Origins of genetic variation Flashcards
Sources of genetic variation
Independent assortment
Crossing over
Random fertilisation
Crossing over
When large multi enzyme complexes cut and join bits of the maternal and paternal chromatids together at the chiasmata
Results in new combinations of alleles
Random fertilisation
Particular combination of the two male and female gametes that ultimately fuse
Completely random
Independent assortment
Components of the maternal and paternal chromosome pairs are randomly distributed into gametes
Each gamete produced has a different mixture of maternal and paternal chromosomes
What are Mendel’s 2 laws of inheritance
Law of segregation - one allele is inherited from each parent
Law of independent assortment - different traits are inherited independently from each other
What is a test cross
A way for breeders to find out genotypes
Pedigree diagrams
Shows offspring in a human family over a long period of time
Why are fruit flies, pea plants, fungi and bacteria used in genetic experiments rather than humans
Easy and cheap to raise
Short life cycle so the results of crosses of mutations can be seen quickly
Produce large numbers of offspring
Clear defined characteristics
Codominance
Where a traits phenotypes can blend (both alleles are expressed)
In this case codominant alleles are written in superscript
Multiple alleles
Some genes have more than two alleles in a population although only 2 are expressed e.g. blood groups
Sex linkage
Genes on the X chromosome are said to be sex linked
Dihybrid inheritance
When two characteristics are determined by two different genes on different chromosomes
What’s a digenic cross
A breeding experiment involving the inheritance of two pairs of contrasting characteristics at the same time
Chi squared steps
Write null hypothesis - there is no statistically significant difference between expected and observed
Work out expected figures based on the ratio
Compare these with the observed figures and apply to equation
Work out degrees of freedom and find p=0.05
If higher, reject null, if lower, accept null
Write conclusion - reject null hypothesis, there is a significant difference, observed results do not support mendels hypothesis
What is Mendel’s expected ratio in chi squared
3:1
Autosomal linkage
When two or more genes being inherited are located on the same autosome ( non sex chromosome)
How to spot an exam question on autosomal linkage
It will state “the genes are on the same chromosomes”
Hardy Weinberg equation for allele frequency
P + Q = 1
Hardy Weinberg equation for genotype frequency
P2 + 2PQ + Q2 = 1
What do the letters in the Hardy Weinberg equations stand for
P = dominant
Q = recessive
PQ = heterozygous
What are the Hardy Weinberg 5 assumptions
Large population
Mating is random
No mutations occur
No immigration or emigration (isolated population)
No natural selection taking place
What are the 3 types of selection
Disruptive selection
Stabilising selection
Directional selection
Stabilising selection
Most common type of selection
Average survives
e.g.
short flowers die - no sunlight
tall flowers die - wind damage
medium flowers live - perfect conditions
Directional selection
One extreme trait is favoured
e.g.
short + medium neck giraffes died - couldn’t reach leaves on tree
long neck giraffes survived - leaves were reachable
