Chromosomal Inheritance II
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Transcript of Chromosomal Inheritance II
Chromosomal Inheritance II
Outline
• Incomplete Dominance, Codominance, and Multiple Allelism
• Interaction of genes
• Pedigree Studies
• Genetics and Ethics
Extending Mendel’s Rules• Incomplete dominance
– heterozygotes have an intermediate phenotype
• Codominance– Heterozygotes displays the phenotype of both alleles
• multiple allelism– Multiple distinct genes versions (i.e., alleles) are present
in the population
• polymorphism – Multiple distinct phenotypes are present in a population
Multiple Alleles and Polymorphism
• ABO blood group in humans are determined by three alleles : IA, IB, and i.
IA
IB
i
A
B
none(a) The three alleles for the ABO blood groups and their associated carbohydrates
Allele Carbohydrate
GenotypeRed blood cellappearance
Phenotype(blood group)
IAIA or IA i A
BIBIB or IB i
IAIB AB
ii O
(b) Blood group genotypes and phenotypes
Pleiotropy• A gene that influences many traits rather than just one is
pleiotropic.– Marfan Syndrome (FBN1): defective fibrillinlimbs, spinal chord, heart– Cystic fibrosis (CFTR): defective salt transportlungs, pancreas,
sebacious glands, etc.
Lung(s)pancreas
healthy
CF
Antagonistic pleiotropy
• Some effects are good; some are bad
• Sickle cell anemia (hemoglobin B)– Codominant trait– HBB/HBB; HBB/hbb; hbb/hbb
Healthy Unhealthy ???
HBB/hbb
Mild sickle cell disease
Malaria protection
Fig. 14-UN2
Degree of dominance
Complete dominanceof one allele
Incomplete dominanceof either allele
Codominance
Description
Heterozygous phenotypesame as that of homo-zygous dominant
Heterozygous phenotypeintermediate betweenthe two homozygousphenotypes
Heterozygotes: Bothphenotypes expressed
Multiple alleles
Pleiotropy
In the whole population,some genes have morethan two alleles
One gene is able toaffect multiplephenotypic characters
CRCR CRCW CWCW
IAIB
IA , IB , i
ABO blood group alleles
Sickle-cell disease
PP Pp
Example
Fig. 14-12
BbCc BbCc
Sperm
EggsBC bC Bc bc
BC
bC
Bc
bc
BBCC
1/41/4
1/41/4
1/4
1/4
1/4
1/4
BbCC BBCc BbCc
BbCC bbCC BbCc bbCc
BBCc BbCc
BbCc bbCc
BBcc Bbcc
Bbcc bbcc
9 : 3 : 4
• A gene at one locus
alters the phenotypic expression of a gene at a second locus
• Coat color in mice– pigment color (B for
black; b for brown)– Pigment deposit (C
for color; c for no color)
Epistasis
Discrete vs. Quantitative Traits• Discrete traits.
– seed color in peas—no intermediate phenotypes• Quantitative traits
– Traits that fall into a continuum• Frequencies
– form a bell-shaped curve (normal distribution) for a population.
A phenotype distribution that forms a bell-shaped curve. Normal distribution—bell-shaped curve
Quantitative Traits Result from the Action of Many Genes
Wheat kernel color is a quantitative trait. Hypothesis to explain inheritance of kernel color
Parentalgeneration
F1generation
F2generation
1 1
6 6
151520
aa bb cc(pure-line white)
AA BB CC(pure-line red)
Aa Bb Cc(medium red)
Self-fertilization
Eggs
Sperm
Phenotypes:Number ofdark-skin alleles: 0 1 2 3 4 5 6
1/646/64
15/6420/64
15/646/64
1/64
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/81/8
1/81/8
1/81/8
1/81/8
AaBbCc AaBbCc
Polygenic Inheritance• Traits that vary in the
population along a continuum
• Additive effect of 2+ genes on a single phenotype
• Skin color in humans is an example of polygenic inheritance
DescriptionRelationship amonggenes
Epistasis One gene affectsthe expression ofanother
Example
Polygenicinheritance
A single phenotypiccharacter isaffected bytwo or more genes
BbCc BbCc
BCBC
bC
bC
Bc
Bc
bc
bc
9 : 3 : 4
AaBbCc AaBbCc
Applying Mendel’s Rules to Humans
• Humans terrible genetic models– Generation time is too long– Parents produce relatively few offspring– Breeding experiments are frowned upon
• Human disorders follow 5 patterns1) Autosomal dominant 2) Autosomal recessive3) X-linked recessive 4) X-linked dominant5) Y-linked
• Pedigrees (family trees) – analyze the human crosses that already exist.
KeyMale
Female
AffectedmaleAffectedfemale
Mating
Offspring, inbirth order(first-born on left)
Human Pedigree Reports
Fig. 14-15b
1st generation(grandparents)
2nd generation(parents, aunts,and uncles)
3rd generation(two sisters)
Widow’s peak No widow’s peak
Is a widow’s peak a dominant or recessive trait?
Ww ww
Ww Wwww ww
ww
wwWw
Ww
wwWW
Wwor
Autosomal Recessive Traits• If a phenotype is due to an autosomal recessive allele
– trait = homozygous – parents (w/o trait) = heterozygous carriers.
• Carriers carry the allele and transmit it even though they do not exhibit the phenotype.
Carrier male Carrier female
Affectedmale
Affectedfemale
I
II
III
IV
Each
row
repr
esen
ts a
gen
erati
on
Carriers (heterozygotes) are indicated with half-filled symbols
Autosomal or Sex-Linked trait?• Equally often in males and females
– likely to be autosomal. • Males more likely to have the trait
– usually X-linked.• Hemophilia is an example of an X-linked trait resulting from a
recessive allele.Queen Victoria Prince Albert
Female carrier of hemophilia allele
I
II
III
IV
Affected male
Frequency of Dominant Alleles• Not necessarily more common (NOT always “WT”)
• one baby out of 400 in the United States is born with extra fingers or toes• Dominant allele; uncommon occurrence
• In this example, the recessive allele is far more prevalent than the population’s dominant allele
What are the Societal Implications of this Knowledge?
Fetal Testing• Tests to determine in utero if a child has a
disorder.• 14th to 16th week of pregnancy• Blood or amniocentesis
• Fetal tests can reveal a serious disorder• Trisomy 21, 18, etc.
• Some testing after birth• eg Type I diabetes
http://www.youtube.com/watch?v=qA25_fiyh_E&feature=related
• Science of “improving the genetic stock” of humans• Old Testament • Plato’s Republic (description of the
ideal society )
• Francis Galton• “National Eugenics Laboratory”• Experimental studies of heredity
• Twins
• Karl Pearson• The higher birth rate of the poor • Supplant by "higher" races
Eugenics
US Propaganda and Policy• The Immigration Act of 1924
– quota for different nationalities – perceived tendencies towards crime etc.
• Forced Sterilization
• Nazi Germany• The Aryan Nation and the Holocaust
• Human races • Ill-founded concept• Populations with overlapping gene pools. • No major difference in the genome sequence
Eugenics and the Third Reich