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Unit 4: Heredity and continuity of life
Quick questions on Mutations and sources of genetic variation (QCE Biology Unit 4)
14short 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 are 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 are point mutations?Show answer
A change at a single base pair.
What are insertions and deletions?Show answer
Bases are added or removed.
What are 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 q1?Show answer
Distinguish between point, frameshift and chromosomal mutations, giving one example of each. [3 marks]
What is q2?Show answer
A DNA sequence 5'-ATG CCG TAT TGA-3' undergoes deletion of the fifth base. Write the new sequence and predict the effect on the polypeptide. [3 marks]
What is q3?Show answer
Refer to sources of genetic variation. (a) Identify three sources arising during meiosis. (b) Explain how fertilisation adds further variation.