Lecture 4: Introduction to Mendelian Genetics

version 7 Jan 2008

The small monastery garden used by Mendel.
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Mendelian Genetics

Gregor Mendel 1865

Key: Particulate Inheritance

Mendel's Experiments

What Mendel Observed

Here we consider his crosses involving Seed color -- Yellow versus Green

Mendel also found that Parental, F1, and F2 Yellow peas behaved quite differently

Crosses with Two traits

The result is a 9 : 3 : 3 : 1 ratio of the 4 different phenotypes

In his crosses, Mendel reported Independent Assortment , where the R any Y allele assort independent of each other

Mendel cheated (or was very lucky)

The seven characters he used to show independent assortment were all on different chromosomes. If they were on the same chromosome they would NOT show independent assortment.

The Key Concepts of Mendelian Genetics

At any given gene, each parent has two copies (two alleles).

Each parent gives you one of their alleles at random

Example

The Genetics of ABO Blood Groups

The ABO blood group is commonly used in crime samples as an inital screen. It is also used as a simple paternity test

The Genetics of Rh Blood Groups

A second blood character is often used, whether you are Rh positive or RH negative

Rh system was named after rhesus monkeys, where the blood from Rh positive humans clumps when interacting with antiserum generated from rhesus blood.

Genetics: Rh negative occurs only in homozygote recessive individuals, with dd = Rh-, and DD, Dd = Rh+.

Example

Consider the offspring from a large family with an A+ father and an O negative mother. They have a large number of offspring:

1/4 are A-

1/4 are A+

1/4 are O-

1/4 are O+

What is the genotype of the mother and father?