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Mendelian
Genetics

Explore the foundational laws of inheritance discovered by Gregor Mendel: how traits are passed from parents to offspring through dominant and recessive alleles.

AP Biology

Who Was Gregor Mendel?

Gregor Mendel was a 19th-century Austrian monk who conducted thousands of crosses with pea plants (Pisum sativum) to discover how traits are inherited. His meticulous experiments revealed predictable patterns of inheritance that form the foundation of modern genetics.

Essential Vocabulary

Gene
A segment of DNA that codes for a specific trait
Allele
An alternative version of a gene (e.g., \(B\) vs. \(b\))
Genotype
The genetic makeup of an organism (e.g., \(Bb\))
Phenotype
The observable physical trait (e.g., purple flowers)

Dominant vs. Recessive Alleles

A dominant allele (uppercase letter, e.g., \(B\)) masks the expression of a recessive allele (lowercase, e.g., \(b\)). Only organisms with two copies of the recessive allele (\(bb\)) express the recessive phenotype.

\(BB\) : Homozygous Dominant
Two dominant alleles: dominant phenotype
\(Bb\) : Heterozygous
One of each: still dominant phenotype
\(bb\) : Homozygous Recessive
Two recessive alleles: recessive phenotype

Mendel's First Law: Segregation

During gamete formation, the two alleles for each gene separate so that each gamete carries only one allele for each trait. This occurs during meiosis I when homologous chromosomes are pulled apart.

Parent Cell
\(Bb\)
 Meiosis I Gamete
\(B\)
 Gamete
\(b\)
 50% chance 50% chance

Mendel's Second Law: Independent Assortment

Genes for different traits assort independently during gamete formation, meaning the allele a gamete receives for one gene does not influence which allele it receives for another gene. This applies when genes are on different chromosomes.

Dihybrid: \(BbRr\)
\(BR\)
\(Br\)
\(bR\)
\(br\)
 25% 25% 25% 25% Each combination equally likely

The Principle of Dominance

In a heterozygote, one allele may be completely dominant over the other. The dominant allele is fully expressed while the recessive allele is masked. This means heterozygous (\(Bb\)) and homozygous dominant (\(BB\)) individuals look identical.

Explain how a controlled cross tests segregation.

Maternal anthers are removed before unintended self-pollination, then pollen is transferred from the selected donor. In the simplified model, PP purple × pp white produces purple F1 Pp; intentional F1 selfing gives expected F2 probabilities ¼ PP, ½ Pp, ¼ pp, not guaranteed family counts.

Two parents PP and pp converge to F1 Pp. An explicit Pp by Pp self cross gives F2 genotype probabilities one-quarter PP, one-half Pp and one-quarter pp.
Maternal anthers are removed before unintended self-pollination, then pollen is transferred from the selected donor. In the simplified model, PP purple × pp white produces purple F1 Pp; intentional F1 selfing gives expected F2 probabilities ¼ PP, ½ Pp, ¼ pp, not guaranteed family counts.

Genotype vs. Phenotype

Genotype Phenotype
Definition Genetic composition (alleles) Observable trait expression
Notation Letters: \(BB\), \(Bb\), \(bb\) Description: purple, white
Determined by Inherited allele combinations Genotype + environment
Can be identical? \(BB\) ≠ \(Bb\) \(BB\) and \(Bb\) look the same
Revealed by Genetic testing or test cross Direct observation

The Punnett Square

A Punnett square is a grid used to predict the genotype and phenotype ratios of offspring from a genetic cross. Each row and column represents the alleles contributed by one parent.

\(B\)
\(b\)
\(B\)
\(b\)
\(BB\)
\(Bb\)
\(Bb\)
\(bb\)
 Parent 2 Parent 1

How to Solve a Monohybrid Cross

1
Identify parent genotypes. Assign allele letters and determine each parent's genotype from the problem.
2
List possible gametes. Each parent produces gametes with one allele each. A \(Bb\) parent produces \(B\) and \(b\) gametes.
3
Set up the Punnett square. Place one parent's gametes across the top and the other's down the side.
4
Fill in the grid. Combine the alleles from each row and column to determine offspring genotypes.
5
Determine ratios. Count genotype and phenotype frequencies. Express as ratios or percentages.

Example 1: \(Bb\) × \(Bb\)

Two heterozygous pea plants (\(Bb\) × \(Bb\)) are crossed. \(B\) = purple flowers (dominant), \(b\) = white flowers (recessive). What are the expected offspring ratios?

\(B\)
\(b\)
\(B\)
\(b\)
\(BB\)
\(Bb\)
\(Bb\)
\(bb\)

Example 2: The Test Cross

A plant with purple flowers (unknown genotype: \(B?\)) is crossed with a homozygous recessive (\(bb\)) plant. If any white offspring appear, the unknown parent must be \(Bb\).

If parent is \(BB\)
\(BB\) × \(bb\) → All offspring \(Bb\) (100% purple). No white offspring appear.
If parent is \(Bb\)
\(Bb\) × \(bb\) → 50% \(Bb\) (purple), 50% \(bb\) (white). White offspring reveal the heterozygous genotype.

Example 3: Dihybrid Cross

Cross two plants heterozygous for seed shape (\(R\) = round, \(r\) = wrinkled) and seed color (\(Y\) = yellow, \(y\) = green): \(RrYy\) × \(RrYy\).

\(RY\)
\(Ry\)
\(rY\)
\(ry\)
\(RY\)
\(Ry\)
\(rY\)
\(ry\)
\(RRYY\)
\(RRYy\)
\(RrYY\)
\(RrYy\)
\(RRYy\)
\(RRyy\)
\(RrYy\)
\(Rryy\)
\(RrYY\)
\(RrYy\)
\(rrYY\)
\(rrYy\)
\(RrYy\)
\(Rryy\)
\(rrYy\)
\(rryy\)

Probability Rules in Genetics

Multiplication Rule
The probability of two independent events both occurring: multiply their individual probabilities. \( P(A \text{ and } B) = P(A) \times P(B) \)
Addition Rule
The probability of either one of two mutually exclusive events occurring: add their probabilities. \( P(A \text{ or } B) = P(A) + P(B) \)

Common Mistakes

✕ Phenotype = Genotype
Assuming that organisms with the same phenotype have the same genotype. Remember: \(BB\) and \(Bb\) both show the dominant phenotype.
✕ Ratios Are Exact Counts
Thinking a 3:1 ratio means exactly 3 dominant and 1 recessive offspring. Ratios are probabilities, not guaranteed outcomes.
✕ Blending Inheritance
Believing alleles blend together. In Mendelian genetics, alleles remain discrete units that segregate intact into gametes.

Summary of Mendel's Laws

Law / Principle States That... Key Cross Expected Ratio
Segregation Two alleles separate during meiosis \(Bb \times Bb\) 3:1
Independent Assortment Genes on different chromosomes sort independently \(RrYy \times RrYy\) 9:3:3:1
Dominance Dominant allele masks recessive \(BB \times bb\) All dominant

Question 1 of 5

In a cross between two heterozygous pea plants (\(Bb \times Bb\)), what fraction of offspring are expected to show the recessive phenotype?

Question 2 of 5

A plant with purple flowers is crossed with a plant with white flowers (\(bb\)). All 40 offspring have purple flowers. The purple parent is most likely:

Question 3 of 5

Mendel's Law of Segregation occurs during which stage of cell division?

Question 4 of 5

In a dihybrid cross \(RrYy \times RrYy\), what is the expected phenotypic ratio of offspring?

Question 5 of 5

Using the multiplication rule, what is the probability of getting a homozygous dominant offspring from a \(Bb \times Bb\) cross, AND also getting a homozygous dominant offspring from a second, independent \(Bb \times Bb\) cross?