Chapter 11 Mendel and the Gene Idea

Character

An observable heritable feature that may vary among individuals

Trait

One of two or more detectable variants in a genetic character

True-breeding

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

Hybridization

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

P generation

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

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

Allele

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

Gene

A discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA (or RNA, in some viruses)

Dominant allele

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

Recessive allele

An allele whose phenotypic effect is not observed in a heterozygote

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

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

Homozygous

Having two identical alleles for a given gene

Heterozygous

Having two different alleles for a given gene

Phenotype

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

Genotype

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

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

Monohybrids

An organism that is heterozygous with respect to a single gene of interest. For example, parents of genotypes AA and aa produce a monohybrid of genotype Aa

Monohybrid cross

A cross between two organisms that are heterozygous fro the character being followed (or the self-pollination of a heterozygous plant)

Dihybrid

An organism that is heterozygous with respect to two genes of interest. For example, parents of genotypes AABB and aabb produce offspring with a genotype of AaBb

Dihybrid cross

A cross between two organisms that are each heterozygous for both of the characters being followed (or the self-pollination of a plant that is heterozygous for both characters)

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

Complete dominance

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

Incomplete dominance

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

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

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

Pleitropy

The ability of a single gene to have multiple effects

Epistasis

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

Quantitative characters

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

Polygenic inheritance

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

Multifactorial

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

Pedigree

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

Carriers

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

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

Sickle-cell disease

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

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