7A - Genetics Flashcards
Define a gene.
A sequence of bases on a DNA molecule that codes for a protein (polypeptide) which results in a characteristic.
Define an allele.
A different version of a gene.
How many alleles can there be for a gene?
Several.
What makes different alleles different?
The order of bases in the gene.
Define genotype.
The alleles an organism has for a certain gene.
Define phenotype.
An organism’s characteristics as a result of the expression of the genotype.
Define dominant.
The allele that is expressed in the heterozygous genotype.
Define recessive.
The allele that is expressed only in the homozygous genotype.
How are dominant alleles represented?
Uppercase letters
How are recessive alleles represented?
Lowercase letters
Define codominant.
Alleles that are both expressed in the heterozygous. Neither one is recessive.
Define locus.
The fixed position of a gene on a chromosome.
Define homozygote.
An organism that carries two copies of the same allele.
Define heterozygote.
An organism that carries two different alleles.
Define carrier.
A person carrying an allele which is not expressed in the phenotype but that can be passed on to offspring.
Are humans haploid or diploid?
Diploid - We have two sets of chromosomes, so we have two alleles for each gene.
What can be used to predict the genotypes and phenotypes of offspring produced in genetic crosses?
Genetic diagrams
What is monohybrid inheritance?
The inheritance of a characteristic controlled by a single gene.
What things must be shown in a Punnet square question?
1) List the parents’ genotypes.
2) List the gametes’ alleles.
3) Draw the square to show the cross between the gametes.
4) Write out the proportions of resulting genotypes and phenotypes.
5) Answer the question.
Remember to revise monohybrid cross diagrams.
Pg 168 and 169 of revision guide.
What are the F1 and F2 phenotypes?
- F1 - First generation phenotypes
* F2 - Second generation phenotypes
Give an example of codominance.
Sickle-cell anaemia
How is codominance represented in genetic diagrams?
- A large, capital letter is given for the gene (e.g. H)
- A superscript capital latter is given for each allele
Example genotypes: HSHN (where S and N are superscript)
Explain genetic inheritance for sickle-cell anaemia.
- Homozygous for normal haemoglobin = HNHN -> Don’t have the disease
- Homozygous for sickle haemoglobin = NSHS -> All blood cells are sickle-shaped
- Heterozygous = HNHS -> Sickle-cell trait with some normal and some sickle haemoglobin
What are multiple alleles?
When there are more than two alleles of the same gene.
Give an example of multiple alleles.
Blood groups (ABO system).
How are multiple alleles represented in genetic diagrams?
- A large, capital letter is given for the gene (e.g. I)
- A superscript capital latter is given for each allele
Example genotypes: IAIB (where A and B are superscript)
Explain genetic inheritance for blood groups.
- IO is the allele for blood group O.
- IA is the allele for blood group A.
- IB is the allele for blood group B.
- IO is recessive, IA and IB are codominant.
What blood group does IAIO code for?
A
What blood group does IBIO code for?
B
What blood group does IAIA code for?
A
What blood group does IBIB code for?
B
What blood group does IAIB code for?
AB
What blood group does IOIO code for?
O
Remember to revise codominance and multiple allele genetic diagrams.
Pg 169 of revision guide.
What is a dihybrid cross?
A diagram that shows how two different genes are inherited at the same time.
Describe how to approach a dihybrid cross diagram question.
Just like monohybrid, except the gametes will contain an allele for each of the two genes.
What is phenotypic ratio?
The ratio of different phenotypes in offspring.
Give the phenotypic ratio for a monohybrid cross between two heterozygous parents.
(e.g. Rr x Rr)
3 : 1
Dominant : Recessive
Give the phenotypic ratio for a dihybrid cross between two heterozygous parents.
(e.g. RrYy x RrYy)
9 : 3 : 3 : 1
Dominant both : Dominant 1st, Recessive 2nd : Recessive 1st, Dominant 2nd : Recessive both
Give the phenotypic ratio for a codominant cross between two heterozygous parents.
(e.g. HNHS x HNHS)
1 : 2 : 1
Homozygous for one allele : Heterozygous : Homozygous for second allele
Give the phenotypic ratios for these crosses of heterozygous parents:
• Monohybrid
• Dihybrid
• Codominant
- Monohybrid - 3 : 1
- Dihybrid - 9 : 3 : 3 : 1
- Codominant - 1 : 2 : 1
For what reasons may the phenotypic ratios not be as expected?
- Sex linkage
- Autosomal linkage
- Epistasis
What sex chromosomes do females have?
XX
What sex chromosomes do males have?
XY
Which sex chromosome is smaller?
Y - It carries fewer genes.
What are sex-linked genes?
Genes that are only found on the X chromosome and not on the Y chromosome.
How many alleles for sex-linked genes do males carry?
1
How does sex-linked gene expression work in males and females?
MALES:
• Have only 1 allele for each sex-linked gene
• This allele is always expressed, whether it is dominant or recessive
FEMALES:
• Have 2 alleles for each sex-linked gene
• Alleles are expressed as with normal genes
Give two examples of sex-linked genetic disorders.
- Colour blindness
* Haemophilia
Which gender is more likely to inherit sex-linked genetic disorders and why?
- Males have only 1 allele for the gene, while females have 2
- This means males only have to inherit one recessive allele to show the recessive phenotype, while females have to inherit two
- Therefore, males are more likely to show the recessive phenotype
Remember to revise colour blindness.
Diagram pg 171 of revision guide
What are the phenotypic ratios for sex-linked genetic disorders?
It depends on the parent genotypes.
Define autosome.
Any chromosome that isn’t a sex chromosome.
Define autosomal genes.
Genes located on the autosomes.
What can be said about genes on the same autosome?
They are linked.
Describe how linkage works.
Genes that are on the same autosome stay together during the independent segregation of chromosomes during meiosis I and their alleles will be passed on to the offspring together.
What can disrupt autosomal linkage?
Crossing over in meiosis I before independent segregation.
What determines how closely genes are linked and why?
- The closer together the genes are on the autosome, the more closely they are linked.
- This is because they are less likely to be separated during crossing over.
What is the result of autosomal linkage?
The phenotypic ratios are not as expected.
Explain how autosomal linkage changes a dihybrid cross of NnRr x NnRr.
- The normal phenotypic ratios are 9 : 3 : 3 : 1
- The actual phenotypic ratios are about 3 : 1
- This is because the two linked alleles are inherited together so it essentially becomes like a monohybrid cross.
- NOTE: The ratio may actually be a bit different because of some crossing over in meiosis I. There will be some unexpected genotypes: Nnrr and nnRr.
Why does a dihybrid autosomal linkage cross not give perfect ratios?
There is some crossing over during meiosis I, giving 4 genotypes instead of the expected 2.
In autosomal linkage questions, which alleles will be linked?
Generally, you can assume that the two dominant alleles are inherited together and the two recessive alleles are inherited together.
In the cross NnRr x nnrr, a ratio of 8 : 1 : 1 : 8 suggests what?
Autosomal linkage
Remember to revise autosomal linkage genetic diagrams.
Pg 172 of revision guide.
What is the name for when one gene masks the expression of another?
Epistasis
Is a characteristic controlled by only 1 gene?
No, it can be many different genes combining to control the same characteristic.
Give an example of epistasis in humans.
- Widow’s peak in hair is controlled by one gene and baldness by others
- If you have the genes that code for baldness, it doesn’t matter whether you have the allele for widow’s peak or not since you have no hair.
- The baldness genes are epistatic to the widow’s peak gene.
If gene A masks the expression of gene B, which gene is epistatic to which?
Gene A is epistatic to gene B.
Remember to revise epistasis examples.
Pg 172 of revision guide
Does epistasis affect phenotypic ratios?
Yes
In a dihybrid cross, how does a recessive epistatic allele work?
Having two copies of that allele masks the expression of the other gene.
In a dihybrid cross, how does a dominant epistatic allele work?
Having at least one copy of the dominant epistatic allele masks the expression of the other gene.
Give the phenotypic ratios for a dihybrid cross involving a recessive epistatic allele and heterozygous parents.
(e.g. YyRr x YyRr)
9 : 3 : 4
Give the phenotypic ratios for a dihybrid cross involving a dominant epistatic allele and heterozygous parents.
(e.g. YyRr x YyRr)
12 : 3 : 1
Remember to revise epistatic genetic diagrams and examples.
Pg 173 of revision guide
What are all the different types of genetic inheritance you need to know about?
- Monohybrid
- Dihybrid
- Codominance
- Multiple alleles
- Sex linkage
- Autosomal linkage
- Epistasis
What type of inheritance does a ratio of 3 : 1 suggest?
Monohybrid (or could be autosomal linkage if dihybrid)
What type of inheritance does a ratio of 9 : 3 : 3 : 1 suggest?
Dihybrid
What type of inheritance does a ratio of 1 : 2 : 1 suggest?
Codominance
What type of inheritance does a ratio of 8 : 1 : 1 : 8 suggest?
Autosomal linkage
What type of inheritance does a ratio of 9 : 3 : 4 suggest?
Recessive epistatic allele
What type of inheritance does a ratio of 12 : 3 : 1 suggest?
Dominant epistatic allele
What is the chi-squared test?
A statistical test used to see if the results of an experiment support a theory.
What is the expected result in the chi-squared test?
The result that is predicted using a theory.
What is the observed result in the chi-squared test?
The result that is actually recorded.
What is the null hypothesis in the chi-squared test and what does it state?
- A hypothesis that states that there is no difference between the observed and expected values.
- Rejecting the null hypothesis shows that the difference is significant.
How is the chi-squared test used in genetics questions?
To see if the observed phenotypic ratios are different to the expected ones for a certain genetic cross. This helps determine the type of inheritance.
What is the equation for the chi-squared test?
x² = Σ((O-E)²/E)
Do you need to learn the chi-squared test equation and be able to use it?
No, you do not, but you need to be able to understand how it works.
"Wing length in fruit flies is controlled by a single gene with two alleles (monohybrid inheritance). The dominant allele (N) gives normal wings and the recessive allele (n) gives vestigial wings. Two heterozygous parents are bred." For this theory, give: • The test • Expected result • Observed result • Null hypothesis
- The test - Chi-squared test
- Expected result - 3 : 1 phentoypic ratio of normal : vestigial wings
- Observed result - The number of each phenotype counted after the cross
- Null hypothesis - There is no significant difference between the observed and expected results.
What does p=0.05 mean?
The 5% probability that a result is due to chance.
What is the critical value?
The value of x² that corresponds to a 5% probability that the difference between observed and expected is due to chance.
What does an x² value greater than the critical value show in the chi-squared test?
- There is a less than 5% probability that the difference between observed and expected is due to chance.
- Null hypothesis can be rejected.
What does an x² value smaller than the critical value show in the chi-squared test?
- There is a greater than 5% probability that the difference between observed and expected is due to chance.
- There is no significant difference between the observed and expected values.
- Null hypothesis cannot be rejected.
How is the critical value in the chi-squared test found?
- Look at a table.
- Find the right degree of freedom by taking away 1 from the number of classes (e.g. the number of phenotypes).
- Find the critical value at the p=0.05 level that corresponds to that degree of freedom.
Remember to practice interpreting chi-squared tests and finding critical values.
Pgs 174 and 175 of revision guide
Remember to practice accepting or rejecting null hypotheses in the chi-squared test, then drawing conclusions.
Exercise books + pgs 174 and 175 of revision guide
