Download Genotypic and Phenotypic Ratios

Genotypic and Phenotypic
Ratios
Law of Segregation and the
Law of Independent Assortment
Genotypic and Phenotypic Ratios
• What were the genotype
and phenotype ratios of
crossing a homozygous
dominant green pea pod
colour with a
homozygous recessive
yellow pea pod colour?
• Cross GG x gg?
• The F1 genotype ratio for the offspring is 0 GG: 4Gg : 0 gg
•The F1 phenotype ratio is 4 green: 0 yellow
Genotypic and Phenotypic Ratios
Vocabulary:
G = dominant allele is green
g = recessive allele is yellow
GG = homozygous dominant genotype
gg = homozygous recessive genotype
Now what happens when the
heterozygous F1 generation is crossed
with itself?
GenotypicCross
and Phenotypic
Ratios
F1 x F1
• The genotypic ratio is 1GG : 2Gg : 1gg
• The phenotypic ratio is 3 green : 1 yellow
Mendelian Laws
• Law of Segregation
• States that inherited traits are determined
by pairs of genes and that each of the
genes separates into separate gametes
• Law of Independent Assortment
• States that inheritance of one trait does
not effect the inheritance of alleles for
another trait
Dihybrid crosses
• Dihybrid crosses are made when
phenotypes and genotypes with 2
independent alleles are analyzed.
• Process is similar to monohybrid crosses.
• The Punnett square now has 16 boxes to
represent the 16 possible genotypes in the
offspring
Steps for solving dihybrid
cross problems
• Step 1: Figure out the genotypes of the parents.
• Step 2: Figure out what kinds of gametes the
parents can produce.
• Step 3: Set up a Punnett Square for your cross.
One set of gametes go across the top and the
other, down the column.
• Step 4: Fill in the offspring inside the table.
• Step 5: Figure out the genotypic ratios for your
predicted offspring.
• Step 5: Figure out the phenotypic ratios for your
predicted offspring.
Example 1
• Free earlobes are controlled by the dominant allele E
and attached earlobes are controlled by the recessive
allele, e.
• A widow’s peak is controlled by the dominant allele H,
while a straight hair line is determined by the recessive
allele h.
• What would be the genotypes and phenotypes of the
offspring of a free earlobed, widow’s peak male and a
free earlobed, widow’s peak female ?
Step 1: Figuring out the genotypes
• What would be the genotypes and phenotypes of the
offspring of a heterozygous free earlobed, widow’s peak
male and a heterozygous free earlobed, widow’s peak
female ?
• Genotypes:
– Male – EeHh
– Female – EeHh
Step 2: Figuring out the Gametes
• Need to use the FOIL rule
Genotype: EeHh
Possible gametes: EH, Eh, eH, eh
Step 3-4: Do Punnett’s Square
P2
P1
EH
Eh
eH
eh
EH
EEHH
EEHh
EeHH
EeHh
Eh
EEHh
EEhh
EeHh
Eehh
eH
EeHH
EeHh
eeHH
eeHh
EeHh
Eehh
eeHh
eehh
eh
Step 5: Figuring out the Genotypic
ratios
Genotypic ratios
•EEHH
1:16
•EEHh
2:16
•EeHH
2:16
•EeHh
4:16
•Eehh
2:16
•eeHH
1:16
•eeHh
2:16
•eehh
1:16
•EEhh
1:16
Step 6: Figuring out the
Phenotypic ratios
Red= free earlobes, widow’s peak,9/16
Black = free earlobes, straight hair line 3/16
Brown = attached earlobes, widow’s peak 3/16
Blue= attached earlobes, straight hair line 1/16
Note that there is a 9:3:3:1 phenotypic ratio.
9/16 showing both dominant traits
3/16 & 3/16 showing one of the recessive traits
1/16 showing both recessive traits.