6.1.2 Patterns of inheritance Flashcards
Explain how an organisms characteristics are related to their genes
The genome of an organism contains their genes
Sexually reproducing organisms have two copies of each gene.
The combination of alleles they have for a particular gene is called their genotype
Different alleles and combinations of alleles result in different gene expression
This is results is particular characteristics of an organism called the phenotype
Summarise the variation that can occur between organisms
The variation in characteristics between different species is called interspecific variation
The variation in characteristics within a species is called intraspecific variation
Intraspecific variation can be caused by genetic factors and/or environmental factors
Intraspecific variation can be continuous or discontinuous
Use eye colour colour to explain how genetics contribute to intraspecific variation
Eye colour is determined by the amount of the pigment melanin in the cells of the iris
Melanin is produced by a metabolic pathway which involves many enzymes
Each enzyme is a result of the expression of particular genes
Depending on the genotype of an individual, the amount of melanin produced will be different
This results is people with different genotypes having different eye colour
Describe how the environment can affect phenotype
Environmental factors can also affect the characteristics of organisms
These may affect members of the populations differently
For example water availability and access to sunlight will affect the height of plants
Those with more access to water and light will grow taller
Environmentally affected variation cannot be passed onto offspring (inherited)
Lifestyle choices such as diet and exercise are environmental factors
Describe how chlorosis in plants is an example of environmental variation
A lack of mineral ion supply (from the soil) such as magnesium ions and iron ions
Cause a reduced production of chlorophyll
A magnesium ion is part of the chlorophyll molecule
Iron ions are cofactors for the enzymes that synthesise chlorophyll
Plants with poor access to mineral ions will have yellow leaves
Not as a result of their genotype, but due to their environment
Describe how etiolation in plants is an example of environmental variation
Plants deprived of light have abnormal production of auxin
This causes weak cell walls
And causes long spindly leaves and stems
As well as a lack of chlorophyll production (chlorosis)
This is caused by an environmental factor, not by the plant’s genotype
Summarise how sexual reproduction can lead to genetic variation in a species
Genetic variation is caused by the range and combinations of alleles
New alleles are produced as a result of mutation (which will result in proteins functioning differently, and new phenotypes)
Meiosis results in gametes with unique and different genetic compositions
And random fertilization produces a zygote or offspring with different genotype to both parents
Describe how events during meiosis increase genetic variation
Adult cells are diploid and have two alleles for each gene
Meiosis produces haploid cells which each have either one of those alleles for each gene
Independent assortment results in further increase in the genotypes of the gametes (2n combinations, where n = number of chromosome pairs)
Crossing over events also increase the possible gamete genotypes
Describe how fertilisation during sexual reproduction contributes to genetic variation in a species
Both parent organisms have their unique genotypes (and phenotypes)
However their gametes (sperm and egg) have only half their alleles
Also the gametes are genetically different from each other
Fertilisation is a random fusion of one sperm and one egg
The genotype of the zygote/offspring will be different from both parents
Describe the keys steps in using genetic diagrams to show patterns of inheritance
State the phenotypes of both parents
state/assign letters to show the genotype of both parents (lowercase, upper case and superscript)
Show the genotypes of all possible gametes for both parents
THEN use a Punnet square to show all the possible offspring phenotypes resulting from random fusion events
Write the proportions of the offspring genotypes (ratios, percentages)
Write the proportions of the offspring phenotypes (these will not always be the same as the genotype proportions)
Describe the features of a monohybrid cross
Monohybrid cross is the analysis of inheritance of one gene, and one characteristic
There will be one phenotype or characteristic being analysed
Each parent will have a genotype referring to one gene
Each genotype will have two letters indicating the alleles present on each of the homologous pair of chromosomes
Each gamete should only have one allele/letter
A 2x2 Punnet square is used
Describe the features of a homozygous cross
Homozygous crosses use individuals whose genotypes are known
They are called ‘pure’ or ‘true’ breeding because they are homozygous for their genes
One parent is homozygous recessive and the other is homozygous dominant
This always results in all the offspring being heterozygous
This offspring is called the F1 generation
Describe the features of a heterozygous cross
The F1 generation (two heterozygous individuals) can be crossed
The offspring are called the F2 generation
The ratio for the dominant phenotype to the recessive phenotype is 3:1
The ratio of offspring produced in the F1 and F2 generation can give information about the alleles
It can tell us which allele/phenotype is dominant and which is recessive
Describe the features of a dihybrid cross
Dihybrid crosses follow the inheritance of two different genes and characteristics
We begin with the assumption that both genes are on separate chromosomes
Each parent will have a phenotype describing two characteristics (hair colour, eye colour)
Each parent will have a genotype of four letters, two for each gene location, using different letters for different genes (e.g. EeHH)
Gametes must have one allele/letter for both genes, and must show all possible allele combinations (e.g. EH, eH)
Offspring should have four-letter genotypes, pairing together alleles of the same gene (EeHh)
Punnets squares will be larger to accommodate more variety in the gametes
Describe how you would use parental genotypes to determine gamete genotypes in a dihybrid cross
Describe how the presence of multiple alleles affects the inheritance of phenotypes
Multiple alleles is the idea that the population (gene pool) can have more than two alleles for a given gene
And that an individual can have any two of those alleles
This results in more genetic variation
There are more possible genotypes
And more possible phenotypes
Describe how codominance affects inheritance of phenotypes
Occurs when a gene has more than one dominant allele
both alleles contribute to the overall phenotype
So as well as the recessive phenotype, there may be more than one dominant phenotype
Codominant alleles are represented by capital letters
With different alleles represented by superscript letters
For example FB and FW could be dominant alleles for fur colour
Explain what sex linked genes are
The 23rd pair of chromosomes are the sex chromosomes (XX or XY)
They are non homologous (they don’t have identical genes)
Alleles on the X don’t have corresponding alleles on the Y and vice-versa
The Y chromosome contains genes that make the organism a male.
Genes for any other characteristics that are on these chromosomes are called sex-linked genes
The non-sex chromosomes are called autosomal
Describe the features of the inheritance of sex-linked phenotypes
Females have two possible alleles for the genotype, whereas males only have one
In genetic diagrams, the genotypes show the sex chromosomes involved as well as the alleles for the phenotype (XAXa)
The Y chromosome of males is assumed not to carry corresponding genes of X chromosomes
In females the phenotype as a result of both alleles is considered
In males, only the allele on the X chromosome affects the phenotype
When reporting the offspring phenotypes, state the sex of the offspring as well as their phenotype
Describe epistasis and its effect on the inheritance of phenotype
A gene at a second, different chromosomal location
Can be expressed that affects the phenotype controlled by the first gene
This is called epistasis
For example two different genes involved in the same metabolic pathway
Or one gene being a transcription factor for another gene
State the four stages of carrying out a chi-squared test
Making a null hypothesis: stating that the differences between observed and expected are due to chance
Calculating a test statistic based on the actual results/observations. You may need to calculate expected numbers based on phenotype ratios.
Calculate degrees of freedom (no. of categories - 1) to determine the critical value at 5% significance. Compare the test statistic to the critical value to determine the likelihood of the result occurring by chance (less or more than 5%).
Make a conclusion, that (a) accepts or rejects null hypothesis and (b) refers to what that means in terms of the question scenario
Explain the genetic influence on discontinuous variation
Discontinuous variation has only a few very distinct phenotypes (eg blood group)
This is because such characteristics are controlled by single or just a few genes
There are not that many different allele combinations possible
Therefore there are not that many different phenotypes produced
However, the possible phenotypes can be very distinct, with no intermediates
(there is little environmental impact)
Explain the genetic influence on continuous variation
Continuous variation has many possible phenotypes, each slightly different from the next (eg height, weight)
This is because multiple genes are affecting the phenotype
There are so many unique allele combinations possible
That this causes many slightly different phenotypes
(because the environment can also influence, this causes even more variation)
Review the key steps in the process of evolution
Evolution occurs by natural selection
There must (genetic) variation in a population
A selection pressure causes a struggle to survive (not all the population will survive)
Certain genetic variations result in some members of the population having a better chance of survival and reproduction than others
Over time, the alleles for these advantageous characteristics (adaptations) become more common in the population (and the population becomes better adapted to survive the selection pressure)
List the factors that can affect the evolution of a species
Anything that causes changes in allele frequency is affecting evolution
Stabilising selection
Directional selection
Genetic drift
Genetic bottleneck
Founder effect
Describe stabilising selection
Stabilising selection is when the selection pressure (environment) stays the same
The most common characteristics (alleles) are still the most advantageous
These become even more common
The extreme phenotypes are selected against
Those alleles become less common
Describe directional selection
When the environment changes
One of the extreme phenotypes may now result in the best chance of survival
The alleles for the extreme phenotype become more common
While alleles for the other phenotypes become less common
Describe genetic drift
Genetic drift is simply a change in frequency of alleles
Due to random, chance events (not selection pressure or adaptation)
This may cause allele frequency to decrease, and even disappear entirely
Or others to increase
Genetic drift is strongest in small populations
Describe genetic bottleneck
A large reduction in population size for at least one generation
This causes a lot of alleles to be lost (randomly)
The surviving members reproduce and rebuild the population
But with significantly lower genetic diversity
Mutation may result in creation of new alleles but this will take time
(and environmental changes may take place faster)
Describe founder effect
Occurs when a small number of individuals from an original population become reproductively separated
And continue to reproduce in isolation
Isolated populations have smaller gene pools (many of the alleles in the original population are not present in the isolated population)
Less genetic variation
Alleles with low frequency may become high frequency in the new population
Explain the significance of calculating allele frequencies
Estimate allele and genotype frequencies in population
This can tell us about the genetic variation in a population
We can estimate how many people in a population may have genetic disorder
Observe changes in allele frequencies
Changes in allele frequency indicate evolution
Recall the assumptions required for a Hardy-Weinberg population
mutation is not occurring
natural selection is not occurring
the population is infinitely large
all members of the population breed
all mating is totally random
everyone produces the same number of offspring
there is no migration in or out of the population
Recall the two parts of the Hardy-Weinberg equation (these will be given to you in an exam)
p2 +2pq+q2 =1
p+q=1
Recall what p, q and the elements of the Hardy-Weinberg represent
p: frequency of dominant allele
q: frequency of recessive allele
p2: frequency of homozygous dominant individuals
2pq: frequency of heterozygous individuals
q2: frequency of homozygous recessive individuals
Describe the process of speciation generally
A subpopulation becomes isolated so that it does not interbreed with the rest of the population
The two populations continue to evolve due to natural selection, accumulating changes in alleles and allele frequencies
Because they do not interbreed (no gene flow) genetic differences begin to add up
Eventually they may no longer be genetically compatible enough to interbreed successfully
They are now two different species
Describe allopatric speciation
Geographical barrier causes separation, and thus, reproductive isolation
The two geographic areas will have different climatic conditions, or other biotic/abiotic factors, therefore there are different selection pressures
Different characteristics will be advantageous in the two areas
Different changes in alleles/allele frequency will occur
There is no gene flow so the gene pools of the two groups become more and more different
Until they cannot (could not) produce fertile offspring
Describe sympatric speciation
A subpopulation may become reproductively isolated
(due to disruptive selection)
They have the same selection pressures
But due to no gene flow (reproduction), they accumulate different adaptations
These differences add up over time, and eventually the two populations become too different to produce fertile offspring
Outline the principle of artificial selection
Is the application of selection pressure
That is not natural (biotic/abiotic)
But rather, for characteristics that are desirable for human interests
Describe the process of selective breeding
Select individuals with desirable characteristics
These are bred, and offspring produced
Select offspring that have the desired characteristics and continue over many generations (inbreeding)
This will increase allele frequencies for desired characteristics in the population
And potentially result in speciation
Describe how genetic variety is maintained in selectively bred plants and animals
Seed banks are stores of wild and domesticated varieties of plants
Gene banks are frozen stores of sperm and eggs (animals)
They are used to preserve genetic variations (individuals with certain alleles) that may otherwise get lost due to artificial selection, genetic drift or some catastrophic event
Outbreeding with these varieties reintroduces genetic variation back into populations
Making populations more heterozygous (less homozygous)
State the ethical arguments in favour of the use of selective breeding
Improved quality of life for humans (pets as emotional support, or helping blind people to travel)
Animals that provide high yields lowering prices for consumers
Plants that are the same size can be harvested through automation (lowering cost of production, reducing requirement for manual labour)
The quality (taste/appearance) of food products can be improved
State the ethical arguments against the use of selective breeding
Inbreeding may cause loss of certain alleles, a reduction in genetic variation called inbreeding depression
Highly related individuals are more likely to have the same alleles and more likely to be homozygous. Greater chance of recessive genetic disorders (suffering of the animal)
