Transcript Monohybrid - World of Teaching

Genetics
The study of inherited
characteristics
Monohybrid inheritance
Let the allele for round seeds be:
Let the allele for wrinkled seeds be:
Parents phenotype
genotype
Gametes
R (dominant allele)
r (recessive allele)
round seeds
RR
R
x
wrinkled seeds
rr
R
r
r
F1 generation
gametes
gametes R
R
r
Rr
Rr
r
Rr
Rr
F1 phenotypes
100% plants producing round seeds
F1 genotypes
100% heterozygotes Rr
• F1 intercross
•
Parents phenotype
genotype
•
Gametes
•
F2 generation
round seeds x round seeds
Rr
Rr
R
r
R
r
gametes
gametes R
r
R
RR
Rr
r
Rr
rr
Phenotype
75% plants producing round seeds
25% plants producing wrinkled seeds
Genotype
25% RR
50% Rr
25% rr
Ratio
3:1 Round seeds: wrinkled seeds
Does the observed ratio match the
theoretical ratio?
• The theoretical or expected ratio of plants producing
round or wrinkled seeds is 3 round :1 wrinkled
• Mendel’s observed ratio was 2.96:1
• The discrepancy is due to statistical error
• The observed ratio is very rarely the same as the
expected ratio
• The larger the sample the more nearly the results
approximate to the theoretical ratio
• Statistical tests such as the chi-squared test can be
used to test the closeness of fit of the observed and
theoretical ratios
What if the parents are not homozygous?
• The use of the term F1 generation is limited to the offspring of
two homozygous parents
• The use of the term F2 is limited to the offspring of
intercrossing the F1 generation
• In all other cases the terms offspring (1) and offspring (2)
should be used
• The complete set of headings will be:
Parents phenotypes
Parents genotypes
Gametes
Offspring (1) genotypes
Offspring (1) phenotypes
Gametes
Offspring (2) phenotypes
Offspring (2) genotypes
Backcross
To test whether a plant producing round seeds is homozygous RR
or heterozygous Rr it can be crossed with a homozygous rr plant
If plant is homozygous dominant RR If plant is heterozygous Rr
Parents
phenotype round x wrinkled
genotype
RR
rr
gametes
R
R
r
Parents
phenotype
genotype
gametes
r
Offspring
round x wrinkled
Rr
rr
R
r
r
r
Offspring
gametes
gametes
R
R
r
Rr
Rr
r
Rr
Rr
Offspring
gametes
gametes
R
r
r
Rr
rr
r
Rr
rr
Offspring
phenotype
100% round
phenotype 50% round 50% wrinkled
Genotype
100% Rr
genotype
50% Rr
50% rr
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