Mendel’s Theory

Mendel: The man, the myth , the legend

Mendel’s Hypotheses Mendel found that there are two separate “heritable factors” for each trait Alleles

Mendel’s Findings in Modern Terms Dominant alleles Recessive alleles Homozygous Heterozygous Genotype Phenotype

Laws of Heredity Law of Segregation Law of Independent Assortment The two alleles for a trait segregate when gametes are formed Law of Independent Assortment Found that inheritance of one trait does not influence inheritance of any other trait

Punnett Squares A diagram that predicts the expected outcome of a genetic cross

The cross of a long-stem (LL or Ll) and short-stem (ll) plants involves a single pair of alternate characteristics. Give the genotype and phenotype ratios of the F1 generation when a homozygous long-stem plant is crossed with a short stem plant.

Cross plants from the F1 generation in the above problem and determine the genotype and phenotype ratios of the F2 generation.

Problem In pea plants, inflated pods are dominant to constricted pods. Show the cross between a true-breeding inflated pod plant with a true-breeding constricted pod plant. What are the expected phenotypes and in what proportion?

Problem In goats, a recessive gene causes the goats to "faint" when they are startled. A farmer breeds two goats (that have never fainted) and their first offspring faints two days after its birth. What must the parent's genotypes have been? Show the cross to prove it.

Determining Unknown Genotypes Perform a test cross An organism with an unknown genotype is crossed with a homozygous recessive individual

Example Y Y Yy A pea plant with yellow seeds has an unknown genotype. It is crossed with a plant with green seeds. y y Y y Yy yy y y

Dihybrid Problem TtRr x TtRr In peas, the allele for tall (T) plants is dominant to the allele for dwarf (t) plants and the allele for round (R) peas is dominant to that for wrinkled (r) peas. Find both phenotypic for the results of the following cross: TtRr x TtRr

Dominant or Recessive?