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Biology 2250
Principles of Genetics
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Mendelian Genetics
Topics:
- Transmission of DNA during cell division
Mitosis and Meiosis
- Segregation
- Sex linkage
- Inheritance and probability
- Independent Assortment
- Mendelian genetics in humans
- Linkage
- Gene mapping
- Tetrad Analysis (mapping in fungi)
- Extensions to Mendelian Genetics
- Gene mutation
- Chromosome mutation
- Quantitative and population genetics
Basic Concepts of Genetics
Cell/nuclear division
Mitosis (somatic tissue):
identical cells
Meiosis (germ tissue):
gametes (variation)
Behaviour of chromosomes can explain
the behaviour of genes (segregation and
independent assortment)
Chromosome and DNA
Replication
1. DNA replication results in
chromosome replication
2. Nuclear and cell division
Mitosis
2n
2n
Meiosis
2n
n
Mitosis
1. one parent cell--------> 2 identical
daughter cells
2. same in all organisms
3. simple:
(a) each chromosome
doubles (identical)
(b) identical halves separate
Meiosis
(overview)
diploid (2N) ---------------> haploid (N) gametes
Chromosome replication once --------> 2 nuclear
divisions (meiosis I, II)
one nucleus -------------------> 4 nuclei
Mitosis
Meiosis I (reductional division)
Pairing of homologous
chromosomes
Continued 
Meiosis II (equational division)
MEIOSIS ANIMATION
(Textbook website)
4 products
Mitosis
Meiosis I (reductional division)
Pairing of homologous
chromosomes
Meiosis II (equational division)
4 products
Comparison (Fig. 4-24)
Mitosis
somatic cells
one doubling
Meiosis
cells of sexual cycle
one doubling
1 division  2 cells
2 divisions  4 cells
same amount of DNA
½ amount of DNA
Comparison (continued)
Mitosis
Meiosis
# chrs doesn’t change
#chrs. halved
No pairing of chr.
Synapsis of homologs
Centromeres divide at
anaphase
Conservative
Not at anaphase I,
but at anaphase II
Variation
Genetic Terminology
Genes: hereditary elements
Alleles: forms of a gene: A , a
b, b+
Genotypes
Heterozygote:
Homozygotes:
Gene pairs
Aa
AA
aa
bb+
bb b+ b+
Genes on Chromosomes
Expect behaviour of genes to correlate with
the behaviour of chromosomes:
genes
chromosomes
Diploid (2n) AA, Aa, aa
pairs
Haploid (n)
A
one set
Genes
Meiosis I
A
Correlation of genes and
Chromosomes during
meiosis
a
1/2
1/2
A
A
A
A
a
a
a
a
A
a
B
Meiosis II
A
b
OR
b
a
B
Mendelian Genetics
Genes - cannot be observed directly
Phenotypes - observed directly
** inheritance of phenotypes used to infer the
inheritance of genes
Mendelian Genetics
Requirements:
1. Attributes of the phenotype that vary among
individuals
2. Phenotypic variation caused by genetic
differences
Mendel’s Experiments
Seven Pea varieties “True Breeding Lines”
Character
Phenotypes
1. seed shape
round, wrinkled
2. seed colour
yellow, green
3. flowers (pods)
axial, terminal
4. pods
full, constricted
5. pods
yellow, green
6. flowers
violet, white
7. stem
tall, dwarf
Advantages:
•
•
•
•
easy to grow
matures in a season
self-fertilizing
easy to cross fertilize
Cross Pollination
Mendel’s
Experimental Approach
•
•
•
•
suitable experimental organism
examine few traits in each experiment
accurate quantitative records
analyzed data------> formulated hypotheses
Genetical Analysis (pea shape)
Parental round
F1 (filial)
F2
3/4 round
X
wrinkled
cross
round
self
1/4 wrinkled
(3:1)
Hypothesis to explain results
1. Hereditary determinants
(genes)
2. Each adult plant has a gene pair (alleles)
F1 plants: one allele dominant phenotype
one allele recessive phenotype
Hypothesis (continued)
3. Alleles of a gene pair segregate equally
into the gametes
4. Each gamete has only one allele of a gene
pair
5. Gametes combine at random to form zygote
Hypothesis
P
AA
X
A
F1
aa
a
gametes
Aa
Self
Aa
X
F2
Aa
Self F1
F2
Aa
X
Aa
equal
segregation
1/2 A
1/2 a
1/2 A 1/4 AA 1/4 Aa
1/2 a 1/4 Aa 1/4 aa
F2 Genotypes 1/4 AA 2/4Aa 1/4 aa (1:2:1)
F2 Phenotypes
3/4 A1/4 aa (3:1)
Test of Equal Segregation
Hypothesis (Test Cross)
round
Rr
wrinkled
X
rr
r
1/2 R 1/2 R r
1/2 r 1/2 r r
round
wrinkled
1:1 round:wrinkled
Mendel’s First Law
Equal segregation of two alleles of a gene pair
during gamete formation
Genetic Terminology
Genes: hereditary elements
Alleles: forms of a gene: A
a
Heterozygote:
Aa
Homozygotes: AA aa
Genotypes
Phenotypes
Dominance AA, Aa same phenotype
different genotypes
Summary
Experimental:
1. Two pure breeding lines
2. Cross --------> F1 hybrid
3. Self F1 ------> F2
Summary
Results:
1. F1 one phenotype
2. F2 3:1 ratio of 2 phenotypes
Summary
Inference:
1. Single major gene
2. dominant phenotype
3. equal segregation
4. existence of genes inferred
P
AA
x
F1
aa ------
Aa ----
F2
¾ AA, Aa
¼ aa
observed
Dominant
Phenotype
Expected
3:1
Fig. 5-2
Mendelian genetics
applies to all organisms
Correlation of genes and
chromosomes
Bb
Bb
Virtual Fly
Parents
F1
X
F2
3 wildtype
..
1 vestigial
Parental
AA x
F1 Hybrid
aa
Aa
Aa x Aa
A
F2
F2 ratio
AA Aa aa
1 : 2 : 1
(¼
½ ¼)
3
:
1
a
A AA Aa
a
Aa
aa
Principle of Segregation
Implications
1. Answer questions on inheritance
2. Explore other questions
Examples:
1. Shell Colour Variation in the molluscs:
Scallop: Hermaphodites self-fertilization
Parent
Offspring
dark
orange
dark
11
0
orange
12
45
Examples
2. Paternity
Families
mother offspring
AA
all AB
AA
26AA, 24 AB
AA
48 AA, 2 AB
father ?
BB
AB
AA + BB ?
Examples:
3. Detecting Hybrids:
Species 1
AA
Hybrids
Species 2
x
BB
Mytilus
Hybrid tross.
edulis
AB
AB
AA
BB
BB
Sex-linked Inheritance
Correlation between inheritance
of genes and sex
Sex Linkage
reciprocal crosses
X
Round
wrinkled
X
wrinkled
Round
Drosophila melanogaster
(T. H. Morgan)
White eye
(mutant)
Red eye
(wild)
X
Drosophila
Red Eye
White eye
Cross A
red female
F1
F2
X
white male
all red
red : white
3 : 1
white all male
red 2 : 1 female : male
Cross B
white female
F1
X
females
F2
red male
males
females
1
:1
males
: 1
:1
Conclusions
1. Sex and eye-colour gene associated
2. Criss-cross inheritance
- daughters inherit father’s phenotype
- sons inherit mother’s phenotype
Interpretation
Sex chromosomes (X, Y)
Females: 3 pairs of autosomes
1 pair of sex chromosomes (XX)
Males:
3 pairs of autosomes
1 heteromorphic pair (XY)
Chromosomes
Male
autosomes
Female
Explanation
Eye colour gene on X chromosome
Wildtype (red) dominant to white
sex chromosomes
Females
Males
XX
XY
eye-colour genotype
WW Ww ww
W
w
Red
white
Alternative Notation
eye-colour genotype
Females
w + w+
Males
w+ Y
Eye colour
w+ w
ww
wY
Cross A
Cross B
FEMALE
Fig. 5-8
MALE
Cross A
Cross B
F1
X
F2
X
Cross B
ww
F1
X
ww+
wY
F2
w
w+
1
w+Y
:1
w
ww
ww+
: 1
Y
wY
w+Y
:1
Conclusion
1. Eye colour gene associated with sex
chromosome (X)
2. Eye colour not related to sexual function.
Genes on sex chromosomes not related to
sexual function
Sex Determination
XX
XY
XXY
XO
*
Drosophila
*
Human
Homogametic
*
klinefelter
Heterogametic
turner
*
Sterile
Sex Determination
Humans: Y chr. -------> maleness
Drosophila: sex -----> X/A ratio
2X/2A = 1.0 ------>
X/2A = 0.5 ------>
Gynandromorphs
Nondisjunction during
Early development
(mitotic cell division)
Normal
mitosis
Xw,m X+,+
Xw,m
X+,+
Xw,m
Wild female
Xw,m X+,+
X+,+
Xw,m
Xw,m
Xw,m X+,+
Xw,m X+,+
Gynandromorphs
Xw,m
Xw,m X+,+
Nondisjunction during
Early development
Other sex determining systems
Birds & Moths
homogametic
ZZ
heterogametic
ZW
Sex Linkage (X Y)
- Y chromosome lacks homology
with X
- alleles on X expressed in males
- hemizygous: a gene present in
one copy
Xw Y
Summary
Traits controlled by a single gene:
- precise Mendelian ratios: 3:1, 1:1, 1:2:1
- ratios due to chromosome segregation
during meiosis
- sex linked inheritance sex chromosomes