Course Overview
Chromosome theory of
inheritance
http://www.erin.utoronto.ca/~w3bio/bio207/index.htm
January 23
Outline
Week
1
2
3
4
5
6
7
8
9
10
11
12
Topic
Course objectives and Introduction to genetics
Human Pedigrees
Patterns of Inheritance: sex-linkage
Chromosomal basis of inheritance
Changes in chromosome number
Gene Mapping
Gene to Phenotype
Modified Mendelian ratios
Model organisms and mutants
Genetics of Plant Development (Arabidopsis)
Genetics of Animal Development (Drosophila)
Behaviour Genetics/Quantitative genetics
Chapter
Ch. 1 & Ch. 2
Ch. 2
Ch. 2
Ch. 3
Ch. 15
Ch. 4 (Ch. 16)
Ch. 6
Ch. 6
Ch. 6 (Ch. 16)
Ch. 18
Ch. 18
Ch. 16 + papers
1:1 ratio?
1:1 ratio?
• What is the probability of having a boy?
• P (boys)= ½
• One report stated that for every 1000 boys born only 927
girls were born.
• Is this an equal sex ratio?
• What is the probability of having a boy?
• P (boys)= ½
• One report stated that for every 1000 boys born only 927
girls were born.
• Is this an equal sex ratio?
– H0: based on Mendel’s first law of equal segregation the
observed progeny fits a 1:1 ratio of girls : boys.
The distribution of boys and girls
Sex O
E
(O-E)2
Girls 927
963.5 (-0.36)2
Boys 1000 963.5 (0.36)2
(O-E)2/E
1.38
1.38
– H0: based on Mendel’s first law of equal segregation the
observed progeny fits a 1:1 ratio of girls : boys.
The distribution of boys and girls
Sex O
E
(O-E)2
Girls 927
963.5 (-0.36)2
Boys 1000 963.5 (0.36)2
(O-E)2/E
1.38
1.38
1
Other 1:1 ratios
Mendel’s laws
• H0: based on Mendel’s first law of equal segregation the observed
progeny fits a 1:1 ratio of girls : boys.
• Chi-square = 2.76, df= 1, p>0.05, rejection level is p=0.05
• Therefore we conclude that for Χ2 =2.76, df=1 we would expect a
deviation from the 1:1 ratio at least this large would occur by chance
alone more than 5% of the time so we fail to reject the null
hypothesis that the observed ratio of progeny fits a 1:1 ratio of girls to
boys.
1. Law of Segregation: alternative trait “factors”
that come together in the offspring separate
again when the offspring produce gametes
2. The law of dominance: hybrids between two
alternate forms of a trait resemble one of the
parental types
3. The law of independent assortment: differences
for one trait are inherited independently of
differences for another trait
Mendel’s laws
Mendel’s laws
1. Law of Segregation: alternative trait “factors”
that come together in the offspring separate
again when the offspring produce gametes
2. The law of dominance: hybrids between two
alternate forms of a trait resemble one of the
parental types
3. The law of independent assortment: differences
for one trait are inherited independently of
differences for another trait
1. Law of Segregation: alternative trait “factors”
that come together in the offspring separate
again when the offspring produce gametes
2. The law of dominance: hybrids between two
alternate forms of a trait resemble one of the
parental types
3. The law of independent assortment: differences
for one trait are inherited independently of
differences for another trait
2
Independent segregation of
gametes
Chromosomes and gametes
• Halving of gene pairs in gametes
• The members of the gene pairs segregate
equally into the gametes
• The union of one gamete from each parent
to form the zygote is random
Text 74
Probability and pedigrees e.g.:
albinism
• Explain with diagrams
how III-5 and III-6
could have an albino
son.
• III-5 and III-6 are both
heterozygous i.e. Aa.
• What is the genotype
of the gametes
produced by an Aa
individual?
Probability and pedigrees e.g.:
albinism
• Explain with diagrams
how III-5 and III-6
could have an albino
son.
• III-5 and III-6 are both
heterozygous i.e. Aa.
• What is the genotype
of the gametes
produced by an Aa
individual?
3
Chromosome theory of heredity
• William Sutton and Theodor Boveri proposed the
chromosome theory of heredity: the idea that
genes are parts of chromosomes.
• Some evidence that supported the theory:
– Nuclei of egg and sperm are equal in size and contain
coloured bodies that are easily visible during cell
division
– The number of chromosomes is consistent from cell
to cell within an organism but varies between
organisms
– The behaviour of the chromosomes at meiosis
correlates with the behaviour of Mendel’s hypothetical
particles
Text p. 75
Chromosome theory of heredity
Chromosome theory of heredity
• William Sutton and Theodor Boveri proposed the
chromosome theory of heredity: the idea that
genes are parts of chromosomes.
• Some evidence that supported the theory:
• William Sutton and Theodor Boveri proposed the
chromosome theory of heredity: the idea that
genes are parts of chromosomes.
• Some evidence that supported the theory:
– Nuclei of egg and sperm are equal in size and contain
coloured bodies that are easily visible during cell
division
– The number of chromosomes is consistent from cell
to cell within an organism but varies between
organisms
– The behaviour of the chromosomes at meiosis
correlates with the behaviour of Mendel’s hypothetical
particles
– Nuclei of egg and sperm are equal in size and contain
coloured bodies that are easily visible during cell
division
– The number of chromosomes is consistent from cell
to cell within an organism but varies between
organisms
– The behaviour of the chromosomes at meiosis
correlates with the behaviour of Mendel’s hypothetical
particles
Text p. 75
Text p. 75
4
Evidence from chromosome
segregation
• Calvin Bridges’ hypothesis
Evidence from chromosome
segregation
• Calvin Bridges’ hypothesis
– If genes are located on chromosomes then
unusual, unexpected genetic results should
correspond to unusual, abnormal
chromosome arrangements
– If genes are located on chromosomes then
unusual, unexpected genetic results should
correspond to unusual, abnormal
chromosome arrangements
• Working with Drosophila melanogaster
Bridges noticed that among the progeny of
the cross w/w;+;+;+ X +/Y;+;+;+ about 1 of
2000 was a white-eyed ♀ or a red-eyed ♂!
• Working with Drosophila melanogaster
Bridges noticed that among the progeny of
the cross w/w;+;+;+ X +/Y;+;+;+ about 1 of
2000 was a white-eyed ♀ or a red-eyed ♂!
Text p. 76
Text p. 76
Drosophila
Drosophila
• Drosophila
melanogaster has 4
chromosomes: 1 pair
of sex chromosomes
and 3 pairs of
autosomes
• semi-colons are used
to indicate when
genes are on different
chromosomes
• Drosophila
melanogaster has 4
chromosomes: 1 pair
of sex chromosomes
and 3 pairs of
autosomes
• semi-colons are used
to indicate when
genes are on different
chromosomes
– Eg: w;+;+;+
1;2;3;4
– Eg: w;+;+;+
4
3
2
1
1;2;3;4
4
3
2
1
5
Morgan’s crosses
Calvin Bridges’ cross
w / w X +/Y
+/+ X w/Y
w / w X +/Y
F1: ½ red : ½ white
F1: red
F1: ½ red : ½ white
mostly
F1X F1 gave red : white, 3:1
But the sex ratio for each eye colour was different
F2: ¾ red : ¼ white
F2: ½ red : ½ white
F2: females but males 1:1 red:white
F2: females AND males 1:1 red: white
Only males flies were white-eyed
Text p. 78
Calvin Bridges’ cross
w / w X +/Y
F1: ½ red : ½ white
mostly
Most red-eyed flies were female and white-eyed flies were male
But occasionally there was a white-eyed female and a red-eyed male.
Primary exceptional progeny occurred about 1 in every 2000 F1
Expected ratio of progeny
• What is the expected ratio of progeny from the
cross:
w/w X +/Y
White-eyed females X red-eyed males ?
X+
Y
Xw Xw
Xw X+ (red ♀)
Xw Y (white ♂)
Xw Xw
Xw X+ (red ♀)
Xw Y (white ♂)
Text p. 78
6
Expected ratio of progeny
• What is the expected ratio of progeny from the
cross:
w/w X +/Y
White-eyed females X red-eyed males ?
X+
Y
Xw Xw
Xw X+ (red ♀)
Xw Y (white ♂)
Xw Xw
Xw X+ (red ♀)
Xw Y (white ♂)
Primary exceptional progeny
• How could Calvin Bridges explain the
primary exceptional progeny: the whiteeyed female and red-eyed male?
Text p. 79
Primary exceptional progeny
• How could Calvin Bridges explain the
primary exceptional progeny: the whiteeyed female and red-eyed male?
– A rare non-disjunction event in the first or
second meiotic division could result in nuclei
that contained either two X chromosomes or
no X chromosome
Text p. 79
Text p. 491
7
Sex determination
• In Drosophila the number of X chromosomes
determines sex:
– 1X:2A is male
– 2X:2A is female
– 2X:3A is intersex
Text p. 79
Drosophila 2n=8
Text p. 87
Drosophila chromosomes
Text p. 87
8
Polytene chromosomes
Chromosome: a single DNA
molecule
Text p. 87
Chromatin
Chromatin
• Chromatin: material of chromosomes composed
of a mixture of DNA and protein
• Chromosomes exposed to Feulgen stain, a
chemical that reacts with DNA, pick up the stain
in particular patterns:
• Chromatin: material of chromosomes composed
of a mixture of DNA and protein
• Chromosomes exposed to Feulgen stain, a
chemical that reacts with DNA, pick up the stain
in particular patterns:
– Euchromatin: stains less intensely and contains most
of the active genes
– Heterochromatin: stains intensely and contains most
of the satellite DNA (multiple tandem repeats of short
nucleotide sequences).
– Euchromatin: stains less intensely and contains most
of the active genes
– Heterochromatin: stains intensely and contains most
of the satellite DNA (multiple tandem repeats of short
nucleotide sequences).
9
Chromosome staining
Chromosome staining
• Giemsa reagent stains DNA and produces
characteristic G-light or G-dark patterns for
each chromosome
• Giemsa reagent stains DNA and produces
characteristic G-light or G-dark patterns for
each chromosome
– G-dark regions have a higher DNA density
– The density of active genes is higher in Glight bands
– G-dark regions have a higher DNA density
– The density of active genes is higher in Glight bands
Text p. 86
Text p. 86
XY
XX
Meiosis
Meiosis
XX
Mitosis
Text p. 86
10
Size of chromosomes
• Chromosomes can be differentiated based
on size
• On a karyotype the chromosomes are
arranged from largest to smallest and
ending with the sex chromosomes
• In human karyotype ch-1 is the largest
autosome
Text p. 84
Text p. 86
Size of chromosomes
Centromere position
• Chromosomes can be differentiated based
on size
• On a karyotype the chromosomes are
arranged from largest to smallest and
ending with the sex chromosomes
• In human karyotype ch-1 is the largest
autosome
• Centromere: the region of the chromosome that
attaches to the spindle fibres
• The position of the centromere
Text p. 86
– Metacentric: in the middle
– Acrocentric: off centre
– Telocentric: at one end
• In human chromosomes the smallest distance to
the centromere is the p region, the larger
distance to the centromere is the q region
Text p. 86
11
Centromere position
• Centromere: the region of the chromosome that
attaches to the spindle fibres
• The position of the centromere
– Metacentric: in the middle
– Acrocentric: off centre
– Telocentric: at one end
• In human chromosomes the smallest distance to
the centromere is the p region, the larger
distance to the centromere is the q region
Text p. 86
12
Drosophila melanogaster
• How would you
classify the
autosomes of
Drosophila?
Nucleolar organizers
• Nucleoli: organelles
within the nucleus that
contain ribosomal RNA
• Nucleolar organizers are
regions of the
chromosome that contain
the genes for ribosomal
RNA. In these regions the
chromatin does not stain
with normal stains
“beads on a string”?
• Eukaryotic DNA is
packaged into
nucleosomes: histone
and DNA
• “String” between the
nucleosome “beads” is
also DNA
• DNA replication involves
copying the parental
strands, disassembling
the nucleosomes in the
parental strands and
reassembling them in the
daughter molecules
Text p. 88
13
“beads on a string”?
• Eukaryotic DNA is
packaged into
nucleosomes: histone
and DNA
• “String” between the
nucleosome “beads” is
also DNA
• DNA replication involves
copying the parental
strands, disassembling
the nucleosomes in the
parental strands and
reassembling them in the
daughter molecules
Text p. 88
Chromosomes of tomatoes
• How many chromosomes
does a tomato have?
– 12
• Where are the
centromeres?
– Constrictions between the
dark regions
• What is the dark region
flanking the centromere
of each chromosome?
– heterochromatin
Text p. 130
Chromosomes of tomatoes
• How many chromosomes
does a tomato have?
– 12
• Where are the
centromeres?
– Constrictions between the
dark regions
• What is the dark region
flanking the centromere
of each chromosome?
– heterochromatin
Text p. 130
14
Genes and chromosomes
• What is a gene?
– Functional regions along the DNA molecule of
a chromosome that are transcribed into RNA
• Even before the structure of DNA was
elucidated it was thought that
chromosomes represented large numbers
of genes in a linear array
Text p. 130
Genes and chromosomes
• What is a gene?
– Functional regions along the DNA molecule of
a chromosome that are transcribed into RNA
• Even before the structure of DNA was
elucidated it was thought that
chromosomes represented large numbers
of genes in a linear array
Text p. 83
15
Introduction to genetic analysis
Griffiths, A., Wessler, S.R., Lewontin,R.C., Gelbart, W.M.,Suzuki, D.T.
and Miller, J.H.
Eighth Edition, W.H. Freeman and Company NY
•
•
•
•
Part I Transmission genetic analysis
Chapter 1: all questions p. 24-26
Chapter 2: all the questions p. 62-72
Chapter 3: questions #1-12,18,19, 22, 25-27, 29,
30, 32, 40-42.
• Chapter 15: .
Text p. 83
Tutorials
Tutorials
• You need to do the assigned questions at the
end of each chapter covered
• As stated on the website you need to present a
doctor’s note to the instructor within 48h of
missing a quiz or a test
• If you have a legitimate reason for missing a
quiz email IN ADVANCE
• NB. Academic conflict is NOT a legitimate
reason for missing a quiz
• Late penalty is 20% per day for quizzes that are
to be handed in to drop boxes.
• You need to do the assigned questions at the
end of each chapter covered
• As stated on the website you need to present a
doctor’s note to the instructor within 48h of
missing a quiz or a test
• If you have a legitimate reason for missing a
quiz email IN ADVANCE
• NB. Academic conflict is NOT a legitimate
reason for missing a quiz
• Late penalty is 20% per day for quizzes that are
to be handed in to drop boxes.
16
Evaluation
• Tutorial Quizzes/Assignments worth a total
of 15%
• 4 Term Tests worth a total of 50%:
– January 30
– February 27
– March 15
– April 3
• Final Exam worth 35%
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