Heredity and Genetics

What is heredity?

Heredity is the passing of traits from parents to their offspring. A trait is a feature of a living thing, such as the color of a flower, the shape of a seed, or the height of a plant. Genetics is the science that studies heredity.

Offspring often look like their parents, but not exactly. Brothers and sisters with the same parents can look quite different, and a trait can skip a generation and then show up again. For a long time, people could not explain why.

In the 1800s, many people believed in blending: the idea that a child's traits are a smooth mix of the parents' traits, the way red and white paint make pink. If that were true, a trait could never vanish and come back. Mendel's peas showed that it can.

Mendel and his peas

Gregor Mendel was a member of the Augustinian Abbey of St. Thomas in Brno. He chose the garden pea, Pisum sativum, for good reasons. Pea plants grow from seed to seed in one season. A pea flower usually fertilizes itself, so a plant that has always made purple-flowered offspring keeps doing so. Scientists call that true-breeding. Mendel could also cross two plants on purpose, moving pollen by hand from one flower to another.

The experiment

Mendel started with true-breeding plants that differed in one trait, such as purple flowers versus white flowers. These were the parents, the P generation. He crossed them and grew their seeds into the first generation of offspring, the F₁ generation. Then he let the F₁ plants fertilize themselves to get the F₂ generation.

The results surprised people who believed in blending. Every F₁ plant had purple flowers, not pale purple. The white trait seemed to be gone. But in the F₂ generation, white flowers came back, in about one plant out of every four.

Mendel did the same experiment with seven different traits. Each time, one form of the trait showed up in every F₁ plant. In the F₂ plants, the two forms came out at close to 3 to 1. Here are his F₂ counts:

DNA, chromosomes, and genes

Mendel never saw a gene. He only knew that something inside the pea seeds carried each trait from parent to offspring. Today we know where to look.

• DNAThe molecule that carries the instructions for building and running a living thing. It is shaped like a twisted ladder, called a double helix. The rungs are made of four chemical bases, written A, T, G, and C. The order of those bases spells out the instructions.
• ChromosomesThread-like structures in the nucleus of a cell. Each one is a long DNA molecule wound tightly around proteins. Most human body cells have 46 chromosomes, in 23 pairs. One chromosome in each pair came from the mother and one from the father.
• GenesA gene is a section of DNA that carries instructions. Many genes hold the instructions for making a protein. Humans have about 19,900 genes that make proteins.

Chromosomes come in pairs, so genes do too. You have two copies of almost every gene, one from each parent. (The X and Y chromosomes are the main exception in males.) A pea plant also has two copies of each gene, one from each parent plant. That is the key to Mendel's results.

Alleles: dominant and recessive

A gene can come in different versions. Each version is called an allele. The pea gene for flower color has a purple allele and a white allele.

When a plant gets two different alleles, often only one of them shows. The allele that shows is dominant. The one that is hidden is recessive. A recessive trait shows only when a living thing has two copies of the recessive allele.

Geneticists use letters for alleles. A capital letter stands for the dominant allele, and the same letter in lowercase stands for the recessive one. For flower color, P is purple and p is white.

• HomozygousThe two alleles are the same, such as PP or pp. (Homo- means "same.")
• HeterozygousThe two alleles are different, such as Pp. (Hetero- means "different.")

This explains Mendel's peas. The true-breeding purple parent was PP and the white parent was pp. Every F₁ plant got one P and one p, so every one was Pp, with purple flowers. The white allele was still there, just hidden. When two Pp plants were crossed, some offspring got p from both parents, and white flowers came back.

Not every gene works this simple way. In snapdragons, a red-flowered parent crossed with a white-flowered parent gives pink offspring, because neither allele fully hides the other. This is called incomplete dominance.

Genotype and phenotype

Your genotype is the pair of alleles you have for a gene. Your phenotype is the trait that actually shows. Two plants can have different genotypes and the same phenotype:

Just by looking, you can't tell whether a purple plant is PP or Pp. Mendel worked out a way to check: cross it with a white (pp) plant. If any white offspring appear, the purple plant must have been carrying a hidden p. This is called a test cross, and plant and animal breeders still use it.

Punnett squares

A Punnett square is a chart for predicting the offspring of a cross. Each parent passes on one of its two alleles, chosen by chance. The square shows every way the alleles can combine.

1. Write one parent's two alleles across the top and the other parent's two alleles down the side.
2. Fill each box with the letter above it and the letter beside it. Write the capital letter first.
3. Count the boxes. Each box stands for a 1 in 4 chance, or 25%.

A Punnett square shows chances, not certainties. If two Pp plants have just four offspring, they won't always get exactly three purple and one white. Each seed is a new roll of the dice. With hundreds of offspring, the numbers get close to 3 to 1, just as Mendel found.

Here is a second example. Cross a Pp plant with a pp plant. The boxes come out Pp, Pp, pp, and pp, so about half of the offspring (2 out of 4) should be purple and half white.

Genes and the environment

Genes are not the whole story. Many traits depend on genes and the environment, meaning where and how a living thing grows.

• Hydrangea flowers. Bigleaf hydrangeas often bloom blue in acidic soil and pink in soil that is neutral or alkaline. In acidic soil, the roots can take up aluminum, and that turns the flowers blue. In between, the flowers can be purple.
• Flamingos. Flamingo chicks hatch with white down. Their pink color comes from their food, such as algae and small shrimp, and it takes several years to develop.
• Human height. Scientists estimate that about 80% of the differences in height between people come from differences in their DNA. More than 700 gene variants are known to play a part. The environment matters too, including nutrition (even a mother's nutrition during pregnancy) and health care.

Each cross uses one of Mendel's pea traits. First fill in all four boxes of the Punnett square, then predict how many offspring out of four are expected to show the dominant trait. A round has five crosses.

How scoring works: each correct box is worth 1 point and a correct prediction is worth 1 more, so each cross is worth 5 points and a perfect round is 25. Your best round is saved in this browser only.