Chapter Eleven- Intro to Genetics

Copyright Pearson Prentice Hall

biology


11-1 The Work of Gregor Mendel11-1 The Work of Gregor Mendel


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- sperm and egg join to produce a new cell.



Pea flowers are self-pollinating.Sperm cells in pollen fertilize the egg cells in the same flower. The seeds that are produced by self-pollination inherit all of their characteristics from the single plant that bore them.



Mendel had true-breeding pea plants.

True-breeding plants, if allowed to self-pollinate, produce offspring identical to themselves.



Mendel wanted to produce seeds by joining male and female reproductive cells from two different plants.He cut away the pollen-bearing male parts of the plant and dusted the plant’s flower with pollen from another plant.



This process is called cross-pollination.

Cross-pollination produces seeds that have two different parents.


Genes and Dominance

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


Genes and Dominance

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.


Genes and Dominance

The offspring of crosses between parents with different traits are called hybrids.The F1 hybrid plants all had the character of only one of the parents.


Mendel’s F1 Crosses on Pea Plants


Mendel’s Seven F1 Crosses on Pea Plants

Mendel’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. = 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

An organism with a dominant allele for a trait will always exhibit that form of the trait.An organism with the recessive allele for a trait will exhibit that form only when the dominant allele for that trait is not present.


Genes and Dominance

Pink x white = all pink

Pink is dominant alleleWhite is recessive allele



SegregationSegregationMendel 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 GenerationP Generation F1 Generation

Tall Tall Tall Tall Tall TallShort Short

F2 Generation

Segregation


Segregation

Mendel assumed that the dominant allele masks the recessive allele in the F1 generation.

The trait controlled by the recessive allele showed up in some of the F2 plants.


Segregation

The reappearance of the recessive trait showed that the allele for shortness had been separated, or segregated, from the allele for tallness.


Segregation

Mendel suggested that the alleles segregate from each other during the formation of the sex cells, or gametes.


SegregationWhen each F1 plant flowers and produces gametes, the two alleles segregate from each other so that each gamete carries only a single copy of each gene. Therefore, each F1 plant produces two types of gametes—those with the allele for tallness, and those with the allele for shortness.


Segregation

Alleles separate during gamete formation.


11-2 Probability and Punnett Squares

11-2 Probability and Punnett Squares


Genetics and Probability

The likelihood that a particular event will occur is called probability.Probability can be used to predict the outcomes of genetic crosses.


Punnett Squares

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


A capital letter represents the dominant allele.A lowercase letter represents the recessive allele.In this example, T = tallt = short

Punnett Squares


Punnett Squares

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


Punnett Squares

Possible gene combinations for the offspring appear in the four boxes.

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.


Punnett Squares

Homozygous organisms are true-breeding for a particular trait. Heterozygous organisms are hybrid for a particular trait.

Punnett Squares

Offspring have the same phenotype if they show the same trait. Offspring have the same genotype if they have the same genetic makeup (alleles).


Punnett Squares

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

TTHomozygous

TtHeterozygousActive art


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).


Probability and Segregation

The ratio of plants showing the dominant phenotype (TT or Tt genotype) to those showing the recessive phenotype(tt genotype) plants is 3:1.The predicted ratio showed up in Mendel’s experiments indicating that segregation did occur.


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.


11-3 Exploring Mendelian Genetics

11–3 Exploring Mendelian Genetics


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.

Independent Assortment



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).•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


For a two-factor F1 hybrid cross, the Punnett square predicts a 9 : 3 : 3 :1 ratio in the F2 generation.


In Mendel’s experiment, the F2 generation produced the following:•9 seeds that were round and yellow•3 seeds that were round and green•3 seeds that were wrinkled and yellow•1 seed that was wrinkled and green



The principle of independent assortment states that alleles for different traits can segregate independently during the formation of gametes.Genes that segregate independently do not influence each other's inheritance.



Mendel's experimental results were very close to the 9 : 3 : 3 : 1 ratio predicted by the Punnett square.Mendel had discovered the principle of independent assortment.


Independent assortment helps account for the many genetic variations observed in plants, animals, and other organisms.


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.


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

•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



•In codominance, both alleles contribute to the phenotype (like spots!).

• 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.


Beyond Dominant and

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 chch AIbino: 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



•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.


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.


Applying Mendel's Principles

Mendel’s principles can be used to study inheritance of human traits and to calculate the probability of certain traits appearing in the next generation.


Genetics and the Environment

Genetics and the Environment•Characteristics of any organism are

determined by the interaction between genes and the environment.


11-4 Meiosis11-4 Meiosis


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 NumberOrganisms 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

Chromosomes come in homologous pairs. Homologous chromosomes contain genes for the same traits.Each of the 4 chromosomes that came from the male parent has a homologous chromosome from the female parent.


Chromosome Number

A cell that contains both sets of homologous chromosomes is said to be diploid (2N).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.

Gametes are haploid (N), with only one set of the homologous chromosomes.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

Meiosis is a process of division in which the number of chromosomes per cell is cut in half.Diploid = 2 sets of each geneHaploid = half = one of each gene


•Meiosis involves two divisions, meiosis I and meiosis II.

•The diploid cell that enters meiosis becomes 4 haploid cells.

Phases of Meiosis

Movie


Phases of Meiosis

Meiosis I

Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis

Interphase IMeiosis I

Movie


Phases of Meiosis

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

Interphase I


Phases of Meiosis

During prophase of meiosis 1, each chromosome pairs with its corresponding homologous chromosome to form a tetrad.There are 4 chromatids in a tetrad.

MEIOSIS I Prophase I


Phases of Meiosis

•When tetrads form in prophase I, they exchange portions of their chromatids in a process called crossing over.

• Crossing-over produces new combinations of alleles.

Movie


Phases of Meiosis

Spindle fibers attach to the chromosomes.

MEIOSIS I Metaphase I


Phases of Meiosis

MEIOSIS I Anaphase IThe 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 parent cell.


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 IMeiosis II

Movie


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

Active art


Gamete Formation

Gamete Formation

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


Gamete Formation

In many female animals, one egg and 3 polar bodies result from meiosis. The three cells, called polar bodies, are usually not involved in reproduction.


Comparing Mitosis and Meiosis


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



Mitosis•Cells produced by mitosis have the

same number of chromosomes and alleles as the original cell.

•Mitosis allows an organism to grow and replace cells.

•Some organisms reproduce asexually by mitosis.



Meiosis•Cells produced by meiosis have half the

number of chromosomes as the parent cell.•These cells are genetically different from the

diploid cell and from each other.

•Meiosis is how sexually-reproducing organisms produce gametes.


Gene Linkage

Thomas Hunt Morgan researched the genetics of fruit flies because they had many offspring in a short time.


11-5 Linkage and Gene Maps

11-5 Linkage and Gene Maps


Gene Linkage

Gene Linkage•Thomas Hunt Morgan’s research on fruit

flies led him to the principle of linkage.•Morgan discovered that many of the

more than 50 Drosophila genes he had identified appeared to be “linked” together.

•They seemed to violate the principle of independent assortment.


Morgan and his associates grouped the linked genes into four linkage groups.Each linkage group assorted independently but all the genes in one group were inherited together.

Each chromosome is actually a group of linked genes ( a linkage group).

Gene Linkage


Gene Linkage

Morgan concluded that Mendel’s principle of independent assortment still holds true. Chromosomes assort independently, not individual genes.Mendel did not observe gene linkage because the genes he studied happened to be on different chromosomes.


Gene Maps

Gene Maps

•Crossing-over during meiosis sometimes separates genes that had been on the same chromosomes onto homologous chromosomes.

•Crossover events can separate linked genes and produce new combinations of alleles.


Alfred Sturtevant, a student of Morgan, reasoned that the farther apart two linked genes are, the more likely they are to be separated due to crossover.Recombination frequencies can be used to determine the distance between genes.

Gene Maps


Sturtevant created a gene map of a Drosophila chromosome. A gene map shows the relative locations of each known gene on a chromosome.

Gene Maps


If two genes are close together, the recombination frequency between them should be low, since crossovers are rare.If they are far apart, recombination rates between them should be high.

Gene Maps


Gene Maps

Aristaless (no bristles on antenna)

Chromosome 2Exact location on chromosome

13.0 Dumpy wing

0.0

48.5 Black body

54.5 Purple eye

67.0 Vestigial (small) wing

99.2 Arc (bent wings)

107.0 Speck wing


Chromosome 2Exact location on chromosome

1.3 Star eye

31.0 Dachs (short legs)

51.0 Reduced bristles

55.0 Light eye

75.5 Curved wing

104.5 Brown eye

Gene Maps


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 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.


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.


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

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.


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.


11-5

According to Mendel's principle of independent assortment, the factors that assort independently are the

a. genes.

b. chromosomes.

c. chromatids.

d. gametes.


11-5

Linkage maps can be produced because the farther apart two genes are on a chromosome,

a. the less likely they are to assort independently.

b. the more likely they are to be linked.

c. the more likely they are to be separated by a crossover.

d. the less likely they are to be separated by a crossover.


11-5

If two genes are close together on the same chromosome, they are more likely to

a. behave as though they are linked.

b. behave independently.

c. move to different chromosomes.

d. belong to different linkage groups.

Education

Chapter Eleven- Intro to Genetics