20 - Patterns of Inheritance and Variation Flashcards
How is closeness of two linked genes on a chromosome linked to the number of recombinant offspring?
More closely linked genes means less separation and so less recombinant offspring
What is a phenotype?
Physical characteristics of an organism
What is a genotype?
Genetic makeup of an organism
What 2 things affect an organism’s phenotype?
Genotype and environment
What is it called when a characteristic is codes for by multiple genes?
Polygenic
What type of characteristics tend to be polygenic?
Ones which vary within a range, such as height
What is continuous variation?
A characteristic that can take any value within a range
What is discontinuous variation?
A characteristic that can only result in certain values
What does it mean if something is monogenic?
Controlled by only 1 gene
What type of characteristics tend to be monogenic?
Ones which exhibit discontinuous variation
What are the 3 things which affect genotype?
- Sexual reproduction 2. Gene mutations 3. Chromosome mutations
What 2 characteristics do all mutations which take place during gamete formation have?
- Persistent (passed through many generations) 2. Random (so not directed by need)
When do chromosome mutations occur?
During meiosis
What happens in a deletion chromosome mutation?
Part of the chromosome, containing genes and regulatory sequences, is lost
What happens in an inversion chromosome mutation?
Sections of chromosome break off, turn around 180 degrees and rejoin
Why might genes not be expressed after an inversion chromosome mutation despite still being present?
They may be too far away from their regulatory nucleotide sequence
What happens in a translocation chromosome mutation?
A piece of chromosome breaks off and reattaches to another chromosome
What happens in a duplication chromosome mutation?
Part of the chromosome is duplicated, which can lead to harmful overexpression of certain genes
What type of mutation causes Downs syndrome?
Non-disjunction chromosome mutation
What happens in a non-disjunction chromosome mutation?
One pair of chromatids fails to separate, leaving one gamete with an extra chromosome
What are 3 ways in which sexual reproduction can cause genetic variation?
- Allele shuffling during crossing over 2. Independent assortment 3. Random fertilisation, where any male gamete can fertilise any female gamete
What is etiolation?
Where plants grow long and spindly due to a lack of light and they grow fast so they can reach the light
What is chlorosis?
The yellowing of leaves due to a lack of Magnesium ions (Mg2+) which means chlorophyll cannot be made
What are 3 examples of environmental factors which can affect phenotype?
- Diet in humans 2. Etiolation 3. Chlorosis
What is a dominant allele?
An allele which will always be expressed when present
What is a recessive allele?
One which will only be expressed when there are 2 copies present
What does it mean if alleles are codominant?
Both are expressed in the phenotype, which becomes sort of a hybrid of the traits coded for by both
What is a dihybrid?
A hybrid that is heterozygous for alleles of two different genes
How do you write the first and second generations when doing homozygous genetic crosses?
First is F1, second is F2
What do you call the parental generation when doing a homozygous genetic cross?
F1 generation
What is a gene locus?
Location of a gene on a chromosome
What are the 3 golden rules of monohybrid crosses?
- Homozygous dominant and homozygous recessive give 100% heterozygous offspring 2. Two heterozygous give a 3:1 ratio of phenotypes 3. Heterozygous and homozygous recessive give a 1:1 ratio of phenotypes
What is a test cross?
Crossing an organism with a heterozygous genotype with one with a homozygous recessive genotype
What would you use a test cross for?
Determining the genotype of an individual showing a dominant characteristic
What happens when multiple (i.e. more than 2) possible alleles are present for a gene?
Only 2 can be present in the offspring’s genotype
Which gamete determines the sex of the offspring?
Sperm, depending on whether it carries a Y allele or not
What does it mean if a gene is sex linked?
Located on one of the sex chromosomes
What is an autosome?
Any chromosome that is not a sex chromosome
What is autosomal linkage?
Gene loci present on the same autosome that are often inherited together
What are 3 examples of sex-linked medical conditions?
- Haemophilia 2. Colour blindness 3. Duchenne’s muscular dystrophy
What is dihybrid inheritance?
The inheritance of two characteristics which are controlled by different genes
What are 2 reasons the actual ratio of phenotypes may differ from the theoretical one?
- Fertilisation of gametes is a random process, so few chance events can lead to a skewed ratio 2. Genes being studied are on the same autosome
What is epistasis?
When one gene masks or suppresses the expression of another
What is an example of epistasis?
Baldness masking the widow’s peak gene
Why are sex-linked recessive conditions more likely to appear in men?
As there is no dominant allele on the X chromosome
What is the usual phenotypic ratio you would expect for a dihybrid cross?
9:3:3:1
What happens to the phenotypic ratio of a dihybrid cross if there is autosomal linkage?
It becomes more like a monogenic one (3:1)
What is the Chi Squared test used to measure?
The size of the difference between the observed and expected results, as well as whether these differences are significant
How do linked genes tend to be inherited?
Together, unless they are separated by chiasmata
What are recombinant offspring?
Recombinant offspring are those that have an arrangement of alleles on the chromosome that was not present in either parent
When are recombinant offspring produced?
When normally linked genes are split during meiosis
What is recombination frequency?
A measure of the amount of crossing over which has occurred during meiosis?
How is recombination frequency calculated?
Number of recombinant offspring/total number of offspring
What does a recombination frequency of 50% indicate?
That there is no linkage and that the genes are on separate chromosomes
What does a recombination frequency of less than 50% indicate?
Gene linkage
What can recombination frequencies be used for?
Mapping genes on a chromosome
When does dominant epistasis occur?
If a dominant allele results in a gene having an effect on another gene
What are the 2 forms of epistasis?
Dominant and recessive epistasis
When would recessive epistasis occur?
If the presence of 2 recessive alleles led to a lack of something
What is a gene pool?
The sum of all genes in a population
What is allele frequency?
The relative frequency of an allele in a population at any given time
What 2 equations make up the Hardy-Weinberg Principle?
- p + q = 1, 2. p² + 2pq + q² = 1
What 2 things is allele frequency not?
- Not linked to whether the allele is dominant or recessive 2. Not fixed
What does evolution involve in terms of alleles?
A long-term change in the allele frequencies within a population
What is p in the Hardy-Weinberg equation?
Frequency of dominant allele
What is q in the Hardy-Weinberg equation?
Frequency of recessive allele
What is p squared in the Hardy-Weinberg equation?
Frequency of homozygous dominant allele
What is 2pq in the Hardy-Weinberg equation?
Frequency of heterogeneous genotype in the population
What is q squared in the Hardy-Weinberg equation?
Frequency of homozygous recessive genotype in the population
What 5 assumptions underpin the Hardy-Weinberg principle?
- No selection 2. No mutation 3. No emigration/immigration 4. Random mating 5. Large population
What does the Hardy-Weinberg principle state?
That allele frequencies in a population should remain constant unless one or more factors cause those frequencies to change
Do populations tend to follow the Hardy-Weinberg principle in the real world? Why/why not?
No as the conditions/assumptions rarely tend to be met
What 2 things can the Hardy-Weinberg equations be used to calculate?
Allele and genotype frequencies
What is genetic drift?
Change in allele frequency between generations caused by random mutations (why we are not clones of parents)
smaller populations will be affected more by changes in allele frequency - leads to evolution happening faster
Why is a large population assumed for the Hardy-Weinberg principle?
Because genetic drift occurs much more in small populations
Upsetting the Hardy-Weinberg equilibrium may eventually lead to what?
Evolution
What are the 2 types of factors which affect population size?
- Density-dependent 2. Density-independent
Why are small populations more likely to become extinct?
They tend to have lower genetic diversity and so are worse at responding to change- for example, a pathogen could wipe out an entire population
What are density-dependent factors dependent on?
Population size
What are 4 examples of density-dependent factors?
- Parasitism 2. Predation 3. Communicable disease 4. Competition
What makes density-independent factors different from density-dependent factors?
They affect populations of all sizes equally
What are 4 examples of density-independent factors?
- Natural disasters 2. Human activities i.e. deforestation 3. Climate change 4. Seasonal change
What is a genetic bottleneck?
When a sudden environmental change drastically reduces the size of the population for at least a generation
What happens to genetic diversity when a genetic bottleneck occurs?
It is greatly reduced for a long period of time
What is a positive effect of a genetic bottleneck?
A beneficial mutation will have a much greater impact and lead to quicker development of a new species
What is the founder effect?
The reduced genetic diversity that results when a population is descended from a small number of colonizing ancestors. The new population can also be very genetically different from the one its founders split off from due to genetic drift and random presence of some alleles in the founders
What is the founder effect an extreme example of?
Genetic drift
What form will the distribution of different variants of characteristics take?
A bell curve (standard distribution)
What happens in stabilising selection?
Average individuals are favoured
What happens in directional selection?
Individuals are favoured in one direction
When does directional selection occur?
When there is a change in the environment and the most common phenotype is no longer the most advantageous, so organisms with more extreme phenotypes are favoured
What happens in disruptive selection?
The extremes are selected for and the average is selected against
What is the opposite of stabilising selection?
Disruptive selection
What is speciation?
The formation of new and distinct species in the course of evolution
What form of speciation is the most common?
Allopatric speciation
What is the sequence of events for speciation?
- Members of a population become isolated and no longer interbreed with the rest of the population, so no gene flow between the groups 2. Random mutation within the groups continues, but each may have different selection pressures 3. Eventually mutations and changes in allele frequency will mean that both populations are so different that they can no longer interbreed and produce fertile offspring
What happens in allopatric speciation?
A population is geographically isolated, and due to different environmental selection pressures speciation occurs
What happens in sympatric speciation?
Speciation occurs despite the two populations sharing the same habitat
What do prezygotic reproductive barriers do?
Prevent fertilisation and formation of a gamete
What often forms postzygotic reproductive barriers?
Hybridisation
What is the wild type allele?
Most common allele in a population
What are 3 examples of problems with artificial selection?
- Reduced gene pool, therefore more susceptible to diseases 2. Breeding to produce perfect offspring can cause problems for them eg: fat chicken makes a lot of meat but too heavy to carry their own body 3. Inbreeding exacerbates genetic conditions and problems
What is artificial selection?
The selective breeding of plants and animals by humans to produce desirable traits
What are 2 reasons artificial selection reducing genetic diversity is a bad thing?
- More susceptible to genetic diseases 2. Potentially useful alleles for the future are lost
What are 2 problems with artificial selection?
- Can cause health problems by exaggerating certain negative traits 2. Reduces genetic diversity
What is a species?
A population of genetically similar individuals whose members can interbreed and produce fertile offspring
What are 3 types of changes which can cause reproductive isolation, with one example for each?
- Seasonal changes i.e. different mating seasons 2. Mechanical changes i.e. changes in genitalia prevent successful mating 3. Behavioural changes i.e. different courtship rituals developing
What is inbreeding?
Mating between closely related individuals
What is outbreeding?
The breeding of distantly related individuals of a species
How can the problems of inbreeding be mitigated?
Using alleles from gene banks to do outbreeding
What 2 things does outbreeding do?
- Reduced incidence of homozygous recessives 2. Increases the potential to adapt to environmental change
What do seed banks do?
Keep seeds from both wild type and domesticated varieties of plants
What do gene banks do?
Similar to seed banks but keep biological samples other than seeds such as sperm or ova, usually frozen
What are both seed and gene banks?
An important genetic resource
typical ratio for monohybrid inheritance?
3:1
typical ratio for dominant epistasis?
12:3:1
typical ratio for recessive epistasis?
9:3:4
in terms of closeness of genes, when is crossing over more likely to occur?
when genes are further away, crossing over is more likely
if they are closer together it is more like monogenic inheritance so crossing over is less likely
what stats test can be used to tell us if there is a sig difference between the observed and expected phenotypic ratios?
chi squared
why are the majority of haemophilia sufferers male?
because if they inherit the recessive allele on their X chromosome, then due to the shorter Y chromosome they will not have the corresponding dominant allele
blood type AB is a universal…
recipient
blood type O is a universal…
donor
what is incomplete dominance?
when both dominant alleles are partially expressed (eg: red and white flowers make pink flowers)
what is codominance?
both alleles are simultaneously expressed (eg: blood groups)
what does homozygous mean?
a pair of homologous chromosomes carrying the same alleles for a single gene
what does heterozygous mean?
a pair of homologous chromosomes carrying two different alleles for a single gene
how to calculate the number of degrees of freedom for chi squared?
the number of categories - 1
for chi squared, what is usually the null hypothesis?
there is no significant difference between the observed and expected frequency of…
in chi squared, if the calculated value is less than the critical value, what does this mean?
there is more than 5% probability that the results are due to chance
SO the null hypothesis is accepted
what is gene pool?
all of the alleles of all the genes in a population
what is the allele frequency?
the number of a certain allele in a gene pool
