Transcript 14.15 chromosome

CHAPTER 11 GENETICS
1
Gregor Mendel
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© Ned M. Seidler/Nationa1 Geographic Image Collection
2
Gregor Mendel
•The garden pea:
• Organism used in Mendel’s experiments
• A good choice for several reasons:
• Easy to cultivate
• Short generation
• Normally self-pollinating, but can be cross-pollinated by
hand
• True-breeding varieties were available
• Simple, objective traits
3
Garden Pea Anatomy
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Flower Structure
stamen
anther
filament
stigma
style
ovules in
ovary
carpel
a.
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Garden Pea Anatomy
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Cutting away
anthers
Brushing
on pollen
from another
plant
All peas are yellow when
one parent produces yellow
seeds and the other parent
produces green seeds.
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Garden Pea Anatomy
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Trait
Stem length
Characteristics
*Dominant
*Recessive
Tall
Short
Pod shape
Inflated
Constricted
Seed shape
Round
Wrinkled
Seed color
Yellow
Green
Flower position
Axial
Terminal
Flower color
Purple
White
Pod color
Green
Yellow
b.
Figure 14.5 Genotype versus phenotype
Figure 14.6 A testcross
Figure 14.2 Mendel tracked heritable characters for three generations
11.2 Mendel’s Laws
• Mendel performed cross-breeding experiments
• Used “true-breeding” (homozygous) plants
• Chose varieties that differed in only one trait
(monohybrid cross)
• Performed reciprocal crosses
• Parental generation = P
• First filial generation offspring = F1
• Second filial generation offspring = F2
• Formulated the Law of Segregation
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Mendel’s Laws
• Law of Segregation:
• Each individual has a pair of factors (alleles) for each
trait
• The factors (alleles) segregate (separate) during gamete
(sperm & egg) formation
• Each gamete contains only one factor (allele) from each
pair of factors
• Fertilization gives the offspring two factors for each trait
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Mendel’s Laws
• Classical Genetics and Mendel’s Cross:
• Each trait in a pea plant is controlled by two alleles
(alternate forms of a gene)
• Dominant allele (capital letter) masks the expression
of the recessive allele (lower-case)
• Alleles occur on a homologous pair of chromosomes
at a particular gene locus
• Homozygous = identical alleles
• Heterozygous = different alleles
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Figure 14.4 Mendel’s law of segregation (Layer 2)
Figure 14.7 Testing two hypotheses for segregation in a dihybrid cross
Mendel’s Laws
• Genotype
• Refers to the two alleles an individual has for a specific
trait
• If identical, genotype is homozygous
• If different, genotype is heterozygous
• Phenotype
• Refers to the physical appearance of the individual
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Mendel’s Laws
• A dihybrid cross uses true-breeding plants differing in two traits
• Mendel tracked each trait through two generations.
• Started with true-breeding plants differing in two traits
• The F1 plants showed both dominant characteristics
• F1 plants self-pollinated
• Observed phenotypes among F2 plants
• Mendel formulated the Law of Independent Assortment
• The pair of factors for one trait segregate independently of the
factors for other traits
• All possible combinations of factors can occur in the gametes
• P generation is the parental generation in a breeding experiment.
• F1 generation is the first-generation offspring in a breeding
experiment.
• F2 generation is the second-generation offspring in a breeding
experiment
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Classical View of Homologous
Chromosomes
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sister chromatids
alleles at a
gene locus
a. Homologous
chromosomes
have alleles for
same genes at
specific loci.
G
g
R
r
S
s
t
T
G
Replication
b. Sister chromatids
of duplicated
chromosomes
have same alleles
for each gene.
R
G
g
g
R
r
r
S
S
s
s
t
t
T
T
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Figure 15.1 The chomosomal basis of Mendel’s laws
Mendel’s Laws
• Punnett Square
• Allows us to easily calculate probability, of genotypes
and phenotypes among the offspring
• Punnett square in next slide shows a 50% (or ½) chance
• The chance of E = ½
• The chance of e = ½
• An offspring will inherit:
• The chance of EE =½  ½=¼
• The chance of Ee =½  ½=¼
• The chance of eE =½  ½=¼
• The chance of ee =½  ½=¼
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Rules of Probability
• Probability scale ranges from 0 to 1
• The possibilities of all outcomes must add up to 1
• In every fertilization involving gametes, the ovum has a
½ chance of carrying a dominant allele and ½ chance of
carrying a recessive allele
• Two basic laws of probability can help are the rule of
multiplication and the rule of addition
Rule of Multiplication
• How can we determine the chance that two or more independent
events will occur together in some combination?
• Compute the probability for each independent event, then
multiply these individual probabilities to obtain the overall
probability of these events occurring together
Rule of Addition
• The probability of an event that can occur in two or more
different ways is the sum of the separate probabilities of those
ways
Mendel’s Laws
• Genetic disorders are medical conditions caused by alleles
inherited from parents
• Autosome - Any chromosome other than a sex chromosome
(X or Y)
• Genetic disorders caused by genes on autosomes are called
autosomal disorders
• Some genetic disorders are autosomal dominant
• An individual with AA has the disorder
• An individual with Aa has the disorder
• An individual with aa does NOT have the disorder
• Other genetic disorders are autosomal recessive
• An individual with AA does NOT have the disorder
• An individual with Aa does NOT have the disorder, but is a
carrier
• An individual with aa DOES have the disorder
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Autosomal Recessive Pedigree
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Generations
I
aa
II
III
IV
A?
Aa
A?
Aa
A?
*
Aa
Aa
aa
aa
A?
A?
A?
Key
aa = affected
Aa = carrier (unaffected)
AA = unaffected
A? = unaffected
(one allele unknown)
Autosomal recessive disorders
• Most affected children have unaffected
parents.
• Heterozygotes (Aa) have an unaffected phenotype.
• Two affected parents will always have affected children.
• Close relatives who reproduce are more likely to have
affected children.
• Both males and females are affected with equal frequency.
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Autosomal Dominant Pedigree
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Generations
Aa
Aa
I
*
II
III
Aa
aa
Aa
Aa
aa
A?
aa
aa
aa
aa
aa
aa
Key
AA = affected
Aa = affected
Autosomal dominant disorders
A? = affected
(one allele unknown)
• Affected children will usually have an
aa = unaffected
affected parent.
• Heterozygotes (Aa) are affected.
• Two affected parents can produce an unaffected child.
• Two unaffected parents will not have affected children.
• Both males and females are affected with equal frequency.
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Non-single gene genetics
• Incomplete dominance: appearance between the
phenotypes of the 2 parents. Ex: snapdragons
• Codominance: two alleles affect the phenotype
in separate, distinguishable ways. Ex: Tay-Sachs
disease
• Multiple alleles: more than 2 possible alleles for a gene.
Ex: human blood types
Figure 14.9 Incomplete dominance in snapdragon color
Blood Types
• Some traits are controlled by multiple alleles (multiple
allelic traits)
• The gene exists in several allelic forms (but each
individual only has two alleles)
• ABO blood types
• The alleles:
• IA = A antigen on red blood cells, anti-B antibody in
plasma
• IB = B antigen on red blood cells, anti-A antibody in
plasma
• i = Neither A nor B antigens on red blood cells, both
anti-A and anti-B antibodies in plasma
• The ABO blood type is also an example of codominance
• More than one allele is fully expressed
• Both IA and IB are expressed in the presence of the other
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ABO Blood Type
Phenotype
A
B
AB
O
Genotype
IAIA, IAi
B
B
B
I I ,I i
IAIB
ii
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Non-single gene genetics
• Pleiotropy: genes with multiple phenotypic
effect. Ex: sickle-cell anemia
• Epistasis: a gene at one locus (chromosomal
location) affects the phenotypic expression of a
gene at a second locus. Ex: mice coat color
• Polygenic Inheritance: an additive effect of
two or more genes on a single phenotypic
character Ex: human skin pigmentation and
height
Figure 14.15 Pleiotropic effects of the sickle-cell allele in a homozygote
Figure 14.11 An example of epistasis
Figure 14.12 A simplified model for polygenic inheritance of skin color
Extending the Range of Mendelian Genetics
•X-Linked Inheritance
•In mammals
• The X and Y chromosomes determine gender
• Females are XX
• Males are XY
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Extending the Range of Mendelian Genetics
• X-Linked Inheritance
• The term X-linked is used for genes that have nothing to
do with gender
• X-linked genes are carried on the X chromosome.
• The Y chromosome does not carry these genes
• Discovered in the early 1900s by a group at Columbia
University, headed by Thomas Hunt Morgan.
• Performed experiments with fruit flies
• They can be easily and inexpensively raised in
simple laboratory glassware
• Fruit flies have the same sex chromosome pattern as
humans
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X – Linked Inheritance
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P generation
XrY
Xr
P gametes
XRXR
XR
Y
F1 generation
XRY
XRXr
eggs
F1 gametes
XR
Xr
F2 generation
sperm
XR
XRXR
XRXr
XRY
XrY
Y
Offspring
Allele Key
R
X = red eyes
Xr = white eyes
Phenotypic Ratio
females:
all red-eyed
males: 1
red-eyed
white-eyed
1
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X – Linked Inheritance
Extending the Range of Mendelian Genetics
• Several X-linked recessive disorders occur in humans:
• Color blindness
• The allele for the blue-sensitive protein is autosomal
• The alleles for the red- and green-sensitive pigments are on the X
chromosome.
• Menkes syndrome
• Caused by a defective allele on the X chromosome
• Disrupts movement of the metal copper in and out of cells.
• Phenotypes include kinky hair, poor muscle tone, seizures, and low body
temperature
• Muscular dystrophy
• Wasting away of the muscle
• Caused by the absence of the muscle protein dystrophin
• Adrenoleukodystrophy
• X-linked recessive disorder
• Failure of a carrier protein to move either an enzyme or very long chain fatty
acid into peroxisomes.
• Hemophilia
• Absence or minimal presence of clotting factor VIII or clotting factor IX
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• Affected person’s blood either does not clot or clots very slowly.
X-Linked Recessive Pedigree
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XbY
XBXB
XBY
XBXb
daughter
grandfather
XBY
XbXb
XbY
XBY
XBXB
XBXb
XbY
grandson
XBXB
XBXb
XbXb
XbY
XbY
Key
= Unaffected female
= Carrier female
= Color-blind female
= Unaffected male
= Color-blind male
X-Linked Recessive
Disorders
• More males than females are affected.
• An affected son can have parents who have the
normal phenotype.
• For a female to have the characteristic, her father must
also have it. Her mother must have it or be a carrier.
• The characteristic often skips a generation from the
grandfather to the grandson.
• If a woman has the characteristic, all of her sons will
have it.
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Expressivity refers to variations in a
phenotype among individuals carrying a
particular genotype.
Penetrance is the proportion of
individuals carrying a particular
variant of a gene (allele or genotype)
that also express an associated trait
(phenotype).
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Chromosomal errors, I
• Nondisjunction: members of a pair of
homologous chromosomes do not separate
properly during meiosis I or sister chromatids fail
to separate during meiosis II
• Aneuploidy: chromosome number is abnormal
• Monosomy ~ missing chromosome
• Trisomy ~ extra chromosome (Down syndrome)
• Polyploidy~ extra sets of chromosomes
Figure 15.11 Meiotic nondisjunction
Chromosomal errors, II
• Alterations of chromosomal structure:
• Deletion: removal of a chromosomal segment
• Duplication: repeats a chromosomal segment
• Inversion: segment reversal in a chromosome
• Translocation: movement of a chromosomal
segment to another
Figure 15.13 Alterations of chromosome structure
Figure 15.14 Down syndrome
Figure 15.x2 Klinefelter syndrome
Figure 15.x3 XYY karyotype
Chromosomal Disorders
• Down syndrome
• XXY Klinefelter syndrome
• XYY
• XXX
• XO Turner syndrome
Extranuclear Genes
• Not all genes are located on the nuclear chromosomes. These
genes do not exhibit Mendelian genetics.
• Mitochondrial DNA which is given from the mother only, can in
some rare cases cause some disorders. If the Mitochondrial
DNA is defected this would reduce the amount of ATP made.