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 purple flowers (dominant) or white flowers; gene B gives yellow seeds (dominant) or green seeds. Click the one cell for an offspring that is shows both white flowers and green seeds.
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
The contractile ring that pinches an animal cell in two during cytokinesis is made of:
- ACellulose fibres
- BActin and myosin
- CKeratin filaments
- DMicrotubules and kinesin
4.Cell biologyHard
A disease is passed from an affected mother to all her children but never from an affected father. Where is the mutation most likely located?
- AIn mitochondrial DNA
- BOn the Y chromosome
- COn the X chromosome, as a dominant trait
- DOn an autosome, as a recessive trait
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(wrinkled seeds) = 0.5
Multiply
P = 0.5·0.5 = 0.25
Answer
P = 0.25
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: B · Actin and myosin
Why this is the answer
A ring of actin filaments and myosin II contracts under the membrane, forming the cleavage furrow that separates the two daughter cells.
Answer
B · Actin and myosin
4.Cell biology
Answer: A · In mitochondrial DNA
Why this is the answer
Mitochondria, and their DNA, are inherited almost only from the egg. Mitochondrial disorders therefore follow maternal inheritance.
Answer
A · In mitochondrial DNA
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