AP Biology Unit 5: Heredity Key Terms

alternation of generations

a life-cycle type in which the diploid and haploid stages alternate

chiasmata

(singular, chiasma) the structure that forms at the crossover points after genetic material is exchanged

cohesin

proteins that form a complex that seals sister chromatids together at their centromeres until anaphase II of meiosis

crossover

exchange of genetic material between non-sister chromatids resulting in chromosomes that incorporate genes from both parents of the organism

diploid-dominant

life-cycle type in which the multi-cellular diploid stage is prevalent

fertilization

union of two haploid cells from two individual organisms

gametophyte

a multicellular haploid life-cycle stage that produces gametes

germ cells

specialized cell line that produces gametes, such as eggs or sperm

haploid-dominant

a life-cycle type in which the multicellular haploid stage is prevalent

interkinesis

(also, interphase II) brief period of rest between meiosis I and meiosis II

life cycle

the sequence of events in the development of an organism and the production of cells that produce offspring

meiosis

a nuclear division process that results in four haploid cells

meiosis I

the first round of meiotic cell division; referred to as reduction division because the resulting cells are haploid

meiosis II

the second round of meiotic cell division following meiosis I; sister chromatids are separated from each other, and the result is four unique haploid cells

recombination nodules

protein assembles formed on the synaptonemal complex that mark the points of crossover events and mediate the multistep process of genetic recombination between non-sister chromatids

reduction division

a nuclear division that produces daughter nuclei each having one-half as many chromosome sets as the parental nucleus; meiosis I is a reduction division

somatic cell

all the cells of a multicellular organism except the gametes or reproductive cells

spore

haploid cell that can produce a haploid multicellular organism or can fuse with another spore to form a diploid cell

sporophyte

a multicellular diploid life-cycle stage that produces haploid spores by meiosis

synapsis

formation of a close association between homologous chromosomes during prophase I

synaptonemal complex

protein lattice that forms between homologous chromosomes during prophase I, supporting crossover

tetrad

two duplicated homologous chromosomes (four chromatids) bound together by chiasmata during prophase I

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 of Inheritance

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

continuous variation

inheritance pattern in which a character shows a range of trait values with small gradations rather than large gaps between them

dihybrid

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

discontinuous variation

inheritance pattern in which traits are distinct and are transmitted independently of one another

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

dominant lethal

inheritance pattern in which an allele is lethal both in the homozygote and the heterozygote; this allele can only be transmitted if the lethality phenotype occurs after reproductive age

epistasis

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

F1

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

F2

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

genotype

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

hemizygous

the 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

the 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

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

model system

a species or biological system used to study a specific biological phenomenon to be applied to other different species

monohybrid

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

P0

the parental generation in a cross

phenotype

observable traits expressed by an organism

product rule

probability of two independent events occurring simultaneously can be calculated by multiplying the individual probabilities of each event occurring alone

Punnett square

a 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

recessive lethal

inheritance pattern in which an allele is only lethal in the homozygous form; the heterozygote may be normal or have some altered, non-lethal phenotype

reciprocal cross

a paired cross in which the respective traits of the male and female in one cross become the respective traits of the female and male in the other cross

sex-linked

any gene on a sex chromosome

sum rule

probability of the occurrence of at least one of two mutually exclusive events is the sum of their individual probabilities

test cross

a 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

aneuploid

an individual with an error in chromosome number; includes deletions and duplications of chromosome segments

autosome

any of the non-sex chromosomes

Centimorgan (cM)

(also, map unit) relative distance that corresponds to a recombination frequency of 0.01

Chromosomal Theory of Inheritance

theory proposing that chromosomes are the vehicles of genes and that their behavior during meiosis is the physical basis of the inheritance patterns that Mendel observed

chromosome inversion

the detachment, 180° rotation, and reinsertion of a chromosome arm

euploid

individual with the appropriate number of chromosomes for their species

homologous recombination

process by which homologous chromosomes undergo reciprocal physical exchanges at their arms, also known as crossing over

karyogram

the photographic image of a karyotype

karyotype

the number and appearance of an individuals chromosomes, including the size, banding patterns, and centromere position

monosomy

otherwise diploid genotype in which one chromosome is missing

nondisjunction

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

nonparental (recombinant) type

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

paracentric

inversion that occurs outside of the centromere

parental types

progeny that exhibits the same allelic combination as its parents

pericentric

inversion that involves the centromere

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 population of progeny

translocation

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

trisomy

otherwise diploid genotype in which one entire chromosome is duplicated

X inactivation

condensation of X chromosome into Barr bodies during embryonic development in females to compensate for the double genetic dose