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Pleiotropy

Pleiotropic effects describe the multiple effects caused by a single allelic disorder. Think of pleiotropy as having a single cause but showing multiple symptoms. Sickle-cell anemia is a good example of a pleiotropic gene.

Genetic imprinting
Who said a mother's allele was equivalent to a father's allele? For many years, this was the standard belief, now it is regarded as a major obstacle to cloning. Imprinting means that the mother and father pass on a "signature" with their alleles that designates the allele as maternal or paternal in origin.

A dramatic example of this is the first discovery of this phenomenon. Prader-Willi syndrome is characterized by mental retardation, obesity, short stature, and unusually small hands and feet.
Angelmann syndrome is characterized by spontaneous (uncontrollable laughter), jerky movements, and other motor and mental defects. Both are caused by the same DNA deletion on chromosome 15. But if the deletion is on the paternal chromosome, the child will have Prader-Willi syndrome; if the deletion is on the maternal chromosome, the child will have Angelmann syndrome.

Epistasis

Epistasis involves two or more genes controlling one character. One gene acts as a "switch". A common epistatic inheritance involves colour. In labrador dogs, coat colour is controlled by two genes. One gene determines whetherpigment in the hair will be present. A dog that inherits the C allele will have melanin. A dog that inherits two copies of the c allele will not have melanin. This latter Lab will be a "golden lab". Dogs with a dominant C allele have their coat colour determined by another gene. Those with a B allele will be black labs. Those with two copies of the b allele will be chocolate labs.

Why isn't it important whether a cc dog has a B allele or two bb alleles?
Modified Punnett squares work very well in these cases. Let's look at coat colour in mice. In these mice, B (black fur) is dominant over b (brown fur). A second gene determines whether pigment is deposited. The dominant allele, C, results in the deposition of pigment; two copies of the recessive allele, c, results in an albino mouse.

Cross two black mice BbCc X BbCc

Start with the C allele since it's the switch.
1/4 of the mice will be albino (think 4/16)
3/4 will have pigment. Of those 3/4 will be black (9/16 of the offspring will be black) and 1/4 will be brown (3/16 of the offspring will be brown)

This is a dihybrid cross with a phenotypic ratio of 9 black : 3 brown : 4 albino.
In fact, this is the first clue that the character in question is under epistatic control. When the F2 ratio diverges from the 9:3:3:1 ratio, it usually means epistasis is at work. Other ratios may be 12:3:1 or 9:6:1 or 12:7.

Lethal alleles

Dominant lethal alleles are usually removed from the population because the organism dies before procreation. There are exceptions. Huntington's chorea is a dominant disorder but a late onset disorder. People who are diagnosed with Huntington's are usually in their 40s or 50s and have already had children or even grandchildren. Genetic screening is a possibility but children whose parents have Huntington's may not want to know, or may fear discrimination.

Lethal recessive alleles are virtually impossible to remove from the population because they may lie hidden in the heterozygote. Only the homozygote with two recessive alleles is affected.

Another twist involves lethal recessive alleles and pleiotropy. Sickle-cell anemia results in a multitude of symptoms in the homozygote including impaired mental function, fatigue, kidney failure, paralysis, and heart failure. It is feasible that the homozygote will not reproduce and pass these alleles onto the next generation. But the carrier - the heterozygote - has an advantage. A single copy of the sickle-cell allele increases resistance to malaria. It seems it also increased fecundity (having more children than average). In tropical populations, it is advantageous to have a high frequency of heterozygotes for the sickle-cell allele (known as "heterozygote advantage" in population genetics).

Discontinuous traits are discrete and can be placed in an "either/or" category. Mendel's pea plants showed seven discontinuous traits.

Continuous traits fall along a spectrum of phenotypes. Human skin colour is a good example of a continuous trait. It is believed that three separate genes control skin colour and their effects are additive. Traits like skin colour, height, IQ, and others are best represented by a "bell curve". Continuous traits are multigenic, or polygenic, since more than one gene is responsible for the character.

How are polygenic inheritance and pleiotropy opposites?

Multifactorial disorders include heart disease, diabetes, cancer, alcoholism, and certain mental disorders. These disorders may have a single or multiple genetic causes, but may also be influenced very heavily by the environment. The environment includes lifestyle choices, nutrition, sanitation, activity, stress, and social interactions.