GREGOR MENDEL Gregor Johann Mendel(born 22nd July 1822, died 6th January 1884) Introduction:"EARLY IDEAS ABOUT HEREDITY"

Until the 19th century people believed that heredity was a BLENDING inheritance and the nature of these

factors was unknown *The resemblance of children was explained by the THEORY OF BLENDING

INHERITANCE. Indeed you do see a little of both parents in a child...

GENETICS- the branch of biology that studies heredity.the scientific study of heredity. HEREDITY- the passing

of traits from parents to their young...biological inheritance. Heredity is the reason we have different

species....cats have kittens, dogs have puppies, oak trees produce acorns... NO!


263 Gregor Mendel , an Austrian monk born in 1822, is known as the "father of genetics".

At the age of 21 Mendel entered a monastery in "Brno"and in 1851 was sent to the Univeristy of Vienna to study science

and math. He spent 2 years at the University and then returned to the monastery and spent

the next 14 years teaching at the monastery. Mendel was a teacher & was also responsible for

tending the garden at the monastery. From this responsibility came the foundation of genetics.

Mendel studied the garden pea plants and conducted experiments that unlocked some of the secrets of heredity.

Teacher Tube Video Clip =2cbe4813cae2f7715336 Mendel found that pea plants have both male

and female parts and that normally, pollen from the male part of a pea flower fertilizes the female egg cells of the same pea flower.

TRUE BREEDING- when allowed to self pollinate, the parents produce offspring identical to the parent plant.

SELF POLLINATIONthe process in which pollen falls from the male part of a flower to the female part of a flower of the same

plant. PUREBRED- belonging to a group of organisms that can produce offspring having only

one form of a trait in each generation. *Mendel used pea plants that had been allowed to self pollinate

for several generations because they were purebred. The purebred pea plants would produce offspring

that were identical to themselves. Purebred pea plants were the basis of Mendel's experiments.

Tall plants produced only tall offspring. Short plants produced only short offspring. Plants that had green seeds

produced offspring with green seeds. Mendels pea plants produced seeds by self pollination. These true

breeding plants were the basis for Mendel's experiments. The seeds that were produced inherited all of

their characteristics from the single plant that "created" them. Mendel prevented selfpollination in some plants and fertilized the

eggs of a flower with the pollen from a different plant. CROSS POLLINATIONthe transfer of pollen from the flower of one

plant to the flower of another plant...fertilization of a plant's eggs by the pollen of another plant.

Cross pollination produces seeds that are the offspring of 2 different plants. Through cross

pollination Mendel was able to cross plants with different characteristics. Mendel studied a few isolated traits...ones

that were easily observed.He studied 7 traits...See figure 11-3 page 264 TRAIT- a specific

characteristic that a living thing can pass on to its young Traits Medel Studied: Seed Shape, Seed Color,

Seed Coat Color, Pod Shape, Pod Color, Flower Position, Plant Height. Studying only a few

traits made measuring the effects of heredity much easier. "GENES AND DOMINANCE" PG 264

Mendel crossed pea plants with different characteristics for the same trait....tall with short.

Flower positions: axial and terminal...axial along the sides...terminal at the end.

HYBRID- an organism that results from crossing parents with differing traits or characteristics

From Mendels crosses he got HYBRIDS. Mendel thought that he would get half short and

half tall plants when he crossed a short and tall plant to his surprise all of the offspring were tall....the

short characteristic had apparently disappeared. Conclusions from Mendels 1st Set of Experiments:

1. The individual factors that do not blend with one another, control the trait of a living thing.

Mendel used the word merkmal to refer to these factors..In German merkmal means character

Merkmal - factors that control traits.Today the word merkmal is replaced with the word GENE.

Bill Nye Gene video Clip GENE-chemical factors that control traits. Each of the traits that Mendel studied was

controlled by one gene that had 2 contrasting forms..tall and short; ALLELES- different forms of a gene.

Allele (conclusions from 1st set of experiments...) 2. PRINCIPLE OF DOMINANCE- some of the

alleles or factors are dominant and some are recessive. The effects of a dominant allele is seen even when the

contrasting recessive allele is present. The effects of a recessive allele are not seen when the dominant allele is

present.In his 1st experiments tall and yellow alleles were dominant while short and green alleles were recessive

Dominance is seen in many traits but does not apply to all genes. "SEGREGATION" PG

265 Mendel wanted to know what happened to the recessive characters.So, he allowed several of

the hybrid plants to reproduce by selfpollination. To keep track of the different groups of seeds he gave them


GENERATION- the first generation of plants produced by cross pollination. The word filius is a Latin

word that means son.The next generation produced from crossing the F one plants would be referred to as the F2 generation and so on.

"THE F1 CROSS" PG 266; When the 1st filial generation was crossed the plants produced (F2) showed the recessive


Mendel assumed that the presence of the dominant tall allele had masked the recessive short allele.In some of

the offspring of the F2 generation the allele was not masked. This segregation or separation puzzled

Mendel He suggested that during the formation of the egg and pollen cells, the tall and short alleles

in the F1 plants were separated from each other. SEGREGATION- the separation of alleles

during gamete formation. 11-2 11-2

PUNNETT SQUARE- a diagram that shows the possible gene combinations in the offspring that result

from a cross. Alleles are represented by letters that serve as symbols.The

DOMINANT allele is represented by a capital letter. The RECESSIVE allele is represented by a

lower case letter that corresponds to the dominant allele symbol. 11-2 page 267 "GENETICS AND

PROBABILITY Mendel applied the math concept of "probability" to biology. PROBABILITY- the

likelihood that a particular event will occur. PROBABILITY = # times a particular event

occurs/ # of trials EX: flipping a coin In probability you only get the expected ratio for large numbers of

trials...the larger the number of trials the closer you get to expected values. Previous events do not

affect future outcomes.Ex: each flip of the coin is a separate independent event; GAMETESreproductive cells

(sex cells);The gametes produced by each parent are placed along the left hand side and the top of the punnett square.

The offspring are represented by each square.The probable results are often expressed as ratios.

PHENOTYPE- physical characteristics; GENOTYPE- genetic make-up

HOMOZYGOUSorganisms that have 2 identical alleles for a particular trait. (TT or tt);Homozygous organisms are PUREBRED.

HETEROZYGOUSorganisms that have two different alleles for a particular trait. (Tt); Heterozygous organisms are hybrids.


Mendel also questioned if the segregation of one pair of alleles affect the segregation of another pair.For example: does

the gene for seed shape have anything to do with seed color? To find out the answer to this Mendel crossed

purebred plants that produce round yellow seeds with purebred plants that produced wrinkled green seeds.

TWO FACTOR CROSS- a cross that involves 2 traits. See the cross in figure 11-9 & 11-10 pages 270 & 271

The F1 plants from the above mentioned cross will produce seeds that are round and yellow...the dominant traits show up in a

hybrid and the recessive ones seem to disappear. Segregation is still not proved to be independent...another

cross is needed.The F1 generation plants need to be crossed to produce an F2 generation.

"THE 2 FACTOR CROSS F2 Mendel concluded from his results of the F1 cross that genes could

segregate independently during the formation of gametes...genes can undergo independent assortment.

The only exception to independent assortment is for genes that are located on the same chromosome...they

cannot undergo independent assortment. A SUMMARY OF MENDEL'S WORK:

-Genes control heredity. In sexually reproductive organisms genes are inherited from each parent.

-When 2 or more forms of one gene exist, some forms of the gene may be dominant and some forms may be recessive.

-Genes of different traits may assort independently of one another.

APPLYING MENDEL'S PRINCIPLES Mendel's ideas about heredity and his applications of

mathematics and statistics to Biology were ahead of their time. Mendel's pioneering

work in genetics remained unappreciated in his lifetime. More than 20 years after his death, Mendel's

experiments and conclusions were recognized as important breakthroughs in Biology.


TEST CROSS- the cross of an organism of unknown genotype with a homozygous recessive individual.


11-4 MEIOSIS PG 275 The sex cells or gametes that carry the genetic information are formed by a special kind

of cell division known as MEIOSIS. The number of chromosomes, the structures that contain

the genetic information, is reduced by half during meiotic division. By fertilization the full number of

chromosomes is restored. These 2 processes Meiosis and fertilizationallow for infinite variety in the selection and

recombination of genetic traits. From Mendel's genetics we know that an organism inherits a

single copy of each gene from each of their parents. These 2 copies are segregated from one

another during the formation of gametes. EX: fruit fly...each body or somatic cell of a fruit fly contains 8

chromosomes. If chromosomes were not separated the offspring would have 16 chromosomestheir

offspring would have 32 and so on. Drosophila melanogaster

CHROMOSOME NUMBER PG 275 The chromosomes of the fruit fly can be divided into 2 sets4

chromosomes from the male parent and 4 chromosomes from the female parent. Each chromosome in the

male set has a corresponding chromosome in the female set. HOMOLOGOUS- a

description of chromosomes that occur in pairs; having a corresponding structurethe chromosomes are

called HOMOLOGS. EX: Each of the chromosomes from the male parent have a corresponding

chromosome from the female parent. A cell that contains both sets of homologous

chromosomes are said to be DIPLOID. A diploid cell contains 2 complete sets of chromosomes and 2

complete sets of genes.The diploid number is sometimes represented by 2Nso for the Drosophila 2N= 8.

DIPLOID- "two sets" a cell that contains both sets of homologous chromosomes.All of

an organisms cells (except for sex cells) contain 2 alleles for a given trait. Gametes or sex cells

contain only a single set of genes because alleles are separated during the process of gamete formation.

The GAMETES of Sexually reproducing organisms contain a single set of chromosomes and genes.

HAPLOID- a cell that contains a single set of chromosomes..represente d by the symbol N.Drosophila, N =4

THE PHASES OF MEIOSIS MEIOSIS- a process of reduction division in

which the number of chromosomes per cell is cut in half and homologous chromosomes that exist in a diploid cell are

separated. In most organisms meiosis takes place in 2 st nd

stages..1 & 2 meiotic divisions. The phases of meiosis are very different from the phases of MITOSIS.

SEE FIG 11-15 page 276 MEIOSIS I.

Special cells in reproductive organs undergo a round of DNA replicationthis resembles mitosis but it is not the same!

In Prophase 1 of meiosis, each chromosome seeks out its corresponding homologous

chromosome to form a special structure called a tetrad. There are 4 chromatids in a tetrad;

CROSSING OVER homologous chromosomes may exchange portions of

their chromatids, an exchange of genes that produces new combinations of genes see figure 11-16 page

METAPHASE 1 Metaphase I Tetrads (rather than individual chromosomes) line up in the center of the cell.

ANAPHASE I The homologous chromosomes separate and (telophase 1) two new cells are formed.

The 2 new cells have sets of chromosomes that are different from the parent cells and different from each other.

MEIOSIS II. The 2 cells produced by meiosis I enter meiosis II.In the

second meiotic division the cells do not under go DNA replication so each cells chromosomes contains 2 chromatids.

In METAPHASE II of meiosis, : chromosomes line up in the middle of the cell. In ANAPHASE II they separateeach of

the 4 daughter cells receives 2 chromatids . The daughter cells contain the HAPLOID number = 2

chromosomes each. *The amount of genetic material has been reduced and the combinations of

chromosomes in each gamete have been made at random. MEIOSIS & GENETICS

Meiosis I results in segregation and independent assortment. GAMETE FORMATION

In males the haploid gametes produced by meiosis are called SPERM.Pollen grains contain haploid sperm

cells. The female gamete is called an egg in animals and an OVULE in higher plants.

In females the cell divisions are uneven, and the egg or ovule gets most of the cytoplasmsee figure

11-17 pg 278; 3 other cells called polar bodies are produced in the female during meiosis..they are called

polar bodies. COMPARING MITOSIS AND MEIOSIS Mitosis results in the

production of 2 genetically identical cells. A diploid cell divides and gives rise to 2

diploid daughter cells that are genetically identical to each other and identical to the original parent cell.

Meiosis begins with a diploid cell but produces 4 haploid cells that are different from the original diploid cell and different from one

another.WHY? Homologous chromosomes are st separated during the 1

meiotic division and crossing over occurs giving new gene combinations on the chromosomes.

*MITOSIS results in the production of 2 genetically identical diploid cells. *MEIOSIS results in the

production of 4 genetically DIFFERENT haploid cells. TEST CH 11

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