Gregor Mendel

Mahitha

Early Life• Johann Gregor Mendel was a Moravian• Born in 1822 in Hyncice, Czechoslovakia on July 22nd.• His father was a peasant and his grandfather was a gardener.

Mendel was initially taught by a local priest but later on he was admitted in an Institute of Philosophy in Olmutz.

• But he was not financially well to do therefore in 1843, he terminated his studies and went back to the monastery in Brunn.

• Mendel thought that monastery was the best place for him to study without worrying about how he’d finance his studies. He was made in charge of the garden at the monastery and named himself Gregor. He became a priest in 1847. After four years he went to University of Vienna where he studied physics, mathematics, chemistry, and botany.

• When he returned to the monastery after completing his studies, he took a position as a teacher of natural sciences at the Technical School at Brno.

The experiments carried out by Gregor Mendel

• He formulated the principles of Inheritance by carrying out experiments with garden peas.

• Mendel chose garden peas because• they were easy to grow, • produced new generations quickly • peas had easily distinguishable characteristics• was also able to strictly control the breeding patterns of his peas.

Mendel examined the following seven characteristics found in peas:• Flower colour - purple or white• Flower position - axial or terminal• Seed colour - yellow or green• Seed shape - round or wrinkled• Pod shape - inflated or constricted• Pod colour - green or yellow• Stem height - tall or short.

• Mendel needed to control the types of fertilisation. • Experiments were done on both self-pollinated and

cross-pollinated pea plants.• Self-fertilization was ensured by placing a bag over

the flowers to make sure pollen from the stamens lands on the carpel of the same flower.• Cross-fertilization was ensured by cutting off

stamens from a flower before pollen was produced, then dusting the carpel of the flower with pollen from another plant. • To ensure reliability, Mendel used thousands of

plants in each experiment.

•Mendel worked with true-breeding plants: self-fertilised plants which produced all offspring identical to the parents. •Mendel first cross-fertilized two true-

breeding plants for one characteristic, • for example tall plants were crossed with

short plants• The offspring produced are called F1 (1st filial)

generation.• The F1 generation were then self-fertilised or

cross-fertilised to produce a second generation, F2.

Each of the seven traits that Mendel studied had a dominant and a recessive factor. When two true-breeding plants were crossed, only the dominant factor appeared in the first generation. The recessive factor appeared in the second generation in a 3:1 (dominant : recessive) relationship.

This 3:1 ratio occurs in later generations as well. Mendel realized that this underlying regularity was the key to understanding the basic mechanisms of inheritance.

• He came to three important conclusions from these experimental results:• the inheritance of each trait is determined by "units" or "factors"

that are passed on to descendants unchanged. These units are now called genes.

• an individual inherits one such unit from each parent for each trait

• a trait may not show up in an individual but can still be passed on to the next generation.

Mendel’s laws

• Mendel formulated 3 principles of genetics with his results of his experiments.• They are:• The Law of Dominance• The Law of Segregation• The Law of Independent Assortment

Law of Dominance

• In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. Offspring that are hybrid for a trait will have only the dominant trait in the phenotype.

•

• While Mendel was crossing his pea plants, he noticed something interesting. When he crossed pure tall plants with pure short plants, all the new pea plants were tall. Similarly, crossing pure yellow seeded pea plants and pure green seeded pea plants produced an F1 generation of all yellow seeded pea plants. The same was true for other pea traits.• The results genotype were more like – Tt, Tt, Tt, Tt

• So what was observed is that when there is a dominant trait present in a genotype, the phenotype is affected by the dominant allele and not the recessive one.

Genotype Phenotype

TT Tall

Tt Tall

tt Short

Law of Segregation

• During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.

• Now, when completing a Punnet Square, we model this "Law of Segregation" every time.

• When you "split" the genotype letters & put one above each column & one in front of each row, you have SEGREGATED the alleles for a specific trait. In real life this happens during a process of cell division called "MEIOSIS".

• Meiosis leads to the production of gametes (sex cells), which are either eggs or sperm.

• You can see from the p-square that any time you cross two hybrids, 3 of the 4 boxes will produce an organism with the dominant trait - "TT", "Tt", & "Tt”.

• 1 of the 4 boxes ends up homozygous recessive, producing an organism with the recessive phenotype - "tt” .

Law of Independent assortment

•Alleles for different traits are distributed to sex cells (& offspring) independently of one another.

• Mendel noticed during all his work that the height of the plant and the shape of the seeds and the color of the pods had no impact on one another.

• The different traits seem to be inherited INDEPENDENTLY.• The genotypes of the parent pea plants will be:• RrGg x RrGg• Where,• "R" = dominant allele for round seeds • "r" = recessive allele for wrinkled seeds • "G" = dominant allele for green pods • "g" = recessive allele for yellow pods

The results from a dihybrid cross are always the same: 9/16 boxes (offspring) show dominant phenotype for both traits (round & green), 3/16 show dominant phenotype for first trait & recessive for second (round & yellow), 3/16 show recessive phenotype for first trait & dominant form for second (wrinkled & green), & 1/16 show recessive form of both traits (wrinled & yellow).

• So, as you can see from the results, a green pod can have round or wrinkled seeds, and the same is true of a yellow pod. • The different traits do not influence the

inheritance of each other. They are inherited INDEPENDENTLY.

Reasons for Mendel’s success

Mendel was successful because :• He used peas, which were easily grown• Peas produced successive generations rapidly• He selected easily observable characteristics• strictly controlled the fertilization process• He used mathematics rigorously to analyze his results• used large numbers of plants.• He studied traits that had two easily identified factors.• One of the main advantages Mendel had over the other

scientists was that he excelled in mathematics which was majorly used in his experiments.

• He maintained an accurate record of all the observations.