alternation of generations
a life-cycle type in which the diploid and haploid stages alternate
The third life-cycle type, employed by some algae and all plants, is called alternation of generations.
diploid-dominant
a life-cycle type in which the multicellular diploid stage is prevalent
Nearly all animals employ a diploid-dominant life-cycle strategy in which the only haploid cells produced by the organism are the gametes.
haploid-dominant
a life-cycle type in which the multicellular haploid stage is prevalent
Haploid-dominant, demonstrated by all fungi and some algae — and alternation of generations, demonstrated by plants and some algae.
gametophyte
a multicellular haploid life-cycle stage that produces gametes
The haploid multicellular plants are called gametophytes because they produce gametes.
germ cell
a specialized cell that produces gametes, such as eggs or sperm
The gametes are produced from diploid germ cells, a special cell line that only produces gametes.
life cycle
the sequence of events in the development of an organism and the production of cells that produce offspring
Fertilization and meiosis alternate in sexual life cycles.
meiosis
a nuclear division process that results in four haploid cells
A type of cell division called meiosis leads to the haploid cells that are part of the sexual reproductive cycle.
sporophyte
a multicellular diploid life-cycle stage that produces spores
The zygote will undergo many rounds of mitosis and give rise to a diploid multicellular plant called a sporophyte.
chiasmata
(singular = chiasma) the structure that forms at the crossover points after genetic material is exchanged
This process is revealed visually after the exchange as chiasmata (singular = chiasma) (Figure 7.3).
crossing over
(also, recombination) the exchange of genetic material between homologous chromosomes resulting in chromosomes that incorporate genes from both parents of the organism forming reproductive cells
An exchange of chromosome segments between non-sister homologous chromatids occurs and is called crossing over.
fertilization
the union of two haploid cells typically from two individual organisms
Sexual reproduction, specifically meiosis and fertilization, introduces variation into offspring that may account for the evolutionary success of sexual reproduction.
interkinesis
a period of rest that may occur between meiosis I and meiosis II; there is no replication of DNA during interkinesis
In some species, cells enter a brief interphase, or interkinesis, that lacks an S phase, before entering meiosis II.
meiosis I
the first round of meiotic cell division; referred to as reduction division because the resulting cells are haploid
Meiosis is the division of the contents of the nucleus that divides the chromosomes among gametes.
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
Meiosis II, in which the second round of meiotic division takes place in a way that is similar to mitosis, includes prophase II, prometaphase II, and so on.
recombinant
describing something composed of genetic material from two sources, such as a chromosome with both maternal and paternal segments of DNA
The genetic information is also mixed during this division to create unique recombinant chromosomes.
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
For this reason, meiosis I is referred to as a reduction division.
somatic cell
all the cells of a multicellular organism except the gamete-forming cells
Somatic cells are sometimes referred to as “body” cells.
synapsis
the formation of a close association between homologous chromosomes during prophase I
The tight pairing of the homologous chromosomes is called synapsis.
tetrad
two duplicated homologous chromosomes (four chromatids) bound together by chiasmata during prophase I
At the end of prophase I, the pairs are held together only at chiasmata (Figure 7.3) and are called tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible.
aneuploid
an individual with an error in chromosome number; includes deletions and duplications of chromosome segments
An individual with an error in chromosome number is described as aneuploid, a term that includes monosomy (loss of one chromosome) or trisomy (gain of an extraneous chromosome).
autosome
any of the non-sex chromosomes
An individual with the appropriate number of chromosomes for their species is called euploid; in humans, euploidy corresponds to 22 pairs of autosomes and one pair of sex chromosomes.
chromosome inversion
the detachment, 180° rotation, and reinsertion of a chromosome arm
A chromosome inversion is the detachment, 180° rotation, and reinsertion of part of a chromosome.
euploid
an individual with the appropriate number of chromosomes for their species
An individual with the appropriate number of chromosomes for their species is called euploid; in humans, euploidy corresponds to 22 pairs of autosomes and one pair of sex chromosomes.
karyogram
the photographic image of a karyotype
To obtain a view of an individual’s karyotype, cytologists photograph the chromosomes and then cut and paste each chromosome into a chart, or karyogram (Figure 7.7).
karyotype
the number and appearance of an individuals chromosomes, including the size, banding patterns, and centromere position
A karyotype is the number and appearance of chromosomes, including their length, banding pattern, and centromere position.
monosomy
an otherwise diploid genotype in which one chromosome is missing
An individual with an error in chromosome number is described as aneuploid, a term that includes monosomy (loss of one chromosome) or trisomy (gain of an extraneous chromosome).
nondisjunction
the failure of synapsed homologs to completely separate and migrate to separate poles during the first cell division of meiosis
They are caused by nondisjunction, which occurs when pairs of homologous chromosomes or sister chromatids fail to separate during meiosis.
polyploid
an individual with an incorrect number of chromosome sets
An individual with more than the correct number of chromosome sets (two for diploid species) is called polyploid.
translocation
the process by which one segment of a chromosome dissociates and reattaches to a different, nonhomologous chromosome
Translocations are implicated in certain cancers, including chronic myelogenous leukemia.
trisomy
an otherwise diploid genotype in which one entire chromosome is duplicated
An individual with an error in chromosome number is described as aneuploid, a term that includes monosomy (loss of one chromosome) or trisomy (gain of an extraneous chromosome).
X inactivation
the condensation of X chromosomes into Barr bodies during embryonic development in females to compensate for the double genetic dose
In part, this occurs because of a process called X inactivation.
These terms were extracted from Concepts of Biology – 1st Canadian Edition by Charles Molnar and Jane Gair, eCampusOntario (originally published by BCcampus) which is licensed under a Creative Commons Attribution 4.0 International License. Chapter content adapted from Concepts of Biology by OpenStax (© Rice University), licensed under CC BY 4.0.