asexual reproduction
a single individual passes genes to its offspring with no fusion of gametes
clone
a group of genetically identical individuals from the same parent
sexual reproduction
two parents give rise to offspring that have unique combinations of genes inherited from the two parents
life cycle
the generation-to-generation sequence of stages in the reproductive history of an organism
fertilization
the union of gametes (the sperm and the egg) - takes place in eukaryotic organisms
zygote
the fertilized egg, has one set of chromosomes from each parent, produces somatic cells by mitosis and develops into an adult
sexual reproduction in animals
gametes are the only haploid cells, which are produced by meiosis and undergo no further cell division before fertilization; gametes fuse to form a diploid zygote that divides by mitosis into a multicellular organism
sexual reproduction in plants and some algae
alternation of generations, which includes both a diploid (sporophyte) and haploid (gametophyte) multicellular stage, fertilization of gametes results in a zygote that grows by mitosis into a diploid sporophyte
sporophyte
diploid organism, grows by mitosis, makes haploid spores by meiosis
gametophyte
haploid organism, makes haploid gametes by mitosis
sexual reproduction in most fungi and some protists
only diploid stage is the single-celled zygote (no multicellular diploid stage), which produces haploid cells by meiosis, each of which grows by mitosis into a haploid multicellular organism (gametophyte)
meiosis
something only diploid cells can undergo, creates four genetically different haploid daughter cells with unduplicated chromosomes
meiosis I
reductional division; homologs pair up and separate, resulting in two haploid daughter cells with replicated chromosomes
meiosis II
equational division; sister chromatids separate
interphase
chromosomes are duplicated to form sister chromatids, which are genetically identical and joined at the centromere, which replicates; centrosome also replicates, resulting in two centrosomes
prophase I
duplicated homologous chromosomes pair and exchange segments, nuclear envelope disintegrates
metaphase I
chromosomes line up by homologous pairs, microtubules attach to kinetochores (located at centromere)
anaphase I
homologous chromosomes separate (sister chromatids remain attached)
telophase I and cytokinesis
two haploid cells form (cleavage furrow forms, nuclear envelope re-forms), each centrosome still consists of two sister chromatids
prophase II
nuclear envelope disintegrates again, microtubules get ready
metaphase II
chromosomes line up, microtubules attach
anaphase II
sister chromatids separate
telophase II and cytokinesis
four haploid daughter cells, containing unduplicated chromosomes, form
mitosis
conserves the number of chromosome sets, produces two daughter cells that are genetically identical to the parent cell
DNA replication (mitosis)
during interphase (before mitosis)
number of divisions (mitosis)
one
synapsis of homologous chromosomes (mitosis)
does not occur
number of daughter cells and genetic composition (mitosis)
two, each diploid and genetically identical to parent cell
role of mitosis in the animal body
enables multicellular adult to arise from zygote; produces cells for growth, repair, and (in some species) asexual reproduction
DNA replication (meiosis)
during interphase (before meiosis I)
number of divisions (meiosis)
two
synapsis of homologous chromosomes (meiosis)
during prophase I along with crossing over between non-sister chromatids; resulting chiasmata hold pairs together
number of daughter cells and genetic composition (meiosis)
four, each a genetically different haploid (containing half as many chromosomes as the parent cell)
role of meiosis in the animal body
produces gametes, reduces number of chromosomes by half and introduces genetic variability among gametes
mutations
original source of genetic diversity, creates different versions of genes called alleles, changes in nucleotide sequences of DNA, can only be passed to offspring if occurring in gametes, rates low in animals and plants
mechanisms contributing to genetic diversity during meiosis and fertilization
independent assortment of chromosomes, crossing over, random fertilization
independent assortment
during metaphase I, each pair of chromosomes sorts maternal and paternal homologs into daughter cells independently of the other pairs, 2^n possible combinations (n is haploid number, ex 23 in humans)
recombinant chromosomes
produced by crossing over, combines DNA inherited from each parent early in prophase I
crossing over
homologous portions of two non-sister chromosomes trade places; contributes to genetic variation by combining DNA from two parents into a single chromosome
random fertilization
fusion of two gametes, adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg), produces a genetically unique zygote
BIG IDEA I
organisms inherit genes, which are modified by evolutionary processes over time, resulting in the past and present diversity of life on earth
ways to transmit genetic information from one generation to another
mitosis, meiosis, and fertilization
Gregor Mendel
Austrian friar, developed a basic model of inheritance long before the discovery of DNA / chromosomes
character
distinct heritable feature
trait
character variant
hybridization
mating two contrasting true-breeding varieties
true-breeding
homozygous for a particular trait
alleles
alternative versions of a gene, which account for variations in inherited characters, resides at a specific locus on a specific chromosome
dominant allele
only one required to display phenotypically
recessive allele
must be homozygous to display phenotypically
Law of Segregation
the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes (an egg or sperm gets only one of the two alleles present in an organism)
homozygous
two identical alleles for a gene controlling a particular character (true breeding)
heterozygous
two different alleles for a gene controlling a particular character (not true breeding)
phenotype
physical appearance, the product of inherited genotype and environmental influences
genotype
genetic makeup
testcross
breeding a "mystery" individual with a homozygous recessive individual to determine genotype
Law of Independent Assortment
genes located near each other on the same chromosome tend to be inherited together (applies to genes on different, nonhomologous chromosomes or far apart on the same chromosome)
dihybrid
the offspring of parents differing in true-breeding for two different characters
dihybrid cross
a cross between first-generation dihybrids to determine whether two characters are transmitted to offspring as a package or independently
incomplete dominance
two alleles are neither dominant nor recessive; the resulting offspring have a phenotype that is a blending of the parental traits
codominance
the phenotypes produced by both alleles are completely expressed
Darwin's three observations
there is unity of life, there is diversity of life, organisms and their environments match
descent with modification
all organisms are related through descent from an ancestor in the remote past (explains unity of life)
variation
domestic species have this, it is heritable, small changes accumulate to make big differences, a natural state, makes taxonomy difficult
struggle for existence
populations tend to increase, competition and predation are everywhere, variation can be related to success, most individuals do not survive
natural selection
the preservation of favorable variations / the rejection of harmful variations, increases the adaptation of organisms to their environment, acts on individuals, can only act of variation with a genetic component
evolution
result of variation, inheritance, and the struggle for existence, acts on populations
artificial selection
human modification of other species by selecting and breeding individuals with desired traits
members of a population often vary in their inherited traits
individuals whose inherited traits increase their chance of surviving and reproducing in a given environment tend to leave more offspring than other individuals (explains match with environment)
all species can produce more offspring than the environment can support, so many of them fail to survive and reproduce
leads to the accumulation of favorable traits in the population over generations
microevolution
a change in allele frequencies in a population over generations
mechanisms causing changes in allele frequencies
natural selection, genetic drift, gene flow
discrete characters
can be classified on an either-or basis, contribute to the population's variation
quantitative characters
vary along a continuum within a population, contribute to the population's variation
average heterozygosity
measures the average percent of loci that are heterozygous in a population, a way to measure gene variation
geographic variation
differences between gene pools of separate populations, exhibited by most species, due to drift (not natural selection)
cline
graded change in a trait along a geographic axis, due to natural selection
population
a localized group of individuals capable of interbreeding and producing fertile offspring
gene pool
all the alleles for all loci in a population
fixed locus
all individuals in a population are homozygous for a particular allele
allele frequency at a locus
(for diploid organisms) total number of individuals * 2
total number of dominant / recessive alleles
(2 * number of homozygous individuals) + number of heterozygous individuals
p + q
1; p = frequency of dominant alleles, q = frequency of recessive alleles
Hardy-Weinberg principle
a population is in equilibrium (not evolving - frequencies of alleles and genotypes remain constant from generation to generation) if it has no mutations, random mating, no natural selection, extremely large population, no gene flow; can be so at some loc
p^2 + 2pq + q^2
1; p^2 is homozygous dominant, 2pq is heterozygous, q^2 is homozygous recessive