Gregor Mendel

Ordained priest

Father of modern genetics

Raised on farm and understood the value of plant breeding.

At 21, entered priesthood and studied plant breeding in a monastery in the Czech Republic.

Loved to read especially about natural sciences and was aware of Darwin’s findings.

Studied the inheritance of traits in pea plants.

• Research with pea plants

• Developed ideas of dominance and trait segregation

• One of the first scientists to use the scientific method and mathematical models

• Pea plants are good for genetic research – available in many varieties with distinct heritable

features with different variations • flower color, seed color, seed shape, etc.

Mendel had strict control over which plants matedwith which

• each pea plant has male & female structures • pea plants can self-fertilize • Mendel could also cross-pollinate plants: moving

pollenfrom one plant to another

• most characters are controlled by a singlegene

• each gene has only 2 alleles, one of which is

completely dominant to the other

• Observed peas to determine what traits were inherited

• He noticed at least 7 characteristics that appeared to be inheritedCharacters investigated

by Mendel

Character –heritable feature Trait – each variant for a character True-breeding – plants that self-pollinate

all offspring are the same variety Monohybrid cross – a cross that tracks

the inheritance of a single character P generation – (parental) true-breeding F1- (first filial) offspring of P generation F2 – (second filial) offspring from F1 cross

Allele- alternate version of a gene Dominate allele – expressed in the

heterozygote Recessive allele – not expressed in the

heterozygote Homozygote – pair of identical alleles for a

character Homozygous dominant- BB Homozygous recessive - bb

Heterozygote – two different alleles for a character (Bb)

Genotype – genetic makeup Phenotype – appearance of an organism

Mendel crossed purebred plants with opposite forms of a trait. He called these plants the parental generation , or P generation. For instance, purebred tall plants were crossed with purebred short plants.

Parent Short Parent TallParent Short Parent Tall P generation P generation P generation P generation

Mendel observed that all of the offspring grew to be tall plants. None resembled the short parent. He called this generation of offspring the first filial , or F1 generation, (The word filial means “son” in Latin.)

Parent Short Parent TallParent Short Parent Tall Offspring Tall Offspring Tall P generation P generation P generation P generation F1 generation F1 generation

Mendel then crossed two of the offspring tall plants (F1) produced from his first experiment.

F1 Tall F1 Tall ¾ Tall & ¼ Short F2 Generation

Mendel called this second generation of plants the second filial, Mendel called this second generation of plants the second filial, F2, generation. To his surprise, Mendel observed that this F2, generation. To his surprise, Mendel observed that this generation had a mix of tall and short plants. This occurred generation had a mix of tall and short plants. This occurred even though none of the F1 parents were short.even though none of the F1 parents were short.

Mendelian GeneticsClassical Punett's Square is a way to determine ways traits can segregate

parental P0 crossA A

a Aa Aaa Aa Aa

F1 crossA a

A AA Aaa Aa aa

backcross: mating F1 generation to the parental (usually recessive) strain

a aA Aa Aaa aa aa

in this backcross, F2 generation gives 50% (1:1) recessive offspringtest cross: mating to a homozygous recessive individual

states that allele pairs separate or segregate during gamete formation, and randomly unite at fertilization. There are four main concepts involved in this idea. They are:

1. There are alternative forms for genes. This means that a gene can exist in more than one form. For example, the gene that determines pod color can either be (G) for green pod color or (g) for yellow pod color.

2. 2. For each characteristic or trait organisms inherit two alternative forms of that gene, one from each parent. These alternative forms of a gene are called alleles

3. When gametes (sex cells) are produced, allele pairs separate or segregate leaving them with a single allele for each trait. This means that sex cells contain only half the compliment of genes. When gametes join during fertilization the resulting offspring contain two sets of alleles, one allele from each parent. For example, the sex cell for the green pod plant had a single (G) allele and the sex cell for the yellow pod plant had a single (g) allele. After fertilization the resulting F1 plants had two alleles (Gg).

4. When the two alleles of a pair are different, one is dominant and the other is recessive.

cross-pollination between true breeding green and yellow pods- all F1 green

F1 GenerationGg =heterozygous

When Mendel performed cross-pollination between a true-breeding yellow pod plant and a true-breeding green pod plant, he noticed that all of the resulting offspring, F1 generation, were green.

- self-pollination of green F1 plants- ¾ in F2 green,

¼ yellow- 3 : 1 ratio in pod colour in F2

G = dominant = greeng = recessive = yellow

GG, gg =homozygous

He then allowed all of the green F1 plants to self-pollinate. He referred to these offspring as the F2 generation. Mendel noticed a 3:1 ratio in pod color. About 3/4 of the F2 plants had green pods and about 1/4 had yellow pods. From these experiments Mendel formulated what is now known as Mendel's law of segregation.

C = dominant = yellow c = recessive = green

phenotype:

genotype:

This law states that allele pairs separate independently during the formation of gametes. Therefore, traits are transmitted to offspring independently of one another.

Mendel performed dihybrid crosses in plants that were true-breeding for two traits. For example, a plant that had green pod color and yellow seed color was cross-pollinated with a plant that had yellow pod color and green seeds. In this cross, the traits for green pod color (GG) and yellow seed color (YY) are dominant. Yellow pod color (gg) and green seed color (yy) are recessive. The resulting offspring (or F1 generation were all heterozygous for green pod color and yellow seeds (GgYy).

Mendel performed dihybrid crosses in plants that were true-breeding for two traits. For example, a plant that had green pod color and yellow seed color was cross-pollinated with a plant that had yellow pod color and green seeds. In this cross, the traits for green pod color (GG) and yellow seed color (YY) are dominant. Yellow pod color (gg) and green seed color (yy) are recessive. The resulting offspring (or F1 generation were all heterozygous for green pod color and yellow seeds (GgYy).

After observing the results of the dihybrid cross, Mendel allowed all of the F1 plants to self-pollinate. He referred to these offspring as the F2 generation. Mendel noticed a 9:3:3:1 ratio About 9 of the F2 plants had green pods and yellow seeds, 3 had green pods and green seeds, 3 had yellow pods and yellow seeds and 1 had a yellow pod and green seeds.

Allows for new gene combinations or genetic recombination

Can mathematically predict the possible combinations Number of possible genotypes = 2n

where n = the number of genes or traits considered

Example: considering 100 traits: 2100 = 1.26765 x 1030

Cross Genotype PhenotypeTT X ttTt X TtTT X TTtt X ttTT X TtTt X tt

Due to presence of recessive allele denoted by HbS.

Causes distortion of red blood cells into long, thin sickles.

Causes pain, tissue damage, and cells to rupture prematurely.

Carriers: have heterozygous genotypes and do not manifest symptoms.

Used to predict possible offspring genotypes

Place alleles for each parent on each side

HbA/ HbA x HbS/ HbS HbA

HbA

HbS HbS

Mother’s genotypeFather’s genotype

Given parents’ genotypes, you can predict offspring’s genotypes and phenotypes

HbS/ HbS = homozygous recessive results in sickle cell anemia

Both HbA/ HbS (heterozygous) and HbA/ HbA (homozygous dominant) are normal.

Law of Independent Assortment: Alleles of one gene are passed to offspring independently of the alleles of other genes. Applies to the

inheritance of two or more genes simultaneously.

Can use the inheritance of traits in fruit flies to illustrate the law of independent assortment.

Dihybrid cross A fertilization in which the parents differ in two

distinct traits or characteristics.

Sources of Genetic Variation Law of Independent Assortment Mutation

Definition = change in DNA sequence resulting the appearance of a new allele

Can occur at any time, but not always beneficial

Some can be harmful and result in death Some can result in the organism having

traits that make them better suited to the environment

When an organism has two different alleles for a gene that does not follow the dominant/recessive pattern

The organism shows a trait that is a blend of the traits represented by the two alleles

Also called INCOMPLETE DOMINANCE

For example:

The gene for the color of some flowers has one allele for red and one for white. When both alleles are present, neither is dominant, and the flower color is pink

Pattern of inheritance in which alleles from both parents are blended

Pattern of inheritance in which both alleles of a gene are expressed