09 lecture presentation

MENDEL’S LAWS

9.1 The science of genetics has ancient roots

Pangenesis was an early explanation for inheritance– It was proposed by Hippocrates

– Particles called pangenes came from all parts of the organism to be incorporated into eggs or sperm

– Characteristics acquired during the parents’ lifetime could be transferred to the offspring

– Aristotle rejected pangenesis and argued that instead of particles, the potential to produce the traits was inherited

Blending was another idea, based on plant breeding– Hereditary material from parents mixes together to form

9.2 Experimental genetics began in an abbey garden

Gregor Mendel discovered principles of genetics in experiments with the garden pea

– Mendel showed that parents pass heritable factors to offspring (heritable factors are now called genes)

– Advantages of using pea plants

– Controlled matings

– Self-fertilization or cross-fertilization

– Observable characteristics with two distinct forms

– True-breeding strains

Stamen

Carpel

Transferredpollen from stamens of whiteflower to carpel of purple flower

StamensCarpel

Parents(P)

Purple

Removedstamens frompurple flower

StamensCarpel

Parents(P)

Purple

Pollinated carpelmatured into pod

StamensCarpel

Parents(P)

Purple

Pollinated carpelmatured into pod

Offspring(F1)

Planted seedsfrom pod

Flower color White

Purple

Flower position Terminal

YellowSeed color Green

RoundSeed shape Wrinkled

InflatedPod shape Constricted

GreenPod color Yellow

TallStem length Dwarf

9.3 Mendel’s law of segregation describes the inheritance of a single character

Example of a monohybrid cross

– Parental generation: purple flowers × white flowers

– F1 generation: all plants with purple flowers

– F2 generation: of plants with purple flowers of plants with white flowers

Mendel needed to explain

– Why one trait seemed to disappear in the F1 generation

– Why that trait reappeared in one quarter of the F2 offspring

3/41/4

P generation(true-breedingparents)

Purple flowers White flowers

F1 generation All plants havepurple flowers

F2 generation

Fertilizationamong F1 plants(F1 × F1)

of plantshave purple flowers

3–4 of plants

have white flowers

Four Hypotheses

1. Genes are found in alternative versions called alleles; a genotype is the listing of alleles an individual carries for a specific gene

2. For each characteristic, an organism inherits two alleles, one from each parent; the alleles can be the same or different

– A homozygous genotype has identical alleles

– A heterozygous genotype has two different alleles

Four Hypotheses

3. If the alleles differ, the dominant allele determines the organism’s appearance, and the recessive allele has no noticeable effect

– The phenotype is the appearance or expression of a trait

– The same phenotype may be determined by more than one genotype

4. Law of segregation: Allele pairs separate (segregate) from each other during the production of gametes so that a sperm or egg carries only one allele for each gene

P plants

Genotypic ratio1 PP : 2 Pp : 1 pp

Phenotypic ratio3 purple : 1 white

F1 plants(hybrids)

Gametes

Genetic makeup (alleles)

All Pp

Gametes

F2 plants

9.4 Homologous chromosomes bear the alleles for each character

For a pair of homologous chromosomes, alleles of a gene reside at the same locus

– Homozygous individuals have the same allele on both homologues

– Heterozygous individuals have a different allele on each homologue

Gene loci

Homozygousfor thedominant allele

Dominantallele

Homozygousfor therecessive allele

Heterozygous

Recessive allele

Genotype:

9.5 The law of independent assortment is revealed by tracking two characters at once

Example of a dihybrid cross

– Parental generation: round yellow seeds × wrinkled green seeds

– F1 generation: all plants with round yellow seeds

– F2 generation: of plants with round yellow seeds of plants with round green seeds

of plants with wrinkled yellow seeds of plants with wrinkled green seeds

Mendel needed to explain– Why nonparental combinations were observed

9/163/163/161/16

9.5 The law of independent assortment is revealed by tracking two characters at once

Law of independent assortment

– Each pair of alleles segregates independently of the other pairs of alleles during gamete formation

– For genotype RrYy, four gamete types are possible: RY, Ry, rY, and ry

P generation

Hypothesis: Dependent assortment Hypothesis: Independent assortment

9––16

3––16

1––16

Gametes

generation

SpermSperm

generation

Gametes

Hypothesized(not actually seen)

Actual results(support hypothesis)

RRYY rryy

rrYYRrYY

RRYyRrYY

Yellowround

Greenround

Greenwrinkled

Yellowwrinkled

RY rY Ry

PhenotypesGenotypes

Mating of heterozygotes(black, normal vision)

Phenotypic ratioof offspring

Black coat, normal visionB_N_

9 black coat,normal vision

Black coat, blind (PRA)B_nn

3 black coat,blind (PRA)

Chocolate coat, normal visionbbN_

3 chocolate coat,normal vision

Chocolate coat, blind (PRA)bbnn

1 chocolate coat,blind (PRA)

Blind Blind

BbNn BbNn

9.6 Geneticists use the testcross to determine unknown genotypes

Testcross

– Mating between an individual of unknown genotype and a homozygous recessive individual

– Will show whether the unknown genotype includes a recessive allele

– Used by Mendel to confirm true-breeding genotypes

Two possibilities for the black dog:

Testcross:

Genotypes

Gametes

Offspring 1 black : 1 chocolateAll black

9.7 Mendel’s laws reflect the rules of probability

The probability of a specific event is the number of ways that event can occur out of the total possible outcomes.

Rule of multiplication

– Multiply the probabilities of events that must occur together

Rule of addit ion

– Add probabilities of events that can happen in alternate ways

F1 genotypes

Formation of eggs

Bb female

F2 genotypes

Formation of sperm

Bb male

BB B B

9.8 CONNECTION: Genetic traits in humans can be tracked through family pedigrees

A pedigree

– Shows the inheritance of a trait in a family through multiple generations

– Demonstrates dominant or recessive inheritance

– Can also be used to deduce genotypes of family members

Freckles

Widow’s peak

Free earlobe

No freckles

Straight hairline

Attached earlobe

Dominant Traits Recessive Traits

Freckles No freckles

Widow’s peak Straight hairline

Free earlobe Attached earlobe

Female MaleAffected

Unaffected

First generation(grandparents)

Second generation(parents, aunts,and uncles)

Third generation(two sisters)

ff ff ff

9.9 CONNECTION: Many inherited disorders in humans are controlled by a single gene

Inherited human disorders show

– Recessive inheritance

– Two recessive alleles are needed to show disease

– Heterozygous parents are carriers of the disease-causing allele

– Probability of inheritance increases with inbreeding, mating between close relatives

– Dominant inheritance

– One dominant allele is needed to show disease

– Dominant lethal alleles are usually eliminated from the population

Parents NormalDd

Offspring

ddDeafd

DdNormal(carrier)

DDNormalD

DdNormal(carrier)

NormalDd×

Genetic testing of parents

Fetal testing: biochemical and karyotype analyses

– Amniocentesis

– Chorionic vi l lus sampling

Maternal blood test

Fetal imaging

– Ultrasound

– Fetoscopy

Newborn screening

9.10 CONNECTION: New technologies can provide insight into one’s genetic legacy

Video: Ultrasound of Human Fetus

Needle insertedthrough abdomen toextract amniotic fluid

Suction tube insertedthrough cervix to extracttissue from chorionic villi

Ultrasoundmonitor

Placenta

Chorionicvilli

UterusCervix

Amniocentesis Chorionic villus sampling (CVS)

Ultrasoundmonitor

Placenta

UterusCervix

Centrifugation

Fetalcells

Amnioticfluid

Severalweeks

Biochemicaltests

Karyotyping

Fetalcells

Severalhours

VARIATIONS ON MENDEL’S LAWS

9.11 Incomplete dominance results in intermediate phenotypes

Incomplete dominance

– Neither allele is dominant over the other

– Expression of both alleles is observed as an intermediate phenotype in the heterozygous individual

P generation

F1 generation

F2 generation

Gametes

PinkRr

Whiterr

HHHomozygous

for ability to makeLDL receptors

hhHomozygous

for inability to makeLDL receptors

HhHeterozygous

LDL receptor

CellNormal Mild disease Severe disease

Genotypes:

Phenotypes:

9.12 Many genes have more than two alleles in the population

Multiple alleles

– More than two alleles are found in the population

– A diploid individual can carry any two of these alleles

– The ABO blood group has three alleles, leading to four phenotypes: type A, type B, type AB, and type O blood

9.12 Many genes have more than two alleles in the population

Codominance

– Neither allele is dominant over the other

– Expression of both alleles is observed as a distinct phenotype in the heterozygous individual

– Observed for type AB blood

BloodGroup(Phenotype) Genotypes

Red Blood Cells

Carbohydrate A

AntibodiesPresent inBlood

Anti-AAnti-B

Reaction When Blood from Groups Below Is Mixedwith Antibodies from Groups at Left

Anti-B

O A B AB

Carbohydrate B

AB IAIB —

Anti-A

BloodGroup(Phenotype) Genotypes

Red Blood Cells

Carbohydrate A

Carbohydrate B

AB IAIB

AntibodiesPresent inBlood

Anti-AAnti-B

Reaction When Blood from Groups Below Is Mixedwith Antibodies from Groups at Left

Anti-B

O A B AB

Anti-A

BloodGroup(Phenotype)

9.13 A single gene may affect many phenotypic characters

*Pleiotropy– One gene influencing many characteristics

– The gene for sickle cell disease

– Affects the type of hemoglobin produced

– Affects the shape of red blood cells

– Causes anemia

– Causes organ damage

– Is related to susceptibility to malaria

Clumping of cellsand clogging of

small blood vessels

Pneumoniaand otherinfections

Accumulation ofsickled cells in spleen

Pain andfever

Rheumatism

Heartfailure

Damage toother organs

Braindamage

Spleendamage

Kidneyfailure

Anemia

ParalysisImpairedmental

function

Physicalweakness

Breakdown ofred blood cells

Individual homozygousfor sickle-cell allele

Sickle cells

Sickle-cell (abnormal) hemoglobin

Abnormal hemoglobin crystallizes,causing red blood cells to become sickle-shaped

9.14 A single character may be influenced by many genes

*Polygenic inheritance– Many genes influence one trait

– Skin color is affected by at least three genes (maybe more)

– Opposite of pleiotropy.

P generation

F1 generation

F2 generation

Skin color

Sperm1–8

aabbcc(very light)

AABBCC(very dark)

AaBbCc AaBbCc

1––64

15––64

6––64

1––64

15––64

6––64

20––64

1––64

15––64

6––64

20––64

P generation

F1 generation

F2 generation

Sperm1–8

aabbcc(very light)

AABBCC(very dark)

AaBbCc AaBbCc

1––64

15––64

6––64

1––64

15––64

6––64

20––64

Skin color

1––64

15––64

6––64

20––64

9.15 The environment affects many characters

Phenotypic variat ions are inf luenced by the environment

– Skin color is affected by exposure to sunlight

– Susceptibility to diseases, such as cancer, has hereditary and environmental components

THE CHROMOSOMAL BASIS OF INHERITANCE

9.16 Chromosome behavior accounts for Mendel’s laws

Mendel’s Laws correlate with chromosome separation in meiosis

– The law of segregation depends on separation of homologous chromosomes in anaphase I.

– The law of independent assortment depends on alternative orientations of chromosomes in metaphase I

F1 generationR

Metaphase Iof meiosis(alternative

arrangements)

All round yellow seeds(RrYy)

F1 generationR

arrangements)

Anaphase Iof meiosis

Metaphase IIof meiosis

F1 generationR

arrangements)

Anaphase Iof meiosis

Metaphase IIof meiosis

1–4 rY

1–4 ry

1–4RY

YGametes

Fertilization among the F1 plants

:39 :3 :1F2 generation

9.17 Genes on the same chromosome tend to be inherited together

Linked Genes

– Are located close together on the same chromosome

– Tend to be inherited together

Example studied by Bateson and Punnett

– Parental generation: plants with purple flowers, long pollen crossed to plants with red flowers, round pollen

– The F2 generation did not show a 9:3:3:1 ratio

– Most F2 individuals had purple flowers, long pollen or red flowers, round pollen

Purple longPurple roundRed longRed round

Explanation: linked genes

Parentaldiploid cellPpLl

Experiment

Purple flower

PpLl Long pollenPpLl

Prediction(9:3:3:1)

ObservedoffspringPhenotypes

284212155

215717124

Mostgametes

Meiosis

Fertilization

Mostoffspring Eggs

3 purple long : 1 red roundNot accounted for: purple round and red long

Purple longPurple roundRed longRed round

Experiment

Purple flower

PpLl Long pollenPpLl

Prediction(9:3:3:1)

ObservedoffspringPhenotypes

284212155

215717124

Explanation: linked genes

Parentaldiploid cellPpLl

Mostgametes

Meiosis

Fertilization

Mostoffspring Eggs

3 purple long : 1 red roundNot accounted for: purple round and red long

9.18 Crossing over produces new combinations of alleles

Linked alleles can be separated by crossing over

– Recombinant chromosomes are formed

– Thomas Hunt Morgan demonstrated this in early experiments

– Geneticists measure genetic distance by recombination frequency

Gametes

Tetrad Crossing over

Ba baa b

A BA B A b

Experiment

Parentalphenotypes

Recombination frequency =

Black vestigial

Black body,vestigial wings

Offspring

Female Male

Gray long

965 944 206 185

Gray vestigial Black long

Gray body,long wings(wild type)

Recombinantphenotypes

391 recombinants2,300 total offspring

Explanation

= 0.17 or 17%

g l g l

g lGgLl

(female)ggll

(male)

G L g l g L

g l g l

SpermEggs

Offspring

Experiment

Parentalphenotypes

Recombination frequency =

Black vestigial

Black body,vestigial wings

Offspring

Female Male

Gray long

965 944 206 185

Gray vestigial Black long

Gray body,long wings(wild type)

Recombinantphenotypes

391 recombinants2,300 total offspring

= 0.17 or 17%

ExplanationG L

g l g l

g lGgLl

(female)ggll

(male)

G L g l g L

g l g l

SpermEggs

Offspring

9.19 Geneticists use crossover data to map genes

Genetic maps

– Show the order of genes on chromosomes

– Arrange genes into linkage groups representing individual chromosomes

Chromosome

Recombinationfrequencies

Mutant phenotypes

Shortaristae

Blackbody(g)

Cinnabareyes(c)

Vestigialwings(l)

Browneyes

Long aristae(appendageson head)

Graybody(G)

Redeyes(C)

Normalwings(L)

Redeyes

Wild-type phenotypes

SEX CHROMOSOMES AND SEX-LINKED GENES

9.20 Chromosomes determine sex in many species

X-Y system in mammals, fruit flies

– XX = female; XY = male

X-O system in grasshoppers and roaches

– XX = female; XO = male

Z-W in system in birds, butterflies, and some fishes

– ZW = female, ZZ = male

Chromosome number in ants and bees

– Diploid = female; haploid = male

(male)

(female)

XYParents’diploidcells

Offspring(diploid)

Sex-linked genes are located on either of the sex chromosomes

– Reciprocal crosses show different results

– White-eyed female × red-eyed male red-eyed females and white-eyed males

– Red-eyed female × white-eyed male red-eyed females and red-eyed males

– X-linked genes are passed from mother to son and mother to daughter

– X-linked genes are passed from father to daughter

– Y-linked genes are passed from father to son

9.21 Sex-linked genes exhibit a unique pattern of inheritance

Female Male

XR XR Xr Y

XR YXR Xr

R = red-eye alleler = white-eye allele

Female Male

XR Xr XR Y

XR YXR XR

Xr XR Xr YXr

Female Male

XR Xr Xr Y

XR YXR XR

Xr Xr Xr YXr

9.22 CONNECTION: Sex-linked disorders affect mostly males

Males express X-linked disorders such as the following when recessive alleles are present in one copy

– Hemophilia

– Colorblindness

– Duchenne muscular dystrophy

QueenVictoria

Albert

Alice Louis

Alexandra CzarNicholas IIof Russia

Alexis

9.23 EVOLUTION CONNECTION: The Y chromosome provides clues about human male evolution

Similarities in Y chromosome sequences

– Show a significant percentage of men related to the same male parent

– Demonstrate a connection between people living in distant locations

09 lecture presentation

purple flowers of plants

pearson education

purple flowers f2 generation

mendels law of segregation

stamens of whiteflower

identical alleles

listing of alleles

mendels lawscopyright

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