Chapter 9

Locus

The specific physical location of a gene on a chromosome.

Gene

A unit of heredity that is transferred from parent to offspring and determines some characteristic of the offspring.

Allele

Different forms of a gene that arise by mutation and are found at the same place on a chromosome.

Homozygous

Having two identical alleles for a particular gene.

Heterozygous

Having two different alleles for a particular gene.

Monohybrid

A genetic cross involving one trait.

Dihybrid

A genetic cross involving two traits.

Dominant

An allele that is expressed in the phenotype even when only one copy is present.

Recessive

An allele that is only expressed in the phenotype when two copies are present.

Co-dominant

Both alleles in a heterozygous individual contribute to the phenotype.

Wild type

The most common phenotype or genotype in a population.

Mutant

An individual or allele that has undergone mutation, resulting in a different phenotype.

Genotype

The genetic constitution of an individual.

Phenotype

The observable characteristics or traits of an individual.

Parental

The original generation from which offspring are derived.

Testcross

Breeding an individual of unknown genotype with a homozygous recessive individual to determine the unknown genotype.

F1

The first generation of offspring resulting from a cross.

F2

The second generation of offspring, produced from a cross of F1 individuals.

Pure-breeding

Organisms that produce offspring identical to themselves when self-fertilized.

Autosome

Any chromosome that is not a sex chromosome.

Sex chromosome

A chromosome that determines the sex of an organism.

Complete dominance

A type of inheritance where an individual with two different genes looks the same as one of the individuals with two identical genes.

Incomplete dominance

A form of inheritance where the phenotype of the heterozygote is intermediate between the phenotypes of the two homozygotes.

Pleiotropy

The ability of a single gene to have multiple effects.

Polygenic inheritance

A trait controlled by multiple genes, often resulting in a range of phenotypes.

Law of Segregation

The principle that alleles for a trait segregate from each other during the formation of gametes.

Punnett square

A diagram used to predict the outcome of a genetic cross by showing all possible combinations of gametes.

Independent Assortment

Mendel's second law stating that alleles of different genes assort independently of one another in gamete formation.

Probability in Genetics

The likelihood of a particular genetic outcome occurring based on the combination of alleles.

Pedigree

A diagram that shows the occurrence and appearance or phenotype of a particular gene or organism and its ancestors.

I gene (blood group)

The gene that controls the ABO blood type, with alleles A, B, and O.

Antigen

A substance on the surface of red blood cells that triggers an immune response if foreign.

Antibody

A protein produced by the immune system in response to a foreign substance; reacts specifically to its corresponding antigen.

Positive/Negative Blood Type

Refers to the presence or absence of the Rh factor; positive indicates the presence of Rh antigen.

SRY gene

A gene on the Y chromosome that triggers male development.

Barr bodies

Inactivated X chromosomes in females which affect gene expression by limiting the amount of gene product from one X chromosome.