Patterns of Inheritance Flashcards
Causes of phenotype variation
The appearance of a living organism (phenotype) is influenced both by its genotype (genetic makeup) and environment
What is a mutation
Change in the genetic material
-Structure of DNA
-Change to structure/ gross number of chromosomes
What is a mutagen
Certain physical/ chemical agents that cause mutations
Physical mutagen agents e.g.
-X-rays
-Gamma rays
-UV light
Chemical mutagen agents e.g.
-Benzopyrene (in tobacco smoke)
-Mustard gas
- Nitrous acid
Biological mutagen agents e.g.
- Some viruses
-Transposons - jumping genes, remnants of viral nucleic acid that have become incorporated into our genome
-Food contaminants i.e. mycotoxins from fungi
Mutations that occur during gamete formation
Can be advantageous/ neutral/ harmful
-Persistent: Can be transmitted through many generations without change
-Random: Not directed by a need on the part of the organism in which they occur
When do chromosome mutations occur
Meiosis
Types of chromosome mutations
-Deletion
-Inversion
-Translocation
-Duplication
-Non-disjunction
Deletion (Chromosome mutation)
Part of a chromosome (containing genes and regulatory sequences) is lost
Inversion (Chromosome mutation)
Section of the chromosome may break off and turn 180 degrees and then join again
=Although all genes are still present, some may now be too far away from their regulatory nucleotide sequence to be expressed
Translocation (Chromosome mutation)
A piece of one chromosome breaks off and attaches to another chromosome
-May also interfere with the regulation of genes on the translocated chromosome
Duplication (Chromosome mutation)
A piece of the chromosome may be duplicated
-Overexpression of genes can be harmful, as too many of certain proteins/ gene regulating nucleic acids, may disrupt metabolism
Non-disjunction (Chromosome mutation)
One pair of chromosomes/chromatids fails to separate, leaving one gamete with an extra chromosome
=When fertilized by a normal haploid gamete, the resulting zygote has one extra chromosome
What diseases are caused by Non-disjunction
Down syndrome / trisomy 21
Two types of Non-disjunction
-Aneuploidy
-Polyploidy
Aneuploidy
The chromosome number is not an exact number of the haploid number for that organism
-Sometimes chromosomes/chromatids fails to separate during meiosis
All in pairs but one set has an extra chromosome i.e. chromosome 21 has an extra chromosome which results in the disease trisomy
Polyploidy
Polyploidy- more than two copies of chromosomes
Diploid - meiosis error = instead of producing haploid egg and sperm it produces diploid egg and sperm
2n+ 2n = tetraploid plant
When diploid+ tetraploid fertilise
n+ 2n = 3n it will produce a triploid plant (non-viable and infertile)
Species
Interbreed; produce fertile offspring
How does sexual reproduction increase genetic variation
- Allele shuffling: P1
-Independent assortment: M1+A1/M2/A2
-Ransom fusing of gametes
Environmental factors that cause variation
-Speaking with a particular regional dialect
-A scar
Genetic factors that cause variation
-Blood group
-Tongue rolling
Genetic and Environmental factors that cause variation
-Skin colour
-Intelligence
-Sporting mass
-Body mass
-Height
How is variation caused by the environment interacting with genes
If plants are kept in dim light after germination/ soil contains insufficient nutrients then leaves do not develop enough chlorophyll
-Plant described as chlorotic + cannot photosynthesise
=Chlorotic plants have the genotype to make chlorophyll but environmental factors prevent them from expressing this gene
Dominant/ Recessive
Dominant: D
Recessive: d
Pure breeding
BB/ bb not Bb
TT
tt
Tt
Homozygous dominant
Homozygous recessive
Heterozygous
F1 generation
Parent phenotypes: Tall Short
Parent genotypes: TT tt
Gametes: T t
T
t = Tt
so F1 generation 100% chance of being tall
F2 generation
Parent phenotypes: Tall Short
Parent genotypes: Tt Tt
Gametes: T or t T or t
T t
T TT Tt
t Tt tt 3: 1 ratio and 75% chance of being tall
Monogenic
Determined by a single gene
The test cross
When you want to find the tall plants original genotype is tall/ short you need to cross-breed it with a plant with the recessive gene (the small plant)
Tall Plant = TT Tt
Short plant = tt
T T t t Tt t Tt tt
all tall 50% chance
Co-dominance
Where both alleles present in the genotype of a heterozygous individual are being expressed
Cow coat colour e.g. of co-dominance
C(R) - red cows
C(W) - white cows
Red White
C(R) C(R) C(W) C(W)
C(R) C(W) C(R) C(W) = F1 generation
Cow coat colour e.g. of co-dominance (F2 generation)
Roan Roan
C(R) C(W) C(R) C(W)
C(R) C(W) C(R) C(W)
Multiple alleles
Characteristic where there are three or more alleles in the population gene pool
Multiple alleles - human blood
3 Blood groups: A;B;O
- A/B dominant over O
Multiple alleles - human blood combinationS
I(B) I(B) = B
I(O) I(O) = O
I (A) I(B) = AB
I(B) I(O) = B
Multiple alleles - human blood e.g.
Female Male
I(B) I(O) I(A) I(O)
I(B) I(O) I(A) I(O)
Sex- linkage
Gene present on (one of) the sex hormones
Autosomes
The first 22 chromosomes
-Fully homologous
-Match for length and contain the same genes at the same loci
23rd chromosome onwards
Sex chromosomes
Female: XX
Males: XY
Male 23rd chromosome
Not fully homologous but a small part of one matches a small part of the other so that the chromosomes can pair before meiosis
Why do males often suffer from genetic diseases via sex hormones and females do not
If a female has an abnormal allele on her X chromosome she probably has a functioning gene on the other X chromosome
If a male inherits from his mother the X chromosome with the abnormality he will suffer from a genetic disease as he won’t have another functioning allele for that gene
Men and X linked genes
Haploid/ hemizygous
Haemophilia A
Unable to clot blood fast enough; injuries may cause an internal haemorrhage
- on the h = recessive allele
How is Haemophilia A caused
Female (carrier) Male
X(H) X(h) X(H) Y
X(H) X(h)
X(H) X(H) X(H) X(H) X(h)
Y X(H) Y X(h) y
Why can males not pass sex-linked diseases to their sons
Gene for the X-linked disease is nearly always on the X -chromosome and males pass their copy of the X chromosome to their daughters and pass their Y chromosomes to their sons
Can haemophilia males pass the disease to their grandsons
Yes. The X with the mutated allele that they pass to their daughters could then pass to their daughters sons
Female Haemophiliacs
Very rare and fatal
Genotype: x(h) x(h)
Phenotype: X(H) X(h) - mum
x(h) y - dad
Sex linkage in cats
C(O) = Orange
C(B) = Black
if it was X(CO) X (CB) = Tortoiseshell
-mainly only females as need 2x X chromosomes
How do females have the same amount of X-linked genes expressed as men and not 2x as many
In every female nucleus, one X chromosome is inactivated
-Determination of which pair will become inactivated is random and happens during early embryonic development
Dihybrid Inheritance
The different genes are inherited independently of each other. They are on different chromosomes
Dihybrid Inheritance e.g.
Yellow-rounded Green-wrinkled
YYRR yyrr
YR yr
YyRr = Yellow-rounded
Dihybrid Inheritance - what to remember about the pairing
Always pair the letters together
Yy Rr
What can you deduce from the results of dihybrid crosses
-The alleles of the two genes are inherited independently of each other, so each gamete has one allele for each gene locus
-During fertilisation, any one of an allele pair can combine with any other allele pair
Autosomal linkage
Gene loci present on the same autosome (non sex chromosomes) that are often inherited together
If linked genes are not affected by crossing over of non-sister chromatids in P1of meiosis
They are always inherited as one unit
Recombinant genes
Chromosomes affected by crossing over
What is epistasis
When one gene masks or suppresses the expression of another gene
How does epistasis reduce genetic variation
Reduces number of crosses in F2 generation
Recessive epistasis
The homozygous presence of a recessive allele at the first locus prevents the expression of another allele at another locus
First locus = epistatic to those at the second locus
Second locus = hypostatic to those at the first locus
Dominant epistasis
Dominant allele at the first locus masks anything at the second locus
Complimentary gene action
The genes working to code for two enzymes that work in succession, catalysing sequential steps of a metabolic pathway
Epistasis vs Dihybrid
Epistasis = one characteristic
Dihybrid = Two characteristics
Epistasis vs Dihybrid
Epistasis = one characteristic
Dihybrid = Two characteristics
Recessive epistasis ratio
9:3:4
Dominant epistasis ratio
12:3:1 / 13:3
Complimentary gene action ratio
9:7 / 9:3:4 / 9:3:3:1
What is the Chi-squared test
Statistical test designed to find out if the difference between observed and expected data is down to chance
What do you need to do before you carry out the Chi-squared test
State a null hypothesis
“There is no statically significant difference between the observed and expected data. Any difference is down to chance”
How do you work out the degrees of freedom
(number of categories -1)
How do you work out whether you should reject/accept the null hypothesis
Read critical values at 95% certainty
If (x)2 is smaller the value in the table we can accept the null hypothesis / if bigger we can reject
Discontinuous variation
Phenotype classes are distinct and discrete, each clearly discernible in a qualitative way’
-No/ few intermediaries
-Monogenic
Discontinuous variation examples
epistasis ; gender ; blood group
How is epistasis an example of Discontinuous variation
Genes at different gene loci may interact to influence one characteristic and produce discontinuous variation
Continuous variation
Where the genetic variation between individuals shows a range with a small gradation between many intermediaries
=Variation that produces phenotypic variation where the quantitative traits vary in small amounts from one group to the next
Continuous variation examples
Foot size; height ; leaf length; hair colour
Continuous variation key features
-Polygenic: many genes involved in determining characteristics
-Alleles have an additive effect: alleles of each gene may contribute a small amount to the phenotype
=As a result the phenotypic categories vary in a quantitative way
How is there more continuous variation
Greater no. of gene loci contributing to the determination of a characteristic
Qualitative vs quantitative
Qualitative research generates non-numerical data
Quantitative research generates numerical data or information that can be converted into numbers
Quantitative genetics
The study of genetics of inherited characteristics
i.e. many characteristics of crop plants are polygenic so plant breeders may need to apply knowledge of quantitative genetics
When does quantitative genetics become more complicated
When gene loci increases to above 2
=trihybrid cross
When does quantitative genetics become more complicated
When gene loci increases to above 2
=trihybrid cross
Does the environment have a greater affect on polygenic/ monogenic characteristics
Polygenic
=Each person has the genetic potential for intelligence/ height but without mental stimulation/ nutrition these potentials will nit be reached
Multiple alleles vs polygenic
Multiple alleles: versions of the same gene
Polygenic: Many gene loci
What can change allele frequencies over time
Natural selection and genetic drift
Natural selection
Individuals, which have mutations, which make them better suited to the environment are more likely to survive a selection pressure than the individuals without the mutation
-Individuals with mutation that survive will pass off advantageous alleles to offspring
What results from Natural selection
- Over time allele frequencies within a population will change
-Can maintain constancy of species or lead to a new species
Types of Natural selection
1) Stabilising
2) Directional
3) Disruptive
Stabilising selection
When an organisms environment remains unchanged it favours intermediate phenotypes
Directional selection
-Environment change may prefer a new phenotype and so results in a new population mean
-Individuals better suited to environment will survive under the selection pressure and pass on advantageous alleles to offspring
-Over several generations, there is a gradual shift in the optimum value for a trait
Disruptive selection
Favours the extreme phenotypes and the intermediate phenotypes are selected against
Genetic drift
A change in the allele frequency within a population over time
-This is caused by a chance event i.e. volcano/ earthquake
Two things that cause a genetic drift
1) Genetic bottleneck
2) The Founder effect
Genetic bottleneck
When an event such as a volcanic eruption greatly reduces the population and then over time the population increases again
Effects of the genetic bottleneck
-Reduces genetic diversity
-Can affect fertility rates if population gets too small
-Could also improve population if surviving population has advantageous alleles to disease etc.
Founder effect
A new population established by a small number of individuals, who originated from a larger parent population
Founder effect affects
-Reduces genetic variation
Important to remember for the Founder effect/ Bottlenecks
Do not cause mutations or the emergence of harmful alleles but they contribute to the increase of the frequency of mutations and harmful alleles within the resulting populations
What is the Hardy Weinberg principle
Describes and predicts a balanced equilibrium int he frequencies of alleles and genotypes within a resulting population
What does the Hardy Weinberg principle assume
1) The population is large enough to make a sampling error negligible
2) Mating within the population occurs at random
3) There is no selective advantage for any genotype and hence no selection
4) There is no mutation; migration; genetic drift
Hardy Weinberg principle calcualtion
P+q=1
P(2) + 2Pq + (q)2 = 1
Speciation
The process at which a new species is formed
-The two species can no longer interbreed
How does speciation occur
-Species must be split into two isolated populations
-This avoids the mutations in one population affecting the other
-In each location the different selection pressures will accumulate different allele frequencies
Sub-species
When during the evolutionary process the two populations are different but still able to freely interbreed
Two types of isolating mechanisms
-Geographical = allopatric speciation
-Reproductive - sympatric speciation
Geographical isolation
If population separated by geographical features i.e. mountains it acts as barrier to gene flow between the populations
How does Geographical isolation work
1) Isolated population undergoes different selection pressures in the two different environments
2) Both undergo independent changes to different allele frequencies or/and chromosome arrangements within their gene pools
3) Genetic mutations can be a result of: Mutations; Natural Selection; genetic drift
Allopatric isolation
Formation of two different species from an original species due to geographical isolation
Reproductive isolation
Biological/ behavioural changes within a species may lead to reproductive isolation of one population from another
Reproductive isolation e.g.
A change in the organisms foraging behaviour at the time of day may enable them to exploit a new niche and they’ll no longer mate with organisms awake at different times in the day
How do genetic changes lead to reproductive isolation
A change in chromosome no. may
- Prevent gamete fusion
-Make zygotes less viable = fail to develop
-Lead to infertile hybrid offspring with an odd no. of chromosomes, so that chromosome pairing during meiosis cannot occur
What happens when members of the reproductively isolated population no longer mate with the original population due to mutations
Leads to changes in:
-Courtship behaviour i.e. time of year of mating
-Animal genitalia / plant flower structure
Sympatric speciation
Formation of two different species from one original species, due to reproductive isolation, whilst populations inhibit the same geographical locations
