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 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 1000 in Hardy–Weinberg equilibrium, 16% show a recessive trait. How many individuals N are expected to be homozygous dominant?

N = 1000q² = 0.16

N =

3.Cell biologyMedium

What separates during anaphase I of meiosis?

AHomologous chromosomes
BWhole nuclei
CCentromeres of each chromatid
DSister chromatids

4.Cell biologyHard

In Michaelis–Menten kinetics, what does the Km of an enzyme represent?

AThe amount of enzyme in the cell
BThe maximum rate of the reaction
CThe substrate concentration at which the rate is half of Vmax
DThe reaction rate when the substrate concentration is close to zero

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 = 360

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 = √0.16 = 0.4

p = 0.6

Homozygous dominant

p² = 0.36

N = 1000·0.36 = 360

Answer

N = 360

3.Cell biology

Answer: A · Homologous chromosomes

Why this is the answer

In anaphase I, homologous chromosomes move to opposite poles, halving the chromosome number. Sister chromatids separate in anaphase II.

Answer

A · Homologous chromosomes

4.Cell biology

Answer: C · The substrate concentration at which the rate is half of Vmax

Why this is the answer

Km is the substrate concentration giving half the maximum velocity. A low Km indicates high apparent affinity for the substrate.

Answer

C · The substrate concentration at which the rate is half of Vmax

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