Biology · Grades 9–12

Genetics & inheritance · Quick reference

Reference sheet for Grades 9 to 12: monohybrid crosses with Punnett squares and DNA base pairing and transcription. Write each parent’s two alleles along the sides of a 2 × 2 grid.

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Rule and key idea

Monohybrid crosses with Punnett squares

Write each parent’s two alleles along the sides of a 2 × 2 grid. Each box is one equally likely offspring genotype. A dominant allele hides a recessive one.

each parent gives one allele · 4 equally likely boxes

In pea plants, the allele for a tall stem (T) is dominant over the allele for a short stem (t). Two plants, TT × Tt, are crossed. What percentage of the offspring are expected to show a short stem? → 0

DNA base pairing and transcription

Bases pair by shape and hydrogen bonding: adenine with thymine (uracil in RNA) and cytosine with guanine. Work one base at a time.

DNA: A-T, C-G · RNA: A-U, C-G

A DNA template strand reads AAGCCT. Which mRNA sequence is transcribed from it, read base by base in the same order? → UUCGGA

Check yourself

Four practice questions on this topic. Try each one before you open the answer.

A DNA template strand reads TGTTTTAG. Which mRNA sequence is transcribed from it, read base by base in the same order?
- ACAAAATC
- ACAAAAUC
- TGTTTTAG
- ACAAAAGC
Show the answer
ACAAAAUC TGTTTTAG → ACAAAAUC (A-U, T-A, C-G, G-C)
A DNA template strand reads CATATG. Which mRNA sequence is transcribed from it, read base by base in the same order?
- GUAUAC
- GTATAC
- GUAAAC
- CATATG
Show the answer
GUAUAC CATATG → GUAUAC (A-U, T-A, C-G, G-C)
Using base-pairing rules, which sequence pairs with the DNA strand CAGTCGC, read base by base in the same order?
- GUCAGCG
- GTCAGCG
- CAGTCGC
- GTCAGCT
Show the answer
GTCAGCG CAGTCGC → GTCAGCG (A-T, C-G)
How many DNA replications precede the two meiotic divisions in the usual model?
- None.
- One.
- Two separate replications, one before each division.
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One. DNA replication occurs once before meiosis I.