patterns of inheritance
Intraspecific variation
Within a species
Causes:
genetic- Mutation, Meiosis, Random fertilisation
Environmental- Diet, Climate, Exercise
Combination of both
Examples: blood group, Height
Interspecific variation
between different species
Basis: Each species has a different gene pool- Large genetic differences, Leads to adaptations
Examples: Broad leaves VS needlelike leaves
phenotype
Observable characteristics of an organism
genetic and environmental factors that affect phenotype
Controlled by alleles inherited from parents
e.g. Blood group, Flower colour
external influences that affect phenotype
e.g. Diet, Body mass, Light availability
continuous variation
Shows a full range of intermediate values between two extremes
Quantitative
Normal distribution (Bell shaped curve)
Controlled by many genes (Polygenic)
genes have Small additive effect
Strong influence from environment
E.g. height
discontinuous variation
Variation that falls into distinct categories with no intermediates
Qualitative
Bar chart
Controlled by a single gene (Monogenic)
No additive effect from genes
Little to know influence from the environment
E.g. blood groups
how does sexual reproduction lead to genetic variation?
Meiosis: Crossing over in prophase 1, Independent assortment in metaphase 1 + 2
Fertilisation: Random fusion of male and female gametes, Produces a diploid zygote with unique allele combinations, Offspring are genetically unique increasing variation
dominant
allele That always expresses
recessive
allele That is not expressed
Codominant
Both alleles are expressed
neither is dominant or recessive
1:2:1
heterozygous and homozygous
Hetero- alleles are different
homo- alleles are the same
monogenic inheritance (Single gene inheritance)
Involves one gene with two alleles
Can be dominant or recessive
Punnet Square
3:1 ratio
multiple alleles
some jeans have more than two alleles
E.g. ABO blood groups
Io is recessive (group O)
sex linkage
Any gene that is carried on the X or Y chromosome
Males only have one copy on the X chromosome
Recessive values on the X chromosome more likely to be expressed in males Because that is no dominant allele on the X
E.g. colour blindness
dihybrid inheritance
Inheritance of two different genes
9:3:3:1
F1 cross
Both parents are homozygous
One is homozygous dominant
One is homozygous recessive
All offspring are heterozygous
f1 generation
F2 cross
Both parents are heterozygous
All possible combinations of alleles
why is ratio 9:3:3:1
During meiosis alleles of different genes are inherited independently
Create genetic variation in gametes
autosomal linkage
Genes found on the same autosome and therefore inherited together, They don't assort independently
Will not be 9:3:3:1
The effect of crossing over in autosomal linkage
During prophase 1 of meiosis
If a crossover occurs between the linked genes, you get recombinant gametes Which usually appear in lower frequencies
epistasis
When one gene affects or masks the expression of another gene
Changes the expected ratio
Both genes are often on different loci
recessive Epistasis
The homozygous recessive masks the expression of another gene
9:3:4
dominant epistasis
The dominant allele masks the expression of another gene
12:3:1
chi squared
used to compare observed results with expected results
Measures the size of difference between observed unexpected
Helps us to determine whether the differences are significant or not
Used to test the null hypothesis
chi squared conclusion
if chisquared < critical = no significant difference, accept null
if chisquared > or equal to critical = significant difference, reject null
process of evolution by natural selection
Selection pressures
New alleles can arise by mutation
Genetic variation
Individuals best adapted to survive reproduce and pass on their beneficial alleles
types of selection
Directional- Individuals favoured in One Direction
Stabilising- Average individuals favoured
Disruptive- the 2 extreme phenotypes of favoured
stablising selection
favours the average phenotype
selects against extremes
occurs in stable environements
reduces genetic diversity
e.g. human birth weight
directional selection
favours one extreme phenotype
shifts the mean of the population in one direction
occurs whem environment changes
e.g. antibiotic resistance in bacteria
disruptive selection
extreme phenotypes are favoured
the average phenotype is selected against
produces a biomod distribution (two peaks in graph)
lead to increased genetic diversity
e.g. darwins finches
genetic drift
random changes in allele frequencies
stronger in small populations
can reduce genetic variation
genetic bottleneck
when population size is dramatically reduced e.g. natural disaster
few surviving individuals=reduced gene pool
leads to loss of rare alleles and less variation
cause different allele frequencies
founder effect
when a small group of individuals becomes isolated
new population has limited genetic variation
allele frequencies may differ from original population
hardy-weinberg principle
predicts allele and genotype frequencies will remain constant from one generation to the next, if no evolutionary forces act on the population
gene pool
allele frquency
all the alleles of all genes
the number of times an allele occurs within a gene pool
conditions for hardy weiberg principle
no mutations aeise
population is isolated
no selection
population is large
mating is random
hardy weiberg-
p^2 - homozygous dominant
2pq - heterozygous
q^2 homozygous recessive
allopatric speciation
when populations of the same species become seperated by a geological barrier, no gene pool
each population experiences different selection pressures
natural selection - advantagous alleles increase the frequency, beneficial alleles are passed on
become reproductively isolated + can no longer interbreed
sympatric speciation
speciation that occurs without geological separation
populations live in same area but become reproductively isolated due to different breedinf seasons, different courtship behaviours etc
genetic changes accumulate, populations become seperate species
artificial selection
humans choose individuals eith desirable traits to reproduce
process: identify individuals with desired characteristics, breed together, select offspring that show trait, repeat over generations
leads to increased frequency of alleles for desirable traits
e.g. high yield, disease resistance in cereals
higher milk yield in cows
advantages and disadvantages of artificial selection
improves yield, produces desirable traits
loss of genetic variation- reduced ability to adapt to environmental change, vulnerable to disease
ethical issues
unintended selection of rare disease genes
importance if maintaining genetic resources
needs to preserve wildlife + rare breeds as sources of alleles for future use
maintains a gene pool
conserved seed banks, gene banks, rare breed programmes