M6, C20 Patterns of Inheritance and Variation Flashcards
define genotype
genetic make-up of an organism
define phenotype
observable characteristics of an organism
define dominant allele
version of the gene that will always be expressed if present
define recessive allele
version of a gene that will only be expressed if 2 copies of this allele are present in the organism
define homozygous
2 identical alleles for a characteristic
define heterozygous
2 different alleles for a characteristic
Describe continuous variation
give an example
continuous variation - individuals in a population vary within a range - there are no distinct categories
eg. height
describe discontinuous variation
give an example
discontinuous variation - when there are to or more distinct categories - each individual falls into only one of these categories
eg. blood group
define codominant
alleles that are both expressed in the phenotype - neither one is recessive
define locus
the fixed position of a gene on a chromosome
what are punnett squares used for
show a genetic diagram
predict genotypes and phenotypes of offspring
define sex linkage
genes when its locus is one of the sex chromosomes, X or Y
determine the possible genotypes of an offspring whose mother is a carrier of haemophilia but the father is unaffected
N is normal
n is faulty
carrier mother’s genotypes: X^N and X^n
unaffected father’s genotypes: X^N and Y
from punnett square the results show the 4 possibilities as: X^NX^N - 25% unaffected female X^NY - 25% unaffected male X^nX^N - carrier female X^nY - affected male
what are the chromosomes of females and males in mammals
what does this mean about inheritance of diseases which are transmitted to offspring during sex linkage
females - 2 X chromosomes
males - 1 X chromosome, 1 Y chromosome
the Y chromosome is smaller than the X chromosome and carries fewer genes. so most genes on the sex chromosomes are only carried on the X chromosome.
this means males often have one allele for sex-linked genes meaning they express the characteristic even if it’s recessive. they can’t be a carrier
determine the possible genotypes for a cow offspring whose parents have genotypes RR and WW
R is red
W is white
what is this an example of?
punnett square shows the possible gneotypes are:
RW, RW, RW, RW
100% red/white cow
example of codominance
define monohybrid inheritance
inheritance of a single characteristic controlled by different alleles
define dihybrid inheritance
inheritance of 2 different characteristics caused by 2 genes
what would you get if you cross two organisms each with 2 homozygous alleles
the offspring would be heterozygous for both alleles
One parent has the genotype of NNGG and the other has the genotype of nngg.
what would the genotype be for the F1 offspring
F1 offspring means first generation
two homozygous parents makes a heterozygous offspring so
NnGg
When you cross two heterozygous individuals what ratio would you expect to find for the phenotypes of the offspring
four possible phenotypes
9:3:3:1
If you cross a heterozygous organism and a homozygous recessive organism, what ratio would you expect for the phenotypes of the offspring
four possible phenotypes
1:1:1:1
two alleles show complete dominance and are located on separate chromosomes
In pea plants the dominant allele for seed shape is round shown by R and the dominant allele for seed colour is green shown by G.
The recessive alleles are r for wrinkled and g for yellow.
A round, yellow seed is crossed with a green, wrinkled seed.
Produce a full genetic cross diagram to show the ratio of 2nd generation offspring.
Two homozygous parents make a heterozygous offspring
So 1st generation offspring = RrGg
On each side of genetic cross diagram there should be the gametes of RG, Rg, rG and rg
You get 16 results of:
RRGG, RRGg, RrGG, RrGg, RRGg, RRgg, RrGg, Rrgg, RrGG, RrGg, rrGG, rrGg, RrGg, Grgg, rrGg, rrgg
Looking at the dominant and recessive alleles, this makes the ratio of 9:3:3:1 of the phenotypes: 9 round and yellow pea plants 3 round and green 3 wrinkled and yellow 1 wrinkled and green
How would you split a parent with the genotypes RrHh into gametes
R with H
R with h
r with H
r with h
SO: RH, Rh, rH and rh
(the alleles come in pairs so Rr and Hh CAN’T go together)
define autosomal linkage
when the genes that are linked are found on one of the other pair of chromosomes
how do recombinants form
hint: meiosis
- When the chromosomes pair up in prophase 1 of meiosis, crossing over occurs between the non sister chromatids, resulting in an exchange of the genes
- Some of the linked genes are separated
- They will have a different combination than either of the parents, so are called recombinants
- The closer the genes are on the chromosome, the less likely they are to separate
what is recombination frequency
the measure of the amount of crossing over in meiosis
what is the equation for recombination frequency
what do the results show
number of recombinant offspring / total number of offspring
50% or more = no linkage (genes are on separate chromosomes)
Less than 50% = there is gene linkage (random process of independent assortment has been hindered)
what is the chi-squared test
tests whether the difference between the observed and expected results are due to chance or whether there is a significant difference and so your experiment or prediction you made must be wrong
χ^2 = Σ(O-E)^2 / E
This is the chi-squared formula. What do all the parts stand for?
χ^2 = chi-squared O = observed results E = expected results
once you have a result for the chi-squared test what do you do?
Work out the degrees of freedom by taking away 1 from the number of categories
Then use the table. Look for the degree of freedom and 5% (0.05)
what does it mean if the chi-squared value is less than the critical value?
there is no significant difference between observed and expected results
more likely to be due to chance
accept null hypothesis
what does it mean if the chi-squared value is more than the critical value?
there is a significant difference between expected and observed results
reject null hypothesis
some other factor is causing the difference between observed and expected results
what are the columns you need to have when completing a chi-squared test
Phenotype Ratio E (expected result) O (observed result) O-E (O-E)^2 (O-E)^2 / E
define epistasis
the interaction of genes at different loci so that one gene locus masks or supresses the expression of another gene locus
What is recessive epistasis?
Homozygous presence of a recessive allele prevents the expression of another allele at a 2nd locus.
So if there is any recessive homozygous set of alleles (eg. aa), the other gene can’t be expressed.
What will the ratio of phenotypes be if you cross two purple flowers that are AaBb?
aa means the flower will be white due to recessive epistasis
when you do the 16 punnet square you get the genotypes of:
AABB, AABb, AaBB, AaBb, AABb, AAbb, AaBb, Aabb, AaBB, AaBb, aaBB, aaBb, AaBb, Aabb, aaBb, aabb
All the ones with aa will be white. All the ones with a dominant A and bb will be pink. All the ones with a dominant A and Bb or BB will be purple.
The ratio of purple:pink:white is 9:3:4
What is the ratio of phenotypes you will get when you cross 2 heterozygous genotypes in recessive epistasis?
9:3:4
What is dominant epistasis?
Dominant allele at one gene locus masks the expression of the alleles at a 2nd gene locus.
So if there is any dominant gene in the set of alleles (eg. Dd or DD) then the other gene can’t be expressed..
What will the ratio of phenotypes be if you cross 2 white flowers (DdEe) in dominant epistasis?
E = yellow flowers
e = green flowers
when you do the 16 punnet square you get the genotypes of:
DDEE, DDEe, DdEE, DdEe, DDEe, DDee, DdEe, Ddee, DdEE, DdEe, ddEE, ddEe, DdEe, Ddee, ddEe, ddee
All the ones with any dominant allele of D mean no colours can be expressed. (Because of the dominant epistasis).
All the ones with dd mean that a colour can be expressed so if the E is present it will be yellow but if it’s ee it will be green.
The ratio of white:yellow:green is 12:3:1
What are the possible ratios of phenotypes you can get when you cross 2 heterozygous genotypes in dominant epistasis?
12:3:1
OR
13:3
Describe the process of evolution by natural selection
- Individuals within a population vary due to different alleles.
- New alleles created due to mutations in their genes.
- Selection pressures like predation, disease or competition create a struggle for survival.
- The individuals that are most suited to the environment (have an allele that increases chance of survival) survive, reproduce and pass on the advantageous allele.
- A greater proportion of the next generation inherit the advantageous allele.
- They in turn are more likely to survive, reproduce and pass on their genes.
- The frequency of advantageous allele increases.
define gene pool
the complete range of alleles present in the population
define population
group of organisms of the same species living in a particular area
What factors affect evolution? (5)
- mutation - forming new alleles
- sexual selection - increase in alleles which code for characteristics that improve mating success
- gene flow - immigration/emigration
- genetic drift in small populations - change in allele frequency due to random mutations
- natural selection - increase in individuals that have characteristics that improve chances of survival
describe the process of genetic drift
- individuals within a population show variation
- by chance, the allele for one genotype is passed on more often than others
- so the number of individuals with that allele increases
- by chance, the same allele is passed on more often again and again
- leads to evolution as allele becomes more common in the population
describe the process of genetic bottleneck
- an ecological event could dramatically reduce population size
- small, surviving populations are unlikely to be representative of the original population
- by chance, alleles may be overrepresented among survivors, some may be eliminated completely
describe the process of the founder effect
- individuals within a population show variation
- some individuals start a new population. by chance, these individuals mostly have one genotype or could be missing some genes completely
- without any further gene flow, the new population will grow with reduced variation
what are the similarities between the founder effect and genetic bottleneck
- Both are followed by genetic drift which results in changes in allele frequencies
- Initially genetic diversity is lost in both systems
- Both involve a small number of individuals breeding with each other, both may involve inbreeding among close relatives
- Both may result in a new population which carries alleles that are unlikely to be a true representation of the original group
what are the differences between the founder effect and genetic bottleneck
In bottlenecks individuals are killed, reducing the choice of mates, in the founder effect individuals are ecologically separated
what is the Hardy-Weinberg principle
predicts that the frequency of alleles in a population won’t change from one generation to the next
This is only true under certain conditions:
- Has to be a large population
- Random mating
- No immigration or emigration
- No mutations or natural selection
If you know frequency of one allele, how can you figure out the frequency of the other allele
p + q = 1
p = frequency of dominant allele
q = frequency of recessive allele
Hardy-Weinberg’s principle
p^2 + 2pq + q^2 = 1
what do all the parts mean
p^2 = frequency of homozygous dominant (eg. BB) q^2 = frequency of homozygous recessive (eg. bb) 2pq = frequency of heterozygotes (eg. Bb)
(p = B and q = b)
Cleft chins are controlled by a single gene with 2 alleles. The allele for cleft chin (C) is dominant over non-cleft chin (c). In a particular population, the frequency of the homozygous dominant for cleft chin is 0.14.
What is the frequency of the recessive allele in the population?
What is the frequency of the homozygous recessive genotype?
What do you want = frequency of the recessive allele (q = c) and frequency of the homozygous recessive (q^2 = cc) What do you have = frequency of the homozygous dominant p^2 = 0.14 So, p^2 = 0.14 p = √0.14 p = 0.37 q = 1 – 0.37 q = 0.63 q^2 = cc q^2 = 0.632 q^2 = 0.39
so freq of recessive allele = 0.63 and freq of homozygous recessive genotype = 0.39
define selection pressure
An environmental factor that gives an organism a greater chance of survival and reproduction
how does the normal distribution curve change when there is no/hardly any change to the environment
why?
Individuals with alleles for characteristics toward the middle of the range are more likely to survive and reproduce
This is called stabilising selection
It reduces the range of possible phenotypes
how does the normal distribution curve change when there are changes to the environment
Individuals with alleles for characteristics of an extreme type are more likely to survive and reproduce
This is called directional selection
Lead to an evolutionary change, therefore said to be an evolutionary force of natural selection
define stabilising selection
A type of natural selection in which the allele and genotype frequency within populations stays the same because organisms are already well adapted to the environment.
define directional selection
a type of natural selection in which an extreme phenotype is favoured over other phenotypes, causing the allele frequency to shift over time in the direction of that phenotype.
how does the normal distribution curve change when there is disruptive/diversifying selection
It looks almost like two separate bell curves. There are peaks at both extremes, and a very deep valley in the middle.
what is disruptive/diversifying selection
A rare type of natural selection that selects against the average individual in a population.
The make up of this type of population would show phenotypes of both extremes, but have very few individuals in the middle.
can lead to speciation which forms 2 or more different species
what is artificial selection
when humans select individuals in a population to breed together to get desirable traits
how does artificial selection work
use the example of cows for milk yield
Farmers select a female with a very high milk yield and a male whose mother had a very high milk yield and breed these 2 together.
Then they select the offspring with the highest milk yields and breed them together.
This is continued over several generations until a very high milk-yielding cow is produced.
Artificial selection reduces the gene pool. Why is this and why is it a problem?
Organisms with similar traits/alleles are bred together. This leads to a reduction in the number of alleles in the gene pool.
This means if a new disease appears, there’s less chance the alleles could offer resistance. Also useful alleles could potentially be lost.
what problems could be created for organisms due to artificial selection
leads to health problems and genetic disorders
eg. pugs and french bulldogs were bred to have squished faces and this trait has become so exaggerated that they suffer from breathing problems.
what are the problems with inbreeding
limits gene pool
decreases genetic diversity
causes many genetic disorders
causes more offspring to have homozygous recessive alleles
over time this reduces the ability of these organisms to survive and reproduce
what is allopatric speciation
when some members of a population are separated from the rest of the group by a physical barrier (eg. river)
They’ve geographically isolated
define adaptive radiation
where rapid organism diversification takes places
part of allopatric speciation
give an example of allopatric speciation
Finches on the Galapagus islands
small groups separated from the main population and got stranded on small islands
they evolved and adapted tot he different environment eventually forming a whole new species
example of adaptive radiation
what is sympatric speciation
when members of 2 different species interbreed and form fertile offspring
the hybrid formed, which is a new species, will have a different number of chromosomes to either parent and may no longer be able to interbreed with members of either parent population
give an example of sympatric speciation
fungus-farming ants supply organic material to keep fungi growing
parasitic ants eat the fungi and reproduce
genetic analysis has shown parasitic ants are descendants of fungus-farming ants
they’ve adapted differently due to a change in behaviour
what are reproductive barriers
barriers to successful interbreeding can form within populations before or after fertilisation has occurred
prezygotic reproductive barriers - prevent fertilisation and formation of a zygote
postzygotic reproductive barriers - reduce the viability of the offspring
define wild type
the allele that codes for the most common phenotype in a natural population
what is the role of a seed bank
keeps samples of seeds from both wild type and domesticated varieties
what is the role of gene banks
how can they lead to an increase of genetic diversity
they store biological samples such as sperm or eggs which are usually frozen
they are used in outbreeding where they can use the alleles to reduce the occurrence of homozygous recessives and increase the potential to adapt to environmental change
