Mitosis & Meiosis
nn
n
n
2nd Nuclear Division
Mendelian Genetics has basically two rules:
- the principle of segregation
- the principle of independent assortment
n
1st Nuclear Division
n
Meiosis
Egg cell
To make it interesting, there are many exceptions like:
-
Gene linkage
Codominance
Sex Linkage
Mitochondrial Linkage
Homozygous Dominant Lethality
Combined Lethality
Maternal Effects
Segregation Distortion
Penetrance
Multiple Alleles
Multiple Genes
Sperm
Meiocyte
Fertilization
2n
2n
2n
Zygote
Diploid
Organism
Mendel’s Rules: the
power of ratios
Products of Mitosis & Meiosis
X
P:
2n
Round
mono-hybrid cross
Wrinkled
principle of segregation
Mitosis
2n
2n
End of
Interphase
Pro-, MetaAna- Telophase
n
n
Mitosis
2n
AA
P:
x aa
F1:
Interphase
All round
Aa x Aa
F1:
n
n
2n
Meiosis
F2:
Separation of hom.
chromosomes
P:
Separation of hom.
chromatids
F1:
F1:
All round, yellow
F2:
F2:
Round, yellow
9
Wrinkled, yellow
3
Round green
3
Wrinkled, green
1
Aa aa
Genotyping
principle of independent
assortment
P: AA BB
a
AA Aa
3 : 1
Gametes
di-hybrid cross
Wrinkled, green
a
1875
Mendel’s Rules:
X
Round, yellow
5474
n
2nd Nuclear Division
A
3 : 1
n
1st Nuclear Division
A
F2:
x aa bb
X
P:
Round
(AA)
Wrinkled
(aa)
Aa Bb x Aa Bb
AB
Ab
aB
ab
AB
AABB AABb
AaBB AaBb
Ab
AABb AAbb
AaBb Aabb
aB
AaBB AaBb
aaBB
ab
AaBb
9 :
Aabb
3
aaBb
F1:
All round (Aa)
Problem: What is the genotype of this
, Aa or AA?
aaBb aabb
: 3 : 1
1
Problem: What is the genotype of this
Genotyping by backcrossing
Testcross: Crossing the unknown (Aa or AA) to the recessive phenotype (aa)
Case 2: Unknown genotype is Aa
P: AA BB
Aa (1:0)
Aa x aa
Aa and aa (1:1)
x aa bb
Expected:
Observed:
F1: Aa Bb x Aa Bb
F2: AB
AA x aa
Case 1: Unknown genotype is AA
Chi-square test: Is my hypothesis correct?
- Aa or AA?
Ab
aB
90
89
30
25
30
31
1 = 160
15 = 160
ab
AB
AABB AABb
AaBB AaBb
Ab
AABb AAbb
AaBb
Aabb
aB
AaBB AaBb
aaBB
aaBb
ab
AaBb
Aabb
aaBb aabb
9 :
3
: 3 : 1
χ2 = SUM
(Obs - Exp) 2
Expected
89 : 25 : 31 : 15
Chi-square test: Is my hypothesis correct?
Expected:
Observed:
90
89
30
25
χ2 = SUM
(89 - 90) 2
90
+
(25 - 30) 2
30
+
30
31
1 = 160
15 = 160
(Obs - Exp) 2
- for example: wing size, wing color, patch color ( usually three)
Expected
(31 - 30) 2
30
+
(15 - 10) 2
10
Step 1: Wake Up!
1. What traits are observed?
= 3.37 = χ2
Step 2: Get Organized!
2. What is the dominant and what is the resessive phenotype for each trait?
- for example: large versus small wing size
x 3!
3. Which butterflies are homozygous dominant/recessive?
Interpretation of χ2-value:
-Degree of freedom (# of classes minus 1)
-p value 0.05 or higher suggests “true”
Step 3: Get Focused!
4. Do the observed traits follow Mendel’s rules?
for example:
- does a mono-hybrid cross give a 3:1 ratio?
- does a two-hybrid cross give a 9:3:3:1 ratio?
Step 4: Get Smart!
5. If there is a “Non-Mendel” trait: What kind of exception is it and why?
2