AP Bio Chapter 14

Mendelian genetics and genes in general

allele

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

genotype

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

amniocentesis

technique associated with prenatal diagnosis in which amniotic fluid is obtained by aspiration from a needle inserted into the uterus. The fluid and the fetal cells it contains are analyzed to detect certain genetic and congenital defects in the fetus.

chorionic villus sampling (CVS)

A technique associated with prenatal diagnosis in which a small sample of the fetal portion of the placenta is removed for analysis to detect certain genetic and congenital defects in the fetus.

trait

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

monohybrid cross

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

phenotype

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

multifactorial

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

quantitative character

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

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.

hybridization

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

dominant allele

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

monohybrid

An organism that is heterozygous with respect to a single gene of interest. All the offspring from a cross between parents homozygous for different alleles are monohybrids. For example, parents of genotypes AA and aa produce a monohybrid of genotype Aa.

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.

sickle-cell disease

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

dihybrid

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 these. For example, parents of genotypes AABB and aabb produce a ______ of genotype AaBb.

true-breeding

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

polygenic inheritance

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

homozygote

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

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.

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.

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.

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.

phage

A virus that infects bacteria; also called a bacteriophage.

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.

incomplete dominance

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

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.

pleiotropy

The ability of a single gene to have multiple effects.

complete dominance

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

homozygous

Having two identical alleles for a given gene.

epistasis

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

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).

character

An observable heritable feature that may vary among individuals.

heterozygous

Having two different 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.