BDE 244: Quantitative Genetics, Selection & Evolution
Written by Tegan Gibaud
Quantitative Genetics
Definitions Mendelian: Relating to Mendel’s theory of heredity.
Mendelian Genetics & Mendelian genetics are not quantitative.
Blood Types They are discrete traits (either/or). This is qualitative.
➔ Example: Red or white petals; no intermediates of pink.
Blood types are a good example of Mendelian Genetics.
Image: Lecture Slides
The genotype of an organism is the genetic code in its cells.
The phenotype is the visible or expressed trait. The phenotype
depends upon the genotype but can also be influenced by
environmental factors.
From the blood type diagram, it can be expected that certain
phenotypes will be more common than others if mating is random
and there is no selection acting upon them.
Blood types are entirely genetically controlled and there are four
distinct phenotypes with no intermediates. It is a 1 locus and 3
allele system.
Multiple Loci When Mendel looked at pea plants, he studied simple traits which
are generally controlled at 1 or 2 single loci, with only a few alleles
at each locus.
➔ But in animals and plants, traits are controlled at multiple
loci and can have many alleles at each locus.
There is a common misconception that tongue rolling is mendelian
and is only controlled by a single locus, but this has been proven
untrue. This is the same for dimples, hair whorls, widow’s peak and
hitchhiker’s thumb.
, However, ear wax is a Mendelian trait. Whether it is wet or dry is
discrete, just like blood types.
Summary Mendelian traits: Traits which are controlled by 1 or 2 loci with just a
few alleles at each locus.
➔ The traits are discrete or categorical in nature with no
intermediate forms.
Dry ear wax and blood types are good examples of Mendelian
Genetics.
Comparison of
Qualitative vs
Quantitative Traits
Note: Gene vs Locus A gene is a sequence of DNA that codes for a particular trait or
function.
A locus is just the location or position on the chromosome where
that gene is found. Alleles are simply genetic variations of the
same gene
, Quantitative Traits Continued
Definitions Multilocus: Relating to more than one genetic locus.
Multilocus Traits Most traits are multilocus.
➔ These are very complex.
Image: https://slideplayer.com/slide/5679851/
Trait Variation When only 2 genes are involved on the single locus, the trait is very
simple.
If the genes effects that control a specific trait are known, the
phenotype can be predicted.
Example: Finger Length
➔ Gene 1
A allele, it contributes 4 cm
a allele contributes 2 cm
➔ Gene 2
B allele contributes 2 cm
b allele contribute 1 cm
See below the list of possible genotypes.
, Image: Lecture Slides
There are 9 possible genetic combinations coding for finger length.
The total finger length for each genetic combination can be
calculated using the individual contributions.
➔ aaBb combination = a (2cm) + a (2cm) + B (3cm) + b (1cm)
= 7cm
Different genetic combinations can result in the same overall trait
length: AAbb & AaBB both result in 10cm.
Histogram of Trait Sizes
● The 9 different genetic combinations result in 7 different
finger lengths.
● The more genes (loci) → the more genotypes → the more
classes (phenotypes) → and the more continuous the trait.
Alleles have the same effect.
● Most traits are continuous and are therefore controlled by
several loci/genes.
Written by Tegan Gibaud
Quantitative Genetics
Definitions Mendelian: Relating to Mendel’s theory of heredity.
Mendelian Genetics & Mendelian genetics are not quantitative.
Blood Types They are discrete traits (either/or). This is qualitative.
➔ Example: Red or white petals; no intermediates of pink.
Blood types are a good example of Mendelian Genetics.
Image: Lecture Slides
The genotype of an organism is the genetic code in its cells.
The phenotype is the visible or expressed trait. The phenotype
depends upon the genotype but can also be influenced by
environmental factors.
From the blood type diagram, it can be expected that certain
phenotypes will be more common than others if mating is random
and there is no selection acting upon them.
Blood types are entirely genetically controlled and there are four
distinct phenotypes with no intermediates. It is a 1 locus and 3
allele system.
Multiple Loci When Mendel looked at pea plants, he studied simple traits which
are generally controlled at 1 or 2 single loci, with only a few alleles
at each locus.
➔ But in animals and plants, traits are controlled at multiple
loci and can have many alleles at each locus.
There is a common misconception that tongue rolling is mendelian
and is only controlled by a single locus, but this has been proven
untrue. This is the same for dimples, hair whorls, widow’s peak and
hitchhiker’s thumb.
, However, ear wax is a Mendelian trait. Whether it is wet or dry is
discrete, just like blood types.
Summary Mendelian traits: Traits which are controlled by 1 or 2 loci with just a
few alleles at each locus.
➔ The traits are discrete or categorical in nature with no
intermediate forms.
Dry ear wax and blood types are good examples of Mendelian
Genetics.
Comparison of
Qualitative vs
Quantitative Traits
Note: Gene vs Locus A gene is a sequence of DNA that codes for a particular trait or
function.
A locus is just the location or position on the chromosome where
that gene is found. Alleles are simply genetic variations of the
same gene
, Quantitative Traits Continued
Definitions Multilocus: Relating to more than one genetic locus.
Multilocus Traits Most traits are multilocus.
➔ These are very complex.
Image: https://slideplayer.com/slide/5679851/
Trait Variation When only 2 genes are involved on the single locus, the trait is very
simple.
If the genes effects that control a specific trait are known, the
phenotype can be predicted.
Example: Finger Length
➔ Gene 1
A allele, it contributes 4 cm
a allele contributes 2 cm
➔ Gene 2
B allele contributes 2 cm
b allele contribute 1 cm
See below the list of possible genotypes.
, Image: Lecture Slides
There are 9 possible genetic combinations coding for finger length.
The total finger length for each genetic combination can be
calculated using the individual contributions.
➔ aaBb combination = a (2cm) + a (2cm) + B (3cm) + b (1cm)
= 7cm
Different genetic combinations can result in the same overall trait
length: AAbb & AaBB both result in 10cm.
Histogram of Trait Sizes
● The 9 different genetic combinations result in 7 different
finger lengths.
● The more genes (loci) → the more genotypes → the more
classes (phenotypes) → and the more continuous the trait.
Alleles have the same effect.
● Most traits are continuous and are therefore controlled by
several loci/genes.
