Patterns of inheritance Flashcards
what is phenotypic variation
difference in phenotypes between organisms of the same species
how does phenotypic variation occur
genetic + environmental factors
example of genes effecting phenotype
blood group
example of environment effecting phenotype
clones with different height due to environmental conditions
example of both genes + environment effecting phenotype
recessive allele that causes sickle cell anaemia has a high frequency in populations where malaria is prevalent due to heterozygous individuals being resistant to malaria
how can diet effect fruit flies
If the larvae of normal grey flies are given a diet of silver salts, they develop the yellow colour regardless of their genotype
flies that should be grey (according to their genes) can become yellow due to an environmental factor (their diet)
what is chlorosis
cells not producing normal amount of chlorophyll
how is chlorosis caused
normal genes – but environmental pressures
lack of light – plants turn off chlorophyll production – conserve resources
mineral deficiencies – lack of iron / magnesium
(iron – cofactor for enzymes that make chlorophyll)
(magnesium – centre of chlorophyll molecule)
virus infections – interfere with metabolism + cant support synthesis of chlorphyll
how is etiolation caused
Plants that are grown in the dark may also develop long stems with small, curled leaves even though, genetically, they should develop normally
when does crossing over occur
prophase 1
describe crossing over
homologous chromosomes pair up and are in very close proximity to each other
non-sister chromatids can cross over and get entangled
crossing points are called chiasmata
entanglement places stress on the DNA molecules
section of chromatid from one chromosome may break and re-join with the chromatid from the other chromosome
when does independent assortment occur
metaphase 1 + 2
what is independant assortment
different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle
how does independant assortment occur
Each pair can be arranged with either chromosome on top, this is completely random
The orientation of one homologous pair is independent / unaffected by the orientation of any other pair
what are the number of different combinations of cells resulting from independant assortment
2 to the power of n
n = number of chromosomes in haploid cell
summarise meoisis
draw a monohybrid cross for Gg
what is codominance
both alleles are dominant
how to represent codominance
one normal letter – two different superscripts
alleles for all blood groups
when is a dihybrid cross used
to show inheritance of 2 different characteristics – caused by 2 different genes
Could be on different pairs of homologous chromosomes
Could have multiple alleles
draw a dihybrid cross
draw a cross with sex linkage
HAVE TO SHOW ALLELE ON SEX CHROMOSOME ITS FOUND ON
why does sex linkage exist
Y chromosome much smaller then X chromosome
Number of genes on the X chromosome + aren’t matched on the same locus on Y
Y chromosome shorter
doesn’t have a homozygous pair
Only have 1 copy
Any characteristic caused by recessive allele on section of x chromosome (missing on the Y) occurs more frequently in males
Females – may also have dominant on other X
recombinant frequency
what is autosomal linkage caused by
Genes found on same chromosome / autosome
Inherited as one unit + no independent assortment
Recombinant offspring – different combinations of alleles than either parents
crossing over
what is epistasis
interaction of genes at different loci
example of epistasis
Lac operon
hypostatic vs epistatic
A gene affected by another gene – hypostatic
Gene that affects the expression of another gene – epistatic
what is recessive epistasis
where the epistatic allele (the allele that masks another gene) is recessive
two copies of epistatic alleles must be present for expression of hypostatic allele to be affected
what is dominant epistasis
epistatic allele is dominant
only one copy of the epistatic allele must be present for expression of the hypostatic allele to be affected
gene
length of DNA that codes for single polypeptide
alleles
genes existing in two or more different forms
locus
position of genes on a chromosome
ratio - dihybrid unlinked
9:3:3:1
ratio - monohybrid
3:1
ratio - dominant epistatic
12:3:1
difference between continuous + discontinuous variation
ratio - recessive epistatic
9:3:4
ratio - dihybrid linked
3:1
what is continuous
differences between individuals of a species where the differences are quantitative (measurable)
at genetic level - continuous
Different alleles at a single locus have a small effect on the phenotype
Different genes can have the same effect on the phenotype and these add together to have an additive effect
If a large number of genes have a combined effect on the phenotype they are known as polygenes
what is discontinuous
differences between individuals of a species where the differences are qualitative (categoric
at genetic level - discontinuous
Different genes have different effects on the phenotype
Different alleles at a single gene locus have a large effect on the phenotype
Factor 8 - heamophillia
what is a selection pressure
environmental factors that effect chances of survival
Factors that affect the size of a population
Density – dependant factors
= Dependant on population size
= Competition / predation / parasitism / communicable disease
Density-independent factors
= Effect populations of all sizes in same way
= Climate change / seasonal change / human activities – deforestation
Factors affecting evolution
mutation
gene flow
genetic drift
genetic bottleneck
founder effect
mutation
- Necessary for the existence of different alleles
gene flow
Movement of alleles between populations
Immigration / emigration result in changes in allele frequency
genetic drift
Chance affects which individuals in population survive / breed + pass on alleles
E.g – when population is small – chance can affect which alleles get passed onto the next generation
Large populations – less likely - any chance variations in allele frequencies usually even out across the whole population
genetic bottlenecking
Occurs when previously large population suffers a dramatic fall in numbers
Reduces genetic diversity = alleles are lost
surviving individuals end up breeding and reproducing with close relatives
genetic diversity greatly reduced
founder effect
extreme example of genetic drift
occurs when only a small number of individuals from a large parent population start a new population
made up of only a few individuals from the original population = only some of the total alleles from the parent population will be present
not all gene pool present
which alleles end up in new population = up to chance
what is stabilising selection
Natural selection that keeps allele frequencies relatively constant over time
Average individuals favoured
Very-low and very-high birth weights are selected against leading to the maintenance of the intermediate birth weights
what is directional selection
Natural selection that produces a gradual change in allele frequencies over several generations
Individuals favoured in one direction
Caused by change in environment / new allele that has appeared in population = advantageous
process of directional selection
Selection pressure that favours phenotype
Phenotype is produced by specific alleles
Individuals with favoured phenotype = fitter / more likely to pass on their alleles to offspring
Frequency of advantageous allele increases
what is disruptive selection
Extremes are selected for
graphs - all selection types
what is the hardy-weinberg principle used for
to calculate allele frequencies in populations
what does the Hardy-Weinberg principle state
– in a stable population with no disturbing factors, the allele frequencies will remain constant from one generation to the next and there will be no evolution
conditions for hardy-weinberg principle
Organisms are diploid
Organisms reproduce by sexual reproduction only
There is no overlap between generations, i.e. parents do not mate with offspring
Mating is random
The population is large
There is no migration, mutation, or selection
Allele frequencies are equal in both sexes
hardy weinberg equations
what is speciation
formation of new species through process of evolution
events leading to speciation
Members of population becomes isolated – no longer interbreed – no gene flow between 2 populations
Alleles within groups – random mutations – different selection pressures
Accumulation of mutations + changes in allele frequency – large difference in phenotype over time
what is allopatric speciation
occurs as a result of geographical isolation
most common type of speciation
describe allopatric speciation
occurs when populations of a species become separated from each other by geographical barriers
creates two populations of the same species who are reproductively separated from each other
no genetic exchange can occur between them
If there are sufficient selection pressures acting to change the gene pools (and allele frequencies) within both populations then eventually these populations will diverge and form separate species
changes in the alleles/genes of each population will affect the phenotypes present in both populations
what is sympatric speciation
takes place with no geographical barrier
A group of same species living in the same place but in order for speciation to take place = exists two populations within that group and no gene flow occurs between them
how can sympatric speciation occur
via:
Ecological separation
Behavioural separation
Polyploidy
what is ecological speciation
Populations are separated because they live in different environments within the same area
example of ecological separation
soil pH can differ greatly in different areas.
Soil pH has a major effect on plant growth and flowering, so a population growing in soil with a slightly different pH may flower at a different time from another population
leading to reproductive separation (and eventually genetic isolation) of the two populations
what is behavioural isolation
separated because they have different behaviours
differences in feeding, communication or social behaviours = courting behaviours to attract a mate
examples of behaviours that lead to sympatric speciation
what is polyploidy
Occurs when an organism has more than two sets of homologous chromosomes
example of polyploidy
When a newly-arisen tetraploid (4n) plant tries to breed with its ancestral species (a backcross), triploid offspring are formed. These are sterile because they cannot form gametes with a balanced assortment of chromosomes. However, the tetraploid plants can breed with each other. So in one generation, a new species has been formed.
process of artificial selection / selective breeding
- The population shows phenotypic variation - there are individuals with different phenotypes
- A breeder selects an individual with the desired phenotype
- Another individual with the desired phenotype is selected
- two selected individuals should not be closely related to each other
- The two selected individuals are bred together
- The offspring produced reach maturity and are then tested for the desirable trait. Those that display the desired phenotype to the greatest degree are selected for further breeding
- continues for many generations until all offspring display the desirable trait
example of selective breeding
farmers have selected female cows that have the highest milk yield and crossed them with male bulls related to high yield females
economical benefit to farmers
what is artificial selection / selective breeding
- process by which humans choose organisms with desirable traits and selectively breed them together to enhance the expression of these desirable traits over time and over many generations
importance of maintaining
a resource of genetic material for use in selective
breeding including wild types.
important to maintain a resource of genetic material that includes types that are close to the original wild type (of the organism you are selectively breeding)
This is important as it ensures that the gene pool for a particular species doesn’t become too small (which can weaken the population by reducing variation)
ethics of selective breeding
lead to inbreeding
results in a reduction in the gene pool
an increased chance of:
- Organisms inheriting harmful genetic defects
- as there is a higher chance of harmful recessive alleles combining in an individual and being expressed in the phenotype
- Organisms being vulnerable to new diseases
- as there is less chance of resistant alleles being present in the reduced gene pool
examples of selective breeding in plants
and animals
