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 affected father, X^bY. The mother's gametes run down the side. Click the cell for a carrier daughter.

♀ 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 10000 in Hardy–Weinberg equilibrium, 1% show a recessive trait. How many individuals N are expected to be homozygous dominant?

N = 10000q² = 0.01

N =

3.Cell biologyMedium

Which motor protein generally carries cargo along microtubules towards the minus end, near the cell centre?

ADynein
BMyosin
CActin
DKinesin

4.Cell biologyHard

In the Hershey–Chase experiment, phages labelled with ³²P and ³⁵S infected bacteria. What was found inside the bacteria?

AEqual amounts of both labels
BMostly ³⁵S, showing protein enters the cell
CNeither label, because phages stay outside
DMostly ³²P, showing DNA enters the cell

3Solutions

1.Genetics

Answer: X^B × X^b

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 × X^b = X^BX^b

Answer

Cell X^B × X^b → X^BX^b

2.Genetics

Answer: N = 8100

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 = 10000·0.81 = 8100

Answer

N = 8100

3.Cell biology

Answer: A · Dynein

Why this is the answer

Kinesins mostly move towards the plus end (out to the periphery) and dyneins towards the minus end (towards the centrosome). Myosin moves along actin.

Answer

A · Dynein

4.Cell biology

Answer: D · Mostly ³²P, showing DNA enters the cell

Why this is the answer

The ³²P-labelled DNA entered the bacteria while most ³⁵S-labelled protein coats stayed outside. This supported DNA as the genetic material of phage.

Answer

D · Mostly ³²P, showing DNA enters the cell

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