Download - Introduction: Mendelian Genetics
Introduction: Mendelian Genetics
Copyright © 2009 Pearson Education, Inc.
Chapter 9
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 theorganism to be incorporated into eggs or sperm
– Characteristics acquired during the parents’ lifetime couldbe transferred to the offspring
– Aristotle rejected pangenesis and argued that instead ofparticles, the potential to produce the traits was inherited
Blending was another idea, based on plant breeding
– Hereditary material from parents mixes together to forman intermediate trait, like mixing paint
Copyright © 2009 Pearson Education, Inc.
Experimental genetics began in an abbeygarden
Gregor Mendel discovered principles of genetics inexperiments with the garden pea
– Mendel showed that parents pass heritable factors tooffspring (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
Copyright © 2009 Pearson Education, Inc.
Flower color White
Axial
Purple
Flower position Terminal
YellowSeed color Green
RoundSeed shape Wrinkled
InflatedPod shape Constricted
GreenPod color Yellow
TallStem length Dwarf
MENDEL’S LAWS
Mendel’s first law describes the inheritance of asingle 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 F1generation
– Why that trait reappeared in one quarter of the F2offspring
Copyright © 2009 Pearson Education, Inc.
3/41/4
P generation(true-breedingparents)
Purple flowers White flowers
P generation(true-breedingparents)
Purple flowers White flowers
F1 generation All plants havepurple flowers
P generation(true-breedingparents)
Purple flowers White flowers
F1 generation All plants havepurple flowers
F2 generation
Fertilizationamong F1 plants(F1 X F1)
of plantshave purple flowers
3–4 of plantshave white flowers
1–4
Mendel’s law of segregation describes theinheritance of a single character
Four Hypotheses
1. Genes are found in alternative versions called alleles;a genotype is the listing of alleles an individualcarries for a specific gene
2. For each characteristic, an organism inherits twoalleles, one from each parent; the alleles can be thesame or different
– A homozygous genotype has identical alleles
– A heterozygous genotype has two different alleles
Copyright © 2009 Pearson Education, Inc.
Mendel’s law of segregation describes theinheritance of a single character
Four Hypotheses
3. If the alleles differ, the dominant allele determinesthe organism’s appearance, and the recessive allelehas no noticeable effect
– The phenotype is the appearance or expression of a trait
– The same phenotype may be determined by more thanone genotype
4. Law of segregation: Allele pairs separate (segregate)from each other during the production of gametes sothat a sperm or egg carries only one allele for eachgene
Copyright © 2009 Pearson Education, Inc.
P plants
1–21–2
Genotypic ratio1 PP : 2 Pp : 1 pp
Phenotypic ratio3 purple : 1 white
F1 plants(hybrids)
Gametes
Genetic makeup (alleles)⇣
All
All Pp
Sperm
Eggs
PP
p
ppPp
Pp
P
pP
pP
P
p
PP pp
All
Gametes
F2 plants
Homologous chromosomes bear the alleles foreach character
For a pair of homologous chromosomes, alleles ofa gene reside at the same locus
– Homozygous individuals have the same allele on bothhomologues
– Heterozygous individuals have a different allele oneach homologue
Gene loci
Homozygousfor thedominant allele
Dominantallele
Homozygousfor therecessive allele
Heterozygous
Recessive allele
Genotype:
P Ba
P
PP
a
aa
b
Bb
Mendel’s second law is revealed by tracking twocharacters at once
Example of a dihybrid cross
– Parental generation: round yellow seeds × wrinkled greenseeds
– 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
Copyright © 2009 Pearson Education, Inc.
9/163/163/161/16
Mendel needed to explain
–Why nonparental combinations were observed
–Why a 9:3:3:1 ratio was observed among the F2offspring
The law of independent assortment is revealedby tracking two characters at once
Law of independent assortment
– Each pair of alleles segregates independently of theother pairs of alleles during gamete formation
– For genotype RrYy, four gamete types are possible:RY, Ry, rY, and ry
P generation
1–2
Hypothesis: Dependent assortment Hypothesis: Independent assortment
1–2
1–2
1–2
1–4
1–4
1–4
1–4
1–41–4
1–4
1–4
9––16
3––16
3––16
1––16
RRYY
Gametes
Eggs
F1generation
SpermSperm
F2generation
Eggs
Gametes
rryy
RrYy
ryRY
ryRY
ry
RY
Hypothesized(not actually seen)
Actual results(support hypothesis)
RRYY rryy
RrYy
ryRY
RRYY
rryy
RrYy
ry
RY
RrYy
RrYy
RrYy
rrYYRrYY
RRYyRrYY
RRYy
rrYy
rrYy
Rryy
Rryy
RRyy
rY
Ry
ry
YellowroundGreenround
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
Mendel’s laws reflect the rules of probability
The probability of a specific event is the numberof ways that event can occur out of the totalpossible outcomes.
Rule of multiplication
– Multiply the probabilities of events that must occurtogether
Rule of addition
– Add probabilities of events that can happen inalternate ways
Copyright © 2009 Pearson Education, Inc.
F1 genotypes
1–2
1–2
1–2
1–2
1–41–4
1–41–4
Formation of eggs
Bb female
F2 genotypes
Formation of sperm
Bb male
B
BB B B
B
b
b
bbbb
Parents NormalDd
Offspring
Sperm
Eggs
ddDeafd
DdNormal(carrier)
DDNormalD
D d
DdNormal(carrier)
NormalDd×
VARIATIONS ON MENDEL’SLAWS
Copyright © 2009 Pearson Education, Inc.
Incomplete dominance results in intermediatephenotypes
Incomplete dominance
– Neither allele is dominant over the other
– Expression of both alleles is observed as anintermediate phenotype in the heterozygousindividual
Copyright © 2009 Pearson Education, Inc.
P generation
1–21–2
1–21–2
1–2
1–2
F1 generation
F2 generation
RedRR
Gametes
Gametes
Eggs
Sperm
RR rR
Rr rr
R
r
R r
R r
PinkRr
R r
Whiterr
HHHomozygous
for ability to makeLDL receptors
hhHomozygous
for inability to makeLDL receptors
HhHeterozygous
LDL receptor
LDL
Normal Cell Mild disease Severe disease
Genotypes:
Phenotypes:
Codominance–Neither allele is dominant over the other
–Expression of both alleles is observed as a distinctphenotype in the heterozygous individual
Many genes have more than two alleles in thepopulation
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 fourphenotypes: type A, type B, type AB, and type Oblood
Copyright © 2009 Pearson Education, Inc.
BloodGroup(Phenotype) Genotypes
O
A
ii
IAIAorIAi
Red Blood Cells
Carbohydrate A
BIBIBorIBi
Carbohydrate B
AB IAIBCodominanceobserved fortype AB blood
A single gene may affect many phenotypiccharacters
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
Copyright © 2009 Pearson Education, Inc.
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
A single character may be influenced by manygenes
Polygenic inheritance - Many genes influenceone trait
– Skin color is affected by at least three genes
– Height
– Intelligence
Copyright © 2009 Pearson Education, Inc.
P generation
1–8
F1 generation
F2 generation
Frac
tion
of p
opul
atio
n
Skin color
Eggs
Sperm1–8
1–81–8
1–81–8
1–81–8
1–8
1–81–8
1–8
1–81–8
1–8
1–8
aabbcc(very light)
AABBCC(very dark)
AaBbCc AaBbCc
1––6415––64
6––641––64
15––646––64
20––64
1––64
15––64
6––64
20––64
The environment affects many characters
Phenotypic variations are influenced by theenvironment
– Skin color is affected by exposure to sunlight
– Susceptibility to diseases, such as cancer, hashereditary and environmental components
Copyright © 2009 Pearson Education, Inc.
Incompletedominance
RedRR
Singlegene
Single characters(such as skin color)
Multiple charactersPleiotropy
PolygenicinheritanceMultiple
genes
Whiterr
PinkRr
THE CHROMOSOMAL BASIS OFINHERITANCE
Copyright © 2009 Pearson Education, Inc.
Sex-linked genes are located on either of thesex chromosomes
– Reciprocal crosses show different results
– White-eyed female × red-eyed male red-eyed femalesand white-eyed males
– Red-eyed female × white-eyed male red-eyed femalesand red-eyed males
– X-linked genes are passed from mother to son andmother to daughter
– X-linked genes are passed from father to daughter
– Y-linked genes are passed from father to son
Sex-linked genes exhibit a unique pattern ofinheritance
Copyright © 2009 Pearson Education, Inc.
Female Male
XR XR Xr Y
XR YXR Xr
YXr
XR
Sperm
Eggs
R = red-eye alleler = white-eye allele
All RED means Red is ?
Female Male
XR Xr XR Y
XR YXR XR
YXR
XR
Sperm
Eggs
Xr XR Xr YXr
3 RED : 1 whitemeans?
Sex-linked disorders affect mostly males
Males express X-linked disorders such as thefollowing when recessive alleles are present inone copy
– Hemophilia
– Red/Green Colorblindness
– Duchenne muscular dystrophy
Copyright © 2009 Pearson Education, Inc.
QueenVictoria
Albert
Alice Louis
Alexandra CzarNicholas IIof Russia
Alexis
SEX CHROMOSOMES ANDSEX-LINKED GENES
Copyright © 2009 Pearson Education, Inc.
Chromosomes determine sex in many species
X-Y system in mammals, fruit flies
– XX = female; XY = male
Copyright © 2009 Pearson Education, Inc.
(male)
Sperm
(female)44+
XYParents’diploidcells
44+
XX
22+X
22+Y
22+X
44+
XY
44+
XX
Egg
Offspring(diploid)
Chromosomes determine sex in many species
Other Systems
1. X-O system in grasshoppers and roaches
1. XX = female; XO = male
Copyright © 2009 Pearson Education, Inc.
22+X
22+
XX
76+
ZZ76+
ZW
2. Z-W in system in birds, butterflies, and some fishes
– ZW = female, ZZ = male
Chromosomes determine sex in many species
Other Systems
3. Chromosome number in ants and bees
– Diploid = female; haploid = male
Copyright © 2009 Pearson Education, Inc.
1632
Nonchromosomal Sex Determination
Reptiles - temperature at a stage of egg development
Shifts dependent of ratio of the sexes in a population
Both sexes in same organism
– Earthworms, some plants
Genes
locatedon
(b)
(a)
at specificlocations called
alternativeversions called
if both same,genotype called
expressedallele called
inheritance when phenotypeIn between called
unexpressedallele called
if different,genotype called
chromosomes
heterozygous
(d)
(c)
(f)
(e)
loci homozygous
alleles
dominant recessive
incomplete dominance
Mutant phenotypesShortaristae
Blackbody(g)
Cinnabareyes(c)
Vestigialwings(l)
Browneyes
Long aristae(appendageson head)
Graybody(G)
Redeyes(C)
Normalwings(L)
Redeyes
Wild-type phenotypes
A few Non-Sex Linked Alleles
1. Explain and apply Mendel’s laws of segregationand independent assortment
2. Distinguish between terms in the followinggroups: allele—gene; dominant—recessive;genotype—phenotype; F1—F2;heterozygous—homozygous; incompletedominance—codominance
3. Explain the meaning of the terms locus, multiplealleles, pedigree, pleiotropy, polygenicinheritance
You should now be able to
Copyright © 2009 Pearson Education, Inc.
4. Describe the difference in inheritance patterns forlinked genes and explain how recombination canbe used to estimate gene distances
5. Describe how sex is inherited in humans andidentify the pattern of inheritance observed forsex-linked genes
6. Solve genetics problems involving monohybridand dihybrid crosses for autosomal and sex-linked traits, with variations on Mendel’s laws
You should now be able to
Copyright © 2009 Pearson Education, Inc.