Probability & Punnett Squares

Genetics & Probability When Mendel crossed two hybrid plants for height, Tt, he found that ¾ of the offspring were tall, and ¼ were short. Probability can be used to explain the results of genetic crosses.

What is Probability? Probability: the chance that an event will occur. If you flip a coin, you have a 50% chance of landing on heads. If you flip three times, each time you have a 50% chance, or ½. The probability of landing on heads three times in a row is: ½ x ½ x ½ = 1/8 Genetic segregation is random, like flipping a coin. The principles of probability can be used to predict the outcomes of genetic crosses.

Punnett Squares Diagrams used to determine the gene combinations from a genetic cross. The letters represent alleles. Can be used to predict and compare the genetic variations in the cross results.

Example

Homozygous vs. Heterozygous Homozygous: have two identical alleles for a trait. TT, aa, ZZ, uu Heterozygous: have two different alleles for the same trait. Tt, Aa, Ww, Cc Homozygous: true breeding Heterozygous: hybrids

Red Flowers Ratio: 1Rr:1rr

Phenotype vs. Genotype Phenotype: physical characteristic. Tall, yellow, Freckled Genotype: genetic makeup Tt, yy, FF Two organisms may have the same phenotype, but not the same genotype. Genotypes Phenotype TT, Tt Tall

Ear Length on Goats L l Ll ll

Questions What are the phenotypes of the parents? Long and Short What are the genotype of the parents? Ll & ll What is the phenotype ratio for the offspring? 1 Long: 1 Short What is the genotype ratios of the offspring? 1 Ll: 1ll