Transcript Biol 178 Lecture 26

Bio 178 Lecture 26
Genetics
Reading
•
Chapters 13 & 14
Quiz Material
•
Questions on P 276-278 & 300
•
Chapters 13 & 14 Quizzes on Text Website
(www.mhhe.com/raven7)
Outline
• Genetics
 Human Genetics (cntd)
 Chromosomes and Genetics
• DNA
 Experiments - DNA is hereditary material
Human Genetic Disorders
• Hemophilia
• Sickle Cell Anemia
• Huntington’s Disease
Dominantly Inherited Disorders Huntington’s Disease
• The Disease
Degenerative disease of the nervous system - progressive
neural cell death caused by build up of the protein
huntingtin (mutated form).
No cure. Terminal 10-30 years post-diagnosis.
• Cause
Associated with a triplet (CAG) repeat in the gene for
huntingtin.
• Genetics
Lethal in heterozygotes.
50% chance of passing the disease to progeny.
Huntington’s Disease (Cntd.)
• Why does the allele persist?
Late onset of disease - symptoms at 35 to 40 years old.
http://www.thesahara.net/huntingtons
_disease.htm
http://www.genephile.com.tw/Tests
/huntington.htm
Basal Ganglia of HD Patient
Caudate Nucleus
http://www.stanford.edu/group/hopes/causes/neuro/d3.html
Chromosomal Theory of Inheritance
Walter Sutton, 1902
• Pre-Sutton
Chromosomes, centrioles, and mitochondria all divide and
segregate in meiosis.
Which of these structures are responsible for heredity
(follow Mendel’s rules)?
• Sutton’s Evidence (Sutton, 1877-1916)
1. Gametes
(a) Must have equal hereditary contributions - sperm
contain little cytoplasm, but do have nuclei. Hereditary
material probably in the nucleus.
(b) Somatic cells have 2 homologous chromosomes, but
gametes have only 1. Consistent with Mendel’s model.
Chromosomal Theory of Inheritance (Cntd.)
• Sutton’s evidence (Cntd.)
2. Mendel’s Laws
Chromosomes segregate and assort independently in
meiosis.
• Response of the Scientific Community
There are more genes than chromosomes!
• The Final Evidence
Thomas Hunt
Morgan, 1910 - eye
color in
Drosophila.
Read P 265-266.
Morgan’s Experiment - Sex Linkage
Morgan’s Experiment - Sex Linkage (Cntd)
• Crossed the F1 females with the male parent.
• What ratio would you predict?
• Obtained a 1:1:1:1
Segregation of white eyed trait had a 1:1 correspondence
with segregation of the X chromosome  Mendel’s
observation of segregation of alternate traits reflected
chromosome behavior.
Autosomes and Sex Chromosomes
• Autosomes
• Sex Chromosomes
Principle function - sex determination.
• Which sex determines progeny sex (human)?
• Features of the Y Chromosome (human)
Carries 78 active genes, including a sex-determining gene
(SRY).
• Sex Linked Genes
Any gene (does not have to be involved with sex) that is
carried on the sex chromosomes (usually on X).
Sex Determination in Different Organisms
Barr Body
1 X chromosome in each female somatic cell is inactivated
early in development  female cells produce the same
amount of protein from the X as do male cells.
http://www.carolguze.com/text/442-4-chromosome_abnormalities.shtml
Nondisjunction
The failure of homologues or sister chromatids to separate
in meiosis  aneuploidy.
• Nondisjunction of Autosomes
Effects are either fatal or severe.
Down Syndrome (Trisomy 21)
Nondisjunction (Cntd.)
• Nondisjunction of Sex Chromosomes
Generally, the effects are not as severe as nondisjunction of
autosomes.
1. X Chromosome
(a) Triple X Syndrome
Usually taller than average females, generally “normal”
and most are *fertile.
(b) Klinefelter Syndrome
XX + Y  XXY
Sterile male with female characteristics and sometimes
mildly impaired intelligence.
Klinefelter Syndrome
http://www.carolguze.com/text/442-4chromosome_abnormalities.shtml
Klinefelter calico cat - evidence that Y chromosome
determines maleness.
Nondisjunction of Sex Chromosomes (Cntd.)
(c) Turner Syndrome (Monosomy X)
X + O  XO
Short females with edema
(resulting in webbed neck) that
are almost always sterile.
http://www.carolguze.com/text/442-4chromosome_abnormalities.shtml
Nondisjunction of Sex Chromosomes (Cntd.)
(d) OY
O (egg) + Y  OY
Lethal - genes on the X chromosome are necessary for
survival.
2. Y Chromosome
X + YY (sperm)  XYY
Tall, fertile males.
Genetic Counseling
Read P 274.
Nondisjunction
Nondisjunction
Abnormal F Meiosis
ity
I
F Meiosis M Meiosis M Meiosis
II
I
II
Trisomies
75-95%
5-25%
45, X
20%
80%
3N
~25%
0-25%
4N
0
Nondisjunction
All
Spontaneous
Abortion (%)
50
Trisomy 16
7.5
Trisomy 13, 18, 21
4.5
XXX, XXY, XYY
0.3
All other trisomies
13.8
45, X
8.7
3N
6.4
4N
2.4
Structural Abnormalities
2.0
Chromosome Abnormality
Pedigree Example
The above pedigree is for a rare kidney disease. Deduce
the inheritance (autosomal, sex-linked, dominant,
recessive).