human genetics phenotype: observed physical and functional traits genotype: complete set of genes...
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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
egg
polarbody
spermatogonium
primaryspermatocyte
secondaryspermatocyte
oogonium
primaryoocyte
secondaryoocyte
polar bodies(will be degraded)
spermatids
meiosis ll
meiosis l
SPERMATOGENESIS OOGENESISa b
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
Figure 19.2
Possible genotypesand their probabilities
Punnett squares show parental gametesand the genotypes of next generation
•Homozygous: BB and bb•Heterozygous: Bb
Law of Independent Assortment
• During gamete formation, genes for different traits separate independently into gametes
• Why? random alignment of homologues at Meiosis I
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Figure 9.17
Chromosome behavior accounts for Mendel’s principles
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A B
ab
Tetrad
Crossing overA B
a
ba
BA 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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• an offspring’s phenotype is intermediate between the phenotypes of its parents
Incomplete dominance
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
VARIATIONS ON MENDEL’S PRINCIPLES
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• Incomplete dominance in human hypercholesterolemia
Figure 9.12B
GENOTYPES:
HHHomozygous
for ability to makeLDL receptors
HhHeterozygous
hhHomozygous
for inability to makeLDL receptors
PHENOTYPES:
LDL
LDLreceptor
Cell
Normal Mild disease Severe disease
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Ex. three alleles for ABO blood type in humans
IA, IB, i
Many genes have more than two alleles in the population
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• Quantitative traits
skin color, height, eye color
Polygenic traits - A single trait may be influenced by many genes
Fra
cti
on
of
po
pu
lati
on
Skin pigmentation
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• The inheritance of many human traits follows Mendel’s principles and the rules of probability
Genetic traits in humans can be tracked through family pedigrees
Figure 9.8A
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• 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
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Inherited Genetic Disorders
• Most mutations usually involve recessive alleles
•Phenylketonuria, PKU
•Tay-Sachs disease
•Cystic fibrosis
Figure 9.9A
D D
d d
NormalDd
NormalDd
DDNormal
DdNormal(carrier)
DdNormal(carrier)
ddDeaf
Eggs Sperm
PARENTS
OFFSPRING
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• A few are caused by dominant alleles
Figure 9.9B
– Examples: achondroplasia, Huntington’s disease
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• Most sex-linked human disorders are due to recessive alleles
– Ex: hemophilia, red-green color blindness
These traits appear mostly in males. Why?
– 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
Sex-linked disorders affect mostly males
Figure 9.23A
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Pedigree Chart: Inheritance Pattern for an X-linked Recessive Disease
Figure 19.12
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• A high incidence of hemophilia has plagued the royal families of Europe
Figure 9.23B
QueenVictoria
Albert
Alice Louis
Alexandra CzarNicholas IIof Russia
Alexis
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Variations on Mendel’s Principles
• Codominance, multiple alleles
• Pleiotropy
• Polygenic traits
• Sex-linked genes
• Environmental effects
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• Abnormal chromosome count is a result of nondisjunction
– homologous pairs fail to separate during meiosis I
Accidents during meiosis can alter chromosome number
Figure 8.21A
Nondisjunctionin meiosis I
Normalmeiosis II
Gametes
n + 1 n + 1 n – 1 n – 1
Number of chromosomes
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– Or sister chromatids fail to separate during meiosis II
Figure 8.21B
Normalmeiosis I
Nondisjunctionin meiosis II
Gametes
n + 1 n – 1 n n
Number of chromosomes
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• 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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• Karyotyping and biochemical tests of fetal cells can help people make reproductive decisions
– Fetal cells can be obtained through amniocentesis
Fetal testing can spot many inherited disorders early in pregnancy
Figure 9.10A
Amnioticfluid
Fetus(14-20weeks)
Placenta
Amnioticfluidwithdrawn
Centrifugation
Fetalcells
Fluid
Uterus Cervix Cell culture
Severalweeks later
Karyotyping
Biochemicaltests
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• 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
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• Examination of the fetus with ultrasound is another helpful technique
Figure 9.10C, D
PGD - Preimplantation Genetic Diagnosisgenetic analysis of embryos from in
vitro fertilization (IVF)before inserting into womb
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Genes and Behavior
• Mechanism
– Product from gene-specific proteins
– Proteins have specific functions leading to phenotypes: hormones, enzymes, transport, neurotransmitters