Word

Definition

Alleles

Different type of gene. May give a different action, as its structure is different to a gene.

Amino acid

Small molecules that make up the proteins. Composed of Carbon Hydrogen Oxygen Nitrogen.

Blended Theory

An incorrect 19th century theory about the inheritance of characteristics. It proposed thatinherited traits blend from generation to generation. Through his plant crossbreeding experiments, Gregor Mendel proved that this was wrong.

Carrier

An individual who is heterozygous for a trait that only shows up in thephenotypeof those who arehomozygousrecessive. Carriers often do not show any signs of the trait but can pass it on to their offspring.

Chromosomes

Thread-like, gene-carrying bodies in the nucleus of a cell.Chromosomes are composed primarily ofDNAandprotein. They are visible only under magnification during certain stages of cell division. Humans have 46 chromosomes in eachsomatic celland 23 in eachsex cell.

Dominant allele

Anallelethat masks the presence of arecessive allelein thephenotype. Dominant alleles for a trait are usually expressed if an individual ishomozygous dominantor heterozygous.

DNA

Deoxyribonucleic acid

A large organic molecule that stores the genetic code for the synthesis ofproteins. DNA is composed of sugars, phosphates and bases arranged in a double helix shaped molecular structure. Segments of DNA inchromosomescorrespond to specificgenes.

Evolution

Genetic change in a population of organisms that occurs over time. The term is also frequently used to refer to the appearance of a new species.

Gene pool

All of thegenesin all of the individuals in a breeding population. More precisely, it is the collectivegenotype

Heterozygous

Agenotypeconsisting of two differentallelesof a gene for a particular trait (Aa). Individuals who are heterozygous for a trait are referred to as heterozygotes. Seehomozygous.

Homozygous

Having the samealleleat the same locus on both members of a pair ofhomologous chromosomes. Homozygous also refers to agenotypeconsisting of two identical alleles of a gene for a particular trait. An individual may be homozygous dominant (AA) or homozygous recessive (aa). Individuals who are homozygous for a trait are referred to as homozygotes. Seeheterozygous.

Huntington disease

A severe genetically inherited fatal degenerative nerve disorder. The symptoms usually do not appear until early middle age. There is a progressive loss of muscle control that inevitably leads to paralysis and death. This disease is also called Huntington's chorea

Meiosis

Cell division in specialized tissues of ovaries and testes that results in the production ofspermorova. Meiosis involves two divisions and results in four daughter cells, each containing only half the original number of chromosomes--23 in the case of humans.

Mendelian genetics

Inheritance patterns which can be explained by simple rules ofdominanceandrecessivenessofgenes.

Monozygotic twin

Identical twins. Twins that come from the samezygoteare essentially the same genetically. Differences between monozygotic twins later in life are virtually always the result of environmental influences rather than genetic inheritance. Fraternal twins may look similar but are not genetically identical

Mutation

An alteration of genetic material such that a new variation is produced. For instance, a trait that has only one allele (A) can mutate to a new form (a). This is the only mechanism ofevolutionthat can produce newallelesof a gene.

Phenotype

The observable or detectable characteristics of an individual organism--the detectable expression of agenotype.

Proteins

Any of a large number of complex organic molecules that are composed of one or more chains ofamino acids. Proteins can serve a wide variety of functions through their ability to bind to other molecules. Proteins may be enzymes, hormones, antibodies, structural components, or gas-transporting molecules

Punnett Square

A simple graphical method of showing all of the potential combinations of offspringgenotypesthat can occur and their probability given the parent genotypes. See example below. Punnet squares are commonly used by genetics counsellors to predict the odds of a couple passing on particular inherited traits.

Recessive Allele

Anallelethat is masked in thephenotypeby the presence of adominant allele. Recessive alleles are expressed in the phenotype when thegenotypeis homozygous recessive (aa).

Sex cell

A gamete, either a sperm or an ovum. Sex cells are produced by themeiosisprocess. Seesomatic cell.

Sex-controlled gene

Agenethat can be inherited by both genders but is usually expressed differently in males and females. Seesex-limited gene.

Sex-limited gene

Agenethat can be inherited by both genders but is usually expressed in only males or females. Seesex-controlled gene.

Sperm

A male sex cell or gamete

Stuttering alleles

Defectiveallelesthat 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. The myotonic form ofmuscular dystrophyis an example. Stuttering alleles are also known as unstable alleles.

Unit inheritance

Gregor Mendel's idea that the characteristics of parents are passed on to descendants unchanged as units. In other words, the hereditary material of any organism is made up of discrete units (now calledgenes).

X-linked

Referring to agenethat is carried by anXsex chromosome.

Zygote

A "fertilized"ovum. More precisely, this is a cell that is formed when aspermand an ovum combine theirchromosomesat conception. A zygote contains the full complement of chromosomes (in humans 46) and has the potential of developing into an entire organism.