Inheritance
Monohybrid Inheritance
Inheritance of characteristics controlled by a single gene.
E.g. Cystic Fibrosis
It is caused by a recessive allele produced by a deletion of 3 bases. The recessive allele codes result in a faulty
CFTR protein being produced. So chloride ions aren’t transported into mucus. This causes the water potential to
not be reduced so water doesn’t move into mucus by osmosis. This sticky mucus blocks the respiratory, digestive
and reproductive systems.
Parents : Ff x Ff
Gametes: F or f and F or f
Genotype ratio : 1:2:1
Phenotype ratio: 3:1
Natural selection changes the allele's frequency. Due to the nature of cystic fibrosis it reduces the reproductive
potential of an individual.
In the parental generation there is an equal ratio of F:f = 1:1.
But in the first generation the frequency of F increases whilst the frequency of f decreases with a ratio of 2:1.
If the disease TB is thrown into the mix it can act as a selection pressure. If an individual is heterozygous they
have the highest chance of survival due to the difference in proteins preventing TB.
E.g. Huntington's Disease
The disease is caused by a gene stutter, CAG is repeated excessively causing the protein to be too big. This causes
dementia and loss of motor control. The symptoms show when a person reaches their 50’s. It is dominant and
most have reproduced by this age.
, Dihybrid Inheritance
Inheritance of characteristics controlled by 2 genes.
E.g. Peas
G= Yellow g= Green
R=Round r=Wrinkled
Parents: GgRr x GgRr
Gametes: GR or Gr or gR or gr
Phenotype Ratio:
Yellow Round: Yellow Wrinkled: Green Round: Green Wrinkled
9:3:3:1
Co-Dominance
Both alleles are expressed in the phenotype.
E.g. Cows with Roan Colouring
CR = Red
CW= White
Parents: CRCW x CRCW
Gametes: CR or CW
Genotype & Phenotype Ratio: 1:2:1
Monohybrid Inheritance
Inheritance of characteristics controlled by a single gene.
E.g. Cystic Fibrosis
It is caused by a recessive allele produced by a deletion of 3 bases. The recessive allele codes result in a faulty
CFTR protein being produced. So chloride ions aren’t transported into mucus. This causes the water potential to
not be reduced so water doesn’t move into mucus by osmosis. This sticky mucus blocks the respiratory, digestive
and reproductive systems.
Parents : Ff x Ff
Gametes: F or f and F or f
Genotype ratio : 1:2:1
Phenotype ratio: 3:1
Natural selection changes the allele's frequency. Due to the nature of cystic fibrosis it reduces the reproductive
potential of an individual.
In the parental generation there is an equal ratio of F:f = 1:1.
But in the first generation the frequency of F increases whilst the frequency of f decreases with a ratio of 2:1.
If the disease TB is thrown into the mix it can act as a selection pressure. If an individual is heterozygous they
have the highest chance of survival due to the difference in proteins preventing TB.
E.g. Huntington's Disease
The disease is caused by a gene stutter, CAG is repeated excessively causing the protein to be too big. This causes
dementia and loss of motor control. The symptoms show when a person reaches their 50’s. It is dominant and
most have reproduced by this age.
, Dihybrid Inheritance
Inheritance of characteristics controlled by 2 genes.
E.g. Peas
G= Yellow g= Green
R=Round r=Wrinkled
Parents: GgRr x GgRr
Gametes: GR or Gr or gR or gr
Phenotype Ratio:
Yellow Round: Yellow Wrinkled: Green Round: Green Wrinkled
9:3:3:1
Co-Dominance
Both alleles are expressed in the phenotype.
E.g. Cows with Roan Colouring
CR = Red
CW= White
Parents: CRCW x CRCW
Gametes: CR or CW
Genotype & Phenotype Ratio: 1:2:1
