Republic of the Philippines
Province of Rizal
Municipality of Montalban
Colegio de Montalban
Kasiglahan Village, San Jose, Montalban, Rizal
colegiodemontalban.official@gmail.com
Contact # 8-296-8667
FACTORS THAT INFLUENCE QUANTITATIVE TRAITS
1.Number of genes controlling a trait
This explains how the number of possible gene combinations (genotypes) increases dramatically as
more genes control a single trait.
Imagine a trait controlled by individual genes, each with two versions (alleles), like A and a.
One gene: You have three possible genotypes: AA, Aa, aa.
Two genes: Now you combine the possibilities for each gene. You'd have 3 (from the first gene) * 3 (from
the second gene) = 9 possible genotypes.
More genes: The number of genotypes keeps multiplying by 3 for every additional gene involved. This is
why the formula is 3n (3 to the power of n, where 'n' is the number of genes).
Example 1: Eye Color
- Imagine eye color is controlled by just one gene with two alleles:
B (brown) and b (blue).
- Possible genotypes: BB (brown), Bb (brown), bb (blue).
- Three possible genotypes lead to two possible phenotypes (brown and blue).
In short, the more genes involved in a trait, the more varied the combinations of those genes can be. This
explains the vast genetic diversity we see in many traits.
2.Additive Gene Action
Additive gene action means that the effects of different genes simply add up to create the final trait. Each
gene contributes a specific amount, and the total trait value is the sum of these contributions.
This table shows how two genes, A and B, can influence a measurable trait (the “metric value”). Each gene
has two versions (alleles), and each allele contributes a specific amount to the trait’s value. The total value
is the sum of the contributions from both genes.
Explanation:
- Genes A and B: Each has two alleles: A (4 units), a (2 units); B (2 units), b (1 unit).
- Genotype: The combination of alleles (e.g., AABB, AaBb).
- Metric Value: The total value of the trait based on the alleles present. For example, AABB has a metric
value of 12 (4+4+2+2).
- Ratio in F2: This refers to how often each genotype would appear in the second generation (F2) of
offspring from a cross. It follows standard Mendelian inheritance patterns.
Example:
Let's say the trait is fruit size in a plant.
- AABB (12 units): This plant has the largest fruit size (maximum contribution from both genes).
- AAbb (8 units): This plant has smaller fruit than AABB because it has the 'bb' alleles, which contribute
less.
- aabb (6 units): This plant has the smallest fruit (minimum contribution from both genes).
3.Dominance
Imagine eye color.
A dominant brown eye allele (B) masks the effect of a blue eye allele (b). So, both BB (brown) and Bb
(brown) genotypes have brown eyes, making the number of phenotypes fewer than the number of
genotypes.
4. Epistasis
Think of fur color in labrador. A gene for black fur (B) can be masked by a gene for a white fur (W) that's
epistatic to the black fur gene. A dog with BbWw will have white fur, even though it carries the black fur
allele.
BbWw: This dog has the black fur allele (B) and the brown fur allele (b), but it also has the
white fur allele (W). So, even though it has the genes to make either black or brown fur,
the white fur allele (W) overrides both, making the dog white.
In short: The white fur gene (W) is “epistatic” to the black fur gene (B), meaning it can mask its effect.
Even if the dog has the genetic potential to be black or brown, the white fur allele “wins” and makes the
dog white.
5. Environmental Influences
This table shows how much wheat different varieties (Roughrider,
Seward, Agassiz) produced each year in North Dakota. The big takeaway
is that even though the varieties have different genes (their genotype),
their output (phenotype – how much wheat they make) changes a lot from
year to year.
Why? Because the weather and other environmental conditions (like the
devastating winter kill in 1990) have a huge impact on how much wheat
each variety produces.
The final amount of wheat isn't just about the genes of the wheat plant;
it's also strongly affected by the environment. This is a key idea in
quantitative genetics—that a trait's final result is a combination of genes
and the environment. The formula at the end summarizes this:
Phenotype = Genes + Environment.
CONCLUSION:
The conclusion is that a trait’s final expression, or phenotype, is the result
of both genetic factors and environmental influences.
