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
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.
1Formulas and methods
2Practice problems
1.GeneticsMedium
Tall (A) is dominant over short (a), and round seeds (B) over wrinkled (b). The genes are on different chromosomes. For the cross AaBb × aabb, what is the probability P that an offspring is tall, round seeds?
AaBb × aabb
P =
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
Which motor protein generally carries cargo along microtubules towards the minus end, near the cell centre?
- AMyosin
- BActin
- CDynein
- DKinesin
4.Cell biologyHard
In Griffith's 1928 experiment, mice injected with heat-killed smooth (S) pneumococci plus live rough (R) bacteria died. What did this show?
- AHeat does not kill S bacteria
- BA substance from the dead S cells transformed R cells into a virulent form
- CR bacteria are virulent on their own
- DMice make S bacteria from R bacteria
3Solutions
1.Genetics
Answer: P = 0.25
Mendelian genetics: each parent passes on one allele of each gene, chosen at random. A Punnett square lists every combination of gametes, each equally likely. A dominant allele (capital letter) shows whenever it is present; a recessive one only in the homozygous form. Genes on different chromosomes are inherited independently, so their probabilities multiply.
Each gene on its own
P(tall) = 0.5
P(round seeds) = 0.5
Multiply
P = 0.5·0.5 = 0.25
Answer
P = 0.25
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 · 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
C · Dynein
4.Cell biology
Answer: B · A substance from the dead S cells transformed R cells into a virulent form
Why this is the answer
Something from the heat-killed S strain changed living R bacteria into the virulent S type. Griffith called this transformation; the "transforming principle" was later shown to be DNA.
Answer
B · A substance from the dead S cells transformed R cells into a virulent form
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