Probability Punnett Squares Genetics Probability When Mendel crossed

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 = 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 R r r Rr rr Ratio: 1 Rr: 1 rr

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 TT, Tt Phenotype Tall

Ear Length on Goats L 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: 1 ll