MENDEL’S EXPERIMENTS. CONTRASTING TRAITS OF PEA PLANTS.
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Transcript of MENDEL’S EXPERIMENTS. CONTRASTING TRAITS OF PEA PLANTS.
MENDEL’S EXPERIMENTS
CONTRASTING TRAITS
OF PEA PLANTS
Mendel’s Principle of Dominance: When an individual is hybrid for a certain trait, only one of the two genes expresses itself. The gene that expresses itself is the dominant gene, and the other whose expression is masked is the recessive gene.
Mendel’s Principle of Segregation:
The two genes that determine a particular trait segregate (separate) when gametes form. Half of the gametes recieve one of the genes from each pair, and the other half recieve the other gene of the pair.
Mendel’s Principle of Independent Assortment: During gamete formation, gene pairs that control different traits separate (segregate) independently of one another. (This is true for genes located on different chromosomes. Linked genes do not obey this principle.)
MONOHYBRID CROSSExample: A cross between a pure round seeded and wrinkled seeded pea plants
Parents: RR X rr
Gametes: R r1/1 1/1
F1 Generation: 1/1 RrPhenotype:round seeded
Genotype:heterozygous round seeded
F1 Cross: Rr X Rr
F1 Gametes: ½ R, ½ r X ½ R, ½ r
fertilization
F2 Generation: ¼ RR; ¼ Rr ; ¼ Rr ; ¼ rr
Phenotypes :Types = 2n = 21 = 2 round and wrinkled(Ratio: ¾ round, ¼ wrinkled)
Genotypes :Types = 3n = 31 = 3 RR ( homozygous round)
Rr (heterozygous round)
rr ( homozygous wrinkled)
1:2:1
3:1
A DIHYBRID CROSSExample: A cross between a pure round yellow seeded and wrinkled green seeded pea plants
Parents: RRYY X rryy
Gametes: RY ry1/1 1/1
F1 Generation: 1/1 RrYy
Phenotype:round yellow seeded
Genotype:heterozygous round yellow seeded
F1 Cross: Rr Yy X Rr Yy
F1 Gametes: 1/4 RY , 1/4 Ry 1/4 RY , 1/4 Ry
X
1/4 rY , 1/ 4 ry 1/4 rY , 1/ 4 ry
fertilization
GAMETES
RY Ry rY ry
RY RRYY RRYy RrYY RrYy
Ry RR Yy RRyy RrYy Rryy
rY RrYY RrYy rrYY rrYy
ry RrYy Rryy rrYy rryy
Phenotypes :
Types = 2n = 22 = 4 types
1.Round yellow (R-Y) 9/16
2.Round green (R-y) 3/16
3.Wrinkled yellow (r-Y) 3/16
4.Wrinkled green (r-y) 1/16
9: 3: 3: 1
PHENOTYPE RATIO
Genotypes :
Types = 3n = 32 = 9 possible genotypes
R-Y R-y r-Y r-y
1/16 RRYY 1/16 RRyy 1/16 rrYY 1/16 rryy
2/16 RRYy 2/16 Rryy 2/16rrYy
2/16 RrYY
4/16 RrYy
1:2.:2:4:1:2:1:2:1
RATIO
EXCEPTIONS TO MENDEL’S PATTERNS OF INHERITANCE
INCOMPLETE DOMINANCE,
CODOMINANCE,
MULTIPLE ALLELES
Example: A cross between red and white flowered snapdragon.
P: RR (red) X WW (white)
F1: RW(pink)
F1 Cross: RW X RW
F2 : RR RW RW WW
¼ RED 2/4 PINK ¼ WHITE
1:2:1 RATIO
INCOMPLETE DOMINANCE
Incomplete Dominance in Snapdragon
RR RW WW
EXAMPLE : Four o’clock flower (akşam sefası)
EXAMPLE : Feather Color Andulisian Chicken
P: BB(black) X WW (white)
F1: BW(blue)
F1 Cross: BW X BW
F2: BB (black); BW(blue);BW(blue); WW(white)
CODOMINANCEIn codominance both alleles for a trait are dominant, and organisms produced from these crosses have both characteristics of the trait.
Example: A cross between M blood-grouped and N blood-grouped couple.
P: MM X NN
F1: MN
F1 Cross: MN X MN
F2: 1/4 MM ; MN ; MN ; 1/4 NN
*2/4
*(Both M and N proteins are produced, they are not dominant over each other)
CODOMINANCE
GENOTYPE PHENOTYPE ANTIGENS ANTIBODIES
MM M M -----
NN N N -----
MN MN M, N -----
NO ANTIBODIES ARE PRODUCED
Codominance in horse coat colorExample: A cross between reddish and white horses results in roan horses which have both red and white hairs in their coat.
P: CR CR (red) X CW CW (white)
F1: CR CW ( roan coat)
F1 Cross: C R C W X C R C W
F2:C R CR ; CR CW; CR CW ; C WC W
red roan coat white 1 : 2 : 1
Codominance in cattle coat color
Phenotype ratio = Genotype ratio = 1: 2 : 1
MULTIPLE ALLELES
Phenotype Genotype Antigens Antibodies
A AA or A0 A Anti-b
B BB or B0 B Anti-a
AB AB A, B None
0 00 None Anti-a, Anti-b
Rh + RR, Rr Rh -
Rh - rr - Anti-Rh
RECIPIENT
A
B
AB
0
DONOR
A,0
B,0
A,B,O
0
0 is the universal donor, AB is the universal receipent.
Rh (+) can receive both Rh(+) and Rh (-) blood but Rh (-) receives only Rh(-).
To calculate the types of genotypes in a population for a trait having multiple alleles, use the formula
n (n+1)
n: number of alleles
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