Genetics: Fundamentals of Mendelian Genetics Classical Genetics

Genetics: Fundamentals of

Mendelian Genetics

Classical Genetics

Inherited characteristics are the result of particulate factors called genes that are transmitted from generation

to generation. The vehicles of transmittance are called

chromosomes. These rules were first described by

Gregor Mendel.

History

• Scientists have looked at animals and their offspring for years .

• They noticed that offspring resemble their parents . – Puppies grow up to look like their

parents, new maple trees grow from seeds of adult maple trees. Offspring resemble their parents because of their HEREDITY .

• For thousands of years, people thought that the heredity of a living thing was merely a blend of the characteristics of its parents.

• Genetics is much more than blending characteristics!

Gregor Mendel• In 1866, around the

time of the civil war, Gregor Mendel was one of the first to recognize that cells passed on traits in distinct hereditary units.

• Mendel lived most of his life in the Czech Republic.

– After becoming a priest, he spent several years studying science and mathematics at the University of Vienna.

– After leaving Vienna, Mendel returned to the monastery in Czech Republic and taught at a local High School. (Yea!)

– Mendel also was a gardener at the monastery.

Mendel and His Peas• In 1856, Mendel

performed his first set of hybridization

experiments with fantastic garden pea. Mendel researched his garden plants until 1868.

• In 1865, Mendel first reported the results of some simple genetic crosses between certain strains of garden pea.

Mendel’s Work• Before arriving at the monastery

the previous gardeners had developed TRUE-BREEDING stocks of pea plants.

• A true-breeding stocks ALWAYS passes its characteristics to the next generation.

• An example of a true breeding plant would be that one would always produce tall plants with green pods, while another stocks would always produce short plants with yellow pods.

• Mendel took these TRUE-BREEDING and mated them and observed their offspring.

• Like many plants, pea plants use parts of their flowers to reproduce.

• One part produces pollen on male parts called anthers, and another part produces egg cells on female parts called stigma .

• When pollen fertilizes egg cells on the same flower the process is called SELF-POLLINATION. In this case, the offspring only has ONE parent. Plants may also have two parents.

• Sometimes pollen from the anthers of one plant fertilizes the stigma of another flower. This process is called cross-pollination.

• To perform his experiment, Mendel had to select the pea plants that mated with each other.

• He needed to prevent the flowers from self-pollinating and to control their cross-pollinating.

•He accomplished this task by first cutting away the male parts of the flower. (Ow!)•Then he dusted the flower with pollen from a second flower.

• To simplify his investigation Mendel chose to study only seven TRAITS in pea plants.

Seed

Shape

Seed

Color

Seed

Coat

Color

Pod

Shape

Pod

Color

Flower

Position

Plant

Height

Round

Yellow

Gray

Smooth

Green

Axial

Tall

Wrinkled

Green

White

Constricted

Yellow

Terminal

Short

Mendel’s Success• One of Mendel’s successes was

that he studied just a small number of traits.

• Also, the traits Mendel studied had contrasting forms only. For example, seed shape is either round or wrinkled and pod color is either green or yellow.

• Using true-breeding stocks for each of the seven traits, Mendel crossed pea plants that showed one form of a trait with pea plants that showed the other form.

• The offspring of these crosses are called HYBRIDS. These are the offspring of parents with different characteristics .

Mendel called the hybrids the F1 cross. The letter F stands for FILIUS, which is Latin for“SON” . Mendel called the true-breeding plants the P generation. This stands for the Latin word parentis, meaning“ of the parent”.

What Mendel Expected:• Mendel expected the traits of the

parents to BLEND in the offspring. • Instead the traits of just one parent

appeared in the offspring. The traits of the other parent seemed to have vanished!.

• If Mendel had stopped with the F1. cross, he might not be remembered today.

• But he was curious about what had happened to the traits that seemed to have disappeared in the F1 generation.

• He decided to take the next logical step. He crossed the plants of the F1 among themselves.

Mendel’s Second Cross

• The second cross he called F2 generation. – In each of the seven traits, the form that

had vanished in the F1 reappeared in the F2 generation.

• These not only reappeared, but reappeared in noticeable ratios. – The traits reappeared in approximately one

fourth of the plants in the F2 generation.

– Because the traits did not blend, Mendel reasoned that some unit of inheritance must determine each of the traits he investigated . Mendel called this unit a MERKMAL , the German word for “Character”. Today the unit that determines traits is called a GENE.

Units of Inheritance:• How many copies of each gene does a

pea plant contain? – Mendel concluded that for each of the

seven traits he investigated, a pea plant must contain at least at least TWO genes, one from each parent.

• Today different forms of a gene are called ALLELES ( uh-LEELZ).

• An example of this is the trait for height in pea plants. One allele produces a tall plant while another allele produces a short plant.

• Some genes may have more than two alleles, many genes have three , four , or even dozens of alleles.

MENDEL’S CONCLUSIONS

Hey, kids! Genetics is fun!

MENDEL’S CONCLUSIONS

• After he crossed two plants, one pure for yellow pod and one pure for green pod, he discovered the F1 generation that demonstrated only the green pod characteristic.

• Not until he self-pollinated the F1 generation did he observe the yellow pod characteristic in the population.

• He noted a three-to-one ratio in the F2 generation.

• After studying thousands of crosses Mendel concluded that there are three principles that govern

inheritance…

Principle One: Dominance & Recessiveness• One factor in a pair may mask the

other factor, preventing it from having an effect. Mendel called the one factor DOMINANT since it masked or dominated the other trait. He called the other factor RECESSIVE.

Principle Two: Segregation

• To explain why traits disappeared in the F1 generation and reappeared in the F2 generation, Mendel reasoned that if each parent had two factors, each offspring must have two factors . Each parent must not be passing two factors because the offspring would have four. The two factors must separate during the formation of egg or sperm.

Principle Three: Independent Assortment• Mendel also noted that traits for dominate

factors did not appear together. Green seed pod (a dominate trait) may appear with a recessive trait of short height. Mendel concluded that factors for different characteristics were not connected. Factors for different characteristics are distributed to reproductive cells independently.

• Today biologists represent a dominant allele with a capital letter and a recessive allele with a lowercase letter. Thus for pea plants, a capital letter T represents the allele for tallness and a lowercase letter t represents the allele for shortness.

The End!