genetics
DESCRIPTION
Genetics. The study of inherited traits: An introduction. Molecular Genetics. Cell . Chromosome. DNA. Nucleus. Nucleotides. History of Genetics. Mid 19 th century (1850) Darwin & Wallace Theories of evolution Lamarck Theories on acquisition of heritable traits Mendel - PowerPoint PPT PresentationTRANSCRIPT
Genetics
The study of inherited traits:An introduction
Molecular GeneticsChromosome
DNA
Nucleotides
Nucleus
Cell
History of Genetics
• Mid 19th century (1850)– Darwin & Wallace • Theories of evolution
– Lamarck• Theories on acquisition of heritable traits
– Mendel• Theories on transmission of traits
History of Genetics• Pioneering work of Mendel was done in ignorance of cell division –
particularly meiosis, and the nature of genetic material – DNA
• 1869 - Friedrich Miescher identified DNA
• 1900-1913 – Chromosomal theory of inheritance – Sutton & Boveri– Genes on chromosomes – TH Morgan– Genes linearly arranged on chromosomes & mapped – AH Sturtevant
• 1941 – George Beadle & Ed Tatum related "gene" to enzyme & biochemical processes
• 1944 – Oswald Avery demonstrated that DNA was genetic material
Early Ideas
• Until the 20th century, many biologists erroneously believed that – characteristics acquired during lifetime could be
passed on – characteristics of both parents blended
irreversibly in their offspring
• Modern genetics began with Gregor Mendel’s quantitative experiments with pea plants
Mendel
Figure 9.2A, B
Stamen
Carpel
• Mendel crossed pea plants that differed in certain characteristics and traced the traits from generation to generation
Figure 9.2C
• This illustration shows his technique for cross-fertilization
1 Removed stamensfrom purple flower
White
Stamens
Carpel
PurplePARENTS(P)
OFF-SPRING
(F1)
2 Transferred pollen from stamens of white flower to carpel of purple flower
3 Pollinated carpel matured into pod
4 Planted seeds from pod
• Mendel studied seven pea characteristics - phenotypes
Figure 9.2D
• He hypothesized that there are alternative forms of genes (although he did not use that term), the units that determine heredity
FLOWER COLOR
FLOWER POSITION
SEED COLOR
SEED SHAPE
POD SHAPE
POD COLOR
STEM LENGTH
Purple White
Axial Terminal
Yellow Green
Round Wrinkled
Inflated Constricted
Green Yellow
Tall Dwarf
• From his experimental data, Mendel deduced that an organism has two genes (alleles) for each inherited characteristic– One characteristic
(phenotype) comes from each parent
Principle of SegregationP GENERATION(true-breedingparents)
F1 generation
F2generation
Purple flowers White flowers
All plants have purple flowers
Fertilization among F1 plants(F1 x F1)
3/4 of plantshave purple flowers
1/4 of plantshave white flowersFigure 9.3A
• A sperm or egg carries only one allele of each pair
– The pairs of alleles separate when gametes form
– This process describes Mendel’s law of segregation
– Alleles can be dominant or recessive
GENETIC MAKEUP (ALLELES)
PLANTS
F1 PLANTS(hybrids)
F2 PLANTS
PP pp
All P All p
All Pp
1/2 P 1/2 p
EggsP
p
P
PPp
Sperm
Pp Pp
pp
Gametes
Gametes
Phenotypic ratio3 purple : 1 white
Genotypic ratio1 PP : 2 Pp : 1 pp
Figure 9.3B
• Alternative forms of a gene (alleles) reside at the same locus on homologous chromosomes
Homologous chromosomes bear the two alleles for each characteristic
GENE LOCI
Figure 9.4
P a B
DOMINANTallele
RECESSIVEallele
P a b
GENOTYPE: PP aa Bb
HOMOZYGOUSfor thedominant allele
HOMOZYGOUSfor therecessive allele
HETEROZYGOUS
• By looking at two characteristics at once, Mendel found that the alleles of a pair segregate independently of other allele pairs during gamete formation– This is known as the principle of independent
assortment
The principle of independent assortment
Figure 9.5A
HYPOTHESIS: DEPENDENT ASSORTMENT
HYPOTHESIS: INDEPENDENT ASSORTMENT
PGENERATION
F1
GENERATION
F2
GENERATION
RRYY rryy
Gametes RY
Yellowround
ry
RrYy
Eggs SpermRY
ry
RY
ry
1/21/2
1/21/2
Actual resultscontradict hypothesis
RRYY rryy
RY ryGametes
RrYy
Eggs RY
rY
1/4
1/4
Ry
ry
1/4
1/4
RY
rY
Ry
ry
1/4
1/4
1/4
1/4
RRYY
RrYYRrYY
RRYy rrYY RrYy
RrYyRrYyRrYyRrYy
rrYy RRyy rrYy
Rryy Rryy
rryy
9/16
3/16
3/16
1/16
Greenround
YellowwrinkledYellowwrinkled
ACTUAL RESULTSSUPPORT
HYPOTHESIS
• Independent assortment of two genes in the Labrador retriever
Figure 9.5B
PHENOTYPES Black coat, normal vision
B_N_
Blind
GENOTYPES
MATING OF HETEROZYOTES(black, normal vision)
PHENOTYPIC RATIO OF OFFSPRING
Black coat, blind (PRA)
B_nn
Chocolate coat, normal vision
bbN_
Chocolate coat, blind (PRA)
bbnn
9 black coat,normal vision
3 black coat,blind (PRA)
3 chocolate coat,normal vision
1 chocolate coat,blind (PRA)
Blind
BbNnBbNn
• The offspring of a testcross often reveal the genotype of an individual when it is unknown
Geneticists use testcross to determine unknown genotypes
TESTCROSS:
B_GENOTYPES bb
BB Bbor
Two possibilities for the black dog:
GAMETES
OFFSPRINGaAll black 1 black : 1 chocolate
B
b
B
b
b
Bb Bb bb
Figure 9.6
• Inheritance follows the rules of probability– The rule of
multiplication and the rule of addition can be used to determine the probability of certain events occurring
Mendel’s principles reflect the rules of probability
F1 GENOTYPES
Bb female
F2 GENOTYPES
Formation of eggs
Bb male
Formation of sperm
1/2
1/2
1/2
1/21/4
1/41/4
1/4
B B
B B
B B
b
b b
b
b b
Figure 9.7
• The inheritance of many human traits follows Mendel’s principles and the rules of probability
Family pedigrees
Figure 9.8A
• Family pedigrees are used to determine patterns of inheritance and individual genotypes
Figure 9.8B
DdJoshuaLambert
DdAbigailLinnell
D_Abigail
Lambert
Female
DdElizabeth
Eddy
D_JohnEddy
? D_HepzibahDaggett
?
?
ddDdDdDdddDdDd
MaleDeaf
Hearing
ddJonathanLambert
• Most such disorders are caused by autosomal recessive alleles– Examples:
cystic fibrosis, sickle-cell disease
Many inherited disorders are controlled by a single gene
Figure 9.9A
D D
d d
NormalDd
NormalDd
DDNormal
DdNormal(carrier)
DdNormal(carrier)
ddDeaf
Eggs Sperm
PARENTS
OFFSPRING
• A few are caused by dominant alleles
Figure 9.9B
– Examples: achondroplasia, Huntington’s disease
Table 9.9
• Karyotyping and biochemical tests of fetal cells and molecules can help people make reproductive decisions– Fetal cells can be obtained through amniocentesis
Fetal testing for inherited disorders
Figure 9.10A
Amnioticfluid
Fetus(14-20weeks)
Placenta
Amnioticfluidwithdrawn
Centrifugation
Fetalcells
Fluid
Uterus Cervix Cell culture
Severalweeks later Karyotyping
Biochemicaltests
• Chorionic villus sampling is another procedure that obtains fetal cells for karyotyping
Figure 9.10B
Fetus(10-12weeks)Placenta
Chorionic villi
Suction
Several hourslater
Fetal cells(from chorionic villi)
Karyotyping
Some biochemical
tests
• Examination of the fetus with ultrasound is another helpful technique
Figure 9.10C, D
• Mendel’s principles are valid for all sexually reproducing species– However, often the genotype does not dictate the
phenotype in the simple way his principles describe
VARIATIONS ON MENDEL’S PRINCIPLESThe relationship of genotype to phenotype is rarely
simple
• When an offspring’s phenotype—such as flower color— is in between the phenotypes of its parents, it exhibits incomplete dominance
Incomplete dominance results in intermediate phenotypes
P GENERATION
F1 GENERATION
F2 GENERATION
RedRR
Gametes R r
Whiterr
PinkRr
R r
R R
r r
1/21/2
1/2
1/21/2
1/2 SpermEggs
PinkRr
PinkrR
Whiterr
RedRR
Figure 9.12A
• Incomplete dominance in human hypercholesterolemia
Figure 9.12B
GENOTYPES:HH
Homozygousfor ability to make
LDL receptors
HhHeterozygous
hhHomozygous
for inability to makeLDL receptors
PHENOTYPES:
LDL
LDLreceptor
Cell
Normal Mild disease Severe disease
• In a population, multiple alleles often exist for a characteristic– The three alleles for ABO blood type in humans is an
example
9.13 Many genes have more than two alleles in the population
Figure 9.13
– The alleles for A and B blood types are codominant, and both are expressed in the phenotype
BloodGroup(Phenotype)
O
Genotypes
AntibodiesPresent inBlood
Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left
O A B AB
A
B
AB
ii
IA IA
orIA i
IB IB
orIB i
IA IB
Anti-AAnti-B
Anti-B
Anti-A
9.14 A single gene may affect many phenotypic characteristics
• A single gene may affect phenotype in many ways– This is called pleiotropy– The allele for sickle-cell disease is an example
Individual homozygousfor sickle-cell allele
Sickle-cell (abnormal) hemoglobin
Abnormal hemoglobin crystallizes,causing red blood cells to become sickle-shaped
Sickle cells
Breakdown of red blood cells
Clumping of cells and clogging of
small blood vesselsAccumulation of
sickled cells in spleen
Physical weakness Anemia Heart
failurePain and
feverBrain
damageDamage to
other organsSpleen damage
Kidney failureRheumatismPneumonia
and other infections
ParalysisImpaired mental
function
Figure 9.14
• Genetic testing can be of value to those at risk of developing a genetic disorder or of passing it on to offspring
Genetic testing to detect disease-causing alleles
Figure 9.15B
Figure 9.15A
• Dr. David Satcher, former U.S. surgeon general, pioneered screening for sickle-cell disease
• This situation creates a continuum of phenotypes– Example: skin color– Polygenic inheritance
A single characteristic may be influenced by many genes
Figure 9.16
P GENERATION
F1 GENERATION
F2 GENERATION
aabbcc(very light)
AABBCC(very dark)
AaBbCc AaBbCc
Eggs Sperm
Frac
tion
of p
opul
atio
n
Skin pigmentation
• Genes are located on chromosomes– Their behavior during meiosis accounts for
inheritance patterns
THE CHROMOSOMAL BASIS OF INHERITANCE
Chromosome behavior accounts for Mendel’s principles
• The chromosomal basis of Mendel’s principles
Figure 9.17
• Certain genes are linked– They tend to be inherited together because they
reside close together on the same chromosome
Genes on the same chromosome tend to be inherited together
Figure 9.18
• This produces gametes with recombinant chromosomes
• The fruit fly Drosophila melanogaster was used in the first experiments to demonstrate the effects of crossing over
Crossing over produces new combinations of alleles
A B
a b
Tetrad Crossing over
A B
a
ba
BA b
Gametes
Figure 9.19A, B
Figure 9.19C
• Crossing over is more likely to occur between genes that are farther apart– Recombination frequencies can be used to map the
relative positions of genes on chromosomes
9.20 Geneticists use crossover data to map genes
g
Figure 9.20B
Chromosome
c l
17%
9% 9.5%
• Alfred H. Sturtevant, seen here at a party with T. H. Morgan and his students, used recombination data from Morgan’s fruit fly crosses to map genes
Figure 9.20A
• A partial genetic map of a fruit fly chromosome
Figure 9.20C
Shortaristae
Blackbody(g)
Cinnabareyes(c)
Vestigialwings(l)
Browneyes
Long aristae(appendageson head)
Graybody(G)
Redeyes(C)
Normalwings(L)
Redeyes
Mutant phenotypes
Wild-type phenotypes
• A human male has one X chromosome and one Y chromosome
• A human female has two X chromosomes• Whether a sperm cell has an X or Y chromosome
determines the sex of the offspring
SEX CHROMOSOMES AND SEX-LINKED GENES
Chromosomes determine sex in many species
Figure 9.21A
X YMale
(male)
Parents’diploidcells
(female)
Sperm
Offspring(diploid)
Egg
• Other systems of sex determination exist in other animals and plants
Figure 9.21B-D
– The X-O system
– The Z-W system
– Chromosome number
• All genes on the sex chromosomes are said to be sex-linked– In many organisms, the X chromosome carries many
genes unrelated to sex– Fruit fly eye
color is a sex-linked characteristic
Sex-linked genes exhibit a unique pattern of inheritance
Figure 9.22A
– Their inheritance pattern reflects the fact that males have one X chromosome and females have two
Figure 9.22B-D
– These figures illustrate inheritance patterns for white eye color (r) in the fruit fly, an X-linked recessive trait
Female Male Female Male Female Male
XrYXRXR
XRXr
XRY
XR Xr
Y
XRXr
XR
Xr XRXR
XR
Y
XRY
XrXR XRY
XrY
XRXr
XR
Xr
Xr
YXRXr
XrXr XRYXrY
XrY
R = red-eye alleler = white-eye allele
• Most sex-linked human disorders are due to recessive alleles– Examples: hemophilia,
red-green color blindness– These are mostly seen in males– A male receives a single X-linked allele from his
mother, and will have the disorder, while a female has to receive the allele from both parents to be affected
Sex-linked disorders affect mostly males
Figure 9.23A
• A high incidence of hemophilia has plagued the royal families of Europe
Figure 9.23B
QueenVictoria
Albert
Alice Louis
Alexandra CzarNicholas IIof Russia
Alexis
DNA
• 1953 - James Watson, Francis Crick, Rosalind Franklin & Maurice Wilkins
• Lead to understanding of mutation and relationship between DNA and proteins at a molecular level
• 1959 – “Central Dogma”– DNARNAprotein
Genetic Concepts
• Chromosome – – double stranded DNA
molecule packaged by histone & scaffold proteins
DNA double helix
nucleosome
30nm fiber
condensed chromosome
Genetic Concepts
• Chromosome numbers– Constant for an organism– n - haploid number – 2n – diploid number
• Karyotype
Genetic Concepts
Y
Genetic Concepts
• Chromosome numbers– Each individual inherits n # of chromosomes from
dad & n # from mom– Humans - 46 chromosomes = 2n– Humans 23 paternal, 23 maternal– Humans n = ____– Each maternal & paternal pair represent
homologous chromosomes - called homologs
Genetic Concepts
(a) Chromosomal composition found in most female human cells (46 chromosomes)
(b) Chromosomal composition found in a human gamete (23 chromosomes)
1 2 3 4 5 6 7
XX
8
9 10 11 12 13 14 15
17 18 19 20 21 22
16
1 2 3 4 5 6 7
X
8
9 10 11 12 13 14 15
17 18 19 20 21 22
16
Diploid Haploid
Genetic Concepts• Homologous Chromosomes– Share centromere position– Share overall size– Contain identical gene sets at matching positions (loci)
gene for color
gene for shape
Genetic Concepts• Gene – sequence of DNA which is transcribed
into RNA – rRNA, tRNA or mRNA
• Locus – the position on a chromosome of a particular DNA sequence (gene)
G Locus – gene for color
W Locus – gene for shape
Genetic Concepts• DNA is mutable• A variation in DNA sequence at a locus is
called an allele– Diploid organisms contain 2 alleles of each locus
(gene)• Alleles can be identical – homozygous• Alleles can be different – heterozygous• If only one allele is present – hemizygous
– Case in males for genes on X and Y chromosomes
Genetic Concepts
Allele – G vs g; W vs w
At the G locus either the G or g allele may be present on a given homologue of a homologous pair of chromosomes
Genetic Concepts• Genome– Collection of all genetic material of organism
• Genotype– Set of alleles present in the genome of an organism
• Phenotype– Result of Gene Expression– Genes (DNA) are transcribed into RNA– mRNA is translated into protein, tRNA & rRNA work in
translation process– Biochemical properties of proteins, tRNAs & rRNAs
determine physical characteristics of organism
DNA
Gene
Transcription
Translation
RNA (messenger RNA)
Protein(sequence ofamino acids)
Functioning of proteins within livingcells influences an organism’s traits.
Gene Expression
Pigmentation gene,dark allele
Pigmentation gene,light allele
Transcriptionand translation
Highly functionalpigmentation enzyme
Poorly functionalpigmentation enzyme
Molecular level
Mutation & Phenotypic Variation
Wing cells
Lots of pigment made Little pigment made
Pigmentmolecule
(b) Cellular level
Pigmentation gene,dark allele
Pigmentation gene,light allele
Transcriptionand translation
Highly functionalpigmentation enzyme
Poorly functionalpigmentation enzyme
(a) Molecular level
Mutation & Phenotypic Variation
Dark butterfly Light butterfly Organismal level
Mutation & Phenotypic Variation
Dark butterflies are usuallyin forested regions.
Light butterflies are usually in unforested regions. Populational level