From Gene to Phenotype
Genes to Phenotypes
"A set of genes represents the individual components of the biological system under scrutiny"
Modifications of the "3:1 F2 monohybrid ratio" and gene interactions are the rules rather than the exceptions"
- the biological system. L-2. Consider interactions: gene x gene, gene x environment
I. Overview - genes and gene products
- One gene - one polypeptide: an oversimplification
II. Allelic variation
A. Many alleles are possible in a population, but in a diploid individual, there are only two alleles....The Vrs1 example, again
B. Mutation is the source of new alleles
C. There are many levels of allelic variation, ranging from DNA sequence changes with no change in phenotype to large differences in phenotype due to effects at the transcriptional, translational, and/or post-translational levels
- Vrs1 example, again. L-2. It is worth a read, again, in the context of allelic variation. How many alleles for 2-row and how many alleles for 6-row?
III. Relationships between alleles at a locus
A. Complete dominance:
Deletion, altered transcription, alternative translation. The interesting case of aroma in rice:. a loss of function makes rice smell great, and patent attorneys salivate....
- aroma in rice. L-1. A clear case of loss of function giving the recessive allle and a novel phenotype (aroma)
B. Incomplete (partial) dominance
Red X ivory gives a pink F1. The F2 phenotypic ratio is 1 red: 2 pink: 1 ivory. Red pigment is formed by a complex series of enzymatic reactions. Plants with the dominant allele at the I locus produce an enzyme critical for pigment formation. Individuals that are ii produce an inactive enzyme and thus no pigment. In this case, II individuals produce twice as much pigment as Ii individuals and ii individuals produce none. The amount of pigment produced determines the intensity of flower color. Note: enzymes are catalytic and heterozygotes usually produce enough enzyme to give normal phenotypes. This is the basis for complete dominance. However, upon closer examination, there are often measurable differences between homozygous dominant and heterozygous individuals. Thus, the level of dominance applies only to a specified phenotype.