7A - genetics Flashcards
(104 cards)
1
Q
what is a chromosome?
A
a long section of DNA wound around a protein called a histone
2
Q
what is a gene?
A
a length of DNA that codes for a single polypeptide or protein
3
Q
what is a locus?
A
the fixed position of a gene on a chromosome
4
Q
what is an allele?
A
a different form of a gene
5
Q
what do different alleles of a gene have?
A
slightly different nucleotide sequences but they still occupy the same position (locus) on the chromosome
6
Q
examples of alleles:
A
-one of the genes for coat colour in horses is Agouti
-this gene for coat colour is found on the same position on the same chromosome for all horses
-hypothetically there are two different forms (alleles) of that gene found in horses: A and a
-each allele can produce a different coat colour:
allele A → black coat
allele a → chestnut coat
7
Q
how do chromosomes of eukaryotic cells occur?
A
in homologous pairs (there are two copies of each chromosome)
↳ as a result cells have two copies of every gene
8
Q
what does having two copies of each gene mean?
A
there can be different allele combinations within an individual
9
Q
what is the genotype?
A
all of an organisms genetic material / the alleles of a gene possessed by that individual
10
Q
homozygous
A
when the two alleles of a gene are the same in an individual
11
Q
heterozygous
A
two different versions of the same gene
12
Q
what is the phenotype?
A
observable characteristics of an organism that is affected by environment and caused by the genotype
13
Q
what is a dominant allele?
A
alleles that are always expressed in the phenotype
14
Q
recessive alleles
A
only expressed in the phenotype if no dominant allele is present
15
Q
codominance
A
when both alleles can be expressed in the phenotype at the same time
16
Q
how to write the codominance genotype:
A
when writing the genotype for codominance the gene is symbolised as the capital letter and the alleles are represented by different superscript letters, for example IA
17
Q
example of codominance:
A
the gene for blood types is represented in the genotype by I and the three alleles for human blood types are represented by A, B and O
allele A results in blood type A (IAIA or IAIO) and allele B results in blood type B (IBIB or IBIO)
if both allele A and allele B are present in a heterozygous individual they will have blood type AB (IAIB)
blood type O (IOIO) is recessive to both group A and group B alleles
18
Q
what happens when a homozygous dominant individual is crossed with a homozygous recessive individual?
A
the offspring are called the F1 generation → all of the F1 generation are heterozygous
19
Q
what happens if two individuals from the F1 generation are then crossed?
A
the offspring they produce are called the F2 generation
20
Q
what are the two types of linkage in genetics?
A
sex linkage and autosomal linkage
21
Q
how many sex chromosomes are there?
A
X and Y
22
Q
male chromosomes and female chromosomes:
A
women: XX
men: XY
23
Q
what is sex linkage?
A
-some genes are found on a region of only one sex
-as the inheritance of these genes is dependent on the sex of the individual they are called sex-linked genes
24
Q
where are most sex linked genes found?
A
on the longer X chromosome
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example of sex linked diseases:
haemophilia
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how are sex-linked genes represented?
-by writing the alleles as superscript next to the sex chromosome
-for example a particular gene that is found only on the X chromosome has two alleles G and g
-the genotype of a heterozygous female would be written as XGXg
-a males genotype would be written as XGY
27
what is autosomal linkage?
-this occurs on the autosomes (any chromosome that isn’t a sex chromosome)
-the alleles for each gene that are linked on the same chromosome will be inherited together
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what does monohybrid inheritance look at?
it looks at how the alleles for a single gene are passed on from one generation to the next
(genetic crosses of single gene combinations)
29
what happens when two individuals sexually reproduce?
there is an equal chance of the zygote inheriting either allele from their parent
30
why are genetic diagrams often used?
present the information of a homologous pair’s reproductionin a clear and precise manner so that predictions can be made
31
examples of a genetic cross diagram for monohybrids:
a punnet square
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genotypes that genetic diagrams produce:
-the predicted genotypes that genetic diagrams produce are all based on chance
-there is no way to predict which gametes will fuse so sometimes the observed or real-life results can differ from the predictions
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three genotypes for females (sex linked)
XAXA = unaffected
XAXa = carrier
XaXa = affected
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two phenotypes for males (sex-linked)
XAY = unaffected
XaY = affected
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can men carry x linked traits?
no, males only pass y chromosomes on to their sons
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EXAMINER TIP:
Make sure to include all of your working out when constructing genetic diagrams. It is not enough just to complete a punnett square, you need to show that you have thought about the possible gametes that can be produced by each parent.Also, remember to state the phenotype as well as the genotype of the offspring that result from the cross. Read the questions carefully when answering sex-linked inheritance questions – is the question asking for a probability for all children or is it asking about a specific gender (boys or girls).
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what do dihybrid crosses look at?
how the alleles of two genes transfer across generations
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how to write the genotypes for a dihybrid genetic cross:
write the two alleles for one gene, followed immediately by the two alleles for the other gene. Do not mix up the alleles from the different genes
if there was a gene with alleles Y and y and another gene with alleles G and g an example genotype for an individual would be YyGg
39
what is epistasis?
when one gene can affect the expression of another gene (two genes on different chromosomes affect the same feature
40
TIP:
genetics questions you may notice that crosses involving autosomal linkage predict offspring that all have the same combination of characteristics as their parents. In reality recombinant offspring are often produced; this is due to crossing over during meiosis. Crossing over breaks the linkage between the genes and recombines the characteristics of the parents. Exam questions often ask students to explain the appearance of recombinant offspring in crosses that involve linkage; crossing over is the most likely explanation
41
recombinant
offspring that have a different combination of characteristics to their parents
42
what is a test cross used for?
to deduce the genotype of an unknown individual that is expressing a dominant phenotype
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results of a monohybrid test cross:
If no offspring exhibit the recessive phenotype then the unknown genotype is homozygous dominant
If at least one of the offspring exhibit the recessive phenotype then the unknown genotype is heterozygous
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results of a dihybrid test cross:
If no offspring exhibit the recessive phenotype for either gene then the unknown genotype is homozygous dominant for both genes
If at least one of the offspring exhibit the recessive phenotype for one gene but not the other, then the unknown genotype is heterozygous for one gene and homozygous dominant for the other
If at least one of the offspring exhibit the recessive phenotype for both genes then the unknown genotype is heterozygous for both genes
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Examiner Tips and Tricks
Make sure before you start a test cross you think about the following: how many genes are there, how many alleles of each gene are there, which is the dominant allele, what type of dominance is it and is there linkage or epistasis between genes?
46
investigating genetic ratios:
known information about genotypes, phenotypes and the process of meiosis can be used to make predictions about the phenotypes of offspring that would result from specific breeding pairs of organisms
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what happens when two individuals sexually reproduce?
-wWhen two individuals sexually reproduce there is an equal chance of either allele from their homologous pair making it into their gametes and subsequently the nucleus of the zygote
-this means there is an equal chance of the zygote inheriting either allele from their parent
48
what are genetic diagrams used for?
to present this information in a clear and precise manner so that predictions can be made
(eg: a punnet square)
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what is used to investigate genetic ratios resulting from genetic crosses?
Drosophila are commonly used
50
why are drosophila commonly used?
-they both reproduce at rapid rates due to their short life cycles
-a single gene determines an easily identifiable physical trait in each organism
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drosophila alleles:
**their wing length is controlled by a single gene → this gene has two alleles**
-the dominant allele L produces the long or wild type wing
-the recessive allele l produces the stunted or vestigial wing
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Examiner Tips and Tricks:
Remember when dealing with genetic diagrams and questions in the exam, you always know the genotype of the individual displaying the recessive phenotype; they have to be homozygous recessive!
53
what do genetic diagrams produce?
ratios that suggest the probability of an offspring having or being a particular phenotype
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common ratio in dihybrid crosses:
9:3:3:1
↳ this ratio results from crossing individuals that are heterozygous for both genes
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meaning of 9:3:3:1 ratio:
The ratio of 9:3:3:1 suggests that there are four possible phenotypes for the offspring
If there were 16 offspring produced
9 would have phenotype W
3 would have phenotype X
3 would have phenotype Y
1 would have phenotype Z
56
what can ratios from genetic diagrams be used to produce?
they can be converted to percentage probabilities
↳ this is the chance that any offspring produced by those two individuals will have that particular phenotype
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when are percentage probabilities often used?
when dealing with the probability of inheriting genetic diseases
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A man (Mike) and a woman (Sarah) want to have their own biological baby. They are concerned as Sarah's father has haemophilia, a blood clotting condition. The allele for a blood clotting factor (F or f) is found on the X chromosome. Females have two copies of the allele whereas men only have one. It is a recessive sex-linked disease.
Neither Mike nor Sarah suffer from the condition.
Calculate the chance that they will have a baby that suffers from haemophilia. Give your answer as a percentage.
**Step 1: Work out Sarah's genotype**
As her father was a sufferer he must have the genotype XfY / Her father must have given her his Xf chromosome for her to be the female sex / As she does not suffer from the recessive disease it can be said that she must also have the dominant allele
Her genotype is X,FX,f
**Step 2: Work out Mike's genotype**
As Mike does not suffer from the disease he must have the dominant allele on his single X chromosome
His genotype is XF, Y
**Step 3: Cross the two genotypes using a genetic diagram**
Parental phenotypes: carrier female x normal male
Parental genotypes:
XFXf XFY
Parental gametes:
XF or Xf XF or Y
The predicted ratio of genotypes in offspring -
1 XFXF : 1 XFXf : 1 XFY : 1 XfY
The predicted ratio of phenotypes in offspring -
1 female with normal blood clotting : 1 carrier female : 1 male with normal blood clotting : 1 male with haemophilia
Ratio 1 : 3
**Step 4: Use the predicted ratio to obtain the percentage probability**
The ratio is 1:3
1 + 3 = 4 so need to find 1 as a percentage of 4
(1 ÷ 4) x 100 = 25
There is a 25% chance or probability that their baby will have haemophilia
59
what statistical test is linked to inheritance?
the chi-squared test
60
what does the chi squared test determine?
whether or not there is a significant difference between the observed and expected results in an experiment
61
what does it mean if the difference is statistically significant?
this suggests the presence of a factor that isn’t being accounted for
(e.g. linkage between genes)
62
what does it mean if the difference isn’t statistically significant?
any differences that are observed can be said to be due to chance alone
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what data is the chi squared test calculated on?
when the data is categorical, i.e. falls into distinct groups
64
chi squared test formula:
the chi-square value (x²) =
x² = sum of
(observed - expected)² / expected
65
chi-squared test steps:
1) obtain the expected (E) and observed (O) results for the experiment
2) calculate the difference between each set of results
3) **square each difference**
it is irrelevant whether the difference is positive or negative
4) divide each squared difference by the expected value
5) add the resulting values together to get a sum of these answers to obtain the chi-squared value
66
analysing chi-squared values:
to work out what the chi-squared value means we need to compare the chi-squared value to a critical value
the critical value is read from a table of critical values and depends on the probability level used and the degrees of freedom
67
what probability level do biologists generally use?
a probability level of 0.05 or 5 %
68
what does a probability level of 0.05 mean?
there is only a 5 % probability that any difference between O and E has occurred by chance
69
what do the degrees of freedom in a chi-squared test take into account?
takes into account the number of comparisons made
70
how to calculate the degreees of freedom in a chi squared test:
degrees of freedom =
number of classes - 1
(e.g. if there are 2 phenotypes then 2 - 1 = 1 and there is 1 degree of freedom)
71
how is the chi-squared value compared to the critical value?
if the chi-squared value is greater than, or equal to, the critical value then there is a significant difference between observed and expected results
↳ the null hypothesis can be rejected
——————————————
if the chi-squared value is smaller than the critical value then there is no significant difference between observed and expected values
↳ the null hypothesis is accepted
72
Examiner Tips and Tricks
When calculating a chi-squared value it is very helpful to create a table like the one seen in the worked example. This will help you with your calculations and make sure you don’t get muddled up!
You should also be prepared to suggest reasons why results might be significantly different. For example, there could be linkage between the genes being analysed.
73
what is a species?
a group of similar organisms that can reproduce to give fertile offspring
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organisms in the same species & chromosomes:
organisms of the same species have the same number of chromosomes in their cells
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why can’t organisms from a different species reproduce?
different species have a different diploid number of chromosomes in their cells
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example of different species reproducing:
a horse has 64 chromosomes in its cells while a donkey has 62
when the haploid gametes from a horse (32) and a donkey (31) combine, the resulting zygote has 63 chromosomes
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cells, chromosome number & viability
cells that have an odd number of chromosomes are not viable, the chromosomes can’t form homologous pairs during meiosis to produce gametes
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members of a species live in…
populations
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how do biologists organise living organisms?
by dividing organisms into species
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which factors need to be taken into consideration when defining a species or determining whether two organisms belong to the same species
-similarities/differences in observable features (morphology)
-similarities/differences in DNA
-similarities/differences in RNA
-similarities/differences in proteins
-the ability to interbreed and produce fertile offspring
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define population:
a group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed
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what is the phenotype of an organism dependent on?
its genotype and the environment
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members of the same species will have the same…
genes, of which there may exist different alleles
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define gene pool:
all of the different alleles in the individuals that make up a population
85
define allele frequency:
how often different alleles occur in the gene pool of a population
86
how can the allele frequency or gene pools in a population change over time?
due to processes such as natural selection
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what happens when the gene pool (or allele frequencies) within a population changes sufficiently over time?
the characteristics of the species population will also change → over time, these changes can become so great that a new species forms
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formula for phenotype frequency:
(total individuals with phenotype ÷ total individuals in population) × 100
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In a population of 9 pea plants, 7 of the plants have purple flowers, whilst 2 have white flowers. Calculate the phenotype frequencies of purple and white flowers. Give your answers as percentages.
1) calculate the phenotype frequency of purple flowers
= (7 ÷ 9) × 100
= 0.78 × 100
= 78%
2) calculate the phenotype frequency of white flowers
= (2 ÷ 9) × 100
= 0.22 × 100
= 22%
90
what does the hardy weinberg principle state?
that if certain conditions are met then the allele frequencies of a gene within a population will not change from one generation to the next
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what does the hardy weinberg equation allow for?
-the calculation of allele and genotype frequencies within populations
- for predictions to be made about how frequencies will change in future generations
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what are the conditions of hardy weinberg?
-no migration to introduce or remove alleles from the population
-no mutations to create new alleles
-no selection favouring particular alleles
-mating is random (no inbreeding)
-the population is large
93
example of hardy weinberg equation:
If the phenotype of a trait in a population is determined by a single gene with only two alleles (we will use B / b as examples throughout this section) then the population will consist of individuals with three possible genotypes:
For example if every individual in the population has the homozygous dominant genotype BB then its frequency will be 1, while if half of the population show this genotype the
Homozygous dominant (BB)
Heterozygous (Bb)
Homozygous recessive (bb)
When using the Hardy-Weinberg equation, frequencies are represented as proportions of the population;
For example if every individual in the population has the homozygous dominant genotype BB then its frequency will be 1, while if half of the population show this genotype then the frequency will be 0.5
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what is a proportion?
a number out of 1
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how is the frequency of allleles represented using hardy weinberg?
as a proportion
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what is the hardy weinberg equation?
p + q = 1
p = the frequency of the dominant allele
q = the frequency of the recessive allele
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hardy weinberg example using
p + q = 1
E.g. in a population of 100 individuals there would be 200 alleles because every individual has two versions of each gene
If 120 of those alleles were the dominant allele then the frequency of the dominant allele would be 120/200
It could be said that p = 120 ÷ 200 = 0.6
If p = 0.6 then
q = 1 - 0.6
= 0.4
98
how is the frequency of genotypes represented using hardy weinberg?
(the proportion of all of the individuals with a particular genotype)
the chance of an individual being homozygous dominant is p²
↳ the offspring would inherit dominant alleles from both parents so p x p = p2
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what is the chance of an individual being heterozygous for a genotype with hardy weinberg?
the chance of an individual being heterozygous is 2pq
↳ offspring could inherit a dominant allele from the father and a recessive allele from the mother
(p x q) or offspring could inherit a dominant allele from the mother and a recessive allele from the father (p x q) so 2pq
100
what is the chance of an individual being recessive for a genotype with hardy weinberg?
the chance of an individual being homozygous recessive is q²
↳ the offspring would inherit recessive alleles from both parents so q x q = q²
101
hardy weinberg equation for genotypes:
p² + q² + 2pq = 1
102
In a population of birds 10% of the individuals exhibit the recessive phenotype of white feathers.
Calculate the frequencies of all genotypes.
1) we will use F / f to represent dominant and recessive alleles for feather colour
those with the recessive phenotype must have the homozygous recessive genotype, ff
therefore q2 = 0.10 (as 10% of the individuals have the recessive phenotype and q2 represents this)
**calculating the frequencies of the homozygous dominant ( p2 ) and heterozygous ( 2pq ):**
2) find p (the frequency of the dominant allele F).
p + q = 1
p = 1 - 0.32
p = 0.68
3) find p² (the frequency of homozygous dominant genotype)
0.68² = 0.46
p2 = 0.46
4) find 2pq = 2 x (p) x (q)
2 x (0.68) x (0.32) = 0.44
5) check calculations by substituting the values for the three frequencies into the equation; they should add up to 1
p² + 2pq + q² = 1
0.46 + 0.44 + 0.10 = 1
103
summary of previous calculation:
In summary:
Allele frequencies:
p = F = 0.68
q = f = 0.32
Genotype frequencies:
p2 = FF = 0.46
q2 = ff = 0.10
2pq = Ff = 0.44
104
Examiner Tips and Tricks
When you are using Hardy-Weinberg equations you must always start your calculations by determining the proportion of individuals that display the recessive phenotype; this is the only phenotype from which you can immediately work out its genotype as it will always be homozygous recessive (the dominant phenotype is seen in both homozygous dominant and heterozygous individuals).
In Hardy-Weinberg questions it is a good idea to begin by establishing what information you have been given in the question (i.e. do you know q2, or do you know p?), and then establishing what the question wants you to work out (i.e. are you calculating 2pq?). You can then work out how to get from one to the other.
Don’t mix up the Hardy-Weinberg equations with the Hardy-Weinberg principle. The equations are used to estimate the allele and genotype frequencies in a population. The principle suggests that there is an equilibrium between allele frequencies and that there is no change in this between generations.
