Skin Color, IQ, and Quantitative Inheritance



I. Characteristics of Quantitative Traits

A. Continuous:  Characters vary in the population along a continuum.

B. Controlled by many genes (polygenic):  an addictive effect of two or more genes on a single
     phenotypic character.

C. Strongly influenced by environment:  sunshine on skin color, school/family on IQ?

D. Controlled by addictive genes with nodominance.

II. An Example of a Quantitative Trait:

A.  Genetic control of color in wheat kernels:

Assume, two loci, each with two alleles involved in the genetic control of color in wheat kernels:
 
 

III. Heritability of Quantitative Traits
 

A.  Heritability:  A measure of the degree to which the variance in the distribution of
                         a phenotype is due to genetic causes.

In the broad sense, it is measured by the total genetic variance divided by the total phenotypic variance.

In the narrow sense, it is measured by the genetic variance due to additive genes divided by the total phenotypic variance.
 
 

B. Quantitative Inheritance in Human Beings

Skin Color.
 

A Biblical View of Inheritance in Human Skin Color.  (for your reference…)
1. Adam and Eve were the first parents of all the races. Adam and Eve contained all the genetic information from which eventually all the races came.

2. From Adam to Noah, all descendants of Adam and Eve were probably all mid-brown color since Adam and Eve were also mid-brown.

3. After the global flood and the tower of Babel incident, descendants of Noah separated into people groups according to their own languages and traveled to different parts of the world.

4. As different "people groups" were exposed to different environments, natural selection occurred resulting in certain genetic traits to be enhanced for adaptability (for example: darker skin pigmentation for environments with more intense sunlight due to the genetic "potential" to increase more melanin).

5. As the "people groups" were isolated and intermarried with each other with a certain group, they eventually lost certain genes that were not needed for adaptability. (That would explain, from this point of view, why African Negroes who move to different northern environments or European Whites who move down to Africa, do not change back to another color because over time they previously lost the genetic potential to do so.)
 

A Modern Molecular Biology Interpretation of this view:

There are at least three and perhaps four genes involved in skin color and several alleles at each gene producing differing amounts of melanin.

Adam and Eve would not necessarily need to possess the entire range of skin gene possibilities since there is some time for accumulation of mutations between them and Noah's sons. With that said, since Adam and Eve would both possess two copies of each gene, that means a possible total of at least 4 different alleles at each gene and if there are 3 different genes, that means 12 different alleles which could be combined 144 different ways. This would seem more than adequate to accommodate the full range of human skin color.
 

                Ray Bohlin
                Probe Ministries
 

A case study showing the effect of three skin color genes (each gene with two alleles):  at each locus, one allele adds a measure of color, whereas the other adds none.

Polygenic inheritance in people showing a cross between two mulatto parents (AaBbCc x AaBbCc). The offspring contain seven different shades of skin color based on the number of capital letters in each genotype.
 
Gametes ABC ABc AbC Abc aBC aBc abC abc
ABC 6 5 5 4 5 4 4 3
ABc 5 4 4 3 4 3 3 2
Abc 5 4 4 3 4 3 3 2
AbC 4 3 3 2 3 2 2 1
aBC 5 4 4 3 4 3 3 2
aBc 4 3 3 2 3 2 2 1
abC 4 3 3 2 3 2 2 1
abc 3 2 2 1 2 1 1 0

6: is the darhest color; 0: is the weakest color.

IQ  (Intelligence Quotient):

IQ is controlled by more than one genes, and is considered as quantitative traits.

A study of monozygotic twins (identical twins):

Bouchard and McCue (1981);
Bouchard, Lykken, McGue, Segal, & Tellegen (1990)

Dr. Bouchard at the U of MN has done extensive research on twins reared apart. He has tested about 86 pairs, all tested in adulthood. Most were separated within the first year of life and some did not even know the other existed until Bouchard got them together. 56 MZ twins reared apart.

RESULTS: there is a correlation between IQ scores of twins reared apart.

   IQ Corr

MZ reared together           0.86
MZ reared apart                0.72

Conclusions:  There is an inherited component to IQ. At most it accounts for about 50% of the variability.
                       The rest will be environmental?

Ten years later (August 8, 2000)…  BBC News Online: Sci/Tech

“Genius of Genes” by Pallab Ghosh
Report:

US researchers believe they have identified the parts of the human genome  involved in developing a person's intelligence.
The researchers, working for the US National Institutes of Health, analyzed the DNA of 200 of the brightest kids in America and compared them with the genetic material from ordinary children.
The team believes more than one gene is involved - and that these genes can  make a big difference to a person's intelligence. The research was led by Professor Robert Plomin.

"I think we need to recognize that genetic influences are important and that we will find genes for intelligence," he told the BBC. "Each may account for a small piece of the action, but together they give us a significant source of prediction for intelligence."

The next step will be to discover what these genius genes do. One theory is that they help make nerve cells. They help transmit signals, our thoughts if you like, from one part of the brain to another.

              The impact of this study…