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Chapter 17 · Part 2 of 10
  1. Chapter overview
  2. 1Mendel's peas and the law of segregation
  3. 2Genotype, phenotype, and Punnett squares
  4. 3Two characters and independent assortment
  5. 4Chromosomes explain Mendel's laws
  6. 5Reading human pedigrees
  7. 6Sex chromosomes and X-linked traits
  8. 7Dominance at the protein level
  9. 8Incomplete dominance, codominance, and the environment
  10. 9Epistasis and polygenic traits
  11. 10Probability and the product rule

Genotype, phenotype, and Punnett squares

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A tall pea plant could be TT or Tt, and you can’t tell by looking. Geneticists keep those two descriptions apart with a few precise words, then use a simple grid to predict what a cross will produce.

What an organism has vs. what it shows

  • An organism’s genotype is its genetic makeup: the alleles it carries, such as TT, Tt, or tt.
  • Its phenotype is its observable characteristics, the traits that result from expressing those genes, such as tall or short.

An individual with two identical alleles of a gene is a homozygote, or is homozygous for that gene. TT and tt plants are both homozygous. An individual with two different alleles, such as Tt, is a heterozygote, or is heterozygous. True-breeding plants are homozygous, which is why they keep producing the same trait when they fertilize themselves.

Genotype Description Phenotype
TT Homozygous dominant Tall
Tt Heterozygous Tall
tt Homozygous recessive Short

Building a Punnett square

A Punnett square is a grid that lists every combination of gametes two parents can make, so you can read off the expected offspring. You need to know the parents’ genotypes first.

  1. Write the genotypes of both parents. For two heterozygous tall plants: Tt × Tt.
  2. List the gametes each parent can make. By the law of segregation, each gamete gets one allele: T or t.
  3. Draw a grid with one column for each male gamete across the top and one row for each female gamete down the side.
  4. Fill each box by combining the allele at the top of its column with the allele at the start of its row.
  5. Count the boxes to get the ratios of genotypes, then use dominance to turn them into phenotypes.

For Tt × Tt the finished square looks like this:

Egg \ Sperm T t
T TT Tt
t Tt tt

The genotype ratio is 1 TT : 2 Tt : 1 tt. Since TT and Tt are both tall, the phenotype ratio is 3 tall : 1 short. Each box is one equally likely outcome, so each is worth one-fourth of the offspring.

Check yourselfA Yy yellow-seeded pea is crossed with a yy green-seeded pea, giving 200 offspring. About how many have green seeds?

About 100. The Yy parent makes Y and y gametes equally and the yy parent makes only y, so half the offspring are yy (green).

The testcross reveals a hidden genotype

A tall plant might be TT or Tt. To find out which, you perform a testcross: cross the individual with the dominant phenotype to a homozygous recessive individual (tt) and look at the offspring.

The tt partner can only give t, so every offspring’s phenotype directly shows which allele came from the mystery parent.

Mystery parent Gametes it makes Offspring Conclusion
TT All T All tall (Tt) Parent is homozygous
Tt Half T, half t 1 tall : 1 short Parent is heterozygous

Even one short offspring settles it: that offspring is tt, so the tall parent must have carried a t allele. If all the offspring are tall, especially across a large number of them, the parent is very likely TT.

Check yourselfYou testcross a tall pea of unknown genotype, and all 40 offspring are tall. What was its genotype?

Almost certainly TT. A Tt parent would give about half short (tt) offspring, so with none, the parent had no t allele to pass on.

Practice questions

  1. 1 of 3

    Which of these describes a phenotype rather than a genotype?

  2. 2 of 3

    To find out whether a tall pea plant is TT or Tt, which plant should you cross it with?

  3. 3 of 3

    A TT pea plant is crossed with a Tt pea plant. What fraction of the offspring will be tall?