What is heredity?
Heredity is the passing of traits from parents to offspring. In the 1860s, an Austrian monk named Gregor Mendel worked out the basic rules of heredity by breeding pea plants (Pisum sativum) and carefully counting the traits of thousands of offspring over many generations. His rules still describe how many traits are inherited today. Every trait Mendel studied was controlled by a gene, a piece of DNA that carries the instructions for that trait. Each gene can come in different versions called alleles. A pea plant's height gene, for example, has a tall allele and a short allele. Every pea plant carries two alleles for each gene, one inherited from each parent.
Why pea plants
Mendel chose pea plants because they have easily observed either/or traits (tall or short, purple or white flowers), they self-pollinate naturally so he could control which plants bred with which, and they produce many offspring quickly, giving him enough data to see clear number patterns.
Alleles, dominant and recessive traits
When an organism carries two different alleles for the same gene, one allele can hide the other. The allele that gets expressed is dominant, the allele that gets hidden is recessive. By convention, biologists write the dominant allele as an uppercase letter and the recessive allele as the same letter lowercase. Example: In pea plants, the tall allele (T) is dominant over the short allele (t). A plant only shows the short trait if it carries two copies of the recessive allele.
| Genotype | Alleles present | Called | Phenotype |
|---|---|---|---|
| TT | two dominant alleles | homozygous dominant | tall |
| Tt | one of each allele | heterozygous | tall (T hides t) |
| tt | two recessive alleles | homozygous recessive | short |
Recessive doesn't mean rare
A recessive allele can still be common in a population. Recessive only describes how the allele behaves when paired with a dominant allele, not how often it shows up. Many common traits, including some blood types and eye colors, involve recessive alleles carried by a large share of people.
Genotype vs phenotype
A genotype is the pair of alleles an organism carries for a gene, written as letters (TT, Tt, or tt). A phenotype is the physical trait that genotype produces, the thing you can actually observe (tall or short). Two different genotypes can produce the same phenotype. Both TT and Tt pea plants look tall, because one dominant T allele is enough to produce the tall phenotype. Only the tt genotype produces the short phenotype, since no dominant allele is present to hide the recessive trait. Example: A pea plant has the genotype Tt. Its phenotype is tall, because the dominant T allele determines the phenotype whenever it's present.
Homozygous vs heterozygous
Homozygous means both alleles match (TT or tt). Heterozygous means the two alleles differ (Tt). You cannot tell a homozygous dominant plant (TT) apart from a heterozygous plant (Tt) just by looking, since both show the dominant phenotype. Only a genetic test or a controlled cross can reveal the genotype.
Reading a Punnett square (monohybrid cross)
A Punnett square is a grid that predicts the possible genotypes of offspring from a cross between two parents. A cross that tracks a single gene, like height alone, is called a monohybrid cross. Each parent contributes one allele to each offspring. Along the top of the square, write the alleles one parent can pass on. Along the side, write the alleles the other parent can pass on. Each inside box shows one possible offspring genotype, formed by combining the allele above it with the allele beside it. Example: Cross two heterozygous tall pea plants, Tt × Tt. Each parent can pass on either a T or a t.
| × | T | t |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
Reading the four boxes: 1 TT, 2 Tt, 1 tt. That's a genotype ratio of 1 : 2 : 1. Since TT and Tt both look tall, the phenotype ratio is 3 tall to 1 short, written 3 : 1.
Genotype ratio vs phenotype ratio
Always report both ratios separately. The genotype ratio for Tt × Tt is 1 TT : 2 Tt : 1 tt. The phenotype ratio, grouping the genotypes that look alike, is 3 tall : 1 short. The two ratios describe the same cross but answer different questions.
Mendel's law of segregation
Mendel's law of segregation states that the two alleles an organism carries for a gene separate from each other during the formation of gametes (sex cells), so each gamete carries only one allele for that gene, not two. This is why a Tt parent can pass on either a T or a t, but never both, to any single offspring. When fertilization happens, the offspring receives one allele from the egg and one allele from the sperm, restoring the normal pair of two alleles per gene. Example: A Tt pea plant produces two kinds of gametes in equal numbers, half carrying T and half carrying t. This 1-to-1 split of gametes is exactly what makes the top row and side row of a Punnett square work.
Segregation happens in every parent
Segregation applies to both parents in a cross, not just one. In the Tt × Tt cross, each parent separately produces T and t gametes in a 1-to-1 ratio, and the Punnett square combines both parents' gametes to predict the offspring.
Mendel's law of independent assortment
Mendel's law of independent assortment states that alleles for different genes are distributed to gametes independently of one another, as long as the genes sit on different chromosomes (or far apart on the same one). Inheriting one trait does not influence which allele gets inherited for a different trait. Example: In pea plants, seed color (Y = yellow, dominant, y = green, recessive) and seed shape (R = round, dominant, r = wrinkled, recessive) are controlled by separate genes. Crossing two plants that are heterozygous for both traits, YyRr × YyRr, produces offspring in a phenotype ratio of 9 : 3 : 3 : 1.
| Phenotype | Ratio |
|---|---|
| yellow, round | 9 |
| yellow, wrinkled | 3 |
| green, round | 3 |
| green, wrinkled | 1 |
Independent assortment needs two genes
A monohybrid cross like Tt × Tt only tests segregation, since it tracks one gene. Independent assortment only shows up when a cross tracks two or more genes at once, like the YyRr × YyRr cross above, because it's specifically about how different genes sort independently of each other.
Try it yourself
pea-plant-cross-counts.csvDownload pea-plant-cross-counts.csv: https://restedteacher.com/practice-files/biology/pea-plant-cross-counts.csv
Offspring phenotype counts from 3 fictional pea plant crosses: two flower-color crosses (Cross 1 and Cross 2) and one two-trait seed cross (Cross 3).
Open it in a spreadsheet
Open it in Google Sheets or Excel. Group the rows by crossid and divide each phenotype's count by the smallest count in that cross to turn the raw numbers into a ratio.
1. A pea plant has the genotype Rr for seed shape, where R (round) is dominant over r (wrinkled). What is its phenotype?
2. Two heterozygous tall pea plants are crossed, Tt × Tt. What is the phenotype ratio of the offspring?
3. Which law explains why a Tt parent passes on only one allele, T or t, to each gamete, never both?
4. A YyRr plant is crossed with another YyRr plant. What phenotype ratio results?
5. Using pea-plant-cross-counts.csv, calculate the phenotype ratio of purple to white offspring for Cross 1. Based on that ratio and what this sheet says about dominant and recessive alleles, what are the most likely genotypes of the two parent plants?
6. Calculate the phenotype ratio of purple to white offspring for Cross 2. A cross between a heterozygous plant and a homozygous recessive plant is called a testcross. What two genotypes, crossed together, produce this exact ratio?
7. Calculate the phenotype ratio for all four phenotypes in Cross 3. Which of Mendel's two laws does this ratio demonstrate, and why?
8. Using a Punnett square and Mendel's law of segregation, predict the phenotype ratio you would expect from crossing a heterozygous purple plant (Pp) with a homozygous recessive white plant (pp). Does Cross 2's data match your prediction?