Lecture 3 notes:
Readings: Chapter 4, pp221-230, 94-95, 519-523
In Mendel’s studies:
- Single gene effects
- 2 alleles only
Not always simple, transmission of many traits don’t produce Mendelian ratios:
- Inheritance of many traits more complex
- Extensions to Mendel, the laws still apply but different ratios
- For non Mendelian inheritance, Mendel’s laws are not followed
Extensions to Mendel for single genes:
Mendel saw complete dominance – in heterozygote only one allele is expressed.
Due to 50% gene product being enough for normal phenotype
Sometimes one allele not completely dominant over the other:
Incomplete dominance: heterozygote has phenotype intermediate between two
Co-dominance – heterozygote exhibits both the phenotypes of the two homozygotes
For both of these you see 1:2:1 F2 Phenotypic ratio (i.e. not Mendelian 3:1)
In this example, the genotype C C IS NOT enough for a red phenotype or white
phenotype and so this is an incomplete dominance pattern where the f1 generation is
intermediate of the two colours producing pink.
- If you then cross the two pink heterozygous genotypes, you will get a
genotypic AND phenotypic ratio in f2 of 1:2:1
- This only happens in incomplete dominance, in complete dominance:
genotypic and phenotypic ratios do not coincide
Both alternative traits are visible in heterozygote; neither phenotype is dominant
over the other.
- Genotypic and phenotypic ratios of F2 coincide: 1:2:1
- E.g. ABO blood system -
- Lethal allele is one that has potential to cause death in an organism
- A lethal allele can produce ratios that deviate from Mendelian ratios
- In mice yellow colour pattern is recessive lethal. Homozygote yellow = death
o In homozygote, no protein results in lethality; therefore it is an
essential gene. NB: this gene is pleiotropic – one gene affects multiple
traits, a visible phenotype and also viability.
Penetrance and Expressivity:
Sometimes individuals carrying a dominant allele do not exhibit the trait – called
Penetrance: proportion of individuals of specified genotype that reveal the expected
Expressivity: degree of expression of phenotype in individual of specified genotype
- Gender can govern the inheritance pattern for certain traits. An allele is
dominant in one sex and recessive in the other!
- E.g. pattern baldness – autosomal trait