Non-Mendelian Inheritance Patterns

Some Traits Don’t Follow Simple Mendelian Rules: Some traits are determined by more than one pair of genes. These are referred to as polygenic, or continuous, traits. (poly = many) Example: Human height. The combined size of all of the body parts from head to foot determines the height of an individual.

Polygenic = many genes Gene 1Gene 2Gene 3 Trait (height)

Some Traits Don’t Follow Simple Mendelian Rules: A single gene can be responsible for more than one trait. This is called pleiotropy. Sickle-cell anemia is an example. A single gene results in irregularly shaped red blood cells that painfully block blood vessels, cause poor overall physical development, as well as related heart, lung, kidney, and eye problems.

Pleiotropy = One gene Many effects

Some Traits Don’t Follow Simple Mendelian Rules: Another pleiotropic trait is albinism. The gene for this trait not only results in the typical albino deficiency of skin, hair, and eye pigmentation but also causes defects in vision.

Some Traits Don’t Follow Simple Mendelian Rules: Regulator genes can either initiate or block the expression of other genes. For example, in order for a human to develop correctly, certain genes must be “turned on” at the right time and “turned off” when they are not needed.

Regulator gene codes for repressor protein Repressor binds to DNA – NO proteins made (gene is “off”) Repressor binds to some other chemical – proteins ARE made (gene is “on”)

Some Traits Don’t Follow Simple Mendelian Rules: Some genes are not expressed unless the right environmental factor is present. For example, you may inherit the genes that are responsible for type 2 diabetes but never get the disease unless you become greatly overweight, persistently stressed psychologically, or do not get enough sleep on a regular basis.

Some Traits Don’t Follow Simple Mendelian Rules: Similarly, the genes that cause the chronic autoimmune disease, multiple sclerosis, may be triggered by the Epstein-Barr virus and possibly other specific environmental stresses. New research suggests that exposure to the sun in childhood can provide some protection from developing MS. So, people who grow up in tropical and subtropical regions of the world have significantly lower rates of MS as adults.

What are Codominance and Incomplete Dominance? Codominance – When BOTH alleles are expressed +  Incomplete dominance – When a BLEND of both traits is expressed + 

Practice problem 1: Longhorns may be white (C W C W ), red (C R C R ) or roan (C R C W ). Roan longhorns have a mixture of both white hairs and red hairs. Is this complete dominance, incomplete dominance, or codominance? ____________ What genotypic and phenotypic ratios would result if a roan longhorn mated with a white longhorn? Genotypic ratio: ____________ Phenotypic ratio: ___________

What are the chances of these two longhorns having a roan offspring? _______

Practice problem 2: A naturalist visiting an island observes a species of small bird with three distinct types of beaks. Those with short, crushing beaks (BB) consume hard shelled nuts, those with long, delicate beaks (bb) pick the seeds from pine cones, and those with intermediate beaks (Bb), consume both types of seeds though they are not as good at either. Is this complete dominance, incomplete dominance, or codominance? ____________ What genotypic and phenotypic ratios would result if a long- beaked bird mated with a short-beaked bird? Genotypic ratio: ____________ Phenotypic ratio: ___________

What are the chances of these two birds having a short-beaked bird?

Practice problem 3: The ability to taste the chemical PTC is determined by a single gene in humans. People that are heterozygous for this allele can taste PTC. People that are homozygous recessive can not taste PTC. Is this complete dominance, incomplete dominance, or codominance? ____________ What genotypic and phenotypic ratios would result if two heterozygous tasters were to marry? Genotypic ratio: ____________ Phenotypic ratio: ___________

What are the chances of this couple having a “taster”? ________________