Monohybrid crosses, dominance and segregation

A question about the family

Look at a family photograph. One child has curly hair like the mother, and a sister has straight hair like the father. How does a trait pass from parents to children? An Austrian monk, Gregor Mendel, asked this question about garden peas, Pisum sativum. He crossed them for seven years, from 1856 to 1863, and he counted every plant. His work is the base of genetics, the study of heredity and variation.

• Heredity is the passing of traits from parents to offspring.
• Variation is the difference between individuals of the same kind.

Why peas?

Mendel chose well. Peas gave him four advantages.

• There were many varieties with clear, contrasting traits, such as tall and dwarf.
• The flower is bisexual and normally pollinates itself, so a variety breeds true. A true-breeding line shows the same trait generation after generation.
• A cross is easy. He removed the anthers of one flower (emasculation) and dusted its stigma with pollen of another.
• The life cycle is short, and each plant gives many seeds, so he could count large numbers.

He picked seven pairs of contrasting traits. In each pair, one trait showed in the F1 and the other seemed to vanish.

The words of genetics

• Gene: Mendel called it a factor. It is the unit of heredity.
• Allele: one of the alternative forms of a gene. The gene for height has the alleles T (tall) and t (dwarf).
• Genotype: the alleles of an individual, for example Tt.
• Phenotype: the trait that can be seen, for example tall.
• Homozygous: two identical alleles, TT or tt. Heterozygous: two different alleles, Tt.
• P, F1 and F2: the parents, their children (the first filial generation) and the grandchildren (the second filial generation).
• Dominant: the allele that shows in the heterozygote. Recessive: the allele that is hidden there.

A capital letter marks the dominant allele and a small letter the recessive one.

A cross with one gene

A cross with one pair of contrasting traits is a monohybrid cross. Mendel crossed a true-breeding tall plant (TT) with a true-breeding dwarf plant (tt).

• Every gamete of the tall parent carries T, and every gamete of the dwarf parent carries t. The F1 plants are all Tt, and all are tall.
• He let the F1 plants pollinate themselves. The dwarf trait came back in the F2. Out of every four plants, three were tall and one was dwarf.

The square in the figure is a Punnett square, named after the British geneticist Reginald Punnett. The gametes of one parent run along the top and those of the other down the side. Each box is one possible zygote. The F2 has the genotypic ratio 1 TT : 2 Tt : 1 tt and the phenotypic ratio 3 tall : 1 dwarf. The two ratios differ because Tt looks the same as TT.

Mendel presented his results in 1865 and printed them in 1866, but nobody took notice. In 1900 three workers, de Vries, Correns and Tschermak, found the same laws again.

What the cross shows

• Law of dominance. Characters are controlled by discrete units called factors, which occur in pairs. When the two factors of a pair are unlike, one (the dominant) hides the other (the recessive).
• Law of segregation. The two alleles of a pair separate when gametes form, so a gamete gets only one of them. The alleles do not blend in the heterozygote. This is why the dwarf trait returns in the F2. Because every gamete is pure for one allele, the law is also called the law of purity of gametes.

You should see that Tt with Tt gives 75 per cent tall plants, and that TT with tt gives only Tt. Now try Tt with tt.

The test cross

A tall plant may be TT or Tt. Its look cannot tell you. A test cross settles it. You cross the plant of unknown genotype with a homozygous recessive plant (tt).

• If the unknown plant is TT, every offspring is Tt and tall.
• If the unknown plant is Tt, half the offspring are tall and half are dwarf, a ratio of 1 : 1.

The recessive parent gives only t gametes, so each offspring shows exactly which allele the unknown parent gave it. A back cross is wider. It is any cross of an F1 plant with one of its parents. A test cross is the back cross with the recessive parent.

Why the numbers are only near 3 : 1

Each fertilisation is a matter of chance. A Tt parent gives T or t with equal chance, like a coin. A family of four Tt children can easily show 2 tall and 2 dwarf. Only a large number of offspring comes close to the ratio. Mendel counted thousands of plants, and that is why he could see the pattern.

Try it in Python

The program pairs every gamete of one parent with every gamete of the other and counts the genotypes with exact fractions.

Practise with new numbers

Common mistakes

• Saying that the F2 has three genotypes in the ratio 3 : 1. The genotypic ratio is 1 : 2 : 1. The ratio 3 : 1 is the phenotypic ratio.
• Thinking that the recessive trait is lost in the F1. It is only hidden. The allele is still there, and it comes back in the F2.
• Saying that a dominant trait is the more common trait. Dominant means that it shows in the heterozygote. It does not mean that it is common.
• Crossing the unknown plant with another heterozygote in a test cross. The test partner must be homozygous recessive.

Remember these

• Mendel worked on Pisum sativum from 1856 to 1863 and chose seven pairs of contrasting traits.
• The monohybrid F2 has the genotypic ratio 1 : 2 : 1 and the phenotypic ratio 3 : 1.
• The law of dominance and the law of segregation come from the monohybrid cross.
• A test cross is a cross with the homozygous recessive. A ratio 1 : 1 shows a heterozygote.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. A true-breeding plant with violet flowers is crossed with a true-breeding plant with white flowers. All F1 plants have violet flowers. Which trait is dominant, and what is the genotype of the F1?

2. The F1 plants of question 1 are crossed with each other. Out of 80 F2 plants, how many are expected to have white flowers? How many of the violet plants are heterozygous?

3. A breeder has a tall plant and does not know its genotype. She crosses it with a dwarf plant, and all 12 offspring are tall. What is the most likely genotype of the tall plant?

4. Why is the F2 genotypic ratio 1 : 2 : 1 but the phenotypic ratio 3 : 1?

5. Which law explains that a recessive trait can come back in the F2 after it vanished in the F1?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 4, sections 4.1 and 4.2.
• Clark M. A., Douglas M. and Choi J. (2018). Biology 2e, OpenStax, chapters 12.1 to 12.3.