1. Monohybrid Inheritance
Section 11

2. Monohybrid Inheritance
Gregor Mendel ( ):
- Used varieties of pea plant
- Ensured large numbers were used
- Studied only 1 characteristic
- Observed 3:1 phenotypic ratio
Principle of segregation: ‘ the alleles of a gene exist in pairs but when gametes are formed, the numbers of each pair pass into different gametes. Thus each gamete contains only one allele of each gene.

3. Homozygous (TT) or (tt) – 2 of the same allele
Heterozygous (Tt) – 2 different alleles

4. Law of probability
Most monohybrid crosses give roughly 3:1 phenotypic ratio
1 in 4 chance of a recessive allele being expressed
200 animals – roughly 150 have 1 phenotype, 50 will have another
Since fertilisation is random – rarely 3:1 exactly

5. Monohybrid Inheritance in humans
Family trees used to predict genetic trends
E.g. tongue rolling
Rhesus D antigen:
D-antigen – found on some blood cells
Present = Rhesus positive blood (Rh+)
Absent = Rhesus negative (Rh-)
Rh- people produce anti-D antibodies –’sensitised’
Antigen-D present (DD/Dd)
Antigen-D absent (dd)

6. Monohybrid Inheritance in humans
Albinism:
Inability to make melanin – pigment for skin, eyes, hair etc.
Is a recessive trait
AA or Aa = normal
aa = albino
Other examples
- Cystic Fibrosis (mucus secretion)
- Phenylketonuria (PKU)
- both are recessive traits

7. Monohybrid Inheritance in humans
Huntingdon’s Chorea:
Determined by a dominant allele
Only expressed after average of 38 years old
50% chance of children of Huntingdon’s sufferer getting the disorder
1 in 20,000 cases

8. Incomplete dominance – sickle cell anaemia
Mutation of haemoglobin gene – haemoglobin S synthesised
Homozygous (SS) – Sickle cell anaemia
- reduced oxygen carrying capacity
- blood cells malformed (sickle-shaped)
Heterozygous – (HS) – Sickle cell trait
- both types of haemoglobin present
- neither H or S are dominant (incomplete dominance)
- sickle cell = malaria resistance

9. Incomplete / Co-Dominance
Incomplete – both alleles expressed in the phenotype, but unevenly
- sickle-cell: more H than S expressed
Co-Dominance - both alleles equally expressed in the phenotype
- AB blood – exactly 50:50 split
- MN blood - another blood type antigen
- no antibodies produced
- equal expression (equal dominance

10. Multiple Alleles 3 or more alleles for a characteristic
3 alleles = 6 possible genotypes
No. of phenotypes depend on the type of dominance
– complete, incomplete or co-dominance
E.g ABO blood – 6 genotypes, 4 phenotypes