Ch 11 Mendel and the Gene Vocab

addition rule

addition rule

A rule of probability stating that the probability of any one of two or more mutually exclusive events occurring can be determined by adding their individual probabilities.

allele

Any of the alternative versions of a gene that may produce distinguishable phenotypic effects.

carrier

In genetics, an individual who is heterozygous at a given genetic locus for a recessively inherited disorder. The heterozygote is generally phenotypically normal for the disorder but can pass on the recessive allele to offspring.

character

An observable heritable feature that may vary among individuals.

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.

complete dominance

The situation in which the phenotypes of the heterozygote and dominant homozygote are indistinguishable.

cystic fibrosis

A human genetic disorder caused by a recessive allele for a chloride channel protein; characterized by an excessive secretion of mucus and consequent vulnerability to infection; fatal if untreated.

dihybrid(dī'-hī'-brid)

An organism that is heterozygous with respect to two genes of interest. All the offspring from a cross between parents doubly homozygous for different alleles are dihybrids. For example, parents of genotypes AABB and aabb produce a dihybrid of genotype AaBb.

dihybrid cross

A cross between two organisms observing two traits

dominant allele

An allele that is fully expressed in the phenotype of a heterozygote.

epistasis

A type of gene interaction in which the phenotypic expression of one gene alters that of another independently inherited gene.

F1 generation

The first filial, hybrid (heterozygous) offspring arising from a parental (P generation) cross.

F2 generation

The offspring resulting from interbreeding (or self-pollination) of the hybrid F1 generation.

genotype

The genetic makeup, or set of alleles, of an organism.

heterozygote

An organism that has two different alleles for a gene (encoding a character).

heterozygous

Having two different alleles for a given gene.

homozygote

An organism that has a pair of identical alleles for a gene (encoding a character).

homozygous

Having two identical alleles for a given gene.

Huntington's disease

A human genetic disease caused by a dominant allele; characterized by uncontrollable body movements and degeneration of the nervous system; usually fatal 10 to 20 years after the onset of symptoms.

hybridization

In genetics, the mating, or crossing, of two true-breeding varieties.

incomplete dominance

The situation in which the phenotype of heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele.

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 pairs of homologous chromosomes or when they are far enough apart on the same chromosome to behave as though they are on different chromosomes.

law of segregation

Mendel's first law, stating that the two alleles in a pair segregate (separate from each other) into different gametes during gamete formation.

monohybrid cross

A cross observing only one trait

multifactorial

Referring to a phenotypic character that is influenced by multiple genes and environmental factors.

multiplication rule

A rule of probability stating that the probability of two or more independent events occurring together can be determined by multiplying their individual probabilities.

pedigree

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

P generation

The true-breeding (homozygous) parent individuals from which F1 hybrid offspring are derived in studies of inheritance. (P stands for parental.)

phenotype

The observable physical and physiological traits of an organism, which are determined by its genetic makeup.

pleiotropy

The ability of a single gene to have multiple effects.

polygenic inheritance

An additive effect of two or more genes on a single phenotypic character.

Punnett square

A diagram used in the study of inheritance to show the predicted genotypic results of random fertilization in genetic crosses between individuals of known genotype.

quantitative character

A heritable feature that varies continuously over a range rather than in an either-or fashion.

recessive allele

An allele whose phenotypic effect is not observed in a heterozygote.

sickle-cell disease

A recessively inherited human blood disorder in which a single nucleotide change in the β-globin gene causes hemoglobin to aggregate, changing red blood cell shape and causing multiple symptoms in afflicted individuals.

Tay-Sachs disease

A human genetic disease caused by a recessive allele for a dysfunctional enzyme, leading to accumulation of certain lipids in the brain. Seizures, blindness, and degeneration of motor and mental performance usually become manifest a few months after birth, followed by death within a few years.

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.

trait

One of two or more detectable variants in a genetic character.

true-breeding

refers to organisms that produce offspring of the same variety over many generations of self-pollination.