biology patterns of inheritance
Terms in this set (42)
more than two alleles of the same gene - leads to dominance hierarchy.
e.g rabbit coat colour, blood type.
Find the unknown genotype via a test cross with homozygous recessive.
where both alleles present in the genotype of a heterozygous individual contribute to an individuals phenotype.
e.g blood groups, cows, flowers.
how do blood groups show codominance?
the blood type AB is the phenotype caused by the presence of both allele A and B.
how do blood groups show multiple alleles?
the four phenotypes are determined by a range of alleles of a single gene on chromosome 9 that codes for isoagglutinogen found in RBCs.
involves the investigation of the simultaneous inheritance of two characteristics from two gene loci (unlinked).
e.g Mendel's peas, flies.
F2 phenotype ratio = 9:3:3:1
a fixed position on a chromosome
controlled by one gene.
F2 phenotype ratio = 3:1
at pair 23, sex chromosomes are not a true homologous pair.
'autosomal' = non-sex chromosomes
- The X chromosome contains genes determining many characteristics e.g metabolic function
- Most do not have a partner on the Y chromosome.
- Therefore, males are homozygous as they have only 1 X chromosome.
e.g haemophilia A, colour blindness, cat colour
NB: notation XⁿY
NB: include gender and carriers in phenotype description and ratio.
what is the relevance of 'inactivation of X chromosome' in females?
- alleles are codominant - both contribute to the phenotype.
- BUT, the orange allele is only expressed due to inactivation of the black allele, producing a tortoiseshell effect
- Therefore, male cats cannot be tortoiseshell because they only have 1 X chromosome.
'autosomal' = non-sex chromosomes
When two genes on the same AUTOSOME (1-22) are inherited together.
F2 phenotype ratio = 12:4 = 3:1
BUT due to independent assortment, and crossing over of non-sister chromatids this may be different.
NB: may be asked to draw recombinants
Sources of genetic variation:
2. meiosis + sexual reproduction
physical = x-rays; UV light
chemical = nitrous acid; aromatic amines
biological = some viruses; mycotoxins from fungi
a section of chromosome may break off and rejoin after turning 180⁰. Some genes may be too far from their regulatory nucleotide to function properly.
part of the chromosome breaks off and attaches to another
a piece of chromosome may be duplicated. Over-expression of a gene may be harmful and may affect metabolism.
too many/ too few chromosomes in one cell if they don't connect properly to spend fibres and separate.
e.g down syndrome
when gametes fertilise each other when one or both are diploid most cultivated plants form tetraploid zygotes
the chromosome number is not an exact multiple of the haploid, so incorrect chromosome number.
what is epistasis?
the interaction of non-linked gene loci, where one masks the expression of the other.
Epistatic gene = capable of making another
hypostatic gene = the one that is masked or blocked.
recessive antagonistic epistasis
homozygous presence of a recessive allele at the first locus prevents the expression of another allele at the second locus.
e.g petal colour (pigment)
F2 phenotype = 9:3:4
dominant antagonistic epistasis
if the epistatic allele is dominant, then having at least one copy of it will mask/block the expression of the other gene. They are working antagonistically.
F2 phenotype ratio = 12:3:1
If x² is higher than the critical value, you ________ the null hypothesis
complementary gene action
genes can work together to code for two enzymes that work in succession e.g catalysing sequential steps in metabolic pathways.
e.g colour in sweet peas
colourless pre cursor molecule → colourless intermediate → end product (purple pigment)
each step catalysed by a different enzyme.
F2 phenotype = 9:7 or 9:3:7
assumptions for complementary gene action with phenotype ratio 9:7
- two gene loci
- one must code for an enzyme which catalyses the production of a colourless intermediate from a colourless precursor
- the other codes for an enzyme that catalyses the production of a purple pigment from the intermediate.
can be placed into distinct categories/groups e.g blood type
caused by one gene
environment has no effect
data display in bar chart (bars mustn't touch!)
non-categorical data. variation where the measured values fall within a range e.g height
polygenic - effected by more than one gene, often alleles have an additive effect.
scatter graph or histogram data display
can be impacted by environment.
there is a change in species over time
natural selection is the...
... mechanism by which evolution occurs
1. organisms over produce offspring
2. but population size remains stable, there must be competition for survival
3. offspring are generally similar to their parents and features are passed through generations
4. variation exists, so some must be better suited to survival than others.
explain the process of natural selection
1. random mutations and migration introduce new alleles into the population
2. there will be variation in phenotypes caused by mutations
3. the environment will impose selection pressures
4. some individuals are better able to survive than others because they are better adapted
5. they will survive, reproduce and pass on their beneficial alleles to their offspring
6. over time, allele frequency of a population will change exp.
7. (type of natural selection: stabilising/disruptive/directional)
natural selection can maintain constancy in a species/ can causes development of new species
members of the same species living in the same place at the same time which can interbreed
population genetics = study of changes in allele frequencies over time
what are the assumptions of the hardy-weinberg principle
- no mutations
- no natural selection
- no genetic drift
- large population
- random mating
- only 2 alleles per gene
calculating allele frequencies
p + q = 1*
p = frequency of dominant allele
q = frequency recessive allele
*if it doesn't add up to 1, they are alleles for different genes
calculating genotype frequency
p² + 2pq + q² = 1
p² = homozygous dominant (AA)
p = frequency of dominant allele (A)
stabilising natural selection
the median is favoured the environment isn't changing.
intermediate phenotype is favoured which reduces the number of phenotypes.
e.g birth weight in humans
directional natural selection
if there is a change in the environment, individuals with the alleles for an extreme phenotype are more likely to be favoured
over generations there is a general shift in the mean.
e.g fur length in changing climate
disruptive natural selection
when extreme values are favoured
what is genetic drift and how is it different to natural selection?
this is when chance dictates which alleles are passed on, not the environment e.g natural disaster.
many individuals would die
alleles may be entirely removed from the population
when the population recovers it will have a lower population diversity than before.
can be bottle neck or founder effect
what is a species?
members of the same species can interbreed freely to produce fertile offspring (BIOLOGICAL)
a group of organisms which share a recent common ancestor (PHYLOGENY)
they have similar physiology, anatomy, behaviour.
the splitting of genetically similar population into two or more populations that undergo genetic differentiation and eventually reproductive isolation, leading to the evolution of two or more new species.
how does speciation occur?
SYMPATRIC ISOLATION: due to biological or behavioural isolation, whilst the population inhabits the same geographical environment
e.g frogs which reproduce at different times of the year so never breed together.
start with 1 species
geographical features prevent gene flow between isolated populations e.g rivers, mountains
individuals are reproductively isolated
differences in selection pressures produces different processes of natural selection.
A new species is formed due to geographical isolation.
having two copies of the same gene OF a gene
how does autosomal linkage affect the F2 genotype ratio
higher proportion of heterozygous offspring like parents