Cell division & inheritanceGCSE Questions

genes

gametes

Y

XX

XY XX

50% or ½

XX XY XY XXFemale Male FemaleMale

Meiosis

23

23

any two from(introduces) variation prevents the risk of all being the sameand a disease wiping out populationor prevent monoculture two parents to raise offspring

both parents carry a recessive allele or gene or are heterozygous

any two from:• other scientists not aware of his work

• chromosomes / DNA / genes not seen / discovered / known

do not accept there was no interest in genetics

• other theories accepted at the time• not considered to be a scientist / not eminent / respected

allow ‘he was just / only a monk’

random selection

19:21 or about 1:1

(genotype / gametes from P / father) D and d (*)1(genotype / gametes from Q / mother) d and d / accept d(*)1offspring genotypes correctly derived from correct gametes(*)1offspring phenotypes R and S identified1(*) eg may be in punnett squareallow own upper and lower case symbols or allow any symbol correctly used with key

For cystic fibrosispros:• termination of pregnancies would reduce number of people with cystic fibrosis (in population)• reduce health-care costs• allows decision / emotional argument,eg allows people to make choices about termination cons:• possible damage / risk to embryo / fetus / baby• possible harm / risk to mother• (may) have to make ethical / moral / religious decisions

For polydactyly:• detects possibility of ‘disfigurement’ in embryo• but condition not life threatening• so risks to fetus / mother unjustified

Aa or aA

allele / gene for vestigial wings / a is recessiveor vestigial is recessive or A is dominant or A would override the effect of a or A present gives long wings

(b) parental genotypes correct – both AaNB can pick up chain of logic at any point correctly derived from candidate’s previous point1gametes correctly derived from P genotypes1offspring genotypes correctly derived from gametes13:1 ratio recognisedwrong cross and not 3:1 ratio = max 2

eggs produced by meiosis

therefore contain only half of mother’s DNA

(child has) mother’s 25 / 28 / 30 / 31 orchild gets 17 / 19 / 22 / 24 from mother 1

(child has) man B’s 10 / 12 / 13 / 14orchild gets 18 / 20 / 21 / 23 from B 1no bars / DNA / lines from man A correspond to child 1

B

D

twins have come from one (fertilised) egg

Y sperm / Y chromosome produces boys

any two from:• (formed from) different egg / 2 eggs

• (formed from) different sperm / 2 sperm

• have different genes / alleles / chromosomes / DNA

(medical) research / named eg growing organsormedical / patient treatmentallow (embryo) cloningdo not allow designer babies / more babies

any one from:• ethical / moral / religious objectionsignore cruel / not natural / playing God• potential harm to embryoallow deformedignore harm to mother

gametes i.e. B b and B b1correct combination of genotypes i.e. BB, Bb, Bb, bb1correct analysis of phenotypes i.e. 3 black fur 1 with brown fur

award one mark for the recognitionthat it is down to chance (which two gametes fuse) and not simply ‘because it’s a prediction’do not accept mutation

(b) (i) B is dominant/ an allele is dominant if it is expressed in the heterozygous phenotypecandidates are likely to use a variety of ways of expressing their ideas1b is recessive/ a recessive allele is not expressed in the presence of itscontrasting alleledo not accept powerfuldo not accept stronger1(ii) alleles are different forms of a gene controlling a characteristic andoccupying the same site on homologous chromosomes (e.g. B or b)1genes are the units of DNA/sites on chromosomes carrying the informationthat determines characteristics (e.g. bB)1

white