Traits and Probability

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

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!

Monohybrid Cross

• Simple test cross looking at just one trait• Predicts the ratio of possible genotypes and

phenotypes in offspring

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

Monohybrid Practice1. 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?

2. 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?

Monohybrid Practice

1. . 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?

2. 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?

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.

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

Dihybrid PracticeCorn 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.1. List the possible genotypes and phenotypes that can be produced by

this cross.2. What is the genotypic ratio resulting from this cross? The phenotypic

ratio?3. 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?

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?

Independent Practice1. About 70% of Americans get a bitter taste from the substance

called phenylthiocarbamide (PTC). It is tasteless to the rest. The "taster" allele is dominant to non-taster. Also, normal skin pigmentation is dominant to albino. A normally pigmented woman who is taste-blind for PTC has an albino-taster father. She marries an albino man who is a taster, though the man's mother is a non-taster. Show the expected offspring of this couple.

2. In pigeons the checkered pattern is caused by a dominant allele. A plain (non-checkered) pattern is recessive. Red color is also caused by a dominant allele and brown color by a recessive allele.

a. Show the expected offspring of a cross between a homozygous checkered red bird and a plain brown one. Carry out this cross through the F2 generation.

b. What fraction of the F2 offspring would be expected to show both recessive traits?

c. What fraction would be expected to show both dominant traits?