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

haemophilia is X-linked recessive (allele h). A carrier mother, X^HX^h, has children with an unaffected father, X^HY. The mother's gametes run down the side. Click the cell for an unaffected son.

♀ X^HX^h♂ X^HY

1X^h × Y
2X^h × X^H
3X^H × Y
4X^H × X^H

2.GeneticsHard

A recessive condition affects 4% of a population in Hardy–Weinberg equilibrium. What fraction f of the population are carriers (heterozygous)?

q² = 0.04

f =

3.Cell biologyMedium

Apoptosis is:

ACell fusion
BCell growth
CProgrammed cell death
DUncontrolled cell division

4.Cell biologyHard

What problem does telomerase solve?

AThe joining of Okazaki fragments
BThe shortening of chromosome ends with each round of replication
CThe removal of introns from mRNA
DThe unwinding of the double helix at each replication fork by helicase

3Solutions

1.Genetics

Answer: X^H × 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^hY, X^hX^H, X^HY, X^HX^H

Find the cell

X^H × Y = X^HY

Answer

Cell X^H × Y → X^HY

2.Genetics

Answer: f = 0.32

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 = √0.04 = 0.2

p = 1 − 0.2 = 0.8

Carriers

2pq = 2·0.8·0.2 = 0.32

Answer

f = 0.32

3.Cell biology

Answer: C · Programmed cell death

Why this is the answer

Apoptosis removes damaged or unneeded cells in a controlled way via caspases; necrosis is uncontrolled death with inflammation.

Answer

C · Programmed cell death

4.Cell biology

Answer: B · The shortening of chromosome ends with each round of replication

Why this is the answer

Telomerase uses its own RNA template to add repeats to telomeres, balancing the end-replication problem. It is active in stem cells and most cancer cells.

Answer

B · The shortening of chromosome ends with each round of replication

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