Mendel crossed pea plants with clear-cut differences, such as round and wrinkled seeds, for years and counted the offspring, and from those counts he found the rules by which traits pass from parents to offspring.
Crossing two pure-breeding plants gives a first generation (F₁) that shows only the dominant trait; crossing F₁ plants with each other brings the two traits back out in the second generation (F₂) in a 3 : 1 ratio.
The genotypes come out as AA : Aa : aa = 1 : 2 : 1, and because a single A is enough for a round seed, the ratio you can see is 3 : 1.
The Punnett square below spreads out, in four boxes, how the reproductive cells from two Aa plants pair up.
Why did Mendel choose peas?
🌱 Clear differences, easy to cross
①Peas have many traits that split cleanly into two with nothing in between, such as seed shape (round or wrinkled) and seed colour (yellow or green)
②A generation is short and each plant bears many seeds, so offspring are easy to count
③The petals wrap around the stamens and pistil so the flower pollinates itself, which lets a person move pollen by hand and make only the crosses they want
④Mendel grew tens of thousands of plants this way and counted the offspring trait by trait
Reading a cross from a Punnett square
seegongsik.com
A four-box Punnett square for a cross between two Aa plants, and a comparison of when the dominant and recessive traits appear
The patterns Mendel found
📚 Dominance, segregation, independence
①Dominance: crossing two pure-breeding plants with contrasting traits gives an F₁ that shows only one of them, the dominant trait
②Law of segregation: the two alleles of a pair separate into different reproductive cells, so the F₂ shows dominant : recessive = 3 : 1
③Law of independent assortment: different traits, such as seed shape and seed colour, are passed on separately without affecting each other
④The picture above follows one trait and gives 3 : 1; following two traits together gives an F₂ ratio of 9 : 3 : 3 : 1 (see the summary below)
Summary
Law of segregation
F₂ phenotype ratio = dominant : recessive = 3 : 1
Aa × Aa → AA : Aa : aa = 1 : 2 : 1
Dominance
AA × aa → F₁ all Aa, showing only the dominant trait
Aa carries the recessive allele a, yet only the dominant trait shows
Law of independent assortment
F₂ phenotype ratio = 9 : 3 : 3 : 1
When two pairs of contrasting traits (for example seed shape and seed colour) are inherited separately
🎯 Points that often come up
①If there is at least one dominant allele A (AA or Aa), the dominant trait appears
②The recessive trait appears only in aa
③Do not mix up the F₂ phenotype ratio 3 : 1 with the genotype ratio 1 : 2 : 1
④AA and aa are pure-breeding, Aa is a hybrid, and AA and Aa cannot be told apart by looks alone
⑤In a cross following two traits, the F₂ ratio is 9 : 3 : 3 : 1
Worked examples and unit test
Example 1
In peas, round seeds (A) are dominant over wrinkled seeds (a). If two Aa hybrids are crossed, what are the phenotype ratio and the genotype ratio of the offspring?
1
Each parent makes reproductive cells carrying A and a in equal numbers. Filling the four boxes of the Punnett square gives one AA, two Aa and one aa.
2
AA and Aa have at least one A and give round peas; only aa gives wrinkled peas, so round : wrinkled = 3 : 1.
▸ Phenotypes round : wrinkled = 3 : 1; genotypes AA : Aa : aa = 1 : 2 : 1
In the Punnett square above, only one of the four boxes shows a wrinkled pea. That one box out of four is the 3 : 1.
Example 2
A pure-breeding round pea (AA) is crossed with a pure-breeding wrinkled pea (aa). What are the genotype and phenotype of the F₁, and why?
1
AA makes only A cells and aa makes only a cells, so every F₁ plant receives one A and one a and is Aa.
2
A is dominant over a, so every Aa plant in the F₁ has round peas.
▸ The F₁ is all Aa, and every plant has round peas (dominance)
There is not a single wrinkled pea in the F₁, but a has not vanished. Cross the F₁ plants with each other and wrinkled peas turn up again in the F₂.
Unit Test
Which statement about Mendel's laws is correct?
①The offspring phenotype ratio of a hybrid Aa × Aa is 1 : 1
②A genotype of Aa shows the recessive trait
③The first hybrid generation of pure AA × aa all show the dominant trait
④The recessive trait appears when the genotype is AA
⑤The phenotype ratio of the second hybrid generation is always 1 : 2 : 1
▸ ③ The first hybrid generation of pure AA × aa all show the dominant trait
1
The phenotype ratio of Aa × Aa offspring is 3 : 1 (① wrong, 1:2:1 is the genotype ratio), and Aa has an A so it shows the dominant trait (② wrong).
2
The recessive trait appears only as aa (④ wrong), and the F₂ phenotype ratio is 3 : 1 (⑤ wrong, 1:2:1 is the genotype); the F₁ of pure AA × aa all show the dominant trait (③ correct, law of dominance).