Quick questions on Mutation, gamete variation and the source of new alleles: HSC Biology Module 6

Meiosis produces haploid gametes from a diploid parent, reducing chromosome number by half. Three processes generate variation.

During metaphase I, the homologous chromosome pairs line up at the spindle equator independently of one another. Either the maternal or the paternal chromosome of each pair can face either pole. With $n$ chromosome pairs, this produces $2^n$ possible combinations.

During prophase I, homologous chromosomes pair up (synapsis) and exchange segments at chiasmata. This recombines maternal and paternal alleles within each chromosome, generating combinations that were not present in either parent. Crossing over essentially makes the $2^n$ figure an underestimate; the actual number of unique gametes is astronomically larger.

Any one of the millions of possible sperm can fertilise any one of the possible eggs, multiplying the variation across the population.

Each daughter DNA molecule retains one original strand and one newly synthesised strand. This conserves the parental sequence.

After replication, mismatch repair proteins recognise base mismatches and excise the wrongly inserted base, reducing the error rate further to about 1 in $10^{10}$.

Meiosis can only shuffle what already exists.

Distinguish between the sources of genetic variation in meiosis and the source of new alleles in a population. [3 marks]

A population of 10 000 antibiotic-susceptible bacteria has a spontaneous mutation rate of 1 in 10 million per replication. Calculate the expected number of resistant mutants after 10 generations, assuming each cell divides once per generation. [3 marks]

Explain why mutation, rather than meiotic shuffling, is the ultimate source of variation in evolution. (a) Define mutation. (b) Distinguish a new mutation from a recombinant gamete.