Organisms pass on their genetic information to the next generation by the transmission of their DNA. The study of the possible outcomes of these genetic recombinations of sexually reproducing organisms is called genetics. The University of Texas has some cool stuff about Gregor Mendel, who came up with a few LAWS of heredity based upon statistical evaluation of the traits of pea plants. Mendel did not know about DNA or chromosomes.

The Law of Dominance

One trait will always be expressed (seen) in the phenotype as long as it is present. (found in the Genotype) ex. Brown Eyes

The Law of Segregation

Each trait will separate or segregate individually. This assures that one allele (trait) will come from each parent. Segregation occurs during meiosis, the formation of the sex cells. (gametes)

The Law of Independent Assortment

Each trait will be inherited independently. They travel in little individual packets unique from other traits.

Mendel Info

Mendel's Laws of Dominance, Segregation and Independent Assortment are illustrated by the Monohybrid Cross (Tt x Tt)


An alternative form of a gene. Examples: all the different eye colors in humans.


The genetic makeup of an organism (ex BB or Bb)


The physical traits we can see

REMEMBER: An organism must be pure (HOMOZYGOUS) in order for a recessive trait to be seen in the phenotype. Example: Blue eyes must be (bb)

About the year 1930 the Gene Chromosome Theory was postulated. Fruitfly , Drosophila melanogaster, showed different traits being transmitted on different chromosomes. This was the birth of modern genetics.

The variation within one species alone is tremendous. Take the species Canis familiaris: there are over 400 recognized breeds within this one species, the dog. Think of all the different types of dogs. It is remarkable that a Great Dane and a Pomeranian are in the same species. In the wild this variety within a single species would be an important factor in allowing the species to be able to survive even if there were drastic changes in the environment.


Both traits are expressed as in the Roan cattle, Red with white spots.

Incomplete Dominanace

Traits are blended as in the 4 o'clock flower. (Red + White = Pink)

Blood Types

AA Ao BB Bo oo AB This will help you to learn why you have a certain blood type.Dennis Oneill's Blood Type Link

Weird Terminology in the Regents Exam where the O allele is represented by i so Type O = ii type Ao = Ai Type Bo = Bi

DNA Structure

It will “split” in half and replicate itself. Half the “ladder” is the old DNA strand half is the new.

DNA and Protein Synthesis

DNA stores the code for each protein that is synthesized on a ribosome.

Step One

  • The DNA code is copied on to the mRNA codon.
  • This is called Transcription.
  • The mRNA codon is then carried from the nucleus out to the ribosome.

Step Two

  • The mRNA codon is attached to the tRNA anticodon by compatable base pairs. A-U and C-G
  • The tRNA also carries the correctly coded amino acid which is linked to form a polypeptide chain. (protien)
  • This is called Translation. The mRNA is the chain pulled through the ribosome. The tRNA "swings" in with the correct amino acid.

Sex Linked Traits Travel on the Sex Chromosomes.
Example is Hemophilia on the "X" chromosome
DNA Used in a Bacterial Transformation to Produce Human Insulin. We can add the human gene to bacteria to “trick” it into producing insulin for us.


This how we find the father in a paternity suit or the murderer from DNA left at a crime scene.

The Human Genome Project

Was designed to map the codes and functions of all the human genes.
Key Findings:

  1. People seem to have only 26,000-40,000 genes which is much less than expected.
  2. Inherited mutations arise about twice as often in men than in women.
  3. Only 1-1.5% of the human DNA carries information for making proteins. This is far less than researchers expected. This DNA which makes up genes tells the cell how to make specific proteins. This is crucial for proper structure and functioning of the cell.
  4. Along stretches of DNA genes appear in "clusters" like villages along a long empty road.
  5. About 200 genes were inserted into the human genome as a result of bacterial infection. This is a real surprise.
  6. We are about 99% genetically identical despite differences in race. This makes us much more fragile as a species than what we thought.

Open ended question:

Describe how mRNA acts as a messenger and as a tRNA acts as a translator during protein synthesis.