genetics chapter 9-11. gregor mendel 1850’s the father of genetics genetic – branch of biology...

Genetics Chapter 9-11

Gregor Mendel 1850’s The father of Genetics

Genetic – Branch of biology that studies heredity

Heredity –biological process whereby genetic factors are transmitted from one generation to the next

Inheritance -attributes acquired via biological heredity from the parents

Mendel

• Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments

• Self pollination -Transfer of pollen from an anther to a stigma of the same flower.

• These were “pure” plants

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 14-1

Mendel’s Experimental, Quantitative Approach

• Advantages of pea plants for genetic study:– There are many varieties with distinct heritable features, or

characters (such as flower color); character variants (such as purple or white flowers) are called traits

– Mating of plants can be controlled

– Each pea plant has sperm-producing organs (stamens) and egg-producing organs (carpels)

– Cross-pollination - fertilization between different plants, can be achieved by dusting one plant with pollen from another


Fig. 14-2

TECHNIQUE

RESULTS

Parentalgeneration(P) Stamens

Carpel

1

2

3

4

Firstfilialgener-ationoffspring(F1)

5

• In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization

• The true-breeding parents are the P generation

• The hybrid offspring of the P generation are called the F1 generation

• When F1 individuals self-pollinate, the F2 generation is produced


• Mendel chose to track only those characters that varied in an either-or manner

• He also used varieties that were true-breeding (plants that produce offspring of the same variety when they self-pollinate)


The Law of Segregation

• When Mendel crossed contrasting, true-breeding white and purple flowered pea plants, all of the F1 hybrids were ?

• When Mendel crossed the F1 hybrids, many of the F2 plants had ?

• Mendel discovered a ratio


Fig. 14-3-1

EXPERIMENT

P Generation

(true-breeding parents) Purple

flowers Whiteflowers

Fig. 14-3-2

EXPERIMENT

P Generation



F1 Generation

(hybrids) All plants hadpurple flowers

Fig. 14-3-3

EXPERIMENT

P Generation



F1 Generation

(hybrids) All plants hadpurple flowers

F2 Generation

705 purple-floweredplants

224 white-floweredplants

• Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids

• Mendel called the purple flower color a dominant trait and the white flower color a recessive trait

• Mendel observed the same pattern of inheritance in six other pea plant characters, each represented by two traits

• What Mendel called a “heritable factor” is what we now call a gene


Table 14-1

Mendel’s Model

• Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring

• Three principles of Mendel’s were formed

• These concepts can be related to what we now know about genes and chromosomes


• The first concept is the principle of dominance and recessiveness the dominant allele determines the organism’s appearance, and the the recessive allele has no noticeable effect on appearance

• In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant


• The second concept, now known as the law of segregation, states that the two factors separate (segregate) during gamete formation and end up in different gametes

• Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of an organism

• This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis


• Mendel’s segregation model accounts for the 3:1 ratio he observed in the F2 generation of his numerous crosses

• A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele


Fig. 14-5-3

P Generation

Appearance:Genetic makeup:

Gametes:

Purple flowers White flowersPP

P

pp

p

F1 Generation

Gametes:

Genetic makeup:Appearance: Purple flowers

Pp

P p1/21/2

F2 Generation

Sperm

Eggs

P

PPP Pp

p

pPp pp

3 1

Fig. 14-6

Phenotype

Purple

Purple3

Purple

Genotype

1 White

Ratio 3:1

(homozygous)

(homozygous)

(heterozygous)

(heterozygous)

PP

Pp

Pp

pp

Ratio 1:2:1

1

1

2

The Law of Independent Assortment

• Mendel derived the law of segregation by following a single character

• Using a dihybrid cross, Mendel developed the law of independent assortment

• The law of independent assortment states that each factors (pair of alleles) segregates independently of each other during gamete formation

• However, genes located near each other on the same chromosome tend to be inherited together


Fig. 14-8

EXPERIMENT

RESULTS

P Generation

F1 Generation

Predictions

Gametes

Hypothesis ofdependentassortment

YYRR yyrr

YR yr

YyRr

Hypothesis ofindependentassortment

orPredictedoffspring ofF2 generation

Sperm

Sperm

YR

YR

yr

yr

Yr

YR

yR

Yr

yR

yr

YRYYRR

YYRR YyRr

YyRr

YyRr

YyRr

YyRr

YyRr

YYRr

YYRr

YyRR

YyRR

YYrr Yyrr

Yyrr

yyRR yyRr

yyRr yyrr

yyrr

Phenotypic ratio 3:1

EggsEggs

Phenotypic ratio 9:3:3:1

1/21/2

1/2

1/2

1/4

yr

1/41/4

1/41/4

1/4

1/4

1/4

1/43/4

9/163/16

3/161/16

Phenotypic ratio approximately 9:3:3:1315 108 101 32

Useful Genetic Vocabulary

• Alleles – an alternative form of a gene,

• For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers

• A capital letter represents a dominant allele which is the gene expressed , and a lowercase letter represents the masked recessive allele


• Because of the different effects of dominant and recessive alleles, an organism’s traits do not always reveal its genetic composition

• Therefore, we distinguish between an organism’s phenotype, or physical appearance, and its genotype, or genetic makeup

• In the example of flower color in pea plants, plants have the same phenotype purple but different genotypes PP and Pp


• An organism with two identical alleles for a character is said to be homozygous ex –PP, pp

• An organism that has two different alleles for a gene is said to be heterozygous ex- Pp

Useful vocab

• The possible combinations of sperm and egg can be shown using a Punnette square, a diagram for predicting the results of a genetic cross between individuals of known genetic makeup

Concept 14.2: The laws of probability govern Mendelian inheritance

• Mendel’s laws of segregation and independent assortment reflect the rules of probability

• Probability -The outcome or likelihood an event will occur

• When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss

• In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles


Fig. 14-4

Allele for purple flowers

Homologouspair ofchromosomes

Locus for flower-color gene

Allele for white flowers

Fig. 14-9

Rr RrSegregation of

alleles into eggs

Sperm

R

R

R RR

R rrr

r

r

r1/2

1/2

1/2

1/2

Segregation ofalleles into sperm

Eggs1/4

1/4

1/41/4

• The rule of addition states that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities + or

• The rule of addition can be used to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous


• The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities x and

• Probability in an F1 monohybrid cross can be determined using the multiplication rule

• Segregation in a heterozygous plant is like flipping a coin: Each gamete has a chance of carrying the dominant allele and a chance of carrying the recessive allele

The Multiplication and Addition Rules Applied to Monohybrid Crosses


1

2

1

2

Lets Practice

• Monohybrid cross one pair of contrasting traits

• TT always 4:0 Tall to short

• Tt x Tt 3:1

• Tt x tt 2:2

• Dihybrid cross two pairs of contrasting traits

• Use the FOIL method

• 16 boxes

The Testcross

• How can we tell the genotype of an individual with the dominant phenotype?

• Such an individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous

• The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual

• If any offspring display the recessive phenotype, the mystery parent must be heterozygous


Fig. 14-7TECHNIQUE

RESULTS

Dominant phenotype, unknown genotype:

PP or Pp?

Predictions

Recessive phenotype, known genotype: pp

If PP If Ppor

Sperm Spermp p p p

P

P

P

p

Eggs Eggs

Pp

Pp Pp

Pp

Pp Pp

pp pp

or

All offspring purple 1/2 offspring purple and1/2 offspring white

Fig. 14-7b

RESULTS

All offspring purple

or

1/2 offspring purple and1/2 offspring white

Meiosis

• At sexual maturity, the ovaries and testes produce haploid gametes

• Gametes are the only types of human cells produced by meiosis, rather than mitosis

• Meiosis results in one set of chromosomes in each gamete

Phases of Meiosis I PMAT

• In the first cell division (meiosis I), homologous chromosomes match up in Pro I and separate in Ana I

• Meiosis I results in two haploid daughter cells with replicated chromosomes; it is called the reductional division

• Haploid (n)

Prophase I 90% of the time required for meiosis

•Chromosomes begin to condense

•In synapsis, homologous chromosomes loosely pair up, aligned gene by gene

•Each pair of chromosomes forms a tetrad, a group of four chromatids

•In crossing over, nonsister chromatids exchange DNA segments

Meiosis II PMAT

• In the second cell division (meiosis II), sister chromatids separate

• Meiosis II results in four haploid daughter cells

• Still haploid

• In males formed 4 sperm,

• In females formed 1 egg (ootid) the others die they are called polar bodies

• 2^23 = 0ver 8 million combination not even including crossing over

Concept 14.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics

• The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied

• Many heritable characters are not determined by only one factor(gene) with two alleles

• However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance


After Mendel

• Walter Sutton (1902) observed the process of meiosis

• Found genes on the chromosomes

• He validated Mendel’s 2 last principles

After Mendel

• Thomas H. Morgan discovers sex-linked genes in fruit flies The Chromosomal Theory developed by Thomas Hunt Morgan Nobel Prize 1933

• XY males are hemizygous [one allele on X and non on Y chromosome]

• XX female

• X linked inherited human conditions are rare.

http://library.thinkquest.org/18258/morgan2.htm

http://www.nobel.se/medicine/laureates/1933/

Extending Mendelian Genetics for a Single Gene

• Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations:

– When alleles are not completely dominant or recessive

– When a gene has more than two alleles

– When a gene produces multiple phenotypes


Degrees of Dominance

• (Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical) (Mendel)

• In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties, active and inactive genes, not dominant or recessive ex-flowers

• In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways ex- sickle cell anemia


Fig. 14-10-1

Red

P Generation

Gametes

WhiteCRCR CWCW

CR CW

Fig. 14-10-2

Red

P Generation

Gametes

WhiteCRCR CWCW

CR CW

F1 GenerationPinkCRCW

CR CWGametes 1/21/2

Fig. 14-10-3

Red

P Generation

Gametes

WhiteCRCR CWCW

CR CW

F1 GenerationPinkCRCW

CR CWGametes 1/21/2

F2 Generation

Sperm

Eggs

CR

CR

CW

CW

CRCR CRCW

CRCW CWCW

1/21/2

1/2

1/2

Sickle-Cell Disease

• Sickle-cell disease affects one out of 400 African-Americans

• The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells

• Symptoms include physical weakness, pain, organ damage, and even paralysis


Multiple Alleles

• Most genes exist in populations in more than two allelic forms

• A person can only have 2 alleles one from mom and one from dad

• For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates (antigen) to red blood cells: IA, IB, and i. = O.

• A,B dominant over O

• RH antigen on the RBC gives blood a + sign, + blood is dominant over -


Fig. 14-11

IA

IB

i

A

B

none(a) The three alleles for the ABO blood groups and their associated carbohydrates

Allele Carbohydrate

GenotypeRed blood cell

appearancePhenotype

(blood group)

IAIA or IA i A

BIBIB or IB i

IAIB AB

ii O

(b) Blood group genotypes and phenotypes

Extending Mendelian Genetics for Two or More Genes

• Some traits may be determined by two or more genes


Polygenic Inheritance

• Quantitative characters are those that vary in the population along a continuum

• Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype

• Skin color in humans is an example of polygenic inheritance


Fig. 14-13

Eggs

Sperm

Phenotypes:

Number ofdark-skin alleles: 0 1 2 3 4 5 6

1/646/64

15/6420/64

15/646/64

1/64

1/8

1/8

1/8

1/8

1/8

1/8

1/8

1/8

1/81/8

1/81/8

1/81/8

1/81/8

AaBbCc AaBbCc

Gene Interactions Continued

• Sex Linked – alleles appear on the X,Y chromosomes ex- Hemophilia and color blindness

• Sex limited – traits limited to one sex

ex – bearded men dominant, female breast also dominant

• Sex influenced – genes behave differently in different sexes ex- baldness

in women only bb = bald

in men BB, Bb = bald

Integrating a Mendelian View of Heredity and Variation

• An organism’s phenotype includes its physical appearance, internal anatomy, physiology, and behavior

• An organism’s phenotype reflects its overall genotype and unique environmental history


Concept 14.4: Many human traits follow Mendelian patterns of inheritance

• Humans are not good subjects for genetic research

– Generation time is too long

– Parents produce relatively few offspring

– Breeding experiments are unacceptable

• However, basic Mendelian genetics endures as the foundation of human genetics


Pedigree Analysis

• A pedigree is a family tree that describes the interrelationships of parents and children across generations

• Inheritance patterns of particular traits can be traced and described using pedigrees


Fig. 14-15Key

Male

Female

AffectedmaleAffectedfemale

Mating

Offspring, inbirth order(first-born on left)

1st generation(grandparents)

2nd generation(parents, aunts,and uncles)

3rd generation(two sisters)

Ww ww ww Ww

Ww ww wwWw Ww ww

ww

Ww

WWor

Widow’s peak No widow’s peak(a) Is a widow’s peak a dominant or recessive trait?




Ff Ff Ff

Ff FfFfFF or ff ff

ff

ff

ff FForFf

Attached earlobe Free earlobe

(b) Is an attached earlobe a dominant or recessive trait?

Fig. 14-15a

KeyMale

Female

AffectedmaleAffectedfemale

Mating

Offspring, inbirth order(first-born on left)

Fig. 14-15b




Widow’s peak No widow’s peak

(a) Is a widow’s peak a dominant or recessive trait?

Ww ww

Ww Wwww ww

ww

wwWw

Ww

wwWW

Wwor

Fig. 14-15c

Attached earlobe




Free earlobe

(b) Is an attached earlobe a dominant or recessive trait?

Ff Ff

Ff Ff Ff

ff Ff

ff ff ff

ff

FF or

orFF

Ff

Fig. 14-UN5

George

Sandra Tom Sam

Arlene

Wilma Ann Michael

Carla

Daniel Alan Tina

Christopher

• Pedigrees can also be used to make predictions about future offspring

• We can use the multiplication and addition rules to predict the probability of specific phenotypes


Recessively Inherited Disorders

• Many genetic disorders are inherited in a recessive manner


The Behavior of Recessive Alleles

• Recessively inherited disorders show up only in individuals homozygous for the allele

• Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal (i.e., pigmented)

• Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair


Fig. 14-16

Parents

Normal Normal

Sperm

Eggs

NormalNormal(carrier)

Normal(carrier) Albino

Aa Aa

A

AAA

Aa

a

Aaaa

a

• If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low

• Consanguineous matings (i.e., matings between close relatives) increase the chance of mating between two carriers of the same rare allele

• Most societies and cultures have laws or taboos against marriages between close relatives


• Mutations are changes in the genetic material of a cell or virus, rare

• Mutagen chemical or physical agent can cause a mutation ex- radiation, uv, disease, pollutants

Chromosome mutations change chromosome structure/ number

• Nondisjunction failure of the chromosome to separate

• Polyploidy total nondisjunction when all the chromosomes fail to separate

3N = 69 chromosomes 4N= 92 chromosomes

fatal in humans plants are hardier and larger

Breakage of a chromosome

• can lead to many changes in chromosome structure:

– Deletion removes a chromosomal segment lethal have no gene

– Inversion reverses a segment within a chromosome

– Translocation moves a segment from one chromosome to another

Fig. 15-15

Deletion(a)

(b)

(c)

(d)

Duplication

Inversion

translocation

A

A B C D E F G H

A B C D E F G H

A D C B E F G H

M N O C D E F G H

M N O P Q R A B P Q R

Gene Mutations replace nitrogen bases in DNA (codon)

• Point mutations are chemical changes in just one base pair of a gene

• The change of a single nucleotide in a DNA template strand can lead to the production of an abnormal protein

• Point mutations within a gene can be divided into two general categories

– Base-pair substitutions

– Base-pair insertions or deletions

• Insertion or deletion of nucleotides may alter the reading frame, producing a frameshift mutation

Fig. 17-23aWild type

3DNA templatestrand

3

355

5mRNA

Protein

Amino end

Stop

Carboxyl end

A instead of G

33

3

U instead of C

55

5

Stop

Silent (no effect on amino acid sequence)

Fig. 17-23bWild type

DNA templatestrand

35

mRNA

Protein

5

Amino end

Stop

Carboxyl end

53

3

T instead of C

A instead of G

33

3

5

5

5

Stop

Missense

Fig. 17-23cWild type

DNA templatestrand

35

mRNA

Protein

5

Amino end

Stop

Carboxyl end

53

3

A instead of T

U instead of A

33

3

5

5

5

Stop

Nonsense

Fig. 17-23dWild type

DNA templatestrand

35

mRNA

Protein

5

Amino end

Stop

Carboxyl end

53

3

Extra A

Extra U

33

3

5

5

5

Stop

Frameshift causing immediate nonsense (1 base-pair insertion)

Genetic Testing and Counseling

• Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease


Counseling Based on Mendelian Genetics and Probability Rules

• Using family histories, genetic counselors help couples determine the odds that their children will have genetic disorders


Fetal Testing

• In amniocentesis, 14th-16th week , liquid that bathes the fetus is removed and tested

• In chorionic villi sampling (CVS), 8th-10th week a sample of the placenta is removed and tested

• Karyotype snap shot picture of metaphase that show size, shape and number of the chromosomes

• Other techniques, such as ultrasound and fetoscopy, allow fetal health to be assessed visually in utero


Video: Ultrasound of Human Fetus IVideo: Ultrasound of Human Fetus I

Fig. 14-18a

Fetus

Amniotic fluidwithdrawn

Placenta

Uterus Cervix

Centrifugation

Fluid

Fetalcells

Severalhours

Severalweeks

Severalweeks

Bio-chemical

tests

Karyotyping

(a) Amniocentesis

Fig. 14-18b

(b) Chorionic villus sampling (CVS)

Bio-chemical

tests

Placenta Chorionicvilli

Fetus

Suction tubeinsertedthroughcervix

FetalcellsSeveral

hours

SeveralhoursKaryotyping

Cystic Fibrosis

• Cystic fibrosis is the most common lethal genetic disease in the United States,striking one out of every 2,500 people of European descent

• The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes

• Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine


Dominantly Inherited Disorders

• Some human disorders are caused by dominant alleles

• Dominant alleles that cause a lethal disease are rare and arise by mutation

• Achondroplasia is a form of dwarfism caused by a rare dominant allele


Fig. 14-17

Eggs

Parents

Dwarf Normal

Normal

Normal

Dwarf

Dwarf

Sperm

Dd dd

dD

Dd dd

ddDd

d

d

• Huntington’s disease is a degenerative disease of the nervous system

• The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age

Huntington’s Disease


Tests for Identifying Carriers

• For a growing number of diseases, tests are available that identify carriers and help define the odds more accurately


Fig. 14-18

Amniotic fluidwithdrawn

Fetus

Placenta

Uterus Cervix

Centrifugation

Fluid

Fetalcells

Severalhours

Severalweeks

Severalweeks

(a) Amniocentesis (b) Chorionic villus sampling (CVS)

Severalhours

Severalhours

Fetalcells

Bio-chemical

tests

Karyotyping

Placenta Chorionicvilli

Fetus

Suction tubeinsertedthroughcervix

Newborn Screening

• Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the United States


Fig. 14-UN2

Degree of dominance

Complete dominanceof one allele

Incomplete dominanceof either allele

Codominance

Description

Heterozygous phenotypesame as that of homo-zygous dominant

Heterozygous phenotypeintermediate betweenthe two homozygousphenotypes

Heterozygotes: Bothphenotypes expressed

Multiple alleles

Pleiotropy

In the whole population,some genes have morethan two alleles

One gene is able toaffect multiplephenotypic characters

CRCR CRCW CWCW

IAIB

IA , IB , i

ABO blood group alleles

Sickle-cell disease

PP Pp

Example

Fig. 14-UN3

DescriptionRelationship amonggenes

Epistasis One gene affectsthe expression ofanother

Example

Polygenicinheritance

A single phenotypiccharacter isaffected bytwo or more genes

BbCc BbCc

BCBC

bC

bC

Bc

Bc

bc

bc

9 : 3 : 4

AaBbCc AaBbCc

Fig. 14-UN4

Fig. 14-UN11

genetics chapter 9-11. gregor mendel 1850’s the father of genetics genetic – branch of biology...

Documents

pearson education

pearson benjamin cummingsfig

truebreeding plants

pearson benjamin cummingstable

f2 plants

purple flower color

truebreeding varieties

white flower color