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, wrinkled seeds?
AaBb × aaBb
P =
2.GeneticsHard
Two AaBb parents are crossed. Gene A gives tall plants (dominant) or short plants; gene B gives purple flowers (dominant) or white flowers. Click the one cell for an offspring that is shows short plants and is homozygous for purple flowers.
AaBb × AaBb
- 1AB × aB
- 2AB × Ab
- 3AB × ab
- 4AB × AB
- 5aB × aB
- 6aB × Ab
- 7aB × ab
- 8aB × AB
- 9ab × aB
- 10ab × Ab
- 11ab × ab
- 12ab × AB
- 13Ab × aB
- 14Ab × Ab
- 15Ab × ab
- 16Ab × AB
3.Cell biologyMedium
Active transport differs from facilitated diffusion because it:
- ANeeds no proteins
- BOnly moves water
- CMoves substances down their gradient faster by using carrier proteins
- DUses energy to move substances against their concentration gradient
4.Cell biologyHard
The endosymbiotic theory is supported by the fact that mitochondria:
- AContain chlorophyll
- BAre made in the nucleus
- CHave their own circular DNA and 70S-like ribosomes
- DHave no membranes
3Solutions
1.Genetics
Answer: P = 0.125
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(wrinkled seeds) = 0.25
Multiply
P = 0.5·0.25 = 0.125
Answer
P = 0.125
2.Genetics
Answer: aB × aB
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.
Genotype asked for
aaBB
Gametes
A homozygous genotype needs the same gamete from both parents, so only one cell in the 16 fits.
aB × aB = aaBB
Answer
Cell aB × aB → aaBB
3.Cell biology
Answer: D · 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
D · Uses energy to move substances against their concentration gradient
4.Cell biology
Answer: C · Have their own circular DNA and 70S-like ribosomes
Why this is the answer
Mitochondria and chloroplasts have circular DNA, bacterial-like ribosomes, double membranes and divide by fission, suggesting a bacterial origin.
Answer
C · Have their own circular DNA and 70S-like ribosomes
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