Biology 1 Notes

Chapter 11 (Introduction to Genetics) Prentice Hall; pages 262-274, 279-285

• 11–1 The Work of Gregor MendelA. Gregor Mendel’s PeasB. Genes and DominanceC. Segregation

1. The F1 Cross2. Explaining the F1 Cross

• 11–2 Probability and Punnett SquaresA. Genetics and ProbabilityB. Punnett SquaresC. Probability and SegregationD. Probabilities Predict Averages

• 11–3 Exploring Mendelian GeneticsA. Independent Assortment

1. The Two-Factor Cross: F1

2. The Two-Factor Cross: F2

B. A Summary of Mendel’s PrinciplesC. Beyond Dominant and Recessive Alleles

1. Incomplete Dominance2. Codominance3. Multiple Alleles4. Polygenic Traits

D. Applying Mendel’s PrinciplesE. Genetics and the Environment

• 11–5 Linkage and Gene MapsA. Gene LinkageB. Gene Maps

Early Concept of Genetics• Every living thing has a set of characteristics inherited from its parents.• People didn’t always understand the process of inheritance, but they had an

idea that certain characteristics are passed from generation to generation• Examples:

– Native Americans developed more than 300 varieties of corn– People of Peru developed potatoes by selecting and breeding wild starchy

plants– The domestic dog came from selectively breeding wolves

• It was not until the mid-nineteenth century that Gregor Mendel, an Austrian monk, carried out important studies of heredity

• Genetics- the scientific study of heredity

• Heredity- the passing on of characteristics from parents to offspring.

• Characteristics that are inherited are called traits.

• Mendel was the first person to succeed in predicting how traits are transferred from one generation to the next.

• Mendel chose to use the garden pea in his experiments for several reasons.

Mendel chose his subject carefully• Garden pea plants reproduce sexually, which means that they produce male

and female sex cells, called gametes (egg and sperm).• In a process called fertilization, the male gamete unites with the female gamete.

• The resulting fertilized cell, called a zygote, then develops into a seed.

• The transfer of pollen grains from a male to a female reproductive organ in a plant is called pollination.

• When he wanted to breed, or cross, one plant with another, Mendel opened the petals of a flower and removed the male organs.

• He then dusted the female organ with pollen from the plant he wished to cross it with. This process is called cross-pollination.

• By using this technique, Mendel could be sure of the parents in his cross.

Remove male parts

Female part

Transfer pollen

Pollen grains

Maleparts

Cross-pollination

Mendel was a careful researcher

• He studied only one trait at a time to control variables, and he analyzed his data mathematically.

• The tall pea plants he worked with were from populations of plants that had been tall for many generations and had always produced tall offspring. Such plants are said to be true breeding for tallness.

• Likewise, the short plants he worked with were true breeding for shortness.

• True-breeding means that if they were allowed to self-pollinate, they would produce offspring identical to themselves.

Mendel’s Research• Mendel studied seven different traits

and each trait had two contrasting characteristics:– Flower color– Flower position– Seed color– Seed shape– Pod shape– Pod color– Stem length

• A trait is a specific characteristic, such as seed color or plant height, that varies from one individual to another

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

• He called each original pair of plants the P (parental) generation.

• He called the offspring the F1, or “first filial,” generation.

Language of Genetics• Mendel called the observed trait dominant and the trait that disappeared

recessive.• Always use the same letter for different forms of the same trait (alleles).

– capital letter shows dominance, lowercase letter shows recessive– Example: Stem Height Tall allele (T) Short allele (t)

• The way an organism looks and behaves is called its phenotype.– Example: tall or short

• The allele combination (genetic make-up) an organism contains is known as its genotype.

– Example: TT, Tt, or tt

• An organism’s genotype can’t always be known by its phenotype.

• An organism is homozygous for a trait if its two alleles for the trait are the same.

– Example: TT or tt

• An organism is heterozygous for a trait if its two alleles for the trait differ from each other.

– Example: Tt

Punnett Squares• In 1905, Reginald Punnett, an English biologist, devised a quick way of

finding the expected genotypes possibilities (Punnett square)

• Punnett Squares are used to make phenotype and genotype predictions Punnett Squares are used to make phenotype and genotype predictions in order to predict/compare the genetic variations that will result from in order to predict/compare the genetic variations that will result from a crossa cross

• A Punnett square for this cross is two boxes tall and two boxes wide because each parent can produce two kinds of gametes for this trait.

• The two kinds of gametes from one parent are listed on top of the square, and the two kinds of gametes from the other parent are listed on the left side.

• It doesn’t matter which set of gametes is on top and which is on the side.

• Each box is filled in with the gametes above and to the left side of that box. You can see that each box then contains two alleles—one possible genotype.

• After the genotypes have been determined, you can determine the phenotypes.

Heterozygous tall parent

T t

T t

T t

T

t

Heterozygous tall parent

Mendel’s Experiment #1

• Parents (P):

• (phenotype) Purebred tall x Purebred short

• (genotype) TT x tt

• F1 all hybrids:

• Phenotype: tall

• Gentoype: Tt

T T

t

t

Tt Tt

Tt Tt

Mendel’s Experiment• Mendel completed similar experiments for all of the seven traits• The offspring of crosses between parents with different traits

are called hybrids.

• All of the offspring had the character of only one of the parents

Seed Shape

Flower Position

Seed CoatColor

Seed Color

Pod Color

Plant Height

PodShape

Round

Wrinkled

Round

Yellow

Green

Gray

White

Smooth

Constricted

Green

Yellow

Axial

Terminal

Tall

Short

Yellow Gray Smooth Green Axial Tall

Conclusions from Experiment #1• Biological inheritance is determined by factors that

are passed from one generation to the next.– factors that determine traits are genes– each trait is controlled by one gene– (genes are found on chromosomes)– each trait has two different forms called alleles

• One form is always dominant over the other – if the dominant allele is present, it will show up

(expressed by a capital letter) – the recessive allele will be exhibited only when the

dominant allele is absent (expressed by a lower case letter)

– This is known as The Principle of Dominance• In Mendel’s experiments, the allele for tall (T)

plants was dominant and the allele for short (t) plants was recessive.

• Pea plants will be tall unless the allele for tallness is absent– TT- Tall plant– Tt- Tall plant– tt- short plant

T T

T

T

t t

t

t

Tall plant Short plant

All tall plants

F1

P

Going further …

• Mendel wondered if the recessive allele had disappeared completely.• So, he crossed the F1 plants to produce the F2 (second filial) generation by

letting the F1 plants self-pollinate.• In every case, he found the recessive trait of the pair seemed to disappear in

the F1 generation, only to reappear unchanged in one-fourth of the F2 plants.

• All of the F2 generation produced plants with the similar results

• The recessive allele reappeared.

• All of the traits (including height) showed a 3:1 ratio of 3 dominant to 1 recessive

Recessive trait

Dominant trait

Seed shape

Seed color

Flower color

Flower position

Pod color

Pod shape

Plant height

round yellow purpleaxial (side) green inflated tall

wrinkled green whiteterminal

(tips) yellow constricted short

Mendel’s Experiment #2

• F1:

• (phenotype) (Hybrid) tall x (Hybrid) Tall

• (genotype) Tt x Tt• F2: TT Tt Tt tt• (tall) (tall) (tall) (short)

• Genotypic ratio: • 1 TT: 2Tt: 1tt• Phenotypic ratio: • 3 tall: 1 short

T t

T

t

TT Tt

Tt tt

Conclusions from Experiment #2

• Mendel concluded from his second experiment that alleles for shortness and tallness had segregated (separated) during the formation of gametes (reproductive cells)

• Each allele is located on different copies of a chromosome, one inherited from each parent.

• During meiosis the two alleles separate so that each gamete carries only a one copy of a gene

• During fertilization, these gametes can randomly pair to produce (up to) four various combinations of alleles.

Homologous Chromosome 4

a A

Terminal Axial

InflatedD Constrictedd

Tall

TShort

t

The Principle of Segregation

• The Principle of Segregation states that every individual has two alleles of each gene and when gametes are produced, each gamete receives one of these alleles.

Law of segregation Tt Tt cross

F1

Tall plant Tall plant

TTT

T t T t

t T t tt

3

Tall Tall Short

1

Tall

F2

Probability• Probability is the likelihood an event will occur• The principles of probability can be used to predict the outcomes of genetic crosses• A Punnett square can be used to determine the probability of the number of desired

outcomes by the total number of possible outcomes

• Example: What is the probability of getting a plant that produces round seeds when two plants that are heterozygous (Rr) are crossed?

• Punnett square shows three plants with round seeds out of four total plants, so the probability is 3/4.

• It is important to remember that the results predicted by probability are more likely to be seen when there is a large number of offspring.

R r

R

r

RR Rr

Rr rr

Monohybrid Cross (one trait)G= green pea pods g= yellow pea pods

1) Cross of homozygous dominant x homozygous recessive

• P _______ X _______ (genotype)

G G

g

g

Gg Gg

Gg Gg

GG gg

F1 genotype: all Gg

F1 phenotype:

all Green

2) Cross two of F1 generation

• F1 ________ X _______ (genotype)

• F2 genotype & ratio:• 1 GG: 2 Gg: 1 gg

• F2 phenotype & ratio:• 3 green: 1 yellow

Gg Gg

G g

G

g

GG Gg

Gg gg

Test Cross - used to distinguish between homozygous and heterozygous organisms

• unknown organism is crossed with a homozygous recessive individual

• if unknown is heterozygous, then 1/2 of offspring will be dominant and 1/2 will be recessive

• if unknown is homozygous, then all offspring will be dominant

Test Cross Example• Example: (unknown vs. homozygous recessive)• If unknown is heterozygous, then ½ the offspring will be dominant

and ½ will be recessive• If unknown is homozygous, then all of the offspring will be

dominant• A breeder has a yellow cat and wants to know if it is a purebred. To

determine what the cat’s genotype is the breeder does a test cross with a brown cat (homozygous recessive) . All of the offspring turn out yellow. What is the genotype of the original yellow cat?

Yellow cat: either YY or Yy

Brown cat: yy

Y Y

y

y

Yy

Yy

Yy

Yy

Y y

y

y

Yy

yy

yy

Yy

Monohybrid vs. Dihybrid Crosses

• Mendel’s first experiments are called monohybrid crosses because mono means “one” and the two parent plants differed from each other by a single trait—height.

• Mendel performed another set of crosses in which he used peas that differed from each other in two traits rather than only one.

• Such a cross involving two different traits is called a dihybrid cross.

Seed shape

Seed color

round yellow

wrinkled green

Plant height

Tall Short

Dihybrid crosses• Mendel took true-breeding pea plants that had round yellow seeds (RRYY) and

crossed them with true-breeding pea plants that had wrinkled green seeds (rryy).

• He already knew the round-seeded (R) trait was dominant to the wrinkled-seeded (r) trait.

• He also knew that yellow (Y) was dominant to green (y).• All of the F1 offspring were round and yellow with the genotype RrYy.

• Mendel then let the F1 plants pollinate themselves.

• He found some plants that produced round yellow seeds and others that produced wrinkled green seeds.

• He also found some plants with round green seeds and others with wrinkled yellow seeds.

• He found they appeared in a definite ratio of phenotypes—9 round yellow: 3 round green: 3 wrinkled yellow: 1 wrinkled green.

Dihybrid Cross round yellow x wrinkled green

Round yellow Wrinkled green

All round yellow

Round yellow Round green Wrinkled yellow Wrinkled green9 3 3 1

P1

F1

F2

Dihybrid Cross (two traits)

• P RRYY x rryy• F1 RrYy x RrYy

R r Y yAll possible

gamete combinations

_______

_______

_______

________

RY

Ry

rY

ry

RY Ry rY ry

RY

Ry

rY

ry

RY Ry rY ry

RY

Ry

rY

ry

RRYY RRYy RrYY RrYy

RRYy RRyy RrYy Rryy

RrYY RrYy rrYY rrYy

RrYy Rryy rrYy rryy

RY Ry rY ry

RY

Ry

rY

ry

RRYY RRYy RrYY RrYy

RRYy RRyy RrYy Rryy

RrYY RrYy rrYY rrYy

RrYy Rryy rrYy rryy

F2 genotype and ratio:

1 RRYY

2 RRYy

1 RRyy

2 RrYY

4 RrYy

2 Rryy

1 rrYY

2 rrYy

1 rryy

RY Ry rY ry

RY

Ry

rY

ry

RRYY RRYy RrYY RrYy

RRYy RRyy RrYy Rryy

RrYY RrYy rrYY rrYy

RrYy Rryy rrYy rryy

F2 phenotype and ratio:

9: round, yellow

3: round, green

3: wrinkled, yellow

1 wrinkled, green

Gametes from RrYy parent

RY Ry rY ry

Ga

me

tes

from

RrY

y p

are

nt

RY

Ry

rY

ry

RRYY RRYy RrYY RrYy

RRYy RRYy RrYy Rryy

RrYY RrYy rrYY rrYy

RrYy Rryy rrYy rryy

The Principle of Independent Assortment

• Mendel’s found that genes for different traits—for example, seed shape and seed color—are inherited independently of each other.

• This conclusion is known as the Principle of Independent Assortment

A Summary of Mendel’s Principles• The inheritance of biological characteristics is

determined by individual units known as genes. Genes are passed from parents to offspring. (Mendel called genes, “factors.”)

• Dominance- if two alleles in a gene pair are different, the dominant allele will control the trait and the recessive allele will be hidden

• Segregation - each adult has two copies of each gene-one from each parent. These genes are segregated from each other when gametes are formed.

• Independent Assortment - genes for different traits are inherited independently of each other.

Exceptions to Mendel’s Principles

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

• Patterns of Inheritance– Incomplete Dominance– Codominance– Multiple Alleles – Polygenic Traits

Incomplete Dominance• Some alleles are neither dominant nor recessive• Heterozygous phenotype is somewhere in between the two

homozygous phenotypes • Phenotype appears to be blended but alleles remain separate &

distinct• There are no dominant or recessive alleles therefore uppercase

and lowercase letters are not used• Example: genotype of four o’clock plants red flowers = FrFr

white flowers = FwFw

pink flowers = FrFw

• Alleles for red and white flowers show incomplete dominance • Heterozygous plants have pink flowers—a mix of red and

white coloring

Incomplete DominanceFlower color of four o’clock plants

FrFw FrFw

FrFw FrFw

Fw

Fw

Fr FrCross a red plant with a white plant.

Genotype: all FrFw

Phenotype: all pink

Fr Fr X Fw Fw

Codominance• both alleles in the heterozygote express themselves

fully and contribute to the phenotype• Example: Feather color in Chickens

– Allele for black feather is codominant with white feathers

– A heterozygous phenotype is “erminette”- speckled black and white feathers

– It is not a blend, the colors appear separate

• Example: Blood Type in Humans– A blood type = IAIA or IAi The letters stand for A– B blood type = IBIB or IBi and B antigens. – AB blood type = IAIB Humans have antigens– O blood type = ii to A, B, both A and B,

or neither A nor B.

Multiple Alleles• Although individuals can’t have more than 2 alleles, more than 2 alleles

can exist in a population

• Example: Coat color in rabbits

– Coat color is determined by a single gene that has four alleles

– Four alleles display a pattern of dominance that produces four possible coat colors

• Example: Blood types in humans– alleles are IA, IB, i

Codominance and Multiple Allele Example (Blood Type in Humans)

• Cross an individual with AB blood and a person with O blood

IAIB X ii

i

i

IA IB

IAi IBi

IAi IBi

Genotype: 2 IAi: 2 IBi

Phenotype: 2 A: 2 B

Polygenic Traits

• results from an interaction of several genes

• traits are controlled by two or more genes

• Example: Eye Color of Fruit Flies

– three genes make the reddish-brown pigment of fruit fly eyes

– wide range of phenotypes can occur

• Example: Eye Color of Humans

– controlled by genes for pigment, tone, amount of pigment, and distribution of pigment

Applying Mendel’s Principles

• Mendel formed hypotheses about inheritance without knowing what genes are or where they are located in cells

• In the 1900’s Thomas Hunt Morgan studied the fruit fly

– (Drosophila melanogaster )

– easy to feed and maintain

– new generation produced every 2 weeks

– produce large numbers of offspring

– have only four pair of chromosomes

• Morgan found that Mendel’s principles

applied to fruit flies as well as plants.

Applying Mendel’s Principles to Humans• Many doctors used Mendel’s principles to study human disorders• Albinism is lack of the pigment melanin that gives human skin its color• Individuals with the dominant allele (A) produce skin coloration • Individuals homozygous for the recessive form of the allele (a) have albinism• If two people with normal skin color have a child with albinism, what are the

odds that a second child will also have albinism?

• If the first child has albinism (aa), then both parents must have at least one allele that is a. Since both parents have normal skin color, they must also have A.

Genotypes of the parents _______ x________Aa Aa

A a

A

a

AA Aa

Aa aa

There is a 25% chance that the second child will also have albinism

25%

Genetics and the Environment

• Characteristics of any organism is not determined by genes alone

• Characteristics are determined by the interaction between genes and the environment

• Genes may affect a sunflower’s height and color of its flowers, but they are influenced by climate, soil, and availability of water (if the sunflower doesn’t have enough water it can’t grow)

• Hydrangea with the same genotype for flower color express different phenotypes depending on the acidity of the soil.

• 1903 - Walter Sutton - stated the chromosome

theory of heredity - the material of inheritance is carried by the chromosomes

- Sutton recognized “factors” were genes on the chromosomes he saw

- both occur in pairs

- both separate during meiosis

- both sort independently

- Sutton realized that there must be many different genes on a chromosome and

that they are inherited together

Gene Linkage• What we know: genes from different chromosomes sort independently

in meiosis• But. . What about genes on the same chromosome?• At first, it seems they would be inherited together, but it is a little

more complicated• Morgan in 1910 noticed that some genes were linked (which violates

the principle of independent assortment)• Morgan and his associates studied 50 genes of the fruit fly and were

able to place all of the genes in 4 linkage groups which assorted independently, but all the genes in one group were inherited together– Linked genes are located on the same chromosome and are inherited together

• 4 linkage groups- fruit fly has 4 chromosomes• It is the chromosomes however, that assort independently, not

individual genes

Gene Linkage• Morgan discovered that chromosomes sort independently and

not genes• Why didn’t Mendel discover this

– 6 of the 7 traits he studied were on different chromosomes– The two genes on the same chromosomes were so far away

from each other they assorted independently • Showing that genes on the same chromosomes are not

forever linked and crossing over occurs• Crossing over can separate and exchange linked genes and

produce new combinations of alleles. • This is important in genetic diversity of a species or

population

An example of gene linkage

• When crossing long, purple sweet peas with round, red peas, the expected F2 ratio (9:3:3:1) did not show up

• new ratio appeared to indicate that the traits did not sort independently

Genetic recombination - the shuffling of genes into new combinations by crossing over during prophase I of

meiosis

• If genes were being sorted together, then crossing over was the only explanation for the appearance of red, long and purple, round peas

• recombinant - an organism or a chromosome with a recombined set of genes

Gene mapping - locating genes on chromosomes

• Geneticists work with two traits at a time and look for the number of recombinants in the offspring

• if two genes are far apart on a chromosome, there are more places between them where crossing over can occur, therefore more recombinants are formed

• if two genes are close together, few recombinants occur

Gene Mapping

• In 1911, Alfred Strutevant hypothesized that the rate at which crossing-over separates linked genes could be used to map were genes are located on a chromosome

• The farther apart genes are, the more likely they were to be separated by a cross-over

• Sturtevant created a gene map showing the relative location of each known gene on a chromosome

How to construct a gene map

• The recombinant rate (frequency of crossing-over between genes) is used to construct genetic maps

• Genetic recombination-the shuffling of genes into new combination by crossing-over during prophase I of meiosis

A 50 B

A 10 D B 5 C

D 10 A C 5 B

D 35 C

C 35 D

or or

or

A 50 B

A 10 D B 5 C

D 10 A C 5 B

D 35 C

C 35 D

or or

or

Chromosome Mapping

• Suppose there are four genes— A, B, C, and D—on a chromosome.

• Geneticists determine that the frequencies of recombination among them are as follows: between A and B—50%; between A and D— 10%; between B and C—5%; between C and D—35%.

• The recombination frequencies can be converted to map units: A-B = 50; A-D =10; B-C = 5; C-D = 35.

A D10 35 C 5 B

50

Chromosome Mapping

• These map units are not actual distances on the chromosome, but they give relative distances between genes. Geneticists line up the genes as shown.

• The genes can be arranged in the sequence that reflects the recombination data.

• This sequence is a chromosome map.

A 50 B

A 10 D B 5 C

D 10 A C 5 B

D 35 C

C 35 D

or or

or

A 50 B

A 10 D B 5 C

D 10 A C 5 B

D 35 C

C 35 D

or or

or

A D10 35 C 5 B

50

Comparative Scale of a Gene Map

Earth

Country

State

City

People

Cell

Chromosome

Chromosome fragment

Gene

Nucleotide base pairs

Mapping of Earth’s Features

Mapping of Cells, Chromosomes, and Genes