During Investigation Two, students determined that organisms have pairs of alleles on each chromosome. The inheritance of specific combinations of alleles results in the expression of dominant and recessive traits. Differences in the combination of alleles are what give variations in traits across populations of organisms.

The biological variation found in humans has a very high level of complexity. This is due, in part, to the large number of allelic combinations that result from the approximately 50,000 genes that are found in the human genome. Each gene has two alleles resulting in a human genome that consists of 100,000 alleles. During sexual reproduction, half of the father’s chromosomes and half of the mother’s chromosomes are passed to their offspring. In humans, this means that 23 chromosomes come from the father and 23 chromosomes come from the mother. This results in an offspring with 46 individual chromosomes or 23 pairs of each chromosome consisting of one each from the father and the mother.

Complete Dominance

Another contributor to the high level of biological variation in humans is the random combination of parental chromosomes that are passed to offspring. We discussed complete dominance in Investigation Two. In complete dominance, an offspring can receive one or the other chromosome of each pair of chromosomes from their father and mother. This random association of parental chromosomes results in a staggering number of possible combinations of chromosomes resulting in great variation among humans. There are approximately 8 million different combinations of parental chromosomes that an offspring can inherit. Considering that there are two alleles of the 50,000 human genes, there are approximately 70 trillion different combinations of alleles that an offspring can inherit. This high potential for biological variation in the human population results in the unique characteristics of every individual.

Incomplete Dominance

The high level of human biological variation is also due to different, more complex mechanisms of dominance and recessiveness. Investigation Two represented traits as the result of dominant and recessive alleles of one gene. The alleles of many traits are not expressed as simply dominant or recessive. One mechanism of expression, termed incomplete dominance, occurs when each of the two alleles is expressed, so the trait is intermediate in appearance between the two different alleles. Incomplete dominance occurs when an offspring inherits one variation of the allele from one parent and another variation of the allele from the other parent. For example, if neck length is the result of incomplete dominance, a human father with two long neck alleles and a mother with two short neck alleles will produce an offspring that has a medium-length neck.

In contrast to the case of complete dominance in which there is a single notation system that represents the dominant and recessive alleles using upper and lower case letters, there are several different notation systems that scientists have developed to represent the different alleles in cases of incomplete dominance. In one notation system, the gene is identified with a letter and the different alleles are represented with different numerals. In the example of neck length described above, the father’s two long neck alleles would be represented as N1N1, and the mother’s two short neck alleles would be represented as N2N2. The offspring would have a medium neck length, with the his/her alleles represented as (N1N2). In a second notation system, the gene is identified by a letter and the different alleles are represented by other letters that describe the trait. Using this notation system, the gene might be assigned the letter N for neck length, with the letter L used to describe the “long” length and the letter S used to describe the “short” length. The father’s alleles would be NLNL, and the mother’s alleles would be NSNS, thus the offspring’s alleles would be NLNS. The NLNS genotype would represent the medium-length neck phenotype. For consistency, the experiments and discussions in Investigation Three will use the letter/number notation system (e.g. N1N2) to represent the alleles in examples of incomplete dominant traits.

Co-Dominance

A second mechanism, termed co-dominance, occurs when the trait is the result of equal contributions of each of the two alleles. Co-dominance occurs when an offspring inherits an allele from one parent and a second, different allele from the other parent, each of which is expressed equally. For example, the AB blood type in humans results from the presence of the A allele and the B allele which are both expressed resulting in the AB blood type. Although there are other traits that are co-dominant, this Investigation will focus only on blood type. As with incomplete dominance, there are several different notation systems associated with co-dominant traits. The notation system used in Investigation Three for the exploration of blood type will be that in which each different allele is represented by a different capital letter.

During Investigation Three, you will explore the high level of biological variation found in the human face. The different traits that contribute to the appearance of the human face will result from simple dominant/recessive pairs of alleles, incomplete dominance, and co-dominance. You will randomly select allele pairs for several facial traits, analyze the alleles that were selected and draw a picture of a human face. You will discover that allele pairs will be different than the allele pairs chosen by other students in the class, resulting in the unique appearance of each face.