Download I A

Extensions to Mendel: Complexities in Relating
Genotype to Phenotype
Outline of extensions to
Mendel’s analysis
• Single-gene inheritance
– In which pairs of alleles show deviations from complete dominance and
recessiveness
– In which different forms of the gene are not limited to two alleles
– Where one gene may determine more than one trait
• Multifactorial inheritance in which the phenotype
arises from the interaction of one or more genes
with the environment, chance, and each other
Dominance is not always complete
• Crosses between true-breeding strains can
produce hybrids with phenotypes different
from both parents
– Incomplete dominance
• F1 hybrids that differ from both parents express an intermediate
phenotype. Neither allele is dominant or recessive to the other
• Phenotypic ratios are same as genotypic ratios
– Codominance
• F1hybrids express phenotype of both parents equally
• Phenotypic ratios are same as genotypic ratios
Three Different Forms of Dominance
Incomplete Dominance for Flower Color in Snapdragon
Codominance of Spotted and Dotted Coat Pattern Alleles
Codominance of IA and IB Blood Group Alleles
There Are Often More Than Two Alleles of a Gene
The ABO blood group system is determined by one gene with three alleles.
There Are Often More Than Two Alleles of a Gene
Note that the ABO blood system shows both complete dominance and codominance.
Multiple Alleles Can be Grouped in a Dominance Series
Dominance series for lentil bean coat color.
Do variations on dominance relations
negate Mendel’s law of segregation?
• Dominance relations affect only the
relationship between genotype and
phenotype
• Alleles still segregate randomly and unite
randomly
• Gene products control expression of
phenotypes
• Interpretation of phenotype/genotype
relationship can be complex