5.3 Mendelian Genetics

Mendelian genetics explains how traits pass from parents to offspring through two main rules: the law of segregation and the law of independent assortment. You use Punnett squares, probability rules, and pedigrees to predict genotypes, phenotypes, and inheritance patterns for single-gene traits. AP Biology questions often ask you to connect these inheritance patterns to meiosis, gamete formation, and probability.

Law of Segregation in AP Bio

The law of segregation says the two alleles for a gene separate during gamete formation, so each gamete receives one allele. When fertilization joins two gametes, the offspring gets two alleles again. This is why a heterozygous parent such as Tt can make T gametes and t gametes, and why Punnett squares can predict offspring ratios.

On the AP Biology exam, connect segregation to meiosis, gamete formation, and probability. If a question asks for a genotypic or phenotypic ratio, start by identifying what alleles each parent can pass on, then use a Punnett square or probability rules to predict the outcome.

Why This Matters for the AP Biology Exam

This topic builds the genetics reasoning you will use across Unit 5 and beyond. On the AP Biology exam, you can expect to calculate genotypic and phenotypic ratios, apply probability rules to single-gene traits, and predict inheritance patterns from data like pedigrees. Mixing up genotypic and phenotypic ratios is a common scoring mistake, so getting comfortable with that difference pays off directly.

These skills also set up the next topic, non-Mendelian genetics, where you compare observed ratios to the ratios Mendel's laws predict and decide whether the data fit or deviate.

Key Takeaways

• The law of segregation: the two alleles for a gene separate during gamete formation, so each gamete gets only one allele.
• The law of independent assortment: genes on different chromosomes are inherited independently of each other.
• Genotype is the set of alleles an organism has; phenotype is the observable trait. Homozygous means two identical alleles, heterozygous means two different alleles.
• Punnett squares predict the possible genotypes and phenotypes of offspring; probability rules let you combine multiple traits or multiple offspring.
• Monohybrid crosses of two heterozygotes give a 3:1 phenotypic ratio; dihybrid crosses of two double heterozygotes give a 9:3:3:1 phenotypic ratio.
• Test crosses, pedigrees, and offspring data help you figure out whether an allele is dominant or recessive and whether a trait is autosomal, sex-linked, or genetically linked.

Mendel's Laws of Inheritance

Gregor Mendel, an Austrian monk in the mid-1800s, worked out the basic rules of inheritance by tracking traits in pea plants across generations. His big insight was that inheritance follows patterns you can predict with math. Two of those patterns became his core laws.

Law of Segregation

During gamete formation, the two alleles for each gene separate so each gamete receives only one allele. When two gametes join at fertilization, the zygote ends up with two alleles per gene again, restoring the diploid number.

For example, a plant with the genotype Tt for height produces gametes carrying either the T allele or the t allele. These gametes combine randomly with gametes from another plant, creating new genotype combinations in the offspring.

Law of Independent Assortment

Genes located on different chromosomes are inherited independently of one another. The alleles for one gene sort into gametes without affecting how the alleles for another gene sort.

This is why you might have one parent's eye color and the other parent's hair texture. A plant heterozygous for two traits (AaBb) produces four types of gametes (AB, Ab, aB, ab) in equal frequency, which increases genetic variation.

Both of these laws apply to genes on different chromosomes. That detail matters later, because genes on the same chromosome do not always assort independently.

Fertilization and Genetic Variation

In most cases, fertilization joins two haploid gametes and restores the diploid chromosome number. Because each gamete carries a unique mix of alleles, fertilization creates new allele combinations in the zygote and increases genetic variation in a population. This is one reason siblings with the same parents can look very different from each other.

Genotype vs. Phenotype

An organism's genotype is the combination of alleles it carries for a gene or genes. For each gene, a genotype is either homozygous (two identical alleles, like AA or aa) or heterozygous (two different alleles, like Aa).

An organism's phenotype is the observable expression of those inherited traits. For example, Yy is a genotype, while yellow seeds is the phenotype. Keeping these straight helps with predicting cross outcomes, because more than one genotype can produce the same phenotype.

Punnett Squares: Predicting Inheritance

Punnett squares are diagrams that lay out every possible allele combination parents can pass to offspring, so you can predict genotypes and phenotypes.

When a heterozygous yellow-seeded plant (Yy) is crossed with a homozygous recessive green-seeded plant (yy):

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    |  Y  |  y  |
----|-----|-----|
 y  | Yy  | yy  |
----|-----|-----|
 y  | Yy  | yy  |

This cross gives:

• 50% heterozygous (Yy) offspring with the dominant yellow phenotype
• 50% homozygous recessive (yy) offspring with the recessive green phenotype

Only the homozygous recessive genotype (yy) shows the recessive phenotype. Both homozygous dominant (YY) and heterozygous (Yy) genotypes show the dominant phenotype.

Probability in Genetics

Probability rules are useful tools for working out cross outcomes, especially when a Punnett square would be huge.

• For mutually exclusive events: P(A or B) = P(A) + P(B)
• For independent events: P(A and B) = P(A) × P(B)

For example, if two heterozygous parents (Aa × Aa) have a child, the probability of a homozygous recessive (aa) child is 1/4, or 25%. If they have four children, the probability that all four are homozygous recessive is (1/4)⁴ = 1/256.

Patterns of Inheritance

Monohybrid Inheritance

Monohybrid inheritance tracks a single gene across generations. When two heterozygotes (Aa × Aa) are crossed, the expected phenotypic ratio is 3:1 (dominant:recessive) and the genotypic ratio is 1:2:1 (homozygous dominant:heterozygous:homozygous recessive). Notice that these two ratios are different numbers for the same cross. A monohybrid cross, paired with observed offspring phenotypes, can help you figure out which allele is dominant and which is recessive.

Dihybrid Inheritance

Dihybrid inheritance tracks two genes at once. When two individuals heterozygous for both traits (AaBb × AaBb) are crossed, the phenotypic ratio follows a 9:3:3:1 pattern:

• 9/16 show both dominant traits (A_B_)
• 3/16 show the first dominant and second recessive traits (A_bb)
• 3/16 show the first recessive and second dominant traits (aaB_)
• 1/16 show both recessive traits (aabb)

This 9:3:3:1 ratio comes directly from the law of independent assortment. It shows how alleles of different genes are inherited independently and form new combinations in offspring. Dihybrid crosses let you track two dominant/recessive relationships at once and test whether two genes assort independently.

Test Crosses

A test cross determines whether an individual showing a dominant trait is homozygous dominant or heterozygous. You cross the organism in question with a homozygous recessive individual. If any recessive offspring appear, the unknown parent must be heterozygous. If all offspring show the dominant trait, the unknown parent is likely homozygous dominant.

Pedigree Analysis

Pedigrees are family-history diagrams used to trace inheritance patterns across generations. From pedigree data, you can often predict whether a trait is autosomal or sex-linked, and whether the allele causing it is dominant or recessive.

Some clues to look for:

• A recessive trait may skip generations.
• A dominant trait often appears in every generation.
• Sex-linked traits often show up differently in males and females.

Pedigrees and Punnett squares work together. Once you identify a likely inheritance pattern from a pedigree or offspring data, you can use Punnett squares both to predict offspring outcomes and to infer the most likely genotypes of the parents.

Autosomal vs. Sex-Linked Inheritance

Autosomal inheritance involves genes on autosomes (non-sex chromosomes), so the trait usually appears in males and females at similar rates. Sex-linked inheritance usually refers to genes on the X chromosome, and those traits often appear more often in males because males have only one X chromosome. Telling autosomal from sex-linked patterns apart helps you interpret pedigrees and predict inheritance.

Genetically Linked Inheritance

Genes located on the same chromosome are genetically linked. Unlike independently assorting genes, linked genes tend to be inherited together unless crossing over separates them during meiosis. Because linked genes do not assort independently, observed offspring ratios deviate from the 9:3:3:1 ratio expected for independent assortment.

In inheritance data, linkage is suggested when parental allele combinations show up more often than recombinant combinations. Crossing over can still produce some recombinant offspring, but linked genes are inherited together more often than not. Practice using offspring data to decide whether two genes assort independently or are genetically linked.

How to Use This on the AP Biology Exam

Problem Solving

• Read carefully to see whether the question asks for a genotypic ratio or a phenotypic ratio, then answer the one it asks for. These are easy points to lose by answering the wrong ratio.
• Set up a Punnett square for one or two genes, but switch to probability rules when the cross involves many genes or many offspring.
• Use P(A and B) = P(A) × P(B) to combine independent traits, and P(A or B) = P(A) + P(B) for outcomes that cannot happen together.

Free Response

• When asked to explain inheritance, name the relevant law (segregation or independent assortment) and connect it to what happens during gamete formation.
• Support claims with the predicted ratios. For example, a 3:1 phenotypic ratio points to a monohybrid cross of two heterozygotes.
• When given offspring data, compare observed numbers to expected Mendelian ratios to argue whether genes assort independently or are linked.

Using Sources Effectively

• For pedigrees, check whether the trait skips generations (often recessive) or appears in every generation (often dominant), and whether it shows up differently in males and females (a clue for sex-linked).
• Use the pedigree pattern to assign likely genotypes, then build a Punnett square to back up your reasoning.

Common Misconceptions

• Genotypic ratio and phenotypic ratio are not the same. A monohybrid cross of two heterozygotes gives a 1:2:1 genotypic ratio but a 3:1 phenotypic ratio.
• Dominant does not mean more common or stronger. It just means that allele's trait shows in the phenotype when present. Recessive traits can be very common in a population.
• The law of independent assortment applies to genes on different chromosomes. Genes on the same chromosome can be linked and may be inherited together.
• Heterozygous and homozygous dominant individuals can have the same phenotype, so you cannot always tell genotype from appearance. That is exactly why test crosses exist.
• A Punnett square shows probabilities, not guarantees. A 3:1 ratio is the expected outcome, not a promise that exactly three of four offspring will show the dominant trait.
• Sex-linked is not the same as sex-limited. Sex-linked means the gene is on a sex chromosome, usually the X, which is why these traits often appear more often in males.

zation restores the two-allele combination in the offspring.

What is the law of independent assortment?

The law of independent assortment says genes on different chromosomes are inherited independently of one another. This is why a dihybrid cross can produce new allele combinations and the classic 9:3:3:1 phenotypic ratio.

What is the difference between genotype and phenotype?

Genotype is the allele combination an organism has, such as AA, Aa, or aa. Phenotype is the observable trait, such as yellow seeds or green seeds. More than one genotype can sometimes produce the same phenotype.

What is the expected ratio for a monohybrid cross?

For a monohybrid cross between two heterozygotes, such as Aa x Aa, the expected genotypic ratio is 1:2:1 and the expected phenotypic ratio is 3:1.

What is the expected ratio for a dihybrid cross?

For a dihybrid cross between two double heterozygotes, such as AaBb x AaBb, the expected phenotypic ratio is 9:3:3:1 if the genes assort independently.

How do I know if genes are linked?

Genes may be linked if offspring data show parental allele combinations much more often than recombinant combinations. Linked genes are on the same chromosome, so they do not follow the independent assortment pattern unless crossing over separates them.

Related AP Biology Guides

• Unit 5 Overview: Heredity
• 5.1 Meiosis
• 5.2 Meiosis and Genetic Diversity
• 5.4 Non-Mendelian Genetics
• Chromosomal Inheritance Review
• 5.5 Environmental Effects on Phenotype