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 short, round seeds?
AaBb × aaBb
P =
2.GeneticsHard
In a population of 10000 in Hardy–Weinberg equilibrium, 1 in 2500 show a recessive trait. How many individuals N are expected to be homozygous dominant?
N = 10000q² = 0.0004
N =
3.Cell biologyMedium
Apoptosis is:
- AUncontrolled cell division
- BProgrammed cell death
- CCell fusion
- DCell growth
4.Cell biologyHard
A plant cell placed in a concentrated salt solution loses water and its membrane pulls away from the cell wall. What is this called?
- AHaemolysis
- BCytolysis
- CPlasmolysis
- DTurgor
3Solutions
1.Genetics
Answer: P = 0.375
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(short) = 0.5
P(round seeds) = 0.75
Multiply
P = 0.5·0.75 = 0.375
Answer
P = 0.375
2.Genetics
Answer: N = 9604
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 = √0.0004 = 0.02
p = 0.98
Homozygous dominant
p² = 0.9604
N = 10000·0.9604 = 9604
Answer
N = 9604
3.Cell biology
Answer: B · 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
B · Programmed cell death
4.Cell biology
Answer: C · Plasmolysis
Why this is the answer
In a hypertonic solution water leaves the vacuole by osmosis. The protoplast shrinks away from the rigid wall, which is plasmolysis.
Answer
C · Plasmolysis
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