Genetics Part 1: Mendelian Genetics

Gregor Mendel:The “Godfather” of Genetics

Gregor Mendel Austrian MonkBorn in 1822 in what is now Czech RepublicEntered Monastery at 21 and was ordained priest

four years laterMonasteries, typically, were the center of

education in this time and area of the worldBecomes a teacher of science and mathematics

at high school after collegeAlso put in charge of monastery garden: this

made him famous

Mendel’s ExperimentsPlants are self-pollinators: they have both male

and female reproductive organs on the same plant

Mendel had purebreds (plants that only produce offspring like themselves) from eariler gardeners (these plants had self-pollinated year after year)

These purebreds form the basis for Mendel’s work Mendel learns to cross-pollinate by cutting out

male and female parts from different plants

Mendel’s Experiments Continued...

Mendel studies only seven traits in order to simplify his research:

Seed shapeSeed colorSeed coat colorPod shapePod colorFlower positionPlant height

Mendel’s Experiments Continued...

Mendel crosses plants with different characteristics for the same trait (i.e., both plants have flowers, but the flowers are positioned differently on the plant)

Mendel develops hybrids, or organisms with traits from two organisms

Mendel sees that the offspring have traits of only one parent and the other has seemingly disappeared.

All plants are tall with yellow seeds!

Mendel’s Conclusions:

Mendel senses that traits are controled by genes and that there must exist two contrasting forms for these genes.

These different forms of genes are called alleles

Other conclusions follow….

Alleles

The Principle of Dominance:

Some alleles are dominant while some are recessive

Dominant alleles will always express themselves if present

Recessive genes will only be expressed if no dominant allele for the same trait is present

Experiment Results:

Tall and yellow alleles are dominant (T & Y)

Short and green alleles are recessive (t & y)

Mendel wants more answers...

Allows all hybrid plants to self-pollinateAssigns names to generation of plants

P generation: Parental generation (First allowed to self-pollinate)

F1 generation: First offspring produced by self-pollination (filial generation)

F2 generation: Offspring of the mixing of F1 generation

P & F1 and F2 Generations

Results of F1 cross:

The resulting F2 plants showed recessive characteristics

How did the recessive trait “get away” from the dominant allele that masked it?

Law of Segregation:

During meiosis, when homologous chromosomes pair-up, one chromosome has one allele whereas the other chromosome has the opposite allele (i.e., one has the tall trait and the other the short)

Question:

Do alleles for one trait have anything to do with other alleles?

In other words, do alleles control alleles for other traits?

Law of Independent Assortment:

Alleles for one gene DO NOT control alleles for other traits.

In other words, having blonde hair doesn’t mean that you will definitely have blue eyes.

Summary of Mendel’s Work:The factors that control heredity are individual units

known as genes. In organisms that reproduce sexually, genes are inherited from each parent.

In cases in which an organism possesses two forms of the gene for a single trait, some forms of the gene may be dominant and others may be recessive.

The two forms of each gene are segregated during the formation or reproductive cells.

The genes for different traits may assort independently of one another.

Basic Genetics Vocabulary Punnett square: A simple scientific tool used to show

the possible combination of alleles in an offspring generation

Gametes: offspring that result from the mixing of parental sex cells

Phenotype: The actual physical appearance of the offspring

Genotype: The actual genetic makeup of the offspring Homozygous: Organisms that have two identical alleles

for a trait Heterozygous: Organisms that have two different

alleles for a trait