Mendel and Genetics

Gregor Mendel
Gregor Mendel

Mendel began his genetics research around 1857, using pea plants.
He was using pea plants because they are available in many varieties and have many different, visible traits.

Law of Segregation- says that two alleles for a character are packaged into seperate gametes.
Mendel created a hypothesis, consisting of four related ideas, to explain thses results.

1. Alternative version of genes account for variations in inherited characters.
2. For each character, an organism inherits 2 alleles. One is recieved from each parent.
- They may be two different alleles.
3. If the alleles are different, the dominant allele is fully expressed in the appearance of the organism. The recessive allele isn't noticeable in the appearance of the organism.
4. Two alleles for each character seperate during gamete production. This segregation corresponds to the distribution of homologous chromosomes to different gametes in meiosis.

Punnett Square- Predicts the result of a genetic cross between individuals of known genotypes.
Mendels model accounts for the 3:1 ratio in the F2 generation.
Homozygous- An organism with 2 identical alleles.
Heterozygous- An organism with 2 different alleles.

Phenotype- Description of organisms traits
Genotype- Description of the organisms genetic makeup.

Test Cross- Breeding a homozygous recessive with dominant phenotype, but unknown genotype, ultimately determining the unknown allele.

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Complete Dominance- The dominant trait has a definite manifestation.
Incomplete Dominance- Heterozygous shows a distinct intermediate phenotype that isn't seen in homozygous.
Codominance- Two alleles affect the phenotype in seperate ways.

Law of Independent Assortment

Many of Mendel's experiments were conducted using Monohybrid Crosses, looking at single characteristics alone.
Dihybrid Crosses look at two alleles at one time.
When Mendel was looking at the dihybrid crosses, he found that all of the results showed a 9:3:3:1 ratio.
He concluded that each character was being inherited independently, giving us the Law of Independent Assortment.

Dihybrid Cross
Dihybrid Cross

Mendel and Probability

Mendel found, using the Law of Independent Assortment and the Law of Segregation, that genetics use the sam rule a the Law of Probabilty, and is much like tossing a coin.
We can use the Rules of Multiplication and Addition in calculating the probabilty of a gene being passed from the parents to the offspring.

Complete Dominace- The dominant trait is the only visible trait.
Incomplete Dominance- A distinct phenotype is shown that isn't seen in homozygotes.
Codominance- Two alleles affect the phenotype in seperate, distinguishable ways.

Many genes have more than two alleles in a poplulation.
------ Blood type is one example of this.

Pleiotropic- Affect more than one phenotypic character.
------ Sickle Cell Disease is due to a single gene.
Epistasis- A gene at one locus that alters the phenotypic expression of a gene at a second locus.
------ Coat color in animals depends on two different genes.
Polygenic Inheritance- The additive effects of 2 or more genes on a single phenotypic character.
------ Skin color in humans is determined by at least 3 different genes.

Pedigree Analysis- Shows the traits of family members of the past, helping us to predict the traits of future generations.


Many of the genetic disorders that are passed from parents to their offspring are inherited as recessive traits.
Heterozygotes have a normal phenotype because one "normal" allele produces enough of the required protein.
The recessively inherited disorder shows up only in homozygous individuals that inherit one recessive allele from each parent.
Although the parent may not have the clear phenotypic effects, they may still be carriers that are able to pass the trait to their offspring.
Most individuals that recieve the disorder are born to those who are just carriers of it.


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Cystic Fibrosis
  • Strikes one of every 2,500 whites of European descent.
  • 1 in 25 whites is a carrier.
  • The channels are either defective or absent, causing high levels of chloride, making the cells stickier than normal.

Tay-Sachs Disease
  • Caused by a disfunctional enzyme that fails to break down specific brain lipids.
    Tay-Sachs Disease
    Tay-Sachs Disease

Sickle Cell Diseaseexternal image sickle.gif
  • The most common disease among blacks.
  • Affects one of 400 African Americans.
  • The red blood cells become deformed, making them sickle shaped.
  • In Africa, where malaria is common, sickle cell disease is both good and bad.

  • Form of dwarfism.
  • One case of every 10,000 people.
  • Heterozygous individuals inherit the trait.

Huntington's Disease
  • Degenerative disease of brain cells in certain parts of the brain.
    Top: Cross section of a brain with Huntington's Disease; Bottom: Cross section of a normal brain.
    Top: Cross section of a brain with Huntington's Disease; Bottom: Cross section of a normal brain.

Many diseases are inherited in Mendelian fashion, but many others have a multifactorial basis.
These have a genetic component plus a significant environmental influence.