Bio 178 Lecture 26Genetics
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 DiseaseDegenerative 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.
• CauseAssociated with a triplet (CAG) repeat in the gene for huntingtin.
• GeneticsLethal 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
http://www.stanford.edu/group/hopes/causes/neuro/d3.html
Caudate Nucleus
Chromosomal Theory of InheritanceWalter Sutton, 1902
• Pre-SuttonChromosomes, 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 CommunityThere are more genes than chromosomes!
• The Final EvidenceThomas 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 ChromosomesPrinciple 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
NondisjunctionThe 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/442-4-chromosome_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/442-4-chromosome_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
Abnormality
F Meiosis I
F Meiosis II
M Meiosis I
M Meiosis II
Trisomies 75-95% 5-25% 0
45, X 20% 80%
3N ~25% 0-25%
4N
Nondisjunction
Chromosome AbnormalitySpontaneous Abortion (%)
All 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
Pedigree Example
The above pedigree is for a rare kidney disease. Deduce the inheritance (autosomal, sex-linked, dominant, recessive).