About this experiment
Repeat Mendel's pea experiments in seconds. Choose the genotypes of two parent plants, read the Punnett square, then grow hundreds of offspring and compare what you get with what the square predicts. Small samples wander away from 3:1 and 9:3:3:1, while large samples settle close to them.
Learning goals
- Build a Punnett square for one-gene and two-gene crosses
- Use the law of segregation and the law of independent assortment to predict offspring
- Tell genotype from phenotype and explain dominant and recessive alleles
- See why observed ratios only approach the expected ratio when the sample is large
How to use it
- 1Pick One gene or Two genes and set the genotype of each parent.
- 2Read the gametes along the edges of the Punnett square and the offspring in each cell.
- 3Press Grow offspring with 10 per click, then with 1000, and compare the bars with the black expected marks.
- 4Try a test cross: set one parent to yy (or yyrr) and see how it reveals the other parent.
The model
Assumptions
- Yellow (Y) is completely dominant over green (y), and round (R) over wrinkled (r).
- The two genes are on different chromosomes, so they assort independently.
- Every gamete is equally likely and every seed survives.
Try this in class
- Q1Why does a cross of two yellow plants sometimes give green seeds?
- Q2Grow only 10 offspring several times. How far from 3:1 can the result be?
- Q3Which parent genotype gives all yellow offspring no matter what the other parent is?
- Q4In a dihybrid cross, what fraction of offspring is green and wrinkled? Check it with 1000 seeds.
Frequently asked questions
What is the difference between genotype and phenotype?
The genotype is the pair of alleles a plant carries, such as Yy. The phenotype is the trait you can see, such as yellow seeds.
Why are my results not exactly 3:1?
Each seed gets its alleles by chance. Ratios are probabilities, so a small sample can differ a lot, and a large sample comes closer to the expected ratio.
What is a test cross?
Crossing an individual with a homozygous recessive partner. If any recessive offspring appear, the individual must carry the recessive allele.