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Unit 4: Heredity and continuity of life
Quick questions on Mutations and sources of genetic variation (QCE Biology Unit 4)
15short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is types of mutation?Show answer
A mutation is a heritable change in the DNA sequence of an organism. Mutations can occur in body cells (somatic, not inherited) or in gametes (germline, passed to offspring).
What is sources of genetic variation in sexually reproducing populations?Show answer
Three processes shuffle existing variation into new combinations every generation, and one process creates new variation.
What is mutations, phenotype and polymorphism?Show answer
The effect of a mutation on phenotype depends on:
What is point mutations?Show answer
A change at a single base pair.
What is insertions and deletions?Show answer
Bases are added or removed.
What is chromosomal mutations?Show answer
Changes affecting whole sections of chromosomes, or whole chromosomes.
What is meiosis: independent assortment?Show answer
During metaphase I, each pair of homologous chromosomes lines up independently. Each gamete therefore inherits a random mix of maternal and paternal chromosomes. With n equals 23 in humans, 2 to the 23 (over 8 million) chromosome combinations are possible per gamete.
What is meiosis: crossing over?Show answer
During prophase I, homologous chromosomes pair (synapsis) and exchange segments at chiasmata. This produces recombinant chromatids carrying allele combinations not present in either parent.
What is random fertilisation?Show answer
Any sperm can fertilise any egg. Combined with independent assortment alone, this produces over 70 trillion genetically distinct offspring possibilities per couple, before considering crossing over.
What is mutation?Show answer
All three of the above only reshuffle existing alleles. A new allele can only arise by mutation. Mutation is therefore the ultimate source of variation; meiosis and fertilisation are the proximate shufflers.
What is polymorphism?Show answer
When two or more alleles of a gene exist in a population above a low frequency (usually defined as one per cent), that gene is polymorphic. Polymorphism is the population-level signature of accumulated mutations that have not been removed by natural selection. The ABO blood group, MN blood group and many single nucleotide polymorphisms (SNPs) used in forensic DNA profiling are examples.
What is beneficial, neutral and harmful?Show answer
Most mutations are neutral (in silent regions or are silent substitutions). A small fraction are harmful, and rarer still are beneficial. Beneficial mutations are the substrate of adaptive evolution.
What is treating all point mutations as harmful?Show answer
The degenerate genetic code means many substitutions are silent.
What is confusing aneuploidy with polyploidy?Show answer
Aneuploidy is one missing or extra chromosome (trisomy 21). Polyploidy is full extra sets of chromosomes (common in flowering plants).
What is calling all chromosomal mutations lethal?Show answer
Many translocations, inversions and duplications are tolerated, especially in heterozygotes.