Punnett Squares and Genetics UPCAT Reviewer: Lesson and Practice

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Genetics and Punnett Squares

Translate parent genotypes into probabilities without confusing genotype and phenotype.

5-10 minute lesson27 original questionsAdaptive practiceSaves progress

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Mendelian Genetics and Punnett Squares

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A Punnett square organizes possible allele combinations

For Tt × Tt, the genotype ratio is 1 TT : 2 Tt : 1 tt, while the phenotype ratio is 3 dominant : 1 recessive.

Each parent contributes one allele. A capital letter commonly represents a dominant allele and a lowercase letter the recessive allele of the same gene.

Genotype

The allele combination, such as TT, Tt, or tt.

Phenotype

The observable trait produced by the genotype and environment.

DO IT FAST

Outside first, inside second

Split each parent genotype into its possible gametes. Put those alleles outside the grid, combine one from each parent inside, then count boxes.

Why it works

The square does not predict four actual children. It displays probabilities for each independent offspring.

WORKED EXAMPLES

Five forms you should recognize

1. TT × tt

Every offspring is Tt and shows the dominant trait.

2. Tt × Tt

Genotypes 1:2:1; dominant phenotype probability 75%.

3. Tt × tt

Half Tt and half tt; this is a test cross.

4. Genotype vs phenotype

Tt and TT share the dominant phenotype but have different genotypes.

5. Independent births

A previous child does not change the allele probability for the next child.

COMMON TRAPS

Check before you commit

• Counting dominant alleles instead of boxes
• Calling Tt homozygous
• Equating dominant with common or better
• Treating 25% as exactly one of every four children
• Mixing genotype and phenotype ratios
• Changing probability after a previous birth

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QUICK ANSWERS

Genetics and Punnett Squares FAQ

Does dominant mean more common?

No. Dominance describes expression in a heterozygote, not population frequency.

Can a recessive trait appear from two dominant-looking parents?

Yes, if both parents are heterozygous carriers.

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