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Human Genetics
• Phenotype: observed physical and
functional traits
• Genotype: complete set of genes and alleles
• Alleles: Different versions of homologous
genes ex. B and b
Human genetics
• How are gametes made?
• How does chromosome behavior affect
inheritance of traits?
• Somatic cells are diploid.
• Gametes are haploid, with only one set of
chromosomes
a
SPERMATOGENESIS
b
OOGENESIS
spermatogonium
oogonium
primary
spermatocyte
primary
oocyte
meiosis l
secondary
spermatocyte
secondary
oocyte
meiosis ll
polar
body
spermatids
polar bodies
(will be degraded)
egg
1st law - segregation of alleles
• Cells contain 2 copies (alleles) of each gene
• Alleles separate during gamete formation
(meiosis)
• gametes carry only one copy of each gene
Punnett squares
show parental
gametes
and the genotypes
of next generation
•Homozygous: BB and bb
•Heterozygous: Bb
Possible genotypes
and
their probabilities
Figure 19.2
Law of Independent Assortment
• During gamete formation, genes for
different traits separate independently
into gametes
• Why? random alignment of homologues at
Meiosis I
Chromosome behavior accounts for Mendel’s
principles
Figure 9.17
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Tetrad
Crossing over
A
A
B
a
b
A
b
a
B
B
a
b
Gametes
• Genes on the same chromosome tend to be
inherited together = linked genes
• Crossing over produces gametes with
recombinant chromosomes
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VARIATIONS ON MENDEL’S PRINCIPLES
P GENERATION
White
rr
Red
RR
Incomplete dominance
Gametes
• an offspring’s
phenotype is
intermediate
between the
phenotypes of its
parents
R
r
Pink
Rr
F1 GENERATION
1/
1/
Eggs
1/
F2 GENERATION
2
2
2
R
1/
2
r
1/
R
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R
Red
RR
r
Pink
Rr
Sperm
1/
Pink
rR
White
rr
Figure 9.12A
2
2
r
• Incomplete dominance in human
hypercholesterolemia
GENOTYPES:
HH
Homozygous
for ability to make
LDL receptors
Hh
Heterozygous
hh
Homozygous
for inability to make
LDL receptors
PHENOTYPES:
LDL
LDL
receptor
Cell
Normal
Mild disease
Figure 9.12B
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Severe disease
Many genes have more than two alleles
in the population
Ex. three alleles for ABO blood type in humans
IA, IB, i
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Polygenic traits - A single trait may be influenced
by many genes
• Quantitative traits
Fraction of population
skin color, height, eye color
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Skin pigmentation
Genetic traits in humans can be tracked
through family pedigrees
• The inheritance of many
human traits follows
Mendel’s principles and
the rules of probability
Figure 9.8A
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• Family pedigrees are used to determine
patterns of inheritance and individual
genotypes
Dd
Joshua
Lambert
Dd
Abigail
Linnell
D_?
Abigail
Lambert
D_?
John
Eddy
dd
Jonathan
Lambert
Dd
Dd
dd
D_?
Hepzibah
Daggett
Dd
Elizabeth
Eddy
Dd
Dd
Dd
dd
Female Male
Deaf
Figure 9.8B
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Hearing
Inherited Genetic Disorders
• Most mutations
usually involve
recessive alleles
Normal
Dd
PARENTS
Normal
Dd
D
D
Eggs
• Phenylketonuria,
PKU
• Tay-Sachs disease
• Cystic fibrosis
Sperm
DD
Normal
d
OFFSPRING
d
Dd
Normal
(carrier)
Dd
Normal
(carrier)
dd
Deaf
Figure 9.9A
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• A few are caused by dominant alleles
– Examples: achondroplasia, Huntington’s disease
Figure 9.9B
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Sex-linked disorders affect mostly males
• Most sex-linked human
disorders are due to
recessive alleles
– Ex: hemophilia,
red-green color blindness
These traits appear mostly in males. Why?
Figure 9.23A
– If a male receives a single X-linked recessive
allele from his mother, he will have the disorder;
while a female has to receive the allele from both
parents to be affected
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Pedigree Chart: Inheritance Pattern for an X-linked
Recessive Disease
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Figure 19.12
• A high incidence of hemophilia has plagued the
royal families of Europe
Queen
Victoria
Albert
Alice
Louis
Alexandra
Czar
Nicholas II
of Russia
Alexis
Figure 9.23B
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Variations on Mendel’s Principles
• Codominance, multiple alleles
• Pleiotropy
• Polygenic traits
• Sex-linked genes
• Environmental effects
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Accidents during meiosis can alter
chromosome number
• Abnormal
chromosome count
is a result of
nondisjunction
– homologous
pairs fail to
separate
during
meiosis I
Nondisjunction
in meiosis I
Normal
meiosis II
Gametes
n+1
n+1
n–1
n–1
Number of chromosomes
Figure 8.21A
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– Or sister chromatids fail to separate during
meiosis II
Normal
meiosis I
Nondisjunction
in meiosis II
Gametes
n–1
n+1
n
Number of chromosomes
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n
Figure 8.21B
• An extra chromosome 21 causes Down syndrome
• The chance of having a Down syndrome child goes up
with maternal age
Figure 8.20C
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Fetal testing can spot many inherited disorders
early in pregnancy
• Karyotyping and biochemical tests of fetal cells
can help people make reproductive decisions
– Fetal cells can be obtained through
amniocentesis
Amniotic
fluid
Amniotic
fluid
withdrawn
Centrifugation
Fluid
Fetal
cells
Fetus
(14-20
weeks)
Biochemical
tests
Placenta
Figure 9.10A
Uterus
Cervix
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Several
weeks later
Cell culture
Karyotyping
• Chorionic villus sampling is another procedure
that obtains fetal cells for karyotyping
Fetus
(10-12
weeks)
Several hours
later
Placenta
Suction
Chorionic villi
Fetal cells
(from chorionic villi)
Karyotyping
Some
biochemical
tests
Figure 9.10B
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• Examination of the fetus with ultrasound is
another helpful technique
PGD - Preimplantation Genetic Diagnosis
genetic analysis of embryos from in vitro fertilization (IVF)
before inserting into womb
Figure 9.10C, D
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Genes and Behavior
• Mechanism
– Product from gene-specific proteins
– Proteins have specific functions leading to
phenotypes: hormones, enzymes, transport,
neurotransmitters
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings