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.

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1Formulas and methods

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

2Practice problems

1.GeneticsMedium
AaBb ♀AaBb ♂ABAbaBabABAbaBab????????????????

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, wrinkled seeds?

AaBb × AaBb
P =
2.GeneticsHard
AA?AaAaaap (A)q (a)p (A)q (a)q² = 0.16N = 2000

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

The Golgi apparatus mainly:

  1. AReplicates DNA
  2. BModifies, sorts and packages proteins for transport
  3. CBreaks down old organelles and engulfed bacteria with enzymes
  4. DProduces ATP
4.Cell biologyHard

What problem does telomerase solve?

  1. AThe joining of Okazaki fragments
  2. BThe shortening of chromosome ends with each round of replication
  3. CThe unwinding of the double helix at each replication fork by helicase
  4. DThe removal of introns from mRNA

3Solutions

1.Genetics

Answer: P = 0.0625

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(wrinkled seeds) = 0.25

Multiply

P = 0.25·0.25 = 0.0625

Answer

P = 0.0625
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: B · Modifies, sorts and packages proteins for transport

Why this is the answer

The Golgi modifies proteins (e.g. glycosylation) and sorts them into vesicles for secretion, membranes or lysosomes.

Answer

B · Modifies, sorts and packages proteins for transport
4.Cell biology

Answer: B · The shortening of chromosome ends with each round of replication

Why this is the answer

Telomerase uses its own RNA template to add repeats to telomeres, balancing the end-replication problem. It is active in stem cells and most cancer cells.

Answer

B · The shortening of chromosome ends with each round of replication

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