The Blending Hypothesis A Trait is a variation of a particular characteristic such as tall and short In the early 1800’s many biologists believed in the Blending Hypothesis They believed offspring were a blend of their parents traits BLACK SQUIRRELL + WHITE SQIRRELL = GREY SQUIRRELL!!

 Thomas Hunt Morgan. “Father of Modern Genetics”  Gregor Mendel “Father of Genetics”

The Father of Genetics- Gregor Mendel Gregor Mendel ( ) Austrian Monk Pea Plant Experiments Mendel studied the inheritance of traits (factors) from one generation to the next These factors are now called genes Genes are sections of DNA that code for a specific trait or a characteristic

Mendel’s Breeding Experiments Mendel bred plants for years to ensure that they were true- breeding True-breeding means that offspring always had the same trait as the parent Mendel Cross-fertilized all his pea plants by hand to control which traits he wanted to test

Mendel’s Breeding Experiments Mendel expected a blending of the traits IE: Tall pea x Short pea resulting in Medium height plants What actually occurred:

Some Mendelian Conclusions: Mendel and Alleles: Mendel developed the hypothesis that genes came in different forms that he called Alleles Tall vs. Short Yellow vs. Green We use letters to represent alleles Tall = T Short = t

Some Mendelian Conclusions: Law of segregation When gametes are made the alleles for a trait separate, so each gamete carries only one allele for each trait Law of dominance: “of the two factors controlling a trait, one may dominate the other”

Punnett Squares Punnett Squares are tools used to determine offspring probabilities from a genetic cross

Punnett Squares Genotype – the Allele combination of an individual (like their I.D. number) Phenotype – the physical characteristic that the individual shows (tall, short, eye color, etc…)

Sample Problem Let’s say you are Mendel. You take a pure-breeding pea plant that is tall (TT) and you combine it with a pure- breeding pea plant that is short (tt). What would their possible offspring be? What would you do first? T T tt Tt

Let’s look a little more closely at the plants Homozygous – An individual that has two copies of the same allele Which of the plants in the sample are homozygous? Heterozygous – An individual that has two different copies of an allele Which of the plants in the sample are heterozygous? Tt T T tt Sample Problem

The offspring would look something like this Why are all the offspring tall? Dominant – an allele that “masked” or covers up another allele Recessive – an allele that does not show up in a heterozygous individual, it is “masked” Sample Problem

Dominant Genes If an individual is Homozygous Dominant the dominant gene will be seen Example: TT = tall If an individual is Heterozygous the dominant gene will be seen Example: Tt = tall If an individual is Homozygous Recessive the recessive gene will be seen Example: tt = short

Some More Sample Problems As a Class: Brown fur = B;Grey fur = b What is the genotype of a Heterozygous individual? What is the genotype of a Homozygous dominant individual? What phenotype would someone who is bb be (haha)? Complete a Punnett square for Bb x BB Bb BB Grey fur B B B b

Key Terms Genotype – Allele combination of an individual, two letters Phenotype – Physical characteristics based on genotype (tall, brown hair, blue eyes, etc…) Dominant – Covers up the other allele, will always be seen in the phenotype (UPPER CASE) Recessive – Gets covered by the dominant, only seen if the individual is homozygous recessive (lower case) Homozygous – Individual has two of the same allele (TT, bb, GG, hh) Heterozygous – Individual has two different alleles (Tt, Bb, Gg, Hh)