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Interpret
Genetics

Learn how traits pass from parents to offspring, and use Punnett squares to predict inheritance patterns.

GED Science

The Language of Heredity

Heredity is the passing of traits from parents to offspring. The instructions for these traits are carried by genes on our chromosomes.

Gene
A segment of DNA that codes for a trait, like eye color.
Allele
A version of a gene. Offspring inherit one allele from each parent.
Trait
An observable characteristic, such as tall or short height.

Dominant vs. Recessive Alleles

Dominant (\(T\))
\(T\)
 masks recessive Shown with even ONE copy
Recessive (\(t\))
\(t\)
 needs two copies
Shown only as \(tt\)

Genotype vs. Phenotype

Every trait has two layers: the genetic code an organism carries, and the physical result you can observe.

Genotype
The allele combination, written as letters: \( TT \), \( Tt \), or \( tt \).
Phenotype
The visible trait that results — for example "tall" or "short".

Homozygous vs. Heterozygous

Homozygous
Two identical alleles: \( TT \) (dominant) or \( tt \) (recessive). Also called "purebred".
Heterozygous
Two different alleles: \( Tt \). Also called a "hybrid" or carrier.

Mendel's Law of Segregation

Gregor Mendel's foundational rule explains how alleles are passed down:

Parent: \(Tt\)
\(T\)
\(t\)
 alleles separate
\(T\)
 sex cell
\(t\)
 sex cell

The Punnett Square

A Punnett square is a grid that predicts the possible genotypes of offspring. One parent's alleles label the top, the other's label the side.

\(T\) \(t\)
\(T\) \(TT\) \(Tt\)
\(t\) \(Tt\) \(tt\)

Genotype vs. Phenotype Ratios

From a \( Tt \times Tt \) cross, the four boxes give us two different kinds of ratios:

Genotype Ratio
\( 1\ TT : 2\ Tt : 1\ tt \) — counts the gene combinations.
Phenotype Ratio
\( 3 \) tall \( : 1 \) short — counts the visible traits.

How to Read a Cross

1
Identify the parent genotypes. Write each parent's two alleles, like \( Tt \) and \( Tt \).
2
Set up the grid. Put one parent's alleles across the top, the other's down the side.
3
Fill each box. Combine the column letter with the row letter, writing the capital first.
4
Count the ratios. Tally the genotypes, then group them into phenotypes.

Example 1: \(Tt \times tt\)

Cross a heterozygous tall plant \( (Tt) \) with a short plant \( (tt) \).

\(T\) \(t\)
\(t\) \(Tt\) \(tt\)
\(t\) \(Tt\) \(tt\)

Example 2: \(Tt \times Tt\)

Cross two heterozygous tall plants. Count the phenotypes carefully.

\(T\) \(t\)
\(T\) \(TT\) \(Tt\)
\(t\) \(Tt\) \(tt\)

Example 3: Carrier Probability

Two parents who are both carriers \( (Cc) \) of a recessive disorder have a child. The disorder appears only in \( cc \) individuals.

\(C\) \(c\)
\(C\) \(CC\) \(Cc\)
\(c\) \(Cc\) \(cc\)

Common Mistakes

✕ Confusing the ratios
A \( Tt \times Tt \) cross gives a \(3:1\) phenotype ratio but a \(1:2:1\) genotype ratio. Don't mix them up.
✕ Ignoring case
Capital \( T \) is dominant; lowercase \( t \) is recessive. Always write the capital letter first.
✕ "Each child = the ratio"
A \( 75\% \) chance is per child. Each new offspring still has its own \( 75\% \) chance.

Genetics Summary

Genotype Phenotype
What it is Allele letters Visible trait
Homozygous dominant \( TT \) Tall
Heterozygous \( Tt \) Tall
Homozygous recessive \( tt \) Short
\( Tt \times Tt \) ratio \( 1:2:1 \) \( 3:1 \)

Ready to Interpret?

You can now read genotypes and phenotypes, build a Punnett square, and predict the chance of inheriting a trait. Let's test your understanding.

Question 1 of 5

An organism with the genotype \( Tt \) is best described as:

Question 2 of 5

In a \( Tt \times Tt \) cross, what is the phenotype ratio of tall to short offspring?

Question 3 of 5

A recessive trait \( (tt) \) can only appear in an offspring when:

Question 4 of 5

Two carrier parents \( (Cc \times Cc) \) have a child. What is the chance the child is a carrier \( (Cc) \)?

Question 5 of 5

The difference between genotype and phenotype is best described as: