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VICBiologyQuick questions
Unit 2: How does inheritance impact on diversity?
Quick questions on Two-gene crosses: linked and unlinked genes: VCE Biology Unit 2
13short 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 independent assortment?Show answer
Mendel's Second Law (the Law of Independent Assortment) says that alleles of one gene segregate into gametes independently of alleles of another gene. This holds when the two genes are on different chromosomes or are far apart on the same chromosome.
What is why independent assortment works?Show answer
During metaphase I of meiosis, each homologous chromosome pair lines up at the equator independently of every other pair. For two pairs, there are two equally likely orientations, producing four equally likely gametes:
What is linkage?Show answer
Two genes on the same chromosome are linked: they tend to be inherited together because they travel as one unit through meiosis.
What is crossing over produces recombinants?Show answer
During prophase I, homologous chromosomes pair up and non-sister chromatids exchange segments at chiasmata. If a crossover happens between two linked loci, it shuffles the alleles:
What is identifying linkage from data?Show answer
A test cross is the cleanest way to detect linkage because it strips away the dominance complication.
What is cis and trans?Show answer
In a dihybrid (AaBb), the dominant alleles can be on the same chromosome (cis, written AB / ab) or on opposite chromosomes (trans, written Ab / aB).
What is why this matters for natural populations?Show answer
Independent assortment and recombination together create enormous genetic diversity (covered in the meiosis-and-genetic-diversity dot point). Linkage limits this diversity for closely-positioned genes: they tend to be inherited as haplotypes.
What is confusing 9:3:3:1 with 1:1:1:1?Show answer
9:3:3:1 is the F2 of a dihybrid self-cross (RrYy × RrYy). 1:1:1:1 is the test cross (RrYy × rryy) result for independent assortment.
What is calling a 9:3:3:1 result "linkage"?Show answer
It is the opposite: 9:3:3:1 is the signature of independent assortment.
What is forgetting that distant linked genes look unlinked?Show answer
Two genes on the same chromosome but very far apart can show RF approaching 50%, indistinguishable from independent assortment in a single cross.
What is saying crossing over always happens between linked genes?Show answer
It happens with a probability proportional to distance. Very close genes rarely recombine.
What is calling a deviation from 9:3:3:1 "always linkage"?Show answer
Other causes include sample size variation, lethal allele combinations, or epistasis. Linkage shows up as parental classes much larger than recombinant classes.
What is forgetting cis vs trans?Show answer
The same set of alleles can be in different parental arrangements; this changes which offspring are parental.