Flashcards for Basic Principles of Genetics
Topic 3:  Exceptions to Simple Inheritance
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The general term for inheritance patterns which can be explained by simple rules of dominance and recessiveness of genes.

Mendelian genetics

A trait that is determined by the combined effect of more than one gene. Human skin and hair color are examples. The result of this kind of inheritance is the perception of gradation in the expression of such traits.

polygenic trait

The inheritance pattern in which a trait is expressed in the phenotype of heterozygous individuals as an apparent blend or an intermediate expression. For instance, in primroses white flowers are homozygous recessive, red ones are homozygous dominant, and pink ones are heterozygous.

incomplete dominance

The inheritance pattern in which two different alleles for a trait are expressed unblended in the phenotype of heterozygous individuals. Type AB human blood is an example.


An inheritance pattern in which a gene has more than two alleles. The human ABO blood type system is an example. It is controlled by at least 3 alleles.

multiple-allele series

Genes that can alter how certain other genes are expressed in the phenotype. Genes causing some kinds of cataracts can be affected by such accompanying genes.

modifying genes

Genes that can either initiate or block the expression of other genes. They control the production of a variety of chemicals in plants and animals. Shortly after conception, some of these genes work as master switches orchestrating the timely development of our body parts. They are also responsible for changes that occur in our bodies as we grow older.

regulator genes

Genes whose effect does not normally occur unless certain environmental factors are present. For example, you may inherit a gene for type II diabetes but never get the disease unless you become overweight, persistently stressed psychologically, or do not get enough sleep on a regular basis.

incompletely penetrant genes

Genes that are inherited by both men and women but are normally only expressed in the phenotype of one of them. The heavy male beard is an example. While women have facial hair it is most often very fine and comparatively sparse.

sex-limited genes

Genes that are expressed in both men and women but differently. An example of this is gout, a disease that causes painfully inflamed joints. If the gene is present, men are nearly eight times more likely than women to have severe symptoms.

sex-controlled genes

An inheritance pattern in which a gene will have a different effect depending on the gender of the parent from whom it is inherited. Diabetes and psoriasis can follow this inheritance pattern.

genome imprinting

The inheritance pattern in which a single allele is responsible for a variety of traits. The collective groups of symptoms known as sickle-cell trait is an example.


Defective alleles that have segments which are doubled in their transmission from generation to generation. In the case of genetically inherited diseases, the result is increasingly severe symptoms each generation.

stuttering alleles or unstable alleles

Twins that come from the same fertilized egg and are, subsequently, genetically the same. They are also known as identical twins.

monozygotic twins

The international research effort designed to discover all human genes and to determine their functions.

Human Genome Project