humans, height, hair color, eye color, and skin color are all the result of polygenic inheritance. Further, traits such as height and skin color are also heavily influenced by environmental factors, and even eye color and hair color can change over a person's lifetime.
Polygenic inheritance can sometimes result from epistasis. In epistasis, one gene "activates" another gene. In other words, the expression of alleles of one gene is dependent on alleles of a completely different gene. When this happens, the "activator" gene is said to be epistatic to the genes it activates. For example, dogs of the Labrador retriever breed have three variants of coat color: black, chocolate (brown), and golden (yellow). The gene that codes for golden fur is epistatic to the gene that codes for black or chocolate fur. The gene for golden fur is denoted E, with the homozygous recessive (ee) giving the gold color. If either homozygous dominant or heterozygous alleles are inherited, the second gene for black (B) or chocolate (b) fur becomes activated. Thus, the Punnett square (which can have more than one set of genes) must show both genes for a single cross.