Patterns of inheritance and variation Flashcards
What are alleles?
Different variations of the same genes
How are gene variants (alleles) produced?
DNA mutations
How does sexual reproduction produce new allele combinations in gametes? How does this establish phenotypic variation?
Crossing over
Independent assortment
Random fertilisation
Different alleles in a population code for different polypeptides
What is genotype?
Genetic composition of an organism, which describes all the alleles it contains. Genotypes for a particular locus can be heterozygous or homozygous
What is phenotype?
An organism’s observable characteristics
What is an example of disease which shows codominance?
Sickle cell anaemia (faulty version of haemoglobin molecule produced)
HAHA - normal haemoglobin production
HSHS - disease
HAHS - both versions of haemoglobin produced because alleles show codominance
When predicting a phenotypic ration of 9:3:3:1 for a dihybrid cross, what assumptions do you have to make about the two gene loci being studied?
Genes do not interact - no epistasis
Genes are not linked - not on the same chromosome
When are 2 genes said to be (autosomally) linked?
When they are located on the same autosome (a chromosome other than X or Y)
Why is autosomal linkage important?
Means linked allele combinations will be inherited together (as a single unit)
Only crossing over during meiosis can separate linked allele combinations
The nearer the 2 genes are on a chromosome, the less likely they are to be separated during crossing over
What are the two forms of epistasis?
Recessive and dominant
What is epistasis?
The effect of one gene on the expression of another
What is recessive epistasis?
The epistatic gene (the gene doing the suppressing) needs to be homozygous recessive to prevent the expression of the other gene - e.g. flower colour in Salvia
What is dominant epistasis?
The epistatic gene needs at least one dominant allele to prevent the expression of the other gene - e.g. fruit colour in summer squash
What is the chi-squared test used for when looking at phenotypes?
To assess whether there is a significant difference between the observed and expected number of offspring phenotypes - in order to see whether there is autosomal linkage or epistasis
What is the chi-squared equation?
x2 = E(O-E)2/E
How do you calculate degrees of freedom?
Number of phenotypes - 1
What does the p value tell us?
The probability of differences being a result of chance
Differences are statistically significant if P = 0.05 (only 5% probability that differences can be attributed to chance)
Suggest a likely molecular mechanism or recessive epistasis
epistatic gene produces enzyme
Homozygous recessive genotype results in no enzyme production
Precursor molecule is not converted
Suggest a likely mechanism for dominant epistasis
Epistatic gene produces inhibitor
modifies other gene product
affects transcription
What phenotypic ratio will recessive epistasis produce?
9:3:4
What phenotypic ratios is dominant epistasis likely to produce?
12:3:1 or 13:3
When does evolution occur?
When genetic mutations and natural selection result in .a change of allele frequencies in a population
How can allele frequencies be calculated?
Using the Hardy-Weinberg equations
What is stabilising selection and give an example?
When selection favours average phenotypes - alleles that produce extreme traits are eliminated. An example is mammals having a fur length close to the mean in an environment with a stable temperature. Individuals with short or long fur are less likely to survive and reproduce - these alleles are therefore eliminited
What shape is a normal distribution?
Bell-shaped curve
What is directional selection and give an example?
Occurs when environmental conditions change - selection favours individuals with extreme phenotypes. An example is mean fur length increasing if the mean temperature decreases, and decreasing if the mean temperature increasing - give higher survival rates so more likely to survive and reproduce. Normal phenotype no longer most favourable.
What is disruptive selection and give an example?
When the extremes are selected for and the norms are selected against. E.g. Darwin’s finches, rabbits with black, white or grey fur (black and white rocks only)
What is a genetic bottleneck?
A drastic reduction in population numbers (e.g. due to a natural disaster or environmental change) - proportions of alleles in surviving population could be very different to those in the original population
What is genetic drift?
A random change in allele frequencies - effects more noticeable in smaller populations - e.g. genetic bottleneck could lead to proportions of alleles in the surviving population being very different to those in the original population
What is the founder effect?
Occurs when a small group breaks away from the original large population to form a new colony (e.g. group of birds migrating to a new island) - can be considered a type of genetic bottleneck
What are the Hardy-Weinberg equations used for?
To calculate the proportions of alleles and genotypes in a population
What is the first Hardy-Weinberg equation and what does it mean?
p + q = 1.0
p is the frequency of the dominant allele, q is the frequency of the recessive allele. If only 2 alleles exist their frequencies must add up to 1.0 (100%)
What is the second Hardy-Weinberg equation and what does it represent?
p2 + 2pq + q2 = 1.0
Considers the frequencies of the possible genotypes in a population
p2 = the homozygous dominant genotype; 2pq = the heterozygous genotype, q2 = the homozygous recessive genotype.
The frequencies of the 3 possible genotypes must add up to 1.0 (100%) since no other genotypes are possible
What assumptions do the Hardy-Weinberg equations make?
No new mutations, no migration, no natural selection for or against alleles, a large population, random mating
- in reality these conditions rarely exist. Nonetheless provides basis for study of gene frequencies
What must occur for a new species to evolve?
Members of a population become isolated from the rest of the population, preventing gene flow between the two groups
Genetic mutations continue to occur in both groups (producing new alleles)
Different groups experience different selection pressures
Different alleles are selected in the two groups
Over generations, the two groups become genetically different and unable to reproduce fertile offspring together
What is allopatric speciation?
Members of a population separated by a physical barrier (geographical reproductive isolation) - e.g. migration to different islands, a mountain range, agricultural activity
What is sympatric speciation?
Speciation occurs within a population that shares the same habitat - rare and theory is controversial
What is an example of temporal reproductive isolation?
Differences in the timing of flowering
What is an example of behavioural reproductive isolatioin?
Different mating rituals or calls
What is an example of mechanical reproductive isolation?
Incompatible reproductive systems
What are prezygotic reproductive barriers?
Prevent fertilisation and formation of a zygote (gametes do not meet)
Geographical, ecological, temporal, behavioural, mechanical isolation
What are postzygotic reproductive barriers?
Reduce viability or reproductive potential of offspring (often as a result of hybridisation)
Hybrid inviability - offspring fail to live to maturity
Hybrid sterility - hybrid offspring are sterile
Give an example of sympatric speciation
Parasitic ants descended from fungus farming ants (change in behaviour at some point in time)
How can selective breeding create problems?
Requires inbreeding (breeding closely related individuals)
- genetic diversity is reduced
- inbred populations less able to adapt to changing environmental conditions
- Homozygous recessive disorders are more likely to occur
What is an important word to use in this topic (p.s. you’re a little bitch, little bitch)
Gene flow
Give an example of the founder effect
Afrikaner population in South Africa (descended mainly from a few dutch settlers) have abnormally high proportion of people with Huntington’s disease in their population - thought that just one of original settlers carried the disease causing allele
Amish in America (Ellis-van Creveld syndrome) - rarely have children outside their own religion and are therefore a closed community . - high proportion of polydactyl, abnormalities of nails and teeth, hole between upper chambers of the heart - caused by one couple who settled their in the 1700s
