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, 16% show a recessive trait. How many individuals N are expected to be homozygous dominant?
N = 2000q² = 0.16
N =
3.Cell biologyMedium
Active transport differs from facilitated diffusion because it:
- ANeeds no proteins
- BOnly moves water
- CUses energy to move substances against their concentration gradient
- DMoves substances down their gradient faster by using carrier proteins
4.Cell biologyHard
Why is DNA replication called semi-conservative?
- AThe new DNA is completely new
- BOnly half of the DNA is copied
- CEach new double helix contains one original strand and one new strand
- DMutations are conserved in half of the daughter cells after every division
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(tall) = 0.75
P(round seeds) = 0.5
Multiply
P = 0.75·0.5 = 0.375
Answer
P = 0.375
2.Genetics
Answer: N = 720
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 = 2000·0.36 = 720
Answer
N = 720
3.Cell biology
Answer: C · Uses energy to move substances against their concentration gradient
Why this is the answer
Active transport uses ATP (or another gradient) to move molecules against their gradient; facilitated diffusion is passive.
Answer
C · Uses energy to move substances against their concentration gradient
4.Cell biology
Answer: C · Each new double helix contains one original strand and one new strand
Why this is the answer
Meselson and Stahl showed that after one replication the DNA had intermediate density, consistent with one old and one new strand.
Answer
C · Each new double helix contains one original strand and one new strand
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