biology chapter 11. introduction to geneticspg 262

BIOLOGY CHAPTER 11

INTRODUCTION TO GENETICSPG 262

11-1 THE WORK OF 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 THIS CANNOT HAPPEN!NO! This CANNOT Happen!

"GREGOR MENDEL" PG 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

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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 POLLINATION- the 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.

Mendel’s 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 self-pollination in some

plants and fertilized the eggs of a flower with the pollen from a different

plant.

CROSS POLLINATION- the 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 self-

pollination.

To keep track of the different groups of

seeds he gave them names.

P GENERATION- PUREBRED PARENTAL

PLANTS

F1 FIRST FILIAL 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 traits. WHY?

"EXPLAINING THE F 1 CROSS" PG 266

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.

11-211-2

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;

GAMETES- reproductive 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

HOMOZYGOUS- organisms that have 2 identical alleles for a particular trait. (TT or

tt);Homozygous organisms are

PUREBRED.

HETEROZYGOUS- organisms that have

two different alleles for a particular trait. (Tt);

Heterozygous organisms are hybrids.

11-3

"INDEPENDENT ASSORTMENT" (page

271)

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.

"USING THE PUNNETT SQUARE"

ONE FACTOR CROSS...crossing one

trait

TEST CROSS- the cross of an organism of

unknown genotype with a homozygous recessive

individual.

"TWO FACTOR CROSS”

SEE FIGURE 11-9 page 270

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 fertilization- allow 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 chromosomes…their

offspring would have 32 and so on.

Drosophila melanogaster

CHROMOSOME NUMBER PG 275

The chromosomes of the fruit fly can be

divided into 2 sets…4 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 structure…the

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 2N…so 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..represented 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 stages..1st & 2nd 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 replication…this

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;

In a process called

“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

276

METAPHASE 1

Metaphase ITetrads (rather than

individual chromosomes) line up in the center of the cell.

ANAPHASE IThe 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 cell’s chromosomes contains

2 chromatids.

In METAPHASE II of meiosis, : chromosomes line up in the middle of the cell. In ANAPHASE II they separate…each 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 cytoplasm…see 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

separated during the 1st 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