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Transcript
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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Severe disease
Many genes have more than two alleles
in the population
Ex. three alleles for ABO blood type in humans
IA, IB, i
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Polygenic traits - A single trait may be influenced
by many genes
• Quantitative traits
Fraction of population
skin color, height, eye color
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• A few are caused by dominant alleles
– Examples: achondroplasia, Huntington’s disease
Figure 9.9B
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Pedigree Chart: Inheritance Pattern for an X-linked
Recessive Disease
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Variations on Mendel’s Principles
• Codominance, multiple alleles
• Pleiotropy
• Polygenic traits
• Sex-linked genes
• Environmental effects
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
– 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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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