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Extra credit problem for
Lecture #4
An agouti mouse is crossed to a white mouse and all the F1
offspring are agouti.
An F1 female is crossed to an F1 male, and the offspring
are:
11 agouti: 5 white: 4 black
Q: Test the hypothesis that the original parental
genotypes were BBCC and bbcc. Give the c2 value,
the df, the P value, and state whether or not you reject
the hypothesis.
Cytogenetics: Chromosome
Mutations, Aberrations &
Evolution
Chromosomes
Prokaryote
4.2 X 106 base pairs
DNA essentially naked
mRNA translated as
it's transcribed
mRNA is often polycistronic
Eukaryote
Has 1000x more DNA
DNA complexed w/ RNA & protein
mRNA is transcribed in nucleus,
translated in the cytoplasm
mRNA is almost never
polycistronic
Eukaryotic chromosomes
Metacentric
Submetacentric
Acrocentric
Human karyotype
Sister chromatids
Homologous pair
autosomes
Sex chromosomes
Why do we care?
Many diseases and birth defects are a
direct result of missing, broken, or extra
chromosomes.
• Down Syndrome
• Cri du chat Syndrome
• Patau Syndrome
Mutations at the level of the
homologous pair
• EUPLOIDY: "true" ploidy, meaning two
members of each homologous pair.
• ANEUPLOIDY: "not true" ploidy, meaning
more or fewer members than two of each
homologous pair.
• MONOSOMY - one homolog; partner is
missing
• TRISOMY - three homologs
• NULLISOMY- one entire homologous pair is
missing.
Monosomy and Trisomy
Down Syndrome
How does it happen? Nondisjunction
Each chrom.
has two
chromatids
Trisomy: Patau Syndrome
• 1/20,000 births
• severe mental
retardation
• heart and organ
defects
• polydactyly
• death by the
age of one year
Chromosomal Abnormalities Occurring in Human Fetuses
% spontaneously aborted
% fetuses with the
fetuses with the
abnormality that
Type of Abnormality
abnormality
survive to term
All abnormalities
50
5
Autosomal Trisomies
16
7.5
0
13, 18, & 21
4.5
15
All others
13.8
0
Trisomies of Sex c hromosomes
XXX, XXY, XYY
0.3
75
Monosomy for X (XO)
8.7
1
Structural A bnormalities
20
45
Structural Changes
•
•
•
•
Deletions (deficiencies)
Duplications
Inversions
Translocations
Deletions (deficiencies)
Homozygotes
lethal
w
Psuedodominance
How can chromosomes break?
Ionizing radiation (production of free radicals,
which act like little atomic "cannon balls",
blasting through strands of DNA or c'somes.
Chemical insult.
Why do they rejoin?
Break points of chromosomes are highly reactive
("sticky"), whereas normal ends of c'somes are
capped by telomeres, which do not readily bond
to other molecules.
S Phase
Dyad
• Breaks that occur before
S phase will affect both
newly formed chromatids,
& all daughter cells
arising from them.
• Breaks that occur when
the chromosome is in
dyad form may affect only
one chromatid.
(Thereafter, only the
progeny carrying the
broken chromatid will be
affected.)
Cri-du-chat Syndrome
1/50,000 births
Deletion short arm chrom 5
Mental retardation
Slow motor skill
development
Low birth weight and slow
growth
Small head (microcephaly)
Partial webbing of fingers or
toes
Wide-set eyes
(hypertelorism)
High-pitched cry
Structural Changes
•
•
•
•
Deletions (deficiencies)
Duplications
Inversions
Translocations
Duplications
Duplication is a source of
new genes over evolutionary time:
e.g., gene families like
globins and MHC genes
Bar eye: caused by duplication
Duplications: source of
evolutionary novelty?
• Ribosomal DNA
• Globins (alpha and
beta)
• Homeobox genes
Structural Changes
•
•
•
•
Deletions (deficiencies)
Duplications
Inversions
Translocations
Inversions