D. Genetic Processes · Genetic Processes

Mendel's Peas and the Law of Segregation

D3.2 D3.3 D2.1 A2.2 A1.8 A1.10About 70 minutes

Learning goals

I can describe Mendel's experiments and why peas were a good choice.
I can explain the P, F1 and F2 generations and the 3 : 1 ratio.
I can state the law of segregation and connect it to meiosis.
I can use the words genotype, phenotype, dominant, recessive, homozygous and heterozygous correctly.

Words to know

true-breeding (purebred)
Always produces offspring with the same trait when self-pollinated; homozygous.

hybrid
Offspring of parents with different traits; for one gene, heterozygous.

P, F1, F2 generations
Parental generation; first filial generation (their offspring); second filial generation (offspring of the F1).

dominant
An allele whose trait shows whenever it is present (written with a capital letter, Y).

recessive
An allele whose trait shows only when there is no dominant allele (written lower-case, y).

genotype
The alleles an organism has for a gene (YY, Yy or yy).

phenotype
The observable trait (yellow or green seeds), produced by the genotype and sometimes the environment.

law of segregation
The two alleles for a trait separate when gametes form, so each gamete carries only one allele for each gene.

monohybrid cross
A cross that follows one gene (one trait).

A friar, a garden and thousands of pea plants

Gregor Mendel (1822 to 1884) was a friar at a monastery in Brno (in what is now the Czech Republic). From about 1856 to 1863 he grew and crossed many thousands of pea plants in the monastery garden, counted the offspring carefully, and found mathematical patterns in inheritance. He published his results in 1866. Almost nobody understood how important they were until 1900, when three other botanists rediscovered the same patterns.

Before Mendel, many people believed in blending inheritance: that offspring are a smooth mix of their parents, like mixing paint. Mendel's peas showed that traits are passed on as separate units that do not blend. Today we call those units genes.

Why peas?

They have many varieties with clear either/or traits: yellow or green seeds, tall or short stems.
They self-pollinate normally, so Mendel could start with true-breeding plants.
He could also cross-pollinate them by hand: remove the pollen-making parts (stamens) from one flower and dust it with pollen from another.
They grow quickly and produce many seeds, giving large samples to count.

Mendel also did something unusual for his time: he followed one trait at a time, kept careful records, and used ratios to analyse his counts. That is good scientific method: control variables, use large samples, and look for patterns in the data.

The experiment

P generation: Mendel crossed a true-breeding yellow-seeded plant with a true-breeding green-seeded plant.
F1 generation: every offspring had yellow seeds. Green seemed to disappear.
F2 generation: he let the F1 plants self-pollinate. Green came back! He counted 6022 yellow and 2001 green seeds: a ratio of about 3 : 1.

He repeated this for seven different traits and got close to 3 : 1 every time.

Mendel's seed-colour cross. Yellow and green parents give an F1 that is all yellow. When the F1 plants self-pollinate, green comes back in about one quarter of the F2. (Shading: yellow and green seeds.)

Trait	Dominant form (F2 count)	Recessive form (F2 count)	Ratio
Seed shape	round (5474)	wrinkled (1850)	2.96 : 1
Seed colour	yellow (6022)	green (2001)	3.01 : 1
Flower colour	purple (705)	white (224)	3.15 : 1
Pod shape	inflated (882)	constricted (299)	2.95 : 1
Pod colour	green (428)	yellow (152)	2.82 : 1
Flower position	axial, along the stem (651)	terminal, at the tip (207)	3.14 : 1
Stem length	tall (787)	dwarf (277)	2.84 : 1

Mendel's F2 results (published 1866).

Mendel's F2 results for all seven traits, as dominant : recessive ratios (for example 5474 round ÷ 1850 wrinkled ≈ 2.96). Every trait comes out close to 3 : 1.

Mendel's explanation (in modern words)

Each trait is controlled by a gene, and each organism has two alleles for each gene: one from each parent.
One allele can be dominant over the other. The recessive allele's trait is hidden when the dominant allele is present.
Law of segregation: the two alleles separate when gametes form. Each gamete carries one allele. At fertilization, the offspring gets one from each parent.

So the yellow parent was YY and the green parent yy. Every F1 plant got Y from one parent and y from the other: Yy, which looks yellow. An F1 plant makes half Y gametes and half y gametes. Combine them at random and you get, on average, 1 YY : 2 Yy : 1 yy. Three of every four look yellow. That is the 3 : 1.

About 35 years later (1902 to 1903), scientists Walter Sutton and Theodor Boveri noticed that chromosomes behave in meiosis exactly as Mendel's "factors" do: in pairs that separate into gametes. This became the chromosome theory of inheritance.

The law of segregation, seen through meiosis: the homologue carrying Y and the one carrying y separate in anaphase I, so each gamete receives only one of the two alleles. Mendel deduced this without ever seeing a chromosome.

Genotype and phenotype

The genotype is the pair of alleles an organism carries; the phenotype is what you observe. Because of dominance, two different genotypes can give the same phenotype.

Genotype	Description	Phenotype (pea seeds)
YY	homozygous dominant	yellow
Yy	heterozygous	yellow
yy	homozygous recessive	green

Two genotypes can give the same phenotype.

Writing alleles

Use one letter for one gene: capital for dominant, lower-case for recessive (Y and y, not Y and g). Pick a letter whose capital and lower-case look different when handwritten (avoid S/s, C/c, O/o, W/w). Always write the dominant allele first: Yy, never yY. Dominant does not mean common or better: it only means the allele's trait shows in a heterozygote.

Worked example

Example: Reading Mendel's data

In pea plants, purple flowers (P) are dominant to white (p). A true-breeding purple plant is crossed with a white plant.

(a) Give the genotypes of the parents and the F1. (b) The F1 self-pollinate and produce 929 plants. About how many would you expect to be white? (c) Mendel actually got 705 purple and 224 white. Do his results support the 3 : 1 prediction?

(a) True-breeding purple: PP. White must be homozygous recessive: pp. F1 gets P from one parent and p from the other: Pp (all purple).
(b) Expected fraction white = 1/4. 929 × 1/4 ≈ 232 white (and about 697 purple).
(c) 705 ÷ 224 ≈ 3.15. This is close to 3, and 224 is close to the expected 232. Small differences are expected by chance, so the data support the 3 : 1 prediction.

PP × pp → all Pp; about 232 white expected; yes, 705 : 224 ≈ 3.15 : 1 supports 3 : 1.

Just for fun

What did the recessive allele say when someone asked why it never shows up?

"I'm not missing. I'm just waiting for the right partner."

Hands-on activity

Pea pod survey (or a kitchen sample)

You need: a bag of dried whole peas, or a bag of mixed dried beans, a tray, and paper.

Mendel's success came from large samples and counting. Pour out a handful (at least 100) of peas or beans.
Choose one either/or trait you can see (for example smooth vs wrinkled skin, or two colours in a mixed-bean bag). Sort and count.
Calculate the ratio: bigger group ÷ smaller group.
Now count only the first 12. Calculate the ratio again.

Think about it: Which ratio is more reliable, the one from 12 or from 100+? Why did Mendel count thousands of seeds? (Bigger samples reduce the effect of chance. A store bag is not from a controlled cross, so your ratio does not have to be 3 : 1; the point is the counting method.)

Your turn

Give two reasons pea plants were a good choice for Mendel's experiments.

What is the difference between genotype and phenotype? Give an example of each.

In peas, tall (T) is dominant to dwarf (t). What is the phenotype of Tt? Of tt?

State the law of segregation.

Why did the green phenotype disappear in the F1 and reappear in the F2?

Which phase of meiosis explains the law of segregation?

More practice: warm-up

What does F1 stand for?

Is Rr homozygous or heterozygous?

Which allele is written with a capital letter?

What ratio did Mendel find in the F2 for every trait?

What idea of inheritance did Mendel's results disprove?

More practice: on your own

Calculate the F2 ratio for stem length: 787 tall and 277 dwarf. Is it close to 3 : 1?

A plant with round seeds could be RR or Rr. Why can't you tell by looking?

Why was it important that Mendel started with true-breeding plants?

A student writes yY and Gg for seed colour. What is wrong?

Mendel counted 428 green pods and 152 yellow pods in an F2. Which pod colour is dominant, and how do you know?

Challenge

In an F2 of 800 pea plants from a Tt × Tt cross, how many would you expect to be (a) TT, (b) Tt, (c) tall?

Of the tall F2 plants from a Tt × Tt cross, what fraction are heterozygous? Explain why this is not 1/2.

Mendel's work was published in 1866 but ignored until 1900. Suggest two reasons, based on how science works.

Check your understanding

Quiz

In Mendel's experiments, the F1 offspring of a true-breeding yellow × true-breeding green cross were:

The law of segregation states that:

Which genotype is homozygous recessive?

Two pea plants have the same phenotype but different genotypes. Their genotypes could be:

Mendel's F2 data were close to:

Which observation from meiosis matches the law of segregation?

For parents and teachers

Big idea: This lesson covers the Mendelian-law part of D3.2 and the core vocabulary of D3.3 (genotype, phenotype, dominance, recessiveness). Mendel is also a great example of scientific method (A1): controlled crosses, large samples, ratios.

How to help: Ask your teen to tell Mendel's story as if explaining it to a younger sibling: what he crossed, what he saw in the F1 and F2, and how he explained it. Check that they can say why YY and Yy look the same.

Common difficulties:

Thinking dominant means "most common" or "stronger". It only describes which trait shows in a heterozygote.
Using two different letters for one gene.
Expecting exact 3 : 1 results; real counts vary by chance.