Copyright Pearson Prentice Hall Gregor Mendel’s Peas Genetics is the scientific study of heredity. Gregor Mendel was an Austrian monk. His work was important

Copyright Pearson Prentice Hall

Gregor Mendel’s Peas

Genetics is the scientific study of heredity.

Gregor Mendel was an Austrian monk. His work was important to the understanding of heredity.

Mendel carried out his work with ordinary garden peas.




Mendel knew that

• the male part of each flower produces pollen, (containing sperm).

• the female part of the flower produces egg cells.



During sexual reproduction, sperm and egg cells join in a process called fertilization.

Fertilization produces a new cell.

Pea flowers are self-pollinating.



Mendel had true-breeding pea plants that, if allowed to self-pollinate, would produce offspring identical to themselves.

Cross-pollination

Mendel was able to produce seeds that had two different parents.


Genes and Dominance

Genes and Dominance

A trait is a specific characteristic that varies from one individual to another.

Mendel studied seven pea plant traits, each with two contrasting characters.

He crossed plants with each of the seven contrasting characters and studied their offspring.


Genes and Dominance

Each original pair of plants is the P (parental) generation.

The offspring are called the F1, or “first filial,” generation.

The offspring of crosses between parents with different traits are called hybrids.


Genes and Dominance

Mendel’s F1 Crosses on Pea Plants


Genes and Dominance

Mendel’s Seven F1 Crosses on Pea PlantsMendel’s F1 Crosses on Pea Plants


Genes and Dominance

Mendel's first conclusion

was that biological inheritance is determined by factors that are passed from one generation to the next.

Today, scientists call the factors that determine traits genes.


Genes and Dominance

Each of the traits Mendel studied was controlled by one gene that occurred in two contrasting forms that produced different characters for each trait.

The different forms of a gene are called alleles.

Mendel’s second conclusion

is called the principle of dominance.


Genes and Dominance

The principle of dominance states that some alleles are dominant and others are recessive.


Genes and Dominance

Mendel’s F1 Crosses on Pea Plants


Segregation

Segregation

Mendel crossed the F1 generation with itself to produce the F2 (second filial) generation.

The traits controlled by recessive alleles reappeared in one fourth of the F2 plants.


Mendel's F2 Generation

P GenerationF1 Generation

Tall Tall Tall Tall Tall TallShort Short

F2 Generation

Segregation


Segregation

The reappearance of the trait controlled by the recessive allele indicated that at some point the allele for shortness had been separated, or segregated, from the allele for tallness.


Segregation

Mendel suggested that the alleles for tallness and shortness in the F1 plants segregated from each other during the formation of the sex cells, or gametes.


Segregation

Alleles separate during gamete formation.


11-1

Gametes are also known as

a. genes.

b. sex cells.

c. alleles.

d. hybrids.


11-1

The offspring of crosses between parents with different traits are called

a. alleles.

b. hybrids.

c. gametes.

d. dominant.


11-1

In Mendel’s pea experiments, the male gametes are the

a. eggs.

b. seeds.

c. pollen.

d. sperm.


11-1

In a cross of a true-breeding tall pea plant with a true-breeding short pea plant, the F1 generation consists of

a. all short plants.

b. all tall plants.

c. half tall plants and half short plants.

d. all plants of intermediate height.


11-1

If a particular form of a trait is always present when the allele controlling it is present, then the allele must be

a. mixed.

b. recessive.

c. hybrid.

d. dominant.


Genetics and Probability

Genetics and Probability

The likelihood that a particular event will occur is called probability.

The principles of probability can be used to predict the outcomes of genetic crosses.


Punnett Squares

Punnett Squares

The gene combinations that might result from a genetic cross can be determined by drawing a diagram known as a Punnett square.

Punnett squares can be used to predict and compare the genetic variations that will result from a cross.


A capital letter represents the dominant allele for tall.

A lowercase letter represents the recessive allele for short.

In this example,

T = tallt = short

Punnett Squares


Gametes produced by each F1 parent are shown along the top and left side.

Punnett Squares


Punnett Squares

Organisms that have two identical alleles for a particular trait are said to be homozygous.

Organisms that have two different alleles for the same trait are heterozygous.

Homozygous organisms are true-breeding for a particular trait.

Heterozygous organisms are hybrid for a particular trait.


Punnett Squares

All of the tall plants have the same phenotype, or physical characteristics.

The tall plants do not have the same genotype, or genetic makeup.

One third of the tall plants are TT, while two thirds of the tall plants are Tt.


Punnett Squares

The plants have different genotypes (TT and Tt), but they have the same phenotype (tall).

TTHomozygous

TtHeterozygous


Probability and Segregation

Probability and Segregation

One fourth (1/4) of the F2 plants have two alleles for tallness (TT).

2/4 or 1/2 have one allele for tall (T), and one for short (t).

One fourth (1/4) of the F2 have two alleles for short (tt).


Probabilities Predict Averages

Probabilities Predict Averages

Probabilities predict the average outcome of a large number of events.

Probability cannot predict the precise outcome of an individual event.

In genetics, the larger the number of offspring, the closer the resulting numbers will get to expected values.


11-2

Probability can be used to predict

a. average outcome of many events.

b. precise outcome of any event.

c. how many offspring a cross will produce.

d. which organisms will mate with each other.


11-2

Compared to 4 flips of a coin, 400 flips of the coin is

a. more likely to produce about 50% heads and 50% tails.

b. less likely to produce about 50% heads and 50% tails.

c. guaranteed to produce exactly 50% heads and 50% tails.

d. equally likely to produce about 50% heads and 50% tails.


11-2

Organisms that have two different alleles for a particular trait are said to be

a. hybrid.

b. heterozygous.

c. homozygous.

d. recessive.


11-2

Two F1 plants that are homozygous for shortness are crossed. What percentage of the offspring will be tall?

a. 100%

b. 50%

c. 0%

d. 25%


11-2

The Punnett square allows you to predict

a. only the phenotypes of the offspring from a cross.

b. only the genotypes of the offspring from a cross.

c. both the genotypes and the phenotypes from a cross.

d. neither the genotypes nor the phenotypes from a cross.


Independent Assortment

To determine if the segregation of one pair of alleles affects the segregation of another pair of alleles, Mendel performed a two-factor cross.




The Two-Factor Cross: F1

Mendel crossed true-breeding plants that produced round yellow peas (genotype RRYY) with true-breeding plants that produced wrinkled green peas (genotype rryy).

RRYY x rryy

All of the F1 offspring produced round yellow peas (RrYy).



The alleles for round (R) and yellow (Y) are dominant over the alleles for wrinkled (r) and green (y).



The Two-Factor Cross: F2

Mendel crossed the heterozygous F1 plants (RrYy) with each other to determine if the alleles would segregate from each other in the F2 generation.

RrYy × RrYy



The Punnett square predicts a 9 : 3 : 3 :1 ratio in the F2 generation.


The alleles for seed shape segregated independently of those for seed color. This principle is known as independent assortment.

Genes that segregate independently do not influence each other's inheritance.



The principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes. Independent assortment helps account for the many genetic variations observed in plants, animals, and other organisms.

Independent

Assortment


A Summary of Mendel's Principles


• Genes are passed from parents to their offspring.

• If two or more forms (alleles) of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive.


a. In most sexually reproducing organisms, each adult has two copies of each gene. These genes are segregated from each other when gametes are formed.

b. The alleles for different genes usually segregate independently of one another.



Beyond Dominant and Recessive Alleles

Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes.



Incomplete Dominance

When one allele is not completely dominant over another it is called incomplete dominance.

In incomplete dominance, the heterozygous phenotype is between the two homozygous phenotypes.


A cross between red (RR) and white (WW) four o’clock plants produces pink-colored flowers (RW).


WW

RR



Codominance

In codominance, both alleles contribute to the phenotype.

In certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers.

Heterozygous chickens are speckled with both black and white feathers. The black and white colors do not blend to form a new color, but appear separately.



Multiple Alleles

Genes that are controlled by more than two alleles are said to have multiple alleles.

An individual can’t have more than two alleles. However, more than two possible alleles can exist in a population.

A rabbit's coat color is determined by a single gene that has at least four different alleles.



Different combinations of alleles result in the colors shown here.

Full color: CC, Ccch, Cch, or CcChinchilla: cchch, cchcch, or cchcHimalayan: chc, or chchAIbino: cc

KEY

C = full color; dominant to all other alleles

cch = chinchilla; partial defect in pigmentation; dominant to ch and c alleles

ch = Himalayan; color in certain parts of the body; dominant to c allele

c = albino; no color; recessive to all other alleles

Epistasis

Epistasis

• A trait involves 2 genes

• The expression of one gene influences the expression of the other

• For example, in Labrador Retrievers one gene determines which color. A different gene determines if you have color.

• E=have color in hair, e=no color in hair

• B=black pigment, b=brown pigment

Fig. 10.13, p.161

EB Eb eB eb

EB

Eb

eB

eb EeBbblack

EeBBblack

EEBbblack

EEBBblack

EEBbblack

EeBBblack

EeBbblack

Eebbchocolate

EeBbblack

EEbbchocolate

EeBbblack

Eebbchocolate

eeBByellow

eeBbyellow

eebbyellow

eeBbyellow



Polygenic Traits

Traits controlled by two or more genes are said to be polygenic traits.

Skin color in humans is a polygenic trait controlled by more than four different genes.

LE 14-12

aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCc AABBCC

AaBbCcAaBbCc

20/64

15/64

6/64

1/64

Fra

ctio

n of

pro

geny


Applying Mendel's Principles

Applying Mendel's Principles

Thomas Hunt Morgan used fruit flies to advance the study of genetics.

Morgan and others tested Mendel’s principles and learned that they applied to other organisms as well as plants.


11–3

In a cross involving two pea plant traits, observation of a 9 : 3 : 3 : 1 ratio in the F2 generation is evidence for

a. the two traits being inherited together.

b. an outcome that depends on the sex of the parent plants.

c. the two traits being inherited independently of each other.

d. multiple genes being responsible for each trait.


11–3

Traits controlled by two or more genes are called

a. multiple-allele traits.

b. polygenic traits.

c. codominant traits.

d. hybrid traits.


11–3

In four o'clock flowers, the alleles for red flowers and white flowers show incomplete dominance. Heterozygous four o'clock plants have

a. pink flowers.

b. white flowers.

c. half white flowers and half red flowers.

d. red flowers.


11–3

A white male horse and a tan female horse produce an offspring that has large areas of white coat and large areas of tan coat. This is an example of

a. incomplete dominance.

b. multiple alleles.

c. codominance.

d. a polygenic trait.


11–3

Mendel's principles apply to

a. pea plants only.

b. fruit flies only.

c. all organisms.

d. only plants and animals.


Each organism must inherit a single copy of every gene from each of its “parents.”

Gametes are formed by a process that separates the two sets of genes so that each gamete ends up with just one set.


Chromosome

Number

Chromosome Number

All organisms have different numbers of chromosomes.

A body cell in an adult fruit fly has 8 chromosomes: 4 from the fruit fly's male parent, and 4 from its female parent.


Chromosome

Number

These sets of chromosomes are homologous.

Each of the 4 chromosomes that came from the male parent has a corresponding chromosome from the female parent.


Chromosome

Number

A cell that contains both sets of homologous chromosomes is said to be diploid.

The number of chromosomes in a diploid cell is sometimes represented by the symbol 2N.

For Drosophila, the diploid number is 8, which can be written as 2N=8.


Chromosome

Number

The gametes of sexually reproducing organisms contain only a single set of chromosomes, and therefore only a single set of genes.

These cells are haploid. Haploid cells are represented by the symbol N.

For Drosophila, the haploid number is 4, which can be written as N=4.


Phases of

Meiosis

Phases of Meiosis

Meiosis is a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell.


Meiosis involves two divisions, meiosis I and meiosis II.

By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells.

Phases of

Meiosis


Phases of

Meiosis

Meiosis I

Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis

Interphase I

Meiosis I


Phases of

Meiosis

Cells undergo a round of DNA replication, forming duplicate chromosomes.

Interphase I


Phases of

Meiosis

Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.

There are 4 chromatids in a tetrad.

MEIOSIS I Prophase II


Phases of

Meiosis

When homologous chromosomes form tetrads in meiosis I, they exchange portions of their chromatids in a process called crossing over.

Crossing-over produces new combinations of alleles.


Phases of

Meiosis

Spindle fibers attach to the chromosomes.

MEIOSIS I Metaphase I


Phases of

Meiosis MEIOSIS I Anaphase I

The fibers pull the homologous chromosomes toward opposite ends of the cell.


Phases of

Meiosis

MEIOSIS I Telophase I and Cytokinesis

Nuclear membranes form.

The cell separates into two cells.

The two cells produced by meiosis I have chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I.


Phases of

Meiosis

Meiosis II

The two cells produced by meiosis I now enter a second meiotic division.

Unlike meiosis I, neither cell goes through chromosome replication.

Each of the cell’s chromosomes has 2 chromatids.


Phases of

Meiosis

Meiosis II

Telophase II and Cytokinesis

Prophase IIMetaphase II Anaphase IITelophase I and

Cytokinesis I

Meiosis II


Phases of

Meiosis

Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell.

MEIOSIS IIProphase II


Phases of

Meiosis

The chromosomes line up in the center of cell.

MEIOSIS II Metaphase II


Phases of

Meiosis

The sister chromatids separate and move toward opposite ends of the cell.

MEIOSIS II Anaphase II


Phases of

Meiosis

Meiosis II results in four haploid (N) daughter cells.

MEIOSIS II Telophase II and Cytokinesis


Gamete Formati

on

Gamete Formation

In male animals, meiosis results in four equal-sized gametes called sperm.


Gamete Formati

on

In many female animals, only one egg results from meiosis. The other three cells, called polar bodies, are usually not involved in reproduction.


Comparing

Mitosis and

Meiosis

Comparing Mitosis and Meiosis

Mitosis results in the production of two genetically identical diploid cells. Meiosis produces four genetically different haploid cells.


Comparing

Mitosis and

Meiosis

Mitosis

a. Cells produced by mitosis have the same number of chromosomes and alleles as the original cell.

b. Mitosis allows an organism to grow and replace cells.

c. Some organisms reproduce asexually by mitosis.


Comparing

Mitosis and

Meiosis

Meiosis

a. Cells produced by meiosis have half the number of chromosomes as the parent cell.

b. These cells are genetically different from the diploid cell and from each other.

c. Meiosis is how sexually-reproducing organisms produce gametes.


11-4

If the body cells of humans contain 46 chromosomes, a single sperm cell should have

a. 46 chromosomes.

b. 23 chromosomes.

c. 92 chromosomes.

d. between 23 and 46 chromosomes.


11-4

During meiosis, the number of chromosomes per cell is cut in half through the separation of

a. daughter cells.

b. homologous chromosomes.

c. gametes.

d. chromatids.


11-4

The formation of a tetrad occurs during

a. anaphase I.

b. metaphase II.

c. prophase I.

d. prophase II.


11-4

In many female animals, meiosis results in the production of

a. only 1 egg.

b. 1 egg and 3 polar bodies.

c. 4 eggs.

d. 1 egg and 2 polar bodies.


11-4

Compared to egg cells formed during meiosis, daughter cells formed during mitosis are

a. genetically different, while eggs are genetically identical.

b. genetically different, just as egg cells are.

c. genetically identical, just as egg cells are.

d. genetically identical, while egg cells are genetically different.

Documents

Copyright Pearson Prentice Hall Gregor Mendel’s Peas Genetics is the scientific study of heredity. Gregor Mendel was an Austrian monk. His work was important