• so far we have been looking a single gene differences with discreet contrasting phenotypes:
◦ eg. Round vs. wrinkled, Yellow vs. green
• many phenotypes have a continuous variation: eg. height, weight and color.
• many cases of continuous variation have a purely environmental basis and little effect by
genetics: eg. height of homozygous plants grown in a field.
• some cases of continuous variation have a genetic component: eg. human skin color, all degrees
of skin darkness can be observed in different parts of the world
• in many cases it is not only one gene but many genes and versions of those genes (alleles)
interact to give an additive effect.
• the interacting genes underlying a genetic basis for continuous variation are called Polygenes or
Quantitative Trait Loci (QTLs)
◦ quantitative= continuous
◦ trait= character or property
◦ locus or loci= the place on the chromosomes= genes
• polygenes or QTLs for the same trait are distributed throughout the genome and in many cases
on different chromosomes and show independent assortment.
Continuous variation in a natural population
Redness in wheat seeds
• assume two independently assorting gene pairs: R1/r1 and R2/r2. Both R1 and R2 contribute to
• R1 or R2 functional allele (i.e. wt) r1 or r2 null allele.
• each “dose” of an R allele of either gene is additive, meaning that it increases the degree of
• in a dihybrid R1/r1; R2/r2. both male and female gametes will show the genotypic proportions
as follows: Polygenes in progeny of a dihybrid self
Branch diagram to show possible
genotypes and corresponding phenotype
Overall in Progeny
• polygenic traits do not follow patterns of Mendelian
inheritance (separated traits).
• instead, their phenotypes typically vary along a
continuous gradient depicted by a normal distribution → in our illustration, we used a dihybrid cross to show how the histogram is produced
What is going on in natural populations?
• if the alleles at each gene pair are approximately equal in frequency in the population (for
example, R1 is about as common as r1), then the dihybrid cross can be said to represent an
average cross for a population in which two polygenes are segregating
• keep in mind example R1 = wild type allele (full function) and r1= null allele (zero function)
• there are many alleles that conceivably span the full spectrum of function
• identifying polygenes and understanding how they act and interact are important challenges for
• polygenes not only control height, skin color and weight but many common human conditions
such as autism and cancer are thought to have a polygenic component.
organelle genes: Inheritance of Independent of the Nucleus
• mitochondria and Chloroplast are specialized organelles that have small circular chromosomes.
• in humans 9