Traits and Probability

1. Traits and Probability

2. Genes • A gene codes for a specific trait (ex: eye color) • Alleles of a gene are the different forms it can take (ex: blue, brown, green) • You inherit two copies of each gene (remember, those are your homologous chromosomes!) • One from each parent • Dominant genes are always expressed • Recessive genes only show up if no dominant genes are present

3. Punnett Squares • Predicts the ratio of possible genotypes that can result from a cross. • Offspring inherit two copies of each gene… one from each parent. • When gametes are made during fertilization, the egg or sperm gets one copy of a gene • This contributes to genetic diversity in the offspring!

4. Monohybrid Cross • Simple test cross looking at just one trait • Predicts the ratio of possible genotypes and phenotypes in offspring

5. Monohybrid Cross Results • Homozygous-Homozygous • All offspring have same genotype and phenotype • Heterozygous-Homozygous • 1:1 ratio of genotypes and phenotypes • Heterozygous-Heterozygous • 1:2:1 genotypic ratio • 3:1 phenotypic ratio

6. Monohybrid Practice • An albino is a person with no pigmentation in his skin, eyes or hair. Normal pigmentation is (A) is dominant over albino (a). A normal pigmented man (whose mother was albino) marries a homozygous normal woman. What would the expected appearances of the children be? • If the man in problem #1 married a normal woman (whose father was albino), what percent of the children would be expected to be albino?

7. Monohybrid Practice • . In chickens, yellow legs (Y) are dominant over white legs (y). A yellow legged rooster was crossed with a white legged hen. Both kinds of offspring were produced. What are the genotypes of the parents and the offspring? • A yellow legged rooster was crossed with a white legged hen. All fifteen of the offspring were yellow legged. What is the genotype of the rooster?

8. Practice Test cross Suppose you work for a company that sells plant seeds. You are studying a plant species in which the dominant phenotype is pink flowers (PP or Pp). The recessive phenotype is white flowers (pp). Customers have been requesting more plants with pink flowers. To meet this demand, you need to determine the genotypes of some of the plants you are currently working with. • You cross a pink plant (Plant A) with a white plant (Plant B). The resulting cross yields 6 pink plants and 6 white plants. Use Punnett squares to determine the genotype of Plant A.

9. Dihybrid Cross • Follow the inheritance of two different traits • Law of Independent Assortment • Allele pairs separate independently during gamete formation • Different traits are inherited separately

10. Dihybrid Practice Corn is bred for traits that improve its usefulness for specific purposes. For example, it may be bred to grow in various climates, to produce more corn, or to be better tasting. These traits depend on the alleles inherited by the corn plant. Suppose you are studying the color and texture of kernels on a cob. Kernels can be either purple (R), which is the dominant color, or yellow (r). Kernels can also be smooth (T) or wrinkled (t). You want to to cross two corn plants with the following genotypes: Rrtt and RrTT. • List the possible genotypes and phenotypes that can be produced by this cross. • What is the genotypic ratio resulting from this cross? The phenotypic ratio? • If the genotypes for kernel texture of two plants are tt and tt, what is the probability of their having offspring with smooth kernels? Why?

11. Independent Practice In summer squash, white fruit color (W) is dominant over yellow fruit color (w) and disk-shaped fruit (D) is dominant over sphere-shaped fruit (d).  If a squash plant true-breeding for white, disk-shaped fruit (WWDD) is crossed with a plant true-breeding for yellow, sphere-shaped fruit (wwdd),  what will the phenotypic and genotypic ratios be for: a. the F1 generation?     b. the F2 generation?