Transcript Ch. 7: Extending Mendelian Genetics

Ch. 7: Extending Mendelian Genetics
7.1 KEY CONCEPT
The chromosomes on which genes are located can
affect the expression of traits.
Ch. 7: Extending Mendelian Genetics
Two copies of each autosomal gene affect phenotype.
• Mendel studied autosomal
gene traits, like hair texture.
Ch. 7: Extending Mendelian Genetics
Rules of Inheritance
• Some traits follow the simple rules of Mendelian
inheritance of dominant and recessive genes.
• Complex traits follow different patterns of
inheritance that may involve multiples genes and
other factors. For example,
–
–
–
–
Incomplete or blended dominance
Codominance
Multiple alleles
Regulatory genes
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Ch. 7: Extending Mendelian Genetics
Incomplete Dominance
• Incomplete dominance results in a phenotype
that is a blend of a heterozygous allele pair.
Ex., Red flower + Blue flower => Purple
flower
If the dragons in Harry Potter have fire-power
alleles F (strong fire) and F’ (no fire) that follow
incomplete dominance, what are the phenotypes
for the following dragon-fire genotypes?
Genotypes
Phenotypes
FF
FF’
F’F’
strong fire
moderate fire (blended trait)
no fire
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Ch. 7: Extending Mendelian Genetics
Codominance
• Codominance results in a phenotype that shows
both traits of an allele pair.
Ex., Red flower + White flower => Red &
White
spotted
flower
• If merpeople have tail color alleles B (blue) and
G (green) that follow the codominance
inheritance rule, what are possible genotypes
and phenotypes?
Genotypes
Phenotypes
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Ch. 7: Extending Mendelian Genetics
• Codominance results in a phenotype that shows
both traits of an allele pair.
Ex., Red flower + White flower => Red &
White
spotted flower (both
traits)
• If merpeople have tail color alleles B (blue) and
G (green) that follow the codominance
inheritance rule, what are possible genotypes
and phenotypes?
Genotypes
Phenotypes
BB
blue tail
GG
green tail
BG
blue & green tail (both
traits)
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Ch. 7: Extending Mendelian Genetics
• Codominant alleles will both be completely expressed.
Ch. 7: Extending Mendelian Genetics
• In incomplete dominance, neither allele is completely
dominant nor completely recessive.
Ch. 7: Extending Mendelian Genetics
• Mendel’s rules of inheritance apply to autosomal
genetic disorders.
– Recessive disorder requires a homozygous recessive
genotype to affect a person. A heterozygote for a
recessive disorder is a carrier.
– Disorders caused by dominant alleles are uncommon.
(dominant)
Ch. 7: Extending Mendelian Genetics
• Mendel’s rule only apply for autosomal genes(chromosome
pairs 1-22). Genes on sex chromosomes are called sexlinked genes.
– Y chromosome genes in mammals are responsible for
male characteristics. Males have an XY genotype.
– X chromosome genes in mammals affect many traits.
Females have an XX genotype.
Ch. 7: Extending Mendelian Genetics
– Because males only
have one copy of
the X chromosome
all of a male’s sexlinked genes are
expressed.
– In females,
expression of sexlinked genes is
similar to autosomal
genes because they
have two copies of
the X chromosome.
Ch. 7: Extending Mendelian Genetics
– Common examples of sex-linked disorders:
• Duchenne’s muscular dystrophy
• Hemophilia
• Color Blindness
Ch. 7: Extending Mendelian Genetics
Ch. 7: Extending Mendelian Genetics
Normal Color Vision
Red-Green Color Blind
Left
Right
Left
Right
Top
25
29
Top
25
Spots
Middle
45
56
Middle
Spots
56
Bottom
6
8
Bottom
Spots
Spots
The test to the left is simpler.
The individual with normal color vision
will see a 5 revealed in the dot pattern.
An individual with Red/Green (the most
common) color blindness will see a 2
revealed in the dots.
Ch. 7: Extending Mendelian Genetics
– X chromosome inactivation randomly “turns off” one X
chromosome, named inactive chromosome a Barr body.
– Stays inactive throughout cell’s lifetime.
– Only seen in cats.
Ch. 7: Extending Mendelian Genetics
Multiple alleles
• Multiple alleles have more than 2 variations.
Ex., human blood type has 3 different allele
variants, A, B, and O.
Genotypes
Phenotypes
AA, AO
A blood type
AB
AB blood type
BB, BO
B blood type
OO
O blood type
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Ch. 7: Extending Mendelian Genetics
• The ABO blood types result from codominant alleles and
multiple alleles.
– Codominant
alleles are
neither
dominant nor
recessive.
– Codominant
alleles will both
be completely
expressed.
• Many genes have more than two alleles.
Ch. 7: Extending Mendelian Genetics
• There are 3 alleles that determine blood type: IA, IB, and i
– IA and IB are dominant to i.
– IA and IB are codominant.
Ch. 7: Extending Mendelian Genetics
Codominance
Ch. 7: Extending Mendelian Genetics
– There are 4 blood types:
A:
• Genotype: IAIA or IAi
• Antigen: A
• Antibody: Anti-B
•
Can receive from: A, O
•
Can donate to: A, AB
Ch. 7: Extending Mendelian Genetics
B:
• Genotype: IBIB or IBi
• Antigen: B
• Antibody: Anti-A
•
Can receive from: B, O
•
Can donate to: B, AB
Ch. 7: Extending Mendelian Genetics
AB :
• Genotype: IAIB
• Antigen: A & B
•
Can receive from: A, B, AB, & O
(universal recipient)
• Antibody: none
•
Can donate to: AB
Ch. 7: Extending Mendelian Genetics
O:
• Genotype: ii
• Antigen: none
• Antibody: Anti-A and Anti-B
•
Can receive from: O
•
Can donate to: A, B, AB, & O
(universal donor)
Ch. 7: Extending Mendelian Genetics
• Rh Factor: Another surface marker which may or may
not be present on the RBC.
Ch. 7: Extending Mendelian Genetics
Rh+ :
• Genotype: Rh+Rh+ or Rh+Rh
• - Can receive from: Rh+ & Rh• Antigen: Rh
• Can donate to: Rh+
• Antibody: none
Ch. 7: Extending Mendelian Genetics
Rh- :
• Genotype: Rh-Rh• Antigen: none
• Antibody: Rh+
•
Can receive from: Rh-
•
Can donate to: Rh- & Rh+
Ch. 7: Extending Mendelian Genetics
• Another example of multiple alleles.
Ch. 7: Extending Mendelian Genetics
• Polygenic traits are
produced by two or
more genes.
Order of dominance:
brown > green > blue.
Ch. 7: Extending Mendelian Genetics
Skin color result
of four genes
that interact to
produce range of
colors
Ch. 7: Extending Mendelian Genetics
Regulatory Genes
• Regulatory genes regulate the expression of other
genes.
• For example, a regulatory gene may ‘silence’ another
gene from expressing its dominant trait. The Manx cat
has no tail because it has a regulatory gene that silences
the gene that expresses the tail. This tail silencing gene
is dominant and has possible alleles:
S = silences tail gene = no tail (Manx cat)
s = doesn’t silence tail gene = has tail (non-Manx cat)
Question: Can 2 Manx cats without tails have a kitten with
a tail? Show your answer using Punnett Square.
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Ch. 7: Extending Mendelian Genetics
Regulatory Genes: Manx Cat
Question: Can 2 Manx cats without tails have a
kitten with a tail? Show your answer using Punnett
Square.
The possible alleles for the tail-silencing gene are:
S = no tail (dominant)
S
s
s = has tail (recessive)
S
SS
Ss
Only if both parent cats have the
s
heterozygous genotype-- Ss,
there is a 25% chance for
their having a kitten with a tail.
Ss
ss
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Ch. 7: Extending Mendelian Genetics
• An epistatic gene can interfere with other genes.
• There are no new phenotypes produced by this type of
gene interaction.
Ch. 7: Extending Mendelian Genetics
Epistatic: recessive trait blocks another allele
Black is dominant to chocolate B or b
Yellow is recessive epistatic (E or e)
Phenotype
Possible
Genotypes
BBEE
BbEE
BBEe
BbEe
bbEE
bbEe
BBee
Bbee
bbee
Ch. 7: Extending Mendelian Genetics
Phenotype can be a combination of genotype and
environment.
Malnourished
Fed and cared for.
Identical twins separated at
birth and treated differently.
Sex of sea turtles depends on genes
and environment. Temperature
when eggs develop determine sex,
warmer, more females, cooler,
more males.
Ch. 7: Extending Mendelian Genetics
7.4 KEY CONCEPT
A combination of methods is used to study human
genetics.
Ch. 7: Extending Mendelian Genetics
A pedigree is a chart for tracing genes in a family.
• Phenotypes are used to infer genotypes on a pedigree.
• Autosomal genes show different patterns on a pedigree
than sex-linked genes.
Ch. 7: Extending Mendelian Genetics
• If the phenotype is more common in males, the gene is
likely sex-linked.
Ch. 7: Extending Mendelian Genetics
7.3 KEY CONCEPT
Genes can be mapped to specific locations on
chromosomes.
Ch. 7: Extending Mendelian Genetics
• Because of their location on a chromosome, some genes
travel together and are considered to be linked.
Wild type
Mutant
Ch. 7: Extending Mendelian Genetics
Linkage maps estimate distances between genes.
• The closer together two genes are, the more likely
they will be inherited together.
• Linkage maps show the relative locations of genes.
Ch. 7: Extending Mendelian Genetics