Inheritance (A Level only) (AQA A Level Biology): Flashcards
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What is a gene?
A gene is a DNA sequence that codes for a polypeptide or protein.
Each gene is located at a specific locus on a chromosome.
What is an allele?
An allele is a different version of a gene.
Alleles have slightly different base sequences but occupy the same locus.
What is meant by the term locus?
A locus is the specific position on a chromosome where a gene is located.
What is the difference between a homozygous and a heterozygous genotype?
A homozygous genotype has two identical alleles of a gene.
A heterozygous genotype has two different alleles of a gene.
Distinguish between an organism's genotype and its phenotype.
The genotype is the alleles an organism possesses.
The phenotype is the observable characteristics, determined by the genotype and the environment.
What is the difference between a dominant and a recessive allele?
A dominant allele is always expressed in the phenotype, in both heterozygous and homozygous individuals.
A recessive allele is only expressed when no dominant allele is present, i.e. in a homozygous individual.
What is meant by codominance?
Codominance occurs when both alleles in a heterozygous genotype are fully expressed.
Features of both alleles are observed in the phenotype.
In the ABO blood group system, which alleles are codominant and which is recessive?
I^A^ and I^B^ are codominant.
I^O^ is recessive.
In the ABO blood group system, the genotype IAIB produces blood group because both alleles are codominant.
In the ABO blood group system, the genotype IAIB produces blood group AB because both alleles are codominant.
What is meant by sex linkage?
Sex-linked genes are located on the X chromosome (rarely on Y).
Males (XY) have only one copy of X-linked genes, so are more likely to express recessive sex-linked traits.
In terms of a recessive X-linked disorder, what is a carrier?
A carrier is a heterozygous female (e.g. X^H^X^h^) who possesses the recessive allele but does not express the disorder in her phenotype.
What is meant by autosomal linkage?
Autosomal linkage occurs when two or more genes are located on the same autosome (a non-sex chromosome).
These genes do not assort independently during meiosis and tend to stay together in the parental combination.
What is monohybrid inheritance?
Monohybrid inheritance involves tracking the inheritance of one gene with two alleles.
In a monohybrid cross between two heterozygotes (e.g. Bb x Bb), what is the expected phenotypic ratio of the offspring?
3 dominant : 1 recessive.
In a monohybrid cross between two heterozygotes (Bb x Bb), what is the expected genotypic ratio of the offspring?
1 BB : 2 Bb : 1 bb.
Why might the observed offspring ratios differ from the predicted ratios in a monohybrid cross?
Predicted ratios are based on probability, not certainty.
Observed ratios may differ due to the random fertilisation of gametes.
During gamete formation, each allele of a pair has an equal chance of being passed on to the offspring.
During gamete formation, each allele of a homologous pair has an equal chance of being passed on to the offspring.
Which type of genetic diagram is used to clearly present the possible offspring of a monohybrid cross?
A Punnett square.
How are codominant alleles represented in a genotype?
Using a capital letter for the gene and superscript letters for the alleles.
For example, I^A^I^A^ for blood group A.
A blood type A person (IAIO) is crossed with a blood type B person (IBIO). What is the expected phenotypic ratio of the offspring?
1 blood type AB : 1 blood type A : 1 blood type B : 1 blood type O.
Why are males more likely than females to express a recessive sex-linked trait?
Sex-linked genes are located on the X chromosome.
Males (XY) have only one copy of the X chromosome, so a single recessive allele is enough for the trait to be expressed.
Haemophilia is a recessive X-linked disorder. What are the genotypes of an affected male and a carrier female?
Affected male: X^f^Y.
Carrier female: X^F^X^f^.
A male cannot pass an X-linked allele to his , because he passes on only his Y chromosome to them.
A male cannot pass an X-linked allele to his sons, because he passes on only his Y chromosome to them.
What is the difference between a monohybrid and a dihybrid cross?
A monohybrid cross tracks the inheritance of one gene.
A dihybrid cross tracks the inheritance of two genes simultaneously.
What is the expected phenotypic ratio when two individuals heterozygous for both genes are crossed in a dihybrid cross?
9 : 3 : 3 : 1.
For a dihybrid genotype, both alleles of one gene are written together, then both alleles of the second gene, for example rather than YGyg.
For a dihybrid genotype, both alleles of one gene are written together, then both alleles of the second gene, for example YyGg rather than YGyg.
Standard dihybrid ratios (9:3:3:1) assume which process occurs during meiosis?
Independent assortment of the two genes.
What is autosomal linkage?
Autosomal linkage occurs when two genes are located on the same chromosome.
Linked genes tend to be inherited together, maintaining the parental allele combination.
How does autosomal linkage affect the expected offspring ratios of a dihybrid cross?
Linked genes tend to be inherited together, so variation is reduced.
This produces offspring ratios that deviate from the standard 9:3:3:1 prediction.
How are the alleles of linked genes written in a genotype?
Linked alleles are written in brackets to show they are inherited together.
For example, (FG)(FG) instead of FFGG.
What is epistasis?
Epistasis occurs when one gene affects the expression of another gene.
It involves two genes on different chromosomes influencing the same trait.
In reality, why are recombinant offspring produced even when genes show autosomal linkage?
Because of crossing over between homologous chromosomes during meiosis.
If the phenotypic ratios of a dihybrid cross deviate from the expected 9:3:3:1, one possible explanation is autosomal between the genes.
If the phenotypic ratios of a dihybrid cross deviate from the expected 9:3:3:1, one possible explanation is autosomal linkage between the genes.
True or False?
In a dihybrid genotype, alleles of different genes should be mixed together, such as YGyg.
False.
Both alleles of one gene are written first, then both alleles of the second gene, e.g. YyGg; alleles of different genes are not mixed.
What is a test cross?
A test cross is used to deduce the genotype of an individual showing the dominant phenotype.
The unknown individual is crossed with an individual expressing the recessive phenotype.
In a test cross, why is the unknown individual crossed with a homozygous recessive individual?
An individual with the recessive phenotype has a known genotype (homozygous recessive, e.g. aa).
This means the offspring phenotypes provide enough information to reveal the unknown genotype.
In a monohybrid test cross, what does it indicate if all offspring show the dominant phenotype?
The unknown individual is homozygous dominant (e.g. AA).
In a monohybrid test cross, what does it indicate if at least one offspring shows the recessive phenotype?
The unknown individual is heterozygous (e.g. Aa).
In a test cross, the individual with the recessive phenotype must be recessive, giving it a known genotype.
In a test cross, the individual with the recessive phenotype must be homozygous recessive, giving it a known genotype.
In a dihybrid test cross, what does it indicate if there are no recessive phenotypes for either gene in the offspring?
The unknown individual is homozygous dominant for both genes.
In a dihybrid test cross, what does at least one recessive phenotype for one gene only indicate about the unknown genotype?
The unknown individual is heterozygous for one gene and homozygous dominant for the other.
In a dihybrid test cross, what does at least one recessive phenotype for both genes indicate about the unknown genotype?
The unknown individual is heterozygous for both genes.
A long-eared rabbit (dominant, DD or Dd) is test crossed and produces some short-eared offspring. What is its genotype?
It must be heterozygous (Dd).
A short-eared offspring (dd) shows that the long-eared parent contributed a recessive d allele.
What do genetic diagrams such as Punnett squares predict?
They predict the genotypes and phenotypes of offspring, using the known parental genotypes and meiosis.
In sexual reproduction, what is the chance of each allele from a homologous pair entering a gamete?
Each allele has an equal chance of entering a gamete and being inherited.
Why might the actual offspring ratios differ from those predicted by a genetic diagram?
Predictions are based on probability.
Actual ratios may differ due to the random fertilisation of gametes.
Why are model organisms such as Fast Plant and Drosophila used to study inheritance?
They have short life cycles, allowing rapid breeding.
Their traits are controlled by single genes that determine easily identifiable traits.
In genetic ratio experiments, it is assumed that gamete fertilisation occurs .
In genetic ratio experiments, it is assumed that gamete fertilisation occurs randomly.
In Fast Plant, a homozygous dominant purple-stemmed plant (AA) is crossed with a homozygous recessive green-stemmed plant (aa). What are the offspring?
All offspring are Aa (heterozygous).
All have purple stems.
In Fast Plant, two heterozygous purple-stemmed plants (Aa x Aa) are crossed. What is the expected phenotypic ratio?
3 purple stem : 1 green stem (3:1).
In Drosophila, wing length is controlled by one gene. What phenotypes do the alleles L and l produce?
L (dominant) produces the long / wild-type wing.
l (recessive) produces the vestigial (stunted) wing.
How do genetic diagrams represent the probability of offspring inheriting a particular phenotype?
As ratios, which represent the probability of each phenotype appearing.
In a dihybrid cross showing a 9:3:3:1 ratio, what percentage probability does the '9' category represent?
56.25% (9 out of 16).
Percentage probabilities are often used when dealing with the probability of inheriting genetic .
Percentage probabilities are often used when dealing with the probability of inheriting genetic diseases.
If the predicted ratio of affected to unaffected offspring is 1 : 3, what is the percentage probability of an affected offspring?
25%.
1 + 3 = 4, so (1 divided by 4) x 100 = 25%.
What does the chi-squared test determine?
Whether there is a significant difference between the observed and expected results in an experiment.
The chi-squared test is used with which type of data?
Categorical data, i.e. data that falls into distinct groups.
In the chi-squared calculation, each squared difference (O - E)^2 is divided by the value before the results are summed.
In the chi-squared calculation, each squared difference (O - E)^2 is divided by the expected value before the results are summed.
Outline the steps to calculate a chi-squared value from observed (O) and expected (E) results.
Calculate the difference between each O and E value.
Square each difference.
Divide each squared difference by the expected value.
Sum these values to obtain the chi-squared value.
How are the degrees of freedom calculated for a chi-squared test?
Degrees of freedom = number of classes - 1.
What probability level do biologists generally use for the chi-squared test, and what does it mean?
A probability level of 0.05 (5%).
This means there is only a 5% probability that any difference between O and E occurred by chance.
In a chi-squared test, what can be concluded if the chi-squared value is greater than or equal to the critical value?
There is a significant difference between observed and expected results, which is not due to chance.
The null hypothesis is rejected.
In a chi-squared test, what can be concluded if the chi-squared value is less than the critical value?
There is no significant difference between observed and expected results; any difference is due to chance.
The null hypothesis is accepted.
The critical value used in a chi-squared test depends on the probability level and the number of degrees of .
The critical value used in a chi-squared test depends on the probability level and the number of degrees of freedom.
If a chi-squared test shows a statistically significant difference in a genetic cross, what might this suggest?
The presence of a factor that is not being accounted for.
For example, linkage between the genes being analysed.
True or False?
A larger chi-squared value indicates a greater difference between observed and expected results.
True.
A very large chi-squared value (e.g. greater than the critical value at p = 0.001) indicates a very low probability (less than 0.1%) that the difference is due to chance, providing strong evidence against the null hypothesis.
In a dihybrid cross with 4 phenotype categories, how many degrees of freedom are used, and what is the critical value at p = 0.05?
Degrees of freedom = 4 - 1 = 3.
The critical value at p = 0.05 is 7.82.
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