Continuity and change practice questions.

The Meselson-Stahl experiment demonstrated that DNA replication is:

• Conservative
• Semi-conservative
• Dispersive
• Fully bidirectional only

Answer: Semi-conservative

Using heavy and light nitrogen isotopes, Meselson and Stahl showed each new DNA molecule contains one original and one newly synthesised strand, confirming the semi-conservative model.

The genetic code is described as 'degenerate' because:

• One codon can code for several amino acids
• Several codons can specify one amino acid
• Codons overlap on the mRNA
• Introns are read as codons

Answer: Several codons can specify one amino acid

Degeneracy means several different codons can specify the same amino acid (e.g. six codons for leucine). This reduces the effect of some point mutations on the resulting protein.

In eukaryotes, which mechanism can switch genes off without changing the DNA base sequence?

• Point mutation
• Methylation of DNA
• Deletion of an exon
• Non-disjunction

Answer: Methylation of DNA

Epigenetic methylation of cytosine bases (and histone modification) can silence genes by preventing transcription, without altering the underlying nucleotide sequence, and can sometimes be inherited.

A dihybrid cross between two heterozygotes for unlinked genes (AaBb x AaBb) gives which phenotypic ratio?

• 3:1
• 1:2:1
• 9:3:3:1
• 1:1:1:1

Answer: 9:3:3:1

With independent assortment of two gene pairs, the offspring show the classic 9:3:3:1 ratio of the four phenotype combinations, reflecting Mendel's law of independent assortment.

A test cross is used to determine whether an organism showing a dominant phenotype is homozygous or heterozygous by crossing it with an individual that is:

• Homozygous dominant
• Homozygous recessive
• Heterozygous
• A different species

Answer: Homozygous recessive

Crossing with a homozygous recessive individual reveals the unknown genotype: if any recessive offspring appear, the parent must be heterozygous; if all offspring show the dominant trait, it is likely homozygous.

A recombination frequency of 10% between two linked genes indicates that they are:

• On different chromosomes
• Close together on the same chromosome
• Exactly 50 map units apart, so unlinked
• Unable to cross over

Answer: Close together on the same chromosome

Recombination frequency reflects distance between loci; 10% recombinants means the genes are relatively close (about 10 map units apart), since crossing over between nearby loci is less likely.