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
Two AaBb parents are crossed. Gene A gives black fur (dominant) or white fur; gene B gives purple flowers (dominant) or white flowers. Click the one cell for an offspring that is homozygous for black fur and shows white 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
Which cell junction lets ions and small molecules pass directly between neighbouring cells?
- ATight junction
- BGap junction
- CDesmosome
- DAdherens junction
4.Cell biologyHard
Avery, MacLeod and McCarty treated the transforming extract with different enzymes. Which result showed that DNA was the transforming principle?
- AOnly RNase destroyed the transforming activity
- BOnly DNase destroyed the transforming activity
- CNone of the enzymes had any effect
- DOnly protease destroyed the transforming activity
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(short) = 0.25
P(round seeds) = 0.5
Multiply
P = 0.25·0.5 = 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: B · Gap junction
Why this is the answer
Gap junctions are channels formed by connexons. They connect cytoplasm of adjacent cells, allowing electrical coupling as in cardiac muscle.
Answer
B · Gap junction
4.Cell biology
Answer: B · Only DNase destroyed the transforming activity
Why this is the answer
Transformation still happened after proteases and RNase, but not after DNase. This showed that DNA, not protein or RNA, carried the hereditary information.
Answer
B · Only DNase destroyed the transforming activity
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