AP Biology Genetics Vocabulary

Recessive allele

An allele that is masked when a dominant allele is present and produces its characteristic phenotype only when its paired with an identical allele.

polyploidy

A chromosomal alteration in which the organism possesses more than two complete chromosome sets (4n) . It is the result of an accident of cell division and usually leads to complications or even cell death.

monosomic

A chromosomal condition in which a particular cell has only one copy of a chromosome, instead of the normal two; the cell is said to be monosomic for that chromosome. If a a cell has three copies instead of the normal two, then it is said to be a trisomic.

Pedigree

A diagram of a family tree showing the occurrence of heritable characters in parents and offspring over multiple generations.

Incomplete dominance

A pattern of inheritance in which two alleles, inherited from the parents, are neither dominant nor recessive. The resulting offspring have a phenotype that is a blending of the parental traits. Example: A snapdragon flower that is pink as a result of cross-pollination between a red flower and a white flower when neither the white or the red alleles are dominant.

Trait

A phenotypic trait, or simply trait, is a distinct variant of a phenotypic characteristic of an organism that may be inherited, be environmentally determined or be a combination of the two. The color of an animal's coat and the shape of a plant's leaves are physical traits. Nesting in birds and burrowing in rodents are examples of behavioral traits.

sex linked

A sex-linked gene is a gene located on a sex chromosome (usually the X chromosome), resulting in a distinctive pattern of inheritance. For example: Men normally have an X and a Y combination of sex chromosomes, while women have two X's. Since only men inherit Y chromosomes, they are the only ones to inherit Y-linked traits. Men and women can get the X-linked ones since both inherit X chromosomes. Males are more likely to be affected by sex-linked traits than females because males have XY chromosomes. The X is always expressed, regardless of it being recessive or dominant. Females, on the other hand, are XX; if they were heterozygous for the trait, the dominant component could overpower it, forcing the female to show no sign of the trait. However, the female is

True-breeding

A true breeding organism, sometimes also called a pure-bred, is when 2 organisms with identical genotypes procreate an organism for the same traits. In other words, to "breed true" happens when two organisms with a particular, inheritable phenotype produce only offspring with that (same) phenotype.

Epistasis

A type of gene interaction in which one gene alters the phenotypic effects of another gene that is independently inherited. Epistasis describes how gene interactions can affect phenotypes. Sometimes genes can mask each other's presence or combine to produce an entirely new trait. For example, the gene for male pattern baldness is epistatic to the hair color gene.

Map units

A unit of measurement of the distance between genes. One map unit is equivalent to a 1% recombination frequency.

Allele

An alternative form of a gene (one member of a pair) that is located at a specific position on a specific chromosome. These DNA codings determine distinct traits that can be passed on from parents to offspring through sexual reproduction. In genetics, different variants of allele gives different phenotype and genotypic outcomes.

Nondisjunction

An error in meiosis or mitosis in which members of a pair of homologous chromosomes or a pair of sister chromatids fail to separate properly from each other. This usually leads to death of the gametes but were the egg to fertilize under such conditions, genetic disorders will likely result as well.

wild type

An individual with the phenotype most commonly observed in natural populations; also refers to the phenotype itself. Any form of that allele other than the wild type is known as a mutant form of that allele. Example: Wild type tigers have orange fur and black stripes. One mutation prevents the deposition of the orange/brown pigment, and the result is a "white tiger" that still has dark stripes. A different mutation prevents any melanin (brown pigment) from forming at all, and the result is an albino tiger.

Heterozygous

An organism that has two different alleles for a gene/trait. Based on the type of inheritance, the heterozygotes can either express the dominant alleles (complete dominance) or both the dominant and recessive alleles (incomplete or codominance).

Genotype

An organism's genetic makeup, or allele combinations is known as a genotype. The genotype ultimately determine the expression of a particular characteristic/trait known as phenotype.

Testcross

Breeding an organism of unknown genotype with a homozygous recessive individual to determine the unknown genotype. The ratio of phenotypes in the offspring reveals the unknown genotype.

genetic recombination

General term for the production of offspring with combinations of traits that differ from those found in either parent. Genetic recombination is the main factor to cause trait difference between generations. For example, the crossing over between homologous chromosomes during meiosis or done artificially by applying genetic engineering techniques.

linked genes

Genes located on the same chromosome that tend to be inherited together in genetic crosses. For example, in fruit flies the genes for eye color and the genes for wing length are on the same chromosome, thus are inherited together. This is not to be confused with sex-linked gene, which refers to a gene located on one of the sex chromosomes.

Hybridization

In genetics, the mating, or crossing of two true-breeding varities. This usuaclly involves crossing dissimilar individuals to bring together the best traits of both organisms

Law of segregation

Mendel's first law, stating that the two alleles in a pair segregate (separate) into different gametes during gamete formation. The segregation is caused by separation of homologous chromosomes, which form individual gametes, during meiosis

Law of independent assortment

Mendel's second law, stating that each pair of alleles segregates, or assorts, independently of each other pair during gamete formation' applies when genes for two characters are located on different pair of homologous chromosomes. When two or more characteristics are inherited, individual hereditary factors assort independently during gamete production, giving different traits an equal opportunity of occurring together.

Multifactorial

Referring to a phenotypic character that is influenced by multiple genes and environmental factors. Example of multifactorial traits include: fingerprint patterns, height, eye color, and skin color, etc.

F1 Generation

The first filial, or hybrid, offspring in a series of genetic crosses. The F1 is the generation resulting from a cross between two parents mating and producing a progeny. That progeny is known as the F1. If those progeny are then mated to one another, their progeny is know as the F2.

Pleiotropy

The phenomenon of one gene being responsible for or affecting more than one seemingly unrelated phenotypic characteristic. Example: Chickens and the Frizzle Trait Chickens that express the dominant frizzle gene produce feathers that curle outward rather than lying flat against their bodies. However, this was not the only phenotypic effect of this gene; the frizzle gene caused the fowl to also have abnormal body temperatures, higher metabolic and blood flow rates, and greater digestive capacity. Furthermore, chickens who had this allele also laid fewer eggs than their wild-type counterparts, further highlighting the pleiotropic nature of the frizzle gene.

Codominance

The situation in which the phenotypes of both alleles are exhibited in the heterozygote because both alleles affect the phenotype in separate, distinguishable ways. For example, in blood, the A and B alleles are codominant because if an A is inherited from one parent and a B from the other, the phenotype will be AB blood.

Polygenic inheritance

Two or more genes contribute to the phenotypic expression of a single characteristic. Because a large number of genes are involved to express just one phenotypic trail, each gene usually has a relatively small effect.