LECTURE                        MENDELIAN GENETICS

"Genetics is the study of heredity - traits inherited from parent to offspring.

In this lecture you will learn about:

Blending theory In ~1850, scientists thought that some fluid substance in the blood of animals or in the sap of plants was the hereditary material. The combination of the parent's characteristics in the offspring was thought to occur by a "blending" of this fluid.

    * If so, a white dog that mated with a brown dog should produce only tan puppies;
    * A tall person who had a child with a short person should produce all "medium-size" children, etc...clearly not the case!
    * Even though people recognized problems with this theory, it was the top theory of the day!
    * Keep in mind, though, that in the mid-1800s, very little was known about cell structure, let alone the concepts of genes and DNA!

A different theory was put forth by Gregor Mendel in 1850. Mendel was an Austrian monk who was interested in plant breeding. He performed careful experiments with the garden pea, Pisum sativum, collected large amounts of data, and in doing so, was able to uncover the basic principles of genetic inheritance that still hold true today!

Mendel discovered that traits are inherited in discrete units (we now know these to be genes). Mendel's discoveries were not understood by other scientists for over 35 years!"
CITATION: (http://www.biology.iupui.edu/biocourses/N100/2k2ch10genetics.html)

Many of the RULES of heredity  (INHERITANCE) had been worked out by a monk named Gregor Mendel.  The rules are called "Mendelian genetics". 

Mendel was fascinated by the question "What is responsible for the variation in plants and animals?"

His monastery sent him to the University of Vienna to get a teaching degree but Mendel basically flunked out.

In order to EXPERIMENT "under the abbot radar" he chose the garden pea. 

For 7 years, 1856 - 1863, Mendel crossed and produced around  28,000 pea plants.  He selected seven sets of differences.

He would plant seed from both "sets", example: white and purple flowers

When the plants flowered he would cover the flowers to prevent bee pollination and remove the male parts of the flower (the stamens) of the seed plant to prevent self fertilization.  He would then pollinate by hand using a brush. 

The matured seeds from this cross are called the F1 generation

He would plant the F1 seeds, raise the plants and record flower color of all the plants. 

Next he would remove the stamens and hand cross pollinate the flowers from the F1 generation to produce the F2 generation. 

Here is the cross of the F1 generation. 

It is not a "blend" of colors. 

All the flowers are purple. 

Here is the result when two plants of the F1 generation are crossed to make the F2 generation. F1 X F1

3 plants have purple flowers

1 plant has white flowers

How is this explained?

"Mendel did this experiment with a total of 7 different traits, studying 22 strains of peas and always using large sample sizes, and he always saw a ~3:1 ratio in the F2 generation."

"Mendel realized that these results were explainable if three things were true. He hypothesized that:

    1. Every trait (like flower color, or seed shape, or seed color) is controlled by two "heritable factors". [We know now that these are GENES - we each have two copies of every gene]. 

    2. If the two alleles differ, one is DOMINANT  (will be observed in the organisms appearance or physiology) and one is RECESSIVE (cannot be observed unless the individual has two copies of 
the recessive allele). Dominant traits mask the appearance of recessive traits. 

    3. Alleles are randomly donated from parents to offspring - the factors (ALLELES) separate when the GAMETES are formed by meiosis, allowing all possible combinations of factors to occur in the gametes."
citation: (http://www.biology.iupui.edu/biocourses/N100/2k2ch10genetics.html)

This is how the F1 cross is written in a "Punnett Square" which is a "device" or method used to record the results. 

We are most aware ofphenotype because we can see the differences in people, the differences in living things. 

OTOH, the vast majority of genes are involved in producing proteins that have no readily apparent appearance. 

Genotype is all the genes an individual has whether they can be "seen" or not. 

Phenotype is what you see. 

Genotype is what you get. 


In procaryotes (bacteria) each cell has only a single 1N number of genetic material.  It is a closed circle.  It divides 1N --> 2N --> 1N.  In cells there is normally only a single gene and it doubles right before binary fission, the method by which bacteria divide.
In eucaryotes each cell has two copies of the same chromosome called HOMOLOGOUS or matching pair. One comes from Mom and one comes from Pop.  If they are lined up in pairs they would be pictured like this.  Division is by mitosis 2N->4N->2N  or meiosis, 2N->4N->2N->1N. 

Each 1N part of the chromosome (2N) is called a chromotid. 


Eucaryotic chromosomes are linear, not circular with a point of attachment between the 2N or pairs of chromosomes called a centromere.  Genes are normally in the 2N number in cells and only duplicate to 4N before the cell divides

In addition, eucaryotes have VARYING numbers of chromosomes.  LIST

In the Ant Myrmecia pilosula, the females have 2 chromosomes and the males have 1 (Other ant species have more chromosomes). cite: wiki

Ophioglossum reticulatum or Adders-tongue has 1200 or 1260 chromosome. cite: wiki

The pairs of chromosomes inherited from mom and pop typically contain DIFFERENT VARIATIONS in the genes.  This is called heterozygous. 

These variations between the same gene are called ALLELES, and the term used to describe alternate forms of the same gene and is referred to as ALLELIC VARIATION. Alleles occur when there is a difference in the DNA sequence between pairs of genes. One example is sickle cell anemia which is caused by a change of only one BASE PAIR. There may be 2, 3 or more alleles or variation for any gene. Strictly speaking, alleles refer to alternate forms of genes, but it is used when referring to alternate forms of non-gene DNA too like in forensic DNA.

The matching pairs of chromosomes contain THE SAME VARIATIONS in the same gene.  This is called homozygous
This is what selection can do to an "original" plant. In this case the selection was done by humans. 

Which was discovered first, DNA or genes?  Explain
Do the Punnett squares for the F1 and F2 generation of white vs purple, white is recessive.
Give the phenotype and genotype for each.
Do the Punnet squares for the F1 and F2 generation of white vs red, both incomplete dominant.
Give the phenotype and genotype for each.
What are ALLELES?
What is the difference between genotype and phenotype.