Classical Mendelian Genetics

Johann Gregor Mendel (1822-1884)

Mendel is considered to be the "Father of Genetics".  In the mid-1800's he carried out systematic studies of the inheritance of traits in the common garden pea.  He then published this body of work in 1866, but it was not well received, as most people simply didn't grasp was he was explaining!!  It wasn't until the early 1900's that the value of his work was recognized.

Mendel chose to work with the garden pea because it grew quickly, it required little space to grow, its seeds were easy to handle, it could self-pollinate or be cross-pollinated, and there were many sub-species.

Mendel focused his studies on 7 different  traits:

Mendel self-pollinated the plants until he got purebreeding strains (offspring always had the same characteristics as the parent plant).  Then he cross-pollinated plants with particular traits and observed the outcome over many generations.

Some of his first observations showed that when he cross-pollinated purebreeding parental plants showing different characteristics (for example, purple flowers or white flowers), all of the offspring were identical (in this example all purple).  The parental generation is refered to as P (parental generation) and the offspring is F1 (or first filial generation).

http://bioserv.fiu.edu/~walterm/GenBio2004/new_chap13_inheritance/pics.htm

He observed this same pattern when he studied the other traits as well.

Parent Pea Plants	F1 Pea Plants
tall stem x short stem	all tall stems
yellow seeds x green seeds	all yellow seeds
green pea pods x yellow pea pods	all green pea pods
round seeds x wrinkled seeds	all round seeds
axial flowers x terminal flowers	all axial flowers

This led Mendel to conclude that certain traits were dominant over other traits and he created the Rule of Dominance  to explain this.

Rule of Dominance:  In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation.  Offspring that are hybrid for a trait will have only the dominant trait in the phenotype.

Mendel then took the F1 generation and let them self-pollinate.

http://bioserv.fiu.edu/~walterm/GenBio2004/new_chap13_inheritance/pics.htm

He consistently observed that in the F2 (second filial generation) there were 3 purple flowers for ever white flower.  He consistently observed this 3:1 ratio with other traits as well.

http://www.biology.iupui.edu/biocourses/N100/2k4ch10genetics.html

Mendel realized that these results could be explained 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 organism's 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.

Mendel's Law of Segregation - The two factors (alleles) separate when the gametes are formed, and only one factor (allele) is present in each gamete.

http://facultylounge.whfreeman.com/?q=node/1438

Punnett Squares

To help in solving genetic problems, Punnett squares are often used.  The genotype of each parent is written on the top and left side, respectively.  The two alleles are separated.  Then the table is filled in with the allele from the respective side and top positions.  Those are the offspring produced by the cross.  

http://kids.britannica.com/comptons/art-90108/Mendels-principle-of-segregation-states-that-during-gamete-formation-the

http://en.wikipedia.org/wiki/Punnett_square

Doing a genetic cross (monohybrid = 1 gene):
Geneticists use letters to represent alleles.

• A capital letter = Dominant trait, a lowercase letter = a recessive trait.
• The same letter is used to indicate both alleles.

Examples

Flower color: P= purple, p= white

Seed color: Y= yellow, y = green

Seed shape: W = wrinkled, w = round

E-Z steps for doing genetics problems:

1. Indicate the genotype of the parents using letters

2. Determine what the possible gametes are

3. Determine the genotype and phenotype of the children after reproduction. To consider every type of offspring possible, use a Punnett Square in which all possible types of sperm are lined up vertically and all types of eggs are lined up horizontally:

4. Fill in the squares by "multiplying" the alleles from mom and dad.

Try to figure out the following genetics problems.  Use a Punnett square.  We will discuss them in class to check answers.

Genetics Problem 1: (a) A man with a widow's peak (WW) marries a woman with a continuous hairline (ww). A widow's peak is dominant over a continuous hairline. What kind of hairline will their children have?

(b) Suppose one of their children (Ww) marries someone who is also heterozygous (Ww). What type of hairline will their children have?

Genetics problem 2: A man and a woman are heterozygous for freckles. Freckles (F) are dominant over no freckles (f). What are the chances that their children will have freckles?

Genetics problem 3: A woman is homozygous dominant for short fingers (SS). She marries a man who is heterozygous for short fingers (Ss). Will any of their children have long fingers (ss)? yes / no   Could any of their grandchildren potentially have long fingers? y / n Why or why not?

Genetics problem 4: Jane and John are expecting a baby and know that they are both carriers (ie heterozygous) of cystic fibrosis (Cc). What is the probability that their child will have cystic fibrosis (cc)? What is the probability that their child will be a carrier of cystic fibrosis?