Mendel Inheritance

allele

gene variations that arise by mutation and exist at the same relative locations on homologous chromosomes

autosomes

any of the non-sex chromosomes

blending theory

hypothetical inheritance pattern in which parental traits are blended together in the offspring to produce an intermediate physical appearance

codominance

in a heterozygote, complete and simultaneous expression of both alleles for the same characteristic

dihybrid

result of a cross between two true-breeding parents that express different traits for two characteristics

dominant

trait which confers the same physical appearance whether an individual has two copies of the trait or one copy of the dominant trait and one copy of the recessive trait

epistasis

antagonistic interaction between genes such that one gene masks or interferes with the expression of another

F1

first filial generation in a cross; the offspring of the parental generation

F2

second filial generation produced when F1 individuals are self-crossed or fertilized with each other

genotype

underlying genetic makeup, consisting of both physically visible and non-expressed alleles, of an organism

hemizygous

presence of only one allele for a characteristic, as in X-linkage; hemizygosity makes descriptions of dominance and recessiveness irrelevant

heterozygous

having two different alleles for a given gene on the homologous chromosome

homozygous

having two identical alleles for a given gene on the homologous chromosome

hybridization

process of mating two individuals that differ with the goal of achieving a certain characteristic in their offspring

incomplete dominance

in a heterozygote, expression of two contrasting alleles such that the individual displays an intermediate phenotype

law of dominance

in a heterozygote, one trait will conceal the presence of another trait for the same characteristic

law of independent assortment

genes do not influence each other with regard to sorting of alleles into gametes; every possible combination of alleles is equally likely to occur

law of segregation

paired unit factors (i.e., genes) segregate equally into gametes such that offspring have an equal likelihood of inheriting any combination of factors

linkage

phenomenon in which alleles that are located in close proximity to each other on the same chromosome are more likely to be inherited together

monohybrid

result of a cross between two true-breeding parents that express different traits for only one characteristic

multiple alleles

genes that have more than one allele governing inheritance. (e.g. blood types) Depending on which two get paired, they can show complete, incomplete, or codominant inheritance.

P

parental generation in a cross

Pedigree

A chart showing genetically related individuals that tracks one trait across generations.

phenotype

observable traits expressed by an organism

Punnett square

visual representation of a cross between two individuals in which the gametes of each individual are denoted along the top and side of a grid, respectively, and the possible zygotic genotypes are recombined at each box in the grid

recessive

trait that appears "latent" or non-expressed when the individual also carries a dominant trait for that same characteristic; when present as two identical copies, the recessive trait is expressed

sex-linked

any gene on a sex chromosome

test cross

cross between a dominant expressing individual with an unknown genotype and a homozygous recessive individual; the offspring phenotypes indicate whether the unknown parent is heterozygous or homozygous for the dominant trait

trait

variation in the physical appearance of a heritable characteristic

X-linked

gene present on the X, but not the Y chromosome

Y-linked

Gene present on the Y, but not the X chromosome

chromosome inversion

detachment, 180° rotation, and chromosome arm reinsertion

Crossing over

process by which homologous chromosomes undergo reciprocal physical exchanges at their arms.

karyotype

an individual's chromosome number and appearance; includes the size, banding patterns, and centromere position

monosomy

otherwise diploid genotype in which one chromosome is missing

nondisjunction

failure of synapsed homologs to completely separate and migrate to separate poles during the meiosis' first cell division

nonparental (recombinant) type

progeny resulting from homologous recombination that exhibits a different allele combination compared with its parents

parental types

progeny that exhibits the same allelic combination as its parents

polyploid

individual with an incorrect number of chromosome sets

recombination frequency

average number of crossovers between two alleles; observed as the number of nonparental types in a progeny's population

translocation

process by which one chromosome segment dissociates and reattaches to a different, nonhomologous chromosome

trisomy

otherwise diploid genotype in which one entire chromosome duplicates