Variation and Mendel's Experiments: Quick Revision

A brief note on Variation and Mendel's Experiments. Read it before a test. Try each question at the end before you open its answer.

Variation

• Heredity passes on a common basic design and small changes in it.
• DNA copying is never perfect, so asexual reproduction makes a little variation.
• Sexual reproduction mixes the DNA of two parents, so it makes far more.
• Variations build up over generations. One that helps survival is more likely to be passed on.

Mendel's peas

• Mendel used the garden pea, Pisum sativum: quick to grow, many seeds, clear contrasting traits.
• He studied seven traits, such as round or wrinkled seeds and tall or short stems, and counted the young.

The monohybrid cross

• Pure tall (TT) crossed with pure short (tt): the F1 is all tall (Tt).
• The F1 selfed: the F2 is $3$ tall : $1$ short, or $1$ TT : $2$ Tt : $1$ tt.
• Each trait has a pair of gene copies, one from each parent. A gamete carries one copy.
• The dominant copy shows and hides the recessive one.
• Genotype is the pair of copies; phenotype is what the plant looks like.
• Homozygous: TT or tt. Heterozygous: Tt.

The dihybrid cross

• Pure round yellow (RRYY) crossed with pure wrinkled green (rryy): the F1 is all round yellow (RrYy).
• An RrYy plant makes four gametes: RY, Ry, rY and ry.
• The F2 is round yellow : round green : wrinkled yellow : wrinkled green $= 9 : 3 : 3 : 1$.
• Round green and wrinkled yellow are new combinations, so the two traits are inherited independently.

A worked example

A pure tall plant is crossed with a pure short one, and the F1 is selfed. The F2 has $400$ plants. How many are tall, and how many of those are Tt?

1. Short. $\dfrac{1}{4} \times 400 = 100$ plants.
2. Tall. $400 - 100 = 300$ plants.
3. Tt. Tt is $2$ parts of $4$: $\dfrac{2}{4} \times 400 = 200$ plants.

So $300$ plants are tall, and $200$ of them are Tt.

Practice with new numbers

A program to try

The program finds the round and wrinkled seeds in an F2 of the dihybrid cross.

Mistakes to avoid

• Saying the F1 is of middle height. It is all tall: tall is dominant.
• Saying a tall plant is always TT. It may be TT or Tt.
• Giving an RrYy plant two kinds of gametes. It makes four.
• Forgetting the new combinations in the dihybrid F2.

Check yourself

1. Why does sexual reproduction make more variation than asexual reproduction?

2. Pure violet-flowered peas are crossed with pure white-flowered ones. What colour are the F1 flowers, and why?

3. What is the difference between genotype and phenotype?

4. A tall plant crossed with a short one gives about half tall and half short young. What is the tall plant's genotype?

5. Which two kinds of F2 seed in the dihybrid cross show independent inheritance?