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All living things pass on their genetic heritage by common processes
Mendelian Genetics - Lecture Outline 11
| A) Gregor Mendel (1822-1884) |
Experiments in the "hybridization" of the garden pea began in 1866. He
identified seven pairs of traits which "bred true" and showed complete
dominance.
Height (Tall or Dwarf)
Seed Color (Yellow or Green)
Seed Shape (Round or Wrinkled)
Flower color (Purple and White)
Flower Position (Along stem or on Tips of stem)
Pod Color (Green or Yellow)
Pod Shape (Inflated or Constricted)
(1) Monohybrid cross. First Law of Segregation.
Emasculated flowers to prevent self-fertilization; crossed by artificial
pollination.
P Generation: Tall homozygous dominant, TT.
Gametes : T
Dwarf homozygous recessive, tt.
Gametes: t
F1 Generation: Artificial cross fertilization
T x t
= Tt (hybrid phenotype = all Tall)
F2 Generation: Allow F1 to self-fertilize. Gametes: T
and t.
Construct a Punnett Square
Phenotypic Ratio: 3 Dominant (1 TT +2 Tt)
1 Recessive (tt).
Conclusions:
The complete dominance shows that inheritance is not due to blending.
Reappearance of the recessive trait in the F2 generation shows that the genes
received from each parent, segregate from each other during the formation of gametes.
(2) Test Cross: To distinguish between phenotypically identical, homozygotes
(TT) and and heterozygotes (Tt). Cross Tall (TT or Tt) with dwarf (tt).
(a) TT x tt = Tt.
Only tall phenotype, therefore, homozygote.
(b) Tt x tt = Tt + tt.
Tall and short (1:1), therefore, heterozygote.
(3) Dihybrid Cross: Second Law of Independent Assortment
P Generation: Smooth seed, Purple flower, SSPP. Gametes: SP
Rough seed, White flower, sspp. Gametes: sp
F1 Generation: SP x sp = SsPp (Smooth, purple flower)
F2 Generation: Allow F1 to self fertilize. Gametes: SP,Sp,
sP, sp.
| |
SP |
Sp |
sP |
sp |
SP |
SSPP |
SSPp |
SsPP |
SsPp |
Sp |
SSPp |
SSpp |
SsPp |
Sspp |
sP |
SsPP |
SsPp |
ssPP |
ssPp |
sp |
SsPp |
Sspp |
ssPp |
sspp |
Phenotypic Ratio: 9 : 3 : 3 : 1
9 Smooth-Purple: 1 SSPP + 2 SSPp + 2 SsPP and 4 SsPp
3 Smooth-white: 1 SSpp + 2 Sspp
3 rough-Purple: 1 ssPP + 2 ssPp
1 rough-white: 1 sspp
(Note: Purple to white = 3:1 and Smooth to Rough = 3.1)
| B. Multiple Alleles: Human ABO Blood Types |
1) Alleles: IA , IB and i.
2) Blood types: Antigens and antibodies.
Blood Type |
Erythrocyte Genotype |
Serum Antibodies |
A |
IA IA |
Ant B |
A |
IA i |
Ant B |
B |
IB IB |
Anti A |
B |
IB i |
Anti A |
AB |
IA IB |
None |
O |
i i |
Anti A and Anti B |
Note: Type AB is a universal acceptor; Type O is a universal donor.
3) Permissible Donor-Recipient combinations.
Blood Donor
Recipient |
O |
A |
B |
AB |
O |
Yes |
No |
No |
No |
A |
Yes |
Yes |
No |
No |
B |
Yes |
No |
Yes |
No |
AB |
Yes |
Yes |
Yes |
Yes |
| C. Polygenic Inheritance: Human skin color. |
1. Three genetic loci, A, B and C.
2. Each locus has two alleles: A or a; B or b; C or c.
3. Color is determined by the total number of maximum pigment genes
Maximum pigment genes: A, B and C
Minimum pigment genes: a, b and c.
Darkest color = AABBCCC; minimum color = aabbcc.
4. Cross between two individuals heterozygous for all three genes.
AaBbCc x AaBbCc + 27 Genotypes
7 Phenotypes
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AaBbCC |
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AABbCc |
aaBBCc |
aaBbCc |
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AaBBCc |
AaBbCc |
AabbCc |
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AaBbCC |
AabbCC |
AaBbcc |
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AaBBCC |
AABBcc |
AaBBcc |
aabbCC |
aabbCc |
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AABbCC |
AAbbCC |
AAbbCc |
aaBBcc |
aaBbcc |
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| AABBCC |
AABBBc |
aaBBCC |
AABbcc |
AAbbcc |
Aabbcc |
aabbcc |
¬ Number of alleles for dark skin®
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