A-Level Biology – Genetics, Variation and Evolution

(a) State the base-pairing rule in DNA.(b) A section of the template (antisense) strand of DNA has the base sequence $\text{TACGGCATT}$. Deduce the sequence of bases on (i) the mRNA transcribed from this strand, and (ii) the complementary DNA strand.

Explain the roles of mRNA and tRNA in translation at a ribosome.

A gene mutation changes one base in a DNA triplet from $\text{CTC}$ to $\text{CTT}$. Both triplets code for the amino acid leucine.(a) State the type of mutation shown.(b) Explain why this mutation is described as ‘silent’ and has no effect on the protein produced.

Explain two ways in which meiosis produces genetic variation among gametes.

Cystic fibrosis is caused by a recessive allele, $f$. Two parents, both unaffected but each a carrier (genotype $Ff$), have children.(a) Complete a genetic cross (Punnett square, described in words) to show the possible genotypes of their offspring.(b) State the probability that a child of these parents is affected by cystic fibrosis, and the probability that a child is an unaffected carrier.

In pea plants, tall ($T$) is dominant to dwarf ($t$), and round seed ($R$) is dominant to wrinkled seed ($r$); the two genes assort independently. A cross between two dihybrid plants ($TtRr \times TtRr$) is expected to produce offspring in a 9:3:3:1 ratio (tall round : tall wrinkled : dwarf round : dwarf wrinkled).Out of 160 offspring, calculate the expected number of each phenotype.

The observed offspring numbers from the cross in the previous question were: tall round $=92$, tall wrinkled $=29$, dwarf round $=32$, dwarf wrinkled $=7$.(a) Calculate the chi-squared ($\chi^2$) value using $\chi^2=\sum\dfrac{(O-E)^2}{E}$.(b) The critical value of $\chi^2$ at $p=0.05$ for 3 degrees of freedom is 7.815. State the conclusion that should be drawn, with a reason.

State the null hypothesis that would be tested by the chi-squared test in the previous question, and explain what is meant by ‘degrees of freedom’ in this context.

In a population of 1000 individuals, a recessive allele $b$ causes a particular disorder when homozygous ($bb$). The frequency of individuals showing the recessive phenotype is 9%.Using the Hardy–Weinberg equations ($p^2+2pq+q^2=1$ and $p+q=1$):(a) Calculate the allele frequency $q$ of allele $b$.(b) Calculate the allele frequency $p$ of the dominant allele $B$.(c) Calculate the percentage of the population that are heterozygous carriers.

State three assumptions that must hold for a population to be in Hardy–Weinberg equilibrium.

Distinguish between allopatric and sympatric speciation, and outline how each can lead to the formation of a new species.

Explain, using the theory of natural selection, how a population of insects could evolve increased resistance to a pesticide after repeated pesticide use, referring to variation, selection pressure and allele frequency.