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 a carrier daughter.

♀ X^HX^h♂ X^HY

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

2.GeneticsHard

In a population of 500 in Hardy–Weinberg equilibrium, 1% show a recessive trait. How many individuals N are expected to be homozygous dominant?

N = 500q² = 0.01

N =

3.Cell biologyMedium

Which cell junction anchors intermediate filaments and gives tissues such as skin mechanical strength?

APlasmodesma
BTight junction
CDesmosome
DGap junction

4.Cell biologyHard

A disease is passed from an affected mother to all her children but never from an affected father. Where is the mutation most likely located?

AOn the Y chromosome
BIn mitochondrial DNA
COn the X chromosome, as a dominant trait
DOn an autosome, as a recessive trait

3Solutions

1.Genetics

Answer: X^h × X^H

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 × X^H = X^hX^H

Answer

Cell X^h × X^H → X^hX^H

2.Genetics

Answer: N = 405

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 = √0.01 = 0.1

p = 0.9

Homozygous dominant

p² = 0.81

N = 500·0.81 = 405

Answer

N = 405

3.Cell biology

Answer: C · Desmosome

Why this is the answer

Desmosomes link the keratin intermediate filaments of neighbouring cells via cadherins, so tissues under stress do not tear apart.

Answer

C · Desmosome

4.Cell biology

Answer: B · In mitochondrial DNA

Why this is the answer

Mitochondria, and their DNA, are inherited almost only from the egg. Mitochondrial disorders therefore follow maternal inheritance.

Answer

B · In mitochondrial DNA

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