Back to the full dot-point answer
VICBiologyQuick questions
Unit 2: How does inheritance impact on diversity?
Quick questions on Monohybrid crosses and test crosses: 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 setting up a monohybrid cross?Show answer
A monohybrid cross considers one gene with two alleles.
What is the four basic monohybrid crosses (autosomal dominant pattern)?Show answer
Using P (purple, dominant) and p (white, recessive).
What is the test cross?Show answer
A test cross is the cross between an organism showing the dominant phenotype (unknown genotype: PP or Pp) and a homozygous recessive organism (pp).
What is worked example?Show answer
A pea plant with round seeds (R dominant, r recessive) is found in a wild population. The grower wants to know if it is RR or Rr. They cross it with a known wrinkled pp plant (rr).
What is why monohybrid crosses matter?Show answer
The 3:1 (and 1:1) ratios are the empirical signature of:
What is beyond Mendel?Show answer
The same approach works with non-classical dominance:
What is 3. Heterozygous times homozygous recessive?Show answer
This is the test cross.
What is sample-size caveat?Show answer
With small numbers of offspring, a heterozygous parent might by chance produce all-dominant offspring just from random gamete sampling. The more offspring observed, the more confident the conclusion.
What is mixing up genotypes and phenotypes in the ratio?Show answer
The Pp × Pp cross gives a genotype ratio of 1 PP : 2 Pp : 1 pp but a phenotype ratio of 3 purple : 1 white. Always state which you mean.
What is forgetting that a test cross requires the recessive homozygote?Show answer
Crossing your unknown with a heterozygote (Pp) does not work cleanly because the heterozygote produces both kinds of gametes; you cannot infer the unknown's genotype from the offspring pattern.
What is treating 3:1 as exact?Show answer
Real offspring counts vary by chance. A 3:1 cross with 100 offspring might give 78:22 or 71:29 and still be consistent with 3:1.
What is saying the test cross "creates" a homozygote?Show answer
The test cross reveals the genotype by producing different offspring patterns from the two possibilities.
What is skipping the gametes step?Show answer
Drawing the Punnett square without listing the gametes is the most common source of errors. Be explicit: PP produces only P gametes; Pp produces 50% P and 50% p; pp produces only p.