AQA A Level Biology - Inheritance

define gene

a sequence of bases on a DNA molecule that codes for a protein resulting in a characteristic

define allele

different variants of the same gene

define locus

a fixed point where the allele of each gene is found on a chromosome

define genotype

genetic constitution of an organism - all the different alleles an organism has

define phenotype

expression of this genetic constitution and its interaction with the environment

define dominant allele

allele that is always expressed in the phenotype, even if only 1 copy is present

define recessive allele

allele that is only expressed in the phenotype if two copies are present

define codominant alleles

when two different alleles of a gene are both dominant so both are expressed in the phenotype

define heterozygous

when two different alleles are present at the same locus (on paired chromosomes)

define homozygous

when the same allele is present at the same locus (on paired chromosomes)

what are the F1 and F2 generations?

• F1 - first generation of offspring

• F2 - second generation of offspring

what must be represented in a genetic diagram?

• parent’s genotypes

• possible gametes

• punnett square

• possible genotypes of offspring and probabilities

• possible phenotypes and probabilities

how are codominant alleles represented?

C^M C^N

what happens if there are multiple alleles for a characteristic?

• there will be an order of dominance

• follow this to establish phenotype - the most dominant allele out of the two present (there will always be 2) will be expressed in the phenotype

• e.g. Blood type - I^A and I^B are codominant, and both are dominant to I^O

what are the differences between the X and Y genes?

• Y chromosome is much shorter than X and carries fewer genes

• Y contains critical SRY gene which causes sex organs to develop when the embryo is at 7 weeks

• X contains some genes that Y does not

describe how sex linkage makes some conditions more common in males than females

• if a gene is located on the X chromosome, males will only inherit 1 copy of the gene so will always display the characteristic coded for whether it is recessive or not (cannot be carriers)

• therefore they are more likely to display the recessive characteristic (if mother is heterozygous, 50% chance)

• females can be carriers of the recessive characteristic and will be less likely to display it than males as they have 2 alleles so both will need to be recessive

how are sex linked alleles represented?

X^D X^r Y

how can sex linkage be identified or estimated from a pedigree diagram?

if a characteristic is only prevalent in males or a lot more prevalent in males, it is likely to be sex linked

describe how to set up a dihybrid punnett square

• parents will have genotypes with 2 sets of 2 alleles. e.g RrYy

• each offspring will inherit one allele from each pair, so each genotype will have 4 possible inheritable allele combinations

what happens if homozygous dominant and homozygous recessive parents are crossed in a dihybrid cross?

• all of F1 offspring will be heterozygous (for both characteristics)

• F2 offspring will have a 9:3:3:1 phenotype ratio

• only true if alleles are not codominant - codominant alleles will have very different phenotype ratios

what is autosomal linkage?

• when two genes are on the same autosome

• they cannot be separated via independent segregation in meiosis 1 so alleles are commonly passed on to offspring together

• they can only be separated by crossing over

what affects how closely two genes are linked?

• how close they are on the autosome - closer = more linked

• this is because the closer they are, the less likely the alleles are to be split up by crossing over

what does autosomal linkage mean in practical terms?

certain combinations of alleles are more likely to be inherited together

what will autosomal linkage do to observed phenotypic ratios?

• they will differ significantly from expected ones

• if there are 4 possible phenotypes, two (each caused by different alleles e.g. dominant and dominant, recessive and recessive) will occur in higher frequencies and two will occur in lower frequencies

• if this is observed, it means autosomal linkage is likely

what is epistasis?

when the expression of one gene depends on and is affected by expression of another gene

what is an example of epistasis?

• flower colour

• gene 1 codes for a yellow pigment (Y) and gene 2 codes for an enzyme that turns the yellow pigment orange (R)

• if you do not have the Y allele, it does not matter if you have R or r as the flower will be colourless

• so gene 1 is epistatic to gene 2

what would be the effect on observed phenotypic ratios of epistasis?

• would not be as expected

• differs if epistatic gene is recessive or dominant

what would be the effect on observed phenotypic ratios of epistasis if the epistatic allele is recessive (include ratio)?

• two copies will block the expression of the other gene

• if homozygous dominant parent and a homozygous recessive parent are crossed, a 9:3:4 phenotypic ratio will be produced in F2 (all of F1 will be hetero) of:

  • 9 - both dominant

  • 3 - epistatic dominant and other recessive

  • 4 - recessive epistatic (other irrelevant)

what would be the effect on observed phenotypic ratios of epistasis if the epistatic allele is dominant (include ratio)?

• having at least one copy will mask the expression of the other gene

• if two homozygous (one D, one r) parents are crossed, are 12:3:1 F2 phenotypic ratio is produced of:

  • 12 - dominant epistatic (other irrelevant)

  • 3 - recessive epistatic, dominant other

  • 1 - recessive both

what is the chi squared test used for?

• used to see if observed phenotypic ratios differ from expected phenotypic ratios

• tests goodness of fit

what is the null hypothesis for the chi squared test?

no significant difference between expected and observed

what does the x² value need to be for H0 to be rejected?

x² >= critical value

what does the x² value need to be for H0 to be accepted?

x² < critical value

how do you find the critical value for the chi squared test?

• found in a table with the degrees of freedom and critical values at different p values

  • degrees of freedom = number of classes - 1