Dominance

An example demonstrating how dominant genes or alleles are passed from generation to generation.

Legend: S=Dominate s=Recessive

• Both parents carry Dominant pairs of alleles. 100% chance of passing the gene.

• Both parents posses a single dominant gene. 75% chance of passing the gene. 1 affected, 2 carriers, 1 unaffected.

• One parent carries a dominate pair. Other parent caries a recessive pair. 100% chance the gene will be passed on. 4 carriers.

• One parent with a single dominant gene the other parent caries a recessive pair. 50% chance of passing the gene. 2 carriers, 2 normal.

[edit] Dominant and Recessive genes

From Wikipedia:

In genetics, dominance describes the effects of different versions of a gene on a trait. Many animals (including humans) and plants have two copies of each gene, one inherited from each parent. genes are located on specific locations called alleles. If the two copies of a gene are different, and their combined effect is determined by one of the genes, than we call that gene dominant. The other gene, which does not determine the outcome, is said to be recessive.

For example, having two copies of one allele of the EYCL3 gene causes the eye's iris to be brown, and having two copies of another allele causes the iris to be blue. But having one copy of each allele leads to a brown iris. Thus the brown allele is said to be dominant over the blue allele (and the blue allele is said to be recessive to the brown allele).

In most cases a dominance relationship is seen when the gene encodes an enzyme, and its recessive counterpart does not. In many cases, a normal function can be maintained with only half the amount of an enzyme. In these cases a single copy of the normal allele produces enough of the gene’s product to give the same effect as two normal copies, and so the normal allele is described as being dominant to the other allele. This is the case for the eye color alleles described above, where a single functional copy of the ‘brown’ allele causes enough melanin to be made in the iris that the eyes appear brown even when paired with the non-melanin-producing ‘blue’ allele.

Dominance was discovered by Mendel, who introduced the use of uppercase letters to denote dominant alleles and lowercase to denote recessive alleles, as is still commonly used in introductory genetics courses (for example, B and b for alleles causing brown and blue eyes). Although this usage is convenient it is misleading, because dominance is not a property of an allele considered in isolation but of a relationship between the effects of two alleles. When geneticists loosely refer to a dominant allele or a recessive allele, they mean that the allele is dominant or recessive to the standard allele.

geneticists often use the term dominance in other contexts, distinguishing between simple or complete dominance as described above, and other relationships. Relationships described as incomplete or partial dominance are usually more accurately described as giving an intermediate or blended phenotype. The relationship described as codominance describes a relationship where the distinct phenotypes caused by each allele are both seen when both alleles are present.

[edit] Other Factors

Dominance of a particular gene does not necessarily mean the trait will manifest. Some traits, while genetic, are also gender specific, baldness, colorblindness and, hemophilia are some examples of gender specific genetic traits. Still other traits are restricted to race, as in sickle cell anemia.

[edit] References

Dominance definition.

Diagram showing Recessive and Dominant inheritance.

Wikipedia

Category: PG