Introduction to Mendelian Genetics
Why Study Mendelian Genetics?
Curiosity
Curiosity of the diverse world around us is sufficient reason to
study Mendelian genetics.
Wow
Diversity!
Wow More
Diversity!
Understanding Inheritance
If we want to understand the mechanisms behind inheritance, then
Mendel's experiments and his conclusions are the place to begin our study.
Inheritance patterns of diseases such as hemophilia, sickle
cell anemia and colorblindness
can be predicted using Punnet
squares. We can determine a plant's genetic make-up for
certain traits using a test cross.
Mendel's experiments are good models of the important
elements of scientific experimentation.
Well Selected Research Subject
Pea plants were
a good choice for inheritance experiments because they are easy to
cultivate and they are available in many varieties (more traits to
study). Pea plants also allowed Mendel to strictly control pollination
because the male and female parts are completely enclosed by the petals
of the flower and prevent air pollination.
This image was created by Lupita Munoz.
Proper Controls
When he wanted to cross two varieties of pea plants, Mendel carefully
removed the stamens (male parts) of the plants with scissors before they
produced pollen. Then with an artist^Òs brush he pollinated the carpels
(female part) with pollen from the other plant.
This image was created by Lupita Munoz.
Large Pool of Subjects and Mathematical Analysis
Mendel understood the statistical nature of inheritance. For example, he
manually cross pollinated hundreds of pea plants as described above. He
counted hundreds of flowers and pods, and thousands of seeds. Sometimes
he even ran out of space to grow his third generations!
This image was created by Lupita Munoz.
Carefully Maintained Records and Mathematical
Comparisons
For almost eight years, Mendel kept detailed numerical records of his
experiments. This allowed him to make mathematical
comparisons. He also simplified his records for greater clarity of
the studied trait by limiting the number of traits studied in a series
of generations to one or only a few. He also simplified his records by
using a single letter of the alphabet, lower and upper case, to denote
the two expressions of a trait. Mendel's
mathematical, statistical approach using punnet squares, test
crosses and probability were a breakthrough for experimental
biology.
This image was created by Lupita Munoz.
Well Selected Traits to Study
Mendel's choice to study
"either/or" type traits was essential to his discoveries. Either/or
traits vary in only one way or another, tall or short, purple or white.
They are do not vary in a continuum or intermediate fashion between the
two parents. These either/or traits follow distinct and measurable
patterns of inheritance that are easy to quantify and describe. Mendel
studied seven contrasting traits.
Mendel developed a well organized plan of research and he followed
it consistently.
To successfully study the inheritance of his
seven traits, Mendel made sure he began with true breeding parent plants
for the trait in question. A True breeding plant for a certain
trait will always produce the same expression of the trait when self
pollinated. Purple flower true breeding plants always produce purple
flowers; white flower true breeding plants always produce white flowers.
Today we know that such a plant is homozygoushomozygous for the trait When he crossed
true breeding, purple and white flower plants for example, he could follow
the trait through several generations to identify how the trait was
inherited. This was essential because in the case of two homozygous
parents the recessive traits do not express until the third generation.
Many times he followed his observations of true breeds for four to six
generations.(from a letter from Mendel to Carl Nageli)
Parental generation--true breeds, homozygous
cross pollination
F1 generation--heterozygous
allowed to self-pollinate
F2 generation--each plant was characterized, counted and
recorded
Mendelian genetics is still the foundation of modern genetics.
Mendelian Genetics Defined
Mendelian genetics refers to the basic patterns of inheritance that
were discovered and described by Mendel. The basic patterns are:
Biography of Mendel
Related Web Sites
General Mendelian Genetics
Study Tools and Practice Qiuzes
Mendelian Statistics and Probability
Back to the Mendelian Genetics Outline
Please send comments to: lupita-m@mail.utexas.edu and
weston@aries.ece.utexas.edu.
