Biology: Genetics and Cell Biology: cram pack

Formula sheets, the key method steps for every topic and 4 practice problems, with full solutions at the back.

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1Formulas and methods

Genetics

Monohybrid cross (Aa × Aa): genotypes 1 : 2 : 1, phenotypes 3 : 1
Dihybrid cross: phenotypes 9 : 3 : 3 : 1
Hardy–Weinberg: p + q = 1, p² + 2pq + q² = 1
Independent genes: multiply the probabilities

2Practice problems

1.GeneticsMedium

red-green colour blindness is X-linked recessive (allele b). A carrier mother, X^BX^b, has children with an unaffected father, X^BY. The mother's gametes run down the side. Click the cell for an unaffected son.

♀ X^BX^b♂ X^BY

1X^B × X^B
2X^B × Y
3X^b × X^B
4X^b × Y

2.GeneticsHard

In a population of 2000 in Hardy–Weinberg equilibrium, 1 in 400 show a recessive trait. How many individuals N are expected to be homozygous dominant?

N = 2000q² = 0.0025

N =

3.Cell biologyMedium

A codon consists of:

AThree amino acids
BOne nucleotide
CThree nucleotides on mRNA that code for an amino acid or a stop signal
DA whole gene

4.Cell biologyHard

A mutant Ras protein cannot hydrolyse GTP. What is the effect on cell signalling?

ARas is broken down faster
BRas moves into the nucleus and blocks transcription
CRas stays inactive and growth signals stop
DRas stays active and sends constant growth signals

3Solutions

1.Genetics

Answer: X^B × Y

Punnett square: one parent's gametes run along the top, the other's down the side. Each cell joins the gamete above it with the gamete beside it, and every cell is equally likely. A recessive trait only shows when both alleles are recessive; for an X-linked trait, sons get their only X from the mother.

Fill the square

X^BX^B, X^BY, X^bX^B, X^bY

Find the cell

X^B × Y = X^BY

Answer

Cell X^B × Y → X^BY

2.Genetics

Answer: N = 1805

Hardy–Weinberg: in a large, randomly mating population with no selection, the allele frequencies p (dominant) and q (recessive), p + q = 1, give genotype frequencies p², 2pq and q².

Allele frequencies

q = √0.0025 = 0.05

p = 0.95

Homozygous dominant

p² = 0.9025

N = 2000·0.9025 = 1805

Answer

N = 1805

3.Cell biology

Answer: C · Three nucleotides on mRNA that code for an amino acid or a stop signal

Why this is the answer

Each triplet of bases specifies an amino acid or stop; the genetic code is redundant (several codons per amino acid).

Answer

C · Three nucleotides on mRNA that code for an amino acid or a stop signal

4.Cell biology

Answer: D · Ras stays active and sends constant growth signals

Why this is the answer

Ras is switched off by hydrolysing its GTP. A mutant that cannot do this stays permanently on, driving proliferation; this is why RAS is a common oncogene.

Answer

D · Ras stays active and sends constant growth signals

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