Punnett Square Notes

1. Punnett Square Notes McDonald/ Francisci SOL LS 12

2. Vocabulary Gamete: A haploid (half # chromosomes) sex cell (sperm or egg) with only ONE set of chromosomes. Genotype: The “types” of genes a person has. Example…TT, Tt, tt Phenotype: The physical characteristics of a person. Example…tall or short. Dominant: The stronger allele Recessive: The weaker allele Homozygous: Two of the same alleles (TT, tt) (purebred) Heterozygous: Two different alleles (Tt) (hybrid)

3. Punnett Square • A diagram that predicts the expected outcome of a genetic cross by considering all combinations of gametes in the cross. • MONOHYBRID CROSS • Involves ONE PAIR of contrasting traits

4. Punnett Squares • Punnettsquares are named for an English geneticist, Reginald Punnett. • Discovered some basic principles of genetics • Used chickens in his genetic experiments

5. Sample Problem • In pea plants (which Gregor Mendel studied), tall pea plants are dominant over short pea plants. • Using Punnett Squares, predict the genotypes and phenotypes of the offspring of a cross between a heterzygous (hybrid) tall pea plant and a homozygous (purebred) short pea plant.

6. Step 1 • Designate letters which will represent the genes/traits. • Capital letters represent dominant traits, and lowercase letters represent recessive traits. T= tall t = short

7. Step 2 • Write down the genotypes (genes) of each parent. These are often given to you or are possible to determine. Tt (tall) X tt(short) – Heterozygous (hybrid) x Homozygous (purebred)

8. Step 3 • List the genes that each parent can contribute. Parent 1 Parent 2Tttt

9. Step 4 • Draw a Punnett square - 4 small squares in the shape of a window. • Write the possible gene(s) of one parent across the top and the gene(s) of the other parent along the side of the Punnett square.

10. Step 5 • Fill in each box of the Punnett square by transferring the letter above and in front of each box into each appropriate box. • As a general rule, the capital letter goes first and a lowercase letter follows.

12. Tall Tall Short Short

13. Step 6 • List the possible genotypes and phenotypes of the offspring for this cross. The letters inside the boxes indicate probable genotypes (genetic makeup), Phenotype (what they look like)

14. Example #1: Yellow color in seeds is dominant over green color in seeds. • What letter would represent yellow gametes • Y (Upper case, dominant trait) • What letter would represent green gametes • y (lower case, recessive trait, use first letter of dominant trait) A plant that is homozygous for yellow seeds () is crossed with a plant that is homozygous for green seeds (). What could their offspring look like? X YY yy yy YY

15. Set up the Punnett Square • Write the genotypes of each parent. Put each allele (gene) in it’s own column. Then cross the gametes to determine the possible genotypes of the offspring Parent #1 Y Y y y Yy Yy Parent #2 Yy Yy

16. Questions • How many of their offspring will be yellow? 4 out of 4 – 100% (the dominant trait is stronger than the recessive trait) • How many of their offspring will be green? 0 • Are the offspring homozygous or heterozygous? heterozygous

17. Example #2: Two plants that are heterozygous for seed color are crossed, what would their offspring look like? Yy Yy X Parent #1 Parent #2 Y y Y y YY Yy yy Yy

18. Questions • What percent of the offspring are homozygous dominant? 1 out of 4 – 25% • What percent of the offspring are heterozygous? 2 out of 4 – 50% • What percentage of the offspring are homozygous recessive? 1 out of 4 – 25% • How many of offspring will be yellow? 75% • How many of the offspring will be green? 25%