inheritance Flashcards
what is a mutagen
substances which increase the likelihood of a mutation occurring
when during the cell cycle is a DNA mutation most likely to occur
S phase (during DNA replication)
when during the cell cycle is a chromosomal mutation most likely to occur
mitosis/meiosis
or S phase
state 4 types of chromosomal mutation
duplication
deletion
inversion
translocation
3 points in meiosis where variation van arise
crossing over (prophase 1)
independent assortment (metaphase 1 and 2)
what is the phenotype of an organism influenced by
genes
environment
genetic factors affecting phenotype: MUTATIONS
what is a mutation
types
where do they occur
effects
mutations are random/spontaneous changes to the DNA
these can be point/substitution mutations which may be silent/missense/nonsense OR InDel mutations which lead to frameshift
usually mutations occur in somatic cells so are not inherited
mutations are neutral, beneficial or harmful
the chance of mutations occurring is increased by mutagens
mutations that occur during gamete formation (meiosis) are….
persistent (carried through many generations)
random (can influence selection of organisms)
what is non disjunction
type of chromosomal mutation
describe non disjunction
1 pair of chromosomes/ one of the sister chromatids fails to correctly separate, leaving 1 gamete with an extra chromosome
what are aneuploidy and polyploidy
types of chromosomal mutation
describe aneuploidy
diploid chromosome number is not an exact multiple of the haploid number so chromosomes fail to evenly separate
what is polyploidy
diploid gametes are fertilised by haploid/diploid gametes, increasing the variation in the phenotype
how does sexual reproduction lead to genetic variation within a species
crossing over during prophase 1 of meiosis
independent assortment of homologous chromosomes in metaphase 1 of meiosis
independent assortment of sister chromatids in metaphase 2 of meiosis
haploid gametes produced undergo random fertilisation with gametes derived from a random organism of the same species
describe crossing over in prophase 1
homologous chromosomes pair up/form bivalents
non-sister chromatids exchange DNA, creating new combinations of alleles
describe independent assortment in metaphase 1
homologous pairs randomly align/orientate at equator
creates new allele combinations
describe independent assortment in metaphase 2
sister chromatids randomly align/orientate at equator
creates new allele combinations
describe random fertilisation
any male gamete could fuse with any female gamete
any male organism could mate with any female organism
do environmental factors affect the phenotype or genotype
ONLY the phenotypes (not inherited)
examples of environment affecting phenotype
diet in animals
sunlight
lack of mineral ions
examples of environment affecting phenotype: diet in animals
overfed = becomes obese
underfed= becomes malnourished
examples of environment affecting phenotype: sunlight
humans use sunlight to produce additional melanin
plants turn yellow in dim light
examples of environment affecting phenotype: ;lack of mineral ions
plants turn yellow without magnesium
humans develop deficiencies e.g. scurvy without vitamin C
examples of variation caused solely by the environment
regional dialects
losing a limb or having a scar
examples of variation caused by the environment interacting w genes
- chlorotic plants
- human height
- E.coli (lac operon)
examples of variation caused by the environment interacting w genes: chlorotic plants
yellow in colour due to a lack of sunlight or minerals
examples of variation caused by the environment interacting w genes: human height
all humans are predisposed to a maximum height BUT factors like diet can influence this
examples of variation caused by the environment interacting w genes: E.coli
lac operon contains structural genes which can only be expressed hen there is insufficient glucose (environment)
describe continuous variation
can take any value within. range (has intermediate values)
is polygenic and significantly affected by environment
can be displayed on histogram or line graph
e.g. animal mass, skin colour
describe discontinuous variation
solely caused by genetic influences
e.g. blood group, sex
involves only discrete categories
presented on bar chart/ pie chart etc
what is monogenic (monohybrid) inheritance
single factor crosses where 1 gene controls 1 characteristic
what did Mendel conclude
features are inherited by offspring via gametes
each gamete contains 1 allele… alleles pair up at fertilisation
what is codominance
when both alleles of a gene in the genotype of a heterozygous individual contribute to the phenotype
neither allele completely dominates
examples of codominant inheritance
coat colour in shorthorn cattle (roan= red and white)
where can >2 alleles exist
more than 2 alleles can exist for a given gene but only 2 alleles can exist at a single locus at any one time ( one allele on each gene locus in a homologous pair)
what is multiple allele inheritance
a characteristic for which where are 3 or more alleles in the population’s gene pool
example of multiple alleles
human blood groups (inheritance of ABO)
A and B codominant and both dominant to O
what is sex determined by in humans
one of the 23 pairs of chromosomes (sex chromosomes)
what are the other 22 pairs (not sex chromosomes) called
autosomes
each autosomal pair is fully homologous (same length and contain same genes at same loci)
what are sex chromosomes in males
XY
what are sex chromosomes in females
XX
are X and Y chromosomes homologous
not fully homologous
what are sex-linked genes
characteristics determined by genes carried on the sex chromosomes
which of X and Y chromosomes are smaller
Y chromosome is much smaller than the X chromosome
what does smaller Y chromosome mean
there are a number of genes in the X chromosomes that males only have 1 copy of
this means that any characteristic caused by a recessive allele on the section of the X chromosome, which is missing in the Y chromosome, occurs more frequently in males
this is because many females will also have a dominant allele present in their cells
examples of sex-linked inheritance
haemophilia A
colourblindness
what is haemophilia A
inability of the blood to clot which leads to slow, persistent bleeding
what is haemophilia A caused by
mutation in the gene which codes for the production of Factor 8, a key blood clotting protein
the gene is located on the non-homologous part of the X chromosome (not on Y) and it is therefore sex-linked
why is colourblindness sex-linked
one of the genes involved in coding for a protein involved in colour vision is on the X chromosome, but not on the Y chromosome
a mutated allele of this gene may result in colourblindness: inability to distinguish between red and green
what are dihybrid crosses used to show
the inheritance of 2 different characteristics, caused by 2 genes, which may be located on different pairs of homologous chromosomes (UNLINKED)
each of these genes can have 2 or more alleles
what does the chi-squared test allow for
to compare the expected outcomes with the observed outcomes
it tests whether there is a significant difference between the 2
when is chi squared test used
when data is discrete/discontinuous
large sample size
we have a strong biological theory we use to predict the expected outcomes
data are raw counts (percentages and ratios cannot be used)
there are no ‘zero’ scores in the raw data
degrees of freedom chi squared
number of categories - 1
when are genes said to be linked
when they are carried on the same chromosome (ie same homologous pair)
what is a linkage group
all the genes carried on the same chromosome
these genes will be inherited together unless pieces of chromosomes break and rejoin onto a different chromosome (within the homologous pair) THIS OCCURS IN CROSSING OVER
when is crossing over and when does it occur
prophase 1
homologous chromosomes per up and form bivalents before crossing over
describe inheritance of autosomal linked genes with NO crossing over
if linked genes are NOT affected by crossing over of non-sister chromatids, then they are ALWAYS inherited as one unit
i.e. one homologous pair accommodates all 4 alleles so gametes have less variation
described inheritance of autosomal linked genes WITH crossing over
crossing over can occur at any point along a chromosome
if the 2 genes are close together, they are less likely to be separated, so the further apart the 2 genes are, the higher the change of recombinant gametes forming
explanation for presence of recombinant gametes
produced by crossing over during prophase 1
what is epistasis
the interaction of unlinked gene loci, where one gene masks the effect of the other
describe epistasis gene interaction
one gene (epistatic gene) affects/inhibits the effect of another locus (the hypostatic gene)
epistasis involves genes working either in a complementary or antagonistic fashion
when are genes assorted independently
metaphase 1 and 2
epistasis effect
reduces the number of possible phenotypes produced in F2 generation of dihybrid crosses, reducing genetic variation
is epistasis dominant or recessive
can be either
types of antagonistic epistasis
recessive epistasis
dominant epistasis
describe recessive epistasis
the homozygous presence of a recessive allele (epistatic) at the 1st locus prevents the expression of another allele (hypostatic) at a 2nd locus
example of recessive epistasis
inheritance of flower colour in Salvia (homozygous recessive aa epistatic to B/b locus)
dominant epistasis example
feather colour in chickens
I allele of epistatic gene I/I, prevents formation of colour at C/c locus
describe synergistic epistasis
2 genes act sequentially in a metabolic pathway
only If a dominant allele of the 1st gene is present will a suitable substrate be formed to be further acted on by a dominant allele of the second locus
genes are said to be complementary since both must be present to produce the particular phenotype
synergistic epistasis examples
flower colour in sweet peas
coat colour in mice
comb shape in domestic chickens
when is a characteristic said to vary continuously
if individuals show a range of phenotypes with a smooth gradation from one extreme to another
phenotypic characteristics vary in a quantitative way
examples of continuous variation
height
mass
length
is continuous variation polygenic or monogenic
elaborate additive effect
generally polygenic, controlled by >2 different (usually unlinked) genes
alleles of each gene may contribute a small amount to the phenotype, so alleles have an additive effect on the phenotype
greater number of gene loci means greater range of variation
when is a characteristic said to vary discontinuously
if it has limited a number of discrete forms which are non-overlapping
phenotypic characteristics vary in a qualitative way
are discontinuous characteristics monogenic or polygenic
therefore, significant??
generally monogenic (controlled by a single gene)
therefore different alleles of the gene have a large effect on the phenotype
different gene loci have quite different effects on the characteristic e.g. leaf shape
sometimes, alleles interact to mask one of them (epistasis)
factors affecting the evolution of a species
the different types of natural selection are:
stabilising selection
directional selection
disruptive/diversifying selection (rare)
describe stabilising selection
occurs in all populations when the environmental conditions remain relatively constant and it tends to eliminate the extremes within a group e.g. intermediate phenotypes are ‘fitter’ than extreme ones
examples of stabilising selection
beetle colour: medium-green beetles are better camouflaged on forest floor surrounded by medium-green leaves, so bell-shaped curve
birth weights in babies displays bell-shaped curve
describe directional selection
when environmental conditions change e.g. by becoming colder, so some organisms may have a selective advantage i.e. one extreme phenotype has greater ‘fitness’ than other normal phenotypes
example of directional selection
if beetle population moves into a new environment w darker soil and vegetation, darker-coloured beetles may be better hidden and survive better than medium-coloured or light-coloured beetles
what happens w directional selection over several generations
a gradual shift in the optimum value for a trait
directional selection can eb manipulated by what
breeders in artificial selection
describe disruptive/diversifying selection
the extremes are selected for and the norms are selected against
example of disruptive/diversifying selection
if a beetle population moved into a new environment with patches of light green moss and dark shrubs, both light and dark beetles may be hidden better
when does genetic drift occur
when a population descends from a small number of parents (see changes in allele frequencies)
what is genetic drift
the random change in allele frequency that occurs without natural secretion
(e.g. due to chance events such as earthquake or flood)
where are effects of genetic drift the greatest
on small populations where chance has a greater influence
in larger populations chance variation in allele frequency tends to even out across the whole population
in some instances, genetic drift may result in a complete elimination of an allele from a population in juts 1 or 2 generations, so could lead to extinction/speciation
2 types of genetic drift
can arise after a genetic bottleneck
founder effect
describe a genetic bottleneck
when a population shrinks and then increases again, it is said to have gone through a genetic bottleneck, leaving a small assortment of random survivors
all subsequent generations will be derived from a few survivors and the variety of alleles at each gene locus in the new reduced gene pool will be very small
what are genetic bottlenecks caused by
natural disasters
excessive hunting by humans
there may be loss of ……. after a genetic bottleneck
some advantageous alleles or a disproportionate frequency of harmful alleles
by chance after a genetic bottleneck, survivors may all have ……..
a particular advantageous allele e.g. resistance to a particular pathogen, so bottleneck may improve the gene pool at the same time as decreasing genetic diversity
when does founder effect occur
when a small population colonises a new area e.g. volcanic island
there is a random assortment of alleles in the new gene pool
due to founder effect, what happens to allele frequencies
they may be very different from the original population
certain alleles, by chance, may have been completely eliminated
extreme example of genetic drift
founder effect
example of founder effect H
huntingdon’s disease
caused by dominant allele for protein huntingtin
huntingtin accumulates in the brain and causes neuronal death
genetic disease are likely to be expressed more frequently within a smaller population
amish example of founder effect
amish people of N America descended from 200 Swiss Germans who settled in the 18th century
they rarely marry outside their religion and have a closed community
they have a high frequency of a rare genetic disorder called Ellis van Creveld syndrome (short stature, polydactyly, hole between atria of hearts)
do genetic bottlenecks and founder effect cause mutations?
NO
do not cause mutations or the emergence of harmful alleles, but do disproportionately change the frequency of harmful alleles
suggest how shrinkage of a population size may affect the reproductive rate of that population
too few females/imbalance of sexs so not all animals can mate, decreasing the number of offspring born
hard for individuals to find mates
v few males so interbreeding may increase the frequency of harmful alleles
why is maple syrup urine disease incidence higher among amish people
founder effect (pop founded by a small number of individuals)
within a small pop, if genetically related individuals interbreed, will carry harmful recessive alleles and probability of offspring inheriting the disorder increases significantly
what is the hardy Weinberg principle used to calulcate?
allele frequencies within a population
list factors that can affect allele frequencies (and hence genetic diversity within a gene pool)
population size
mutation rate
migration
natural selection (directional, stabilising, diversifying)
environmental changes (causing genetic bottlenecks)
founder effect
non-random mating (artificial selection)
gene flow between populations
what is speciation
if 2 populations of the same species become so genetically different that they can no longer interbreed and produce fertile offspring, they have undergone speciation and formed 2 new species
what does the hardy Weinberg principle assume
the population is large enough to make a sampling error negligible (large sample size)
mating within the population occurs at random
there is no selective advantage for any genotype and hence no selection
there is no mutation, migration or genetic drift
in the hardy Weinberg principle, what do p and q represent
frequency of dominant allele A is represented by the symbol p
frequency of recessive allele a is represented by the symbol q
hardy Weinberg principle equation from the fact that there are only 2 alleles in a population’s gene pool
p+q=1
probability of both sperm and egg containing dominant allele
p^2
probability of both sperm and egg containing recessive allele
q^2
probability that sperm and egg will each carry different alleles of a gene
2pq
hardy Weinberg principle longer equation
p^2 + 2pq + q^2 = 1
species definition
a group of organisms with similar anatomical, physiological and behavioural adaptations
members can interbreed to produce fertile offspring
speciation definition
the splitting of a genetically similar population into 2 or more distinct populations
each of these populations can undergo genetic differentiation which leads to evolution
what must happen for a species to evolve into 2 new species
it must be split into 2 isolated populations which cannot interbreed so there’s no gene flow
any mutations that occur in 1 population are not transmitted to the other
each population will have different selection pressures and so each population will accumulate different allele frequencies
what are subspecies/when are they formed
during speciation:
initially the 2 populations will be different but could still interbreed if not reproductively isolated: they are called subspecies
when can species be classified as separate during speciation
when there have been sufficient genetic, behavioural and physiological changes in the 2 populations so they can no longer interbreed to produce fertile offspring
list the 2 isolating mechanisms
geographical isolation
reproductive isoaltion
geographical isolation: what do barriers to gene flow between populations include?
rivers, mountains, roads, oceans
describe geographical isolation
barriers to gene flow e.g. oceans, mountains
as a result of natural selection, each population adapts to its environment
what is speciation resulting from geographical isolation called
allopatric speciation
what may lead to reproductive isolation of one population from another
biological and behavioural changes within a species
what is speciation resulting from reproductive isolation called
sympatric speciation
(same place)
example of genetic change leading to reproductive isolation and what does this lead to
e.g. mutations leading to a change in chromosome number
may lead to:
preventing gamete fusion (or meiosis cannot occur so gametes cannot form)
making zygotes less viable, so they fail to develop (Hox genes fail)
infertile hybrid offspring with an odd number of chromosomes, so that chromosome pairing during meiosis cannot occur
describe how behavioural changes as a form of reproductive isolation with examples
a mutation may lead to some organisms in a population changing their foraging behaviour (e.g. foraging at dawn/dusk to foraging during daytime/nighttime)
mutation may lead to changes in courtship behaviour e.g. songbirds in different towns may/may not be able to be heard above traffic
suggest how certain human activities may speed up the process of evolution among other species
habitat destruction, climate change, pollution levels, monoculture, migration
stages of artificial selection
human selects male/female animal/plant w/ the desired characteristic and allows them to reproduce
select offspring w/ best combination of characteristics and interbreed again
continued over many generations
does artificial or natural selection involve fewer generations
fewer generations involved in artifical selection compared to natural selection
describe marker-assisted selection
use a section of DNA as a marker to recognise a desired characteristic
often involves fluorescence and offspring containing the sequence can be selected at an early stage
what uses marker-assisted selection
artifical selection
what is important to preserve in artificial selection
rare varieties in case they are needed in the future (e.g. in seed banks as ex-situ)
example methods of artificial selection
artificial insemination
genetic engineering
cloning
general desired characteristics in artificial selection
high yield/faster growth rates
increased litter size
docility/ability to be trained
resistance to pests/pathogens
increased tolerance to surroundings e.g. weather
uniform plant size (easier to harvest)
specific examples of artificial selection desired characteristics
dairy cows which produce >40L of milk per day
salmon w a high growth rate (sustainable fishing practices)
describe inbreeding depression
at each stage of selective breeding, the individuals with the desirable characteristics and no/few undesirable characteristics are selected
this can lead to decreased genetic diversity
e.g. if related individuals are crossed , this can lead to inbreeding depression bc the number of homozygous recessive traits increases (homozygosity increases)
what is hybrid vigour
breeders sometimes outcross individuals belonging to 2 different varieties to promote heterozygosity (moves away from homozygosity)
explain the value of gene banks in helping breeders introduce hybrid bigger into a variety of crop plant
gene banks store genomes, but in their organism
act as a store of alleles to introduce back into artificially bred strains which increases hybrid vigour
e.g. Millennium Seed Bank, rare breed forms, sperm banks, botanic gardens/zoos
outline the ethical considerations surrounding the use of artificial selection
artifical selection limits the number of alleles in the gene pool
domesticated animals can retain juvenile characteristics e.g. docility, friendliness, good temperament but they would be less able to defend themselves so nervous prey
e.g. pigs bred to have more meat/less fat so a drop in temps means they are less resilient
(no longer able to survive in the wild bc traits desired by humans put the at a selective disadvantage)
outline the ethical considerations surrounding the use of artificial selection: DOGS
Kennel Club tries to ensure that dog breeders retain some element of heterozygosity in the gene pool
often inbreeding between related individuals can promote inbreeding depression
