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Anth 125 Introduction to Biological Anthropology
Dr. Darlene Applegate
Spring 2008
GROUP LAB PROJECT 1:
INHERITANCE OF SIMPLE TRAITS

OBJECTIVES


TERMS

BASIC PRINCIPLES OF GENETICS AND INHERITANCE

Inheritance has been an important component of evolutionary thinking since Charles Darwin (1809-1882) formulated his theory of “descent with modification through natural selection.” In order to lead to evolutionary change in sexually reproducing organisms, traits or alternate expressions of traits – some of which would provide an adaptive advantage and others which would not – have to be transmitted from parent to offspring, from one generation to the next.

Scientists at the time of Darwin lacked an adequate explanation of how traits are inherited. One hypothesis, known as blending inheritance, posited that the traits of both parents are blended together and then passed to their children. Another hypothesis, pangenesis, proposed that all cells of the body contribute particles, known as pangenes, to the sex cells or gametes (egg, sperm), which are then passed to offspring. Unbeknownst to Darwin and his contemporaries, in eastern Europe a monk named Gregor Mendel (1822-1884) developed an alternate explanation of inheritance based on breeding experiments with pea plants.

Like the pangenesists, Mendel argued that discrete particles of inheritance are passed from parents to offspring; however, these discrete particles are not contributed by body cells and, instead, derive only from the sex cells. Mendel formulated two laws or principles to explain how these particles are inherited. According to Mendel’s law of segregation, both parents contribute equally to their offspring. In other words, half of the particles of inheritance derive from one’s mother and half from one’s father. According to Mendel’s law of independent assortment, the particles of inheritance for one trait or alternate expressions of that trait are passed independently of the particles for other traits/trait expressions. In other words, just because an offspring inherited the particle for sticky ear wax from his/her mother doesn’t necessarily mean the offspring will inherit the particle for attached ear lobes from his/her mother.

dna helixSubsequent research by scientists provided overwhelming empirical evidence to support Mendel’s work. Further, research by molecular biologists and chemists identified the discrete particles of inheritance, which Mendel was unable to do. The particles of inheritance are chromosomes, which are found in the nuclei of sexually reproducing organisms. (There is genetic material in other parts of the cell, but this genetic material is not organized as chromosomes.) Different species have different numbers of chromosomes. For instance, humans have a total of 46 (23 pairs) chromosomes in the nuclei of each body cell (but not the sex cells), while chimpanzees have 48 (24 pairs). The number of chromosomes is not a direct indication of the species’ complexity.

Chromosomes are long strands of very tightly coiled deoxyribonucleic acid or DNA. The DNA molecule is composed of two bonded, parallel strands of nucleotides arranged in a double helix structure that resembles a twisted ladder. Nucleotides, in turn, are composed of phosphates, sugars, and nitrogenous bases. There are four bases: adenine, thymine, guanine, and cytosine.

Segments of DNA, or a series of adjacent nucleotides, that control the expression of a particular trait are called genes. Alternate expressions of a gene, which depend on the specific bases comprising the segment of DNA corresponding to the gene, are referred to as alleles. For example, there is a gene that codes for type of ear wax, and there are two alleles for that gene, one for gray/dry ear wax and one for yellow/sticky ear wax. The location of the gene within the DNA sequence is the locus. Though the genes for many human traits have two alleles, some genes have more than two alleles. For example, the gene for the ABO blood group has three alleles (A, B, O).

gene on chromosome
Since sexually reproducing organisms have a pair of each chromosome in the cell nuclei – one inherited from each parent, as predicted by Mendel’s law of segregation – they also have a pair of alleles for each gene on the chromosomes. If an individual has two of the same alleles for a trait, the individual is said to be homozygous for the trait. If an individual has different alleles for a trait, the individual is said to be heterozygous for the trait. The genetic makeup of an individual, or the alleles the individual has for each trait, is referred to as the genotype.

Some alleles are expressed in the outward physical appearance or phenotype of the individual, while some alleles are not expressed phenotypically. Alleles that are expressed phenotypically are called dominant and alleles that are masked or hidden from expression by dominant alleles are called recessive. An individual with two dominant alleles for a trait are referred to as homozygous dominant for that trait, while an individual with two recessive alleles for a trait are referred to as homozygous recessive for that trait. In order for a recessive allele to be expressed phenotypically, the individual must have two of the recessive alleles (homozygous recessive). There also are codominant alleles, which are both expressed phenotypically in individuals with both alleles.

For instance, in the ABO blood group, A and B are codominant alleles and O is recessive. Therefore, an individual with either AO or AA genotypes has blood type A phenotypically; an individual with either BO or BB genotypes has blood type B phenotypically; an individual with an OO genotype has type O blood phenotypically; and an individual with an AB genotype has blood type AB phenotypically. As illustrated in the previous example, in inheritance studies alleles are represented by single letters. Dominant alleles are represented by upper-case letters, while most but not all (like the O allele in the ABO blood group) recessive alleles are represented by lower-case letters. By convention, the dominant allele is always listed before the recessive allele when notating the genotype of a particular trait.

To reiterate, an individual has two alleles for each trait, one allele inherited from mom and one from dad. These alleles are carried on the pairs of chromosomes that the individual has, with one chromosome of each pair inherited from mom and one from dad. Since mom and dad each, themselves, have pairs of chromosomes and therefore pairs of alleles that they inherited from their parents, there has to be some process by which those pairs of chromosomes split in order that each parent can pass on just one of each chromosome pair to their offspring. That process is meiosis, or the process of cell division in the gametes.

Simply, the process of meiosis involves duplication of the chromosomes in the nucleus of the egg or sperm cell, an initial division of the cell into two cells, and a second division of the two cells into four. In humans, an egg or sperm has 23 single chromosomes (half of the typical compliment of 46 in human cell nuclei) in the cell nucleus. These duplicate to create 46 chromosomes. Then the egg or sperm separates into two cells, each with half of the duplicated chromosomes or 23 different chromosomes; but these chromosomes are two-stranded, meaning they have two copies of the DNA. In the second division, the two cells and their double-stranded chromosomes each divide, producing four cells, each with 23 chromosomes with a single copy of the DNA.

Meiosis, then, results in the production of four gametes, each with half of the genetic material of the mother or father. Further, the resulting gametes are not necessarily identical copies of the original sex cell. Upon fertilization of an egg by a sperm cell, the full compliment of chromosome pairs is restored in the fertilized egg or zygote. The process of meiosis is more complicated than the description provided here – for example, the production of egg cells actually results in the formation of just one viable cell rather than four – so refer to the Relethford textbook for more details.

It is important to emphasize that the preceding discussion relates to the inheritance of simple or monogenic traits, which are traits coded by the base sequence at a single gene locus. Examples of simple traits are ear wax type, ear lobe attachment, tongue rolling, and hair whorl direction. There are other laws for the inheritance of complex or polygenic traits, which are traits coded by the base sequences at two or more gene loci. Examples of complex traits are pigmentation (skin color, hair color, eye color), stature or height, and body proportions. The principles of inheritance for complex traits are beyond the scope of this introductory course, but are discussed in upper-level courses like Anth 450 Modern Human Biological Variation.

INHERITANCE STUDIES

Study of the inheritance of simple traits, at the most fundamental level, is a three-step process that involves applying Mendel’s two laws of particulate inheritance. The first step involves determining what alleles for a trait will be present in the parents’ gametes following the process of meiosis. The second step involves determining the possible combinations of alleles for a trait in a zygote, which will depend on which egg is fertilized by which sperm; this step involves using graphical representations known as Punnett squares. The third step involves determining the resulting phenotypic expression of the trait in the zygote offspring.

Two examples serve to illustrate the three-step process. The first example involves inheritance of one simple trait, while the second example involves inheritance of two simple traits. Remember, however, that humans have tens of thousands of simple traits, so the possible combinations of genotypes in the offspring is huge (hence the great diversity seen in human phenotypes!).

Inheritance of One Simple Trait

In the first example we will use the trait of tongue rolling. The ability to roll the edges of one’s tongue upward is inherited as a dominant (R), while the inability to roll one’s tongue is recessive (r). Therefore, an individual who can roll his/her tongue (phenotype) must have at least one dominant allele, which would result from either a heterozygous genotype (Rr) or a homozygous dominant genotype (RR). An individual who cannot roll his/her tongue (phenotype) must have two recessive alleles or a homozygous recessive genotype (rr).

Here is a sample problem. A woman who is heterozygous for tongue rolling mates with a man who is heterozygous for tongue rolling. What are the possible genotypes and genotype proportions for their offspring? What are the possible phenotypes and phenotype proportions of their offspring?

Remember that the solution involves three steps. First, we must determine what alleles are passed by each parent in the gametes. The genotype for the woman is Rr. Though her body cells contain both alleles, her egg cells only contain one – either R or r. So, she will pass either an R allele or an r allele to her offspring. The genotype for the man is Rr. Though his body cells contain both alleles, his sperm cells only contain one – either R or r. So, he, too, will pass either an R allele or an r allele to his offspring.

Second, a Punnett square is used to determine the possible combinations of alleles in the offspring. The alleles contributed by each parent are placed along the two axes of the square and a matrix is created. Each cell of the matrix contains the combination of alleles or genotypes that could appear in the offspring. The proportion of each genotype is determined by dividing the number of cells with a particular genotype by the number of cells in the matrix; genotype proportions may be expressed as fractions, decimals, or percentages. The Punnett square for this sample problem is given below.


 R
 r
R
 RR
 Rr
r
 Rr
 rr

The genotypes and genotype proportions for this sample problem are given below. In other words, there is a 25% chance the child produced by the mating will be homozygous dominant, a 50% chance the child will be heterozygous, and a 25% the child will be homozygous recessive. Note that the genotype proportions must always add to 1 or 100%.

RR    1/4 or 0.25 or 25%

Rr    2/4 or 0.50 or 50%

Rr    1/4 or 0.25 or 25%

Third, the possible genotypes are translated into possible phenotypes and phenotype proportions, as shown below. Since offspring with at least one dominant allele will be able to tongue roll, there is a 75% the offspring will be a tongue roller. There is a 25% chance the offspring will not be a tongue roller. Note that the phenotype proportions, too, must always add to 1 or 100%.

tongue roller  →  RR + Rr  →  25% + 50%  →  75%

non-tongue roller  →  rr  →  25%

Inheritance of Two Simple Traits

In the second example we will use the traits of tongue rolling and ear lobe attachment. Ear lobes in humans are either free hanging, or they are attached to the skin of the head. Free-hanging ear lobes are inherited as a dominant (E), while attached ear lobes are inherited as recessive (e). Therefore, an individual who has free-hanging ear lobes (phenotype) must have at least one dominant allele, which would result from either a heterozygous genotype (Ee) or a homozygous dominant genotype (EE). An individual with attached ear lobes (phenotype) must have two recessive alleles or a homozygous recessive genotype (ee).

Here is a sample problem. A woman who is heterozygous for tongue rolling and heterozygous for ear lobe attachment mates with a man who also is heterozygous for both traits. What are the possible genotypes and genotype proportions for their offspring? What are the possible phenotypes and phenotype proportions of their offspring?

First, we must determine what alleles are passed by each parent in the gametes. The genotype for both the woman and the man is RrEe. Since one allele from each pair will be passed in each egg and sperm, the gametes could contain one of the following combinations of alleles:  RE, Re, rE, or re. Note that one must list the same trait first – regardless of whether it is a dominant or recessive allele – when writing the possible combinations of genotypes for two or more traits.

Second, a Punnett square is used to determine the possible combinations of alleles in the offspring. The egg will have RE, Re, rE, or re genotype, as will the sperm. The resulting Punnett square for will have 16 cells.


 RE Re rE re
RE RREE RREe RrEE RrEe
Re RREe RRee RrEe Rree
rE RrEE RrEe rrEE rrEe
re RrEe Rree rrEe rree

The possible genotypes and genotype proportions for this sample problem are given below.

RREE    1/16 or 0.0625 or 6.25%
RREe    2/16 or 0.125 or 12.50%
RrEE    2/16 or 0.125 or 12.50%
RrEe    4/16 or 0.25 or 25%
RRee    1/16 or 0.0625 or 6.25%
Rree    2/16 or 0.125 or 12.50%
rrEE    1/16 or 0.0625 or 6.25%
rrEe    2/16 or 0.125 or 12.50%
rree    1/16 or 0.0625 or 6.25%

Third, the possible genotypes are translated into four possible phenotypes with the following phenotype proportions.

tongue roller, free-hanging ear lobes  → RREE + RREe + RrEE +  RrEe  →  6.25% + 12.50% + 12.50% + 25%  →  56.25%

tongue roller, attached ear lobes  →  RRee + Rree  →  6.25% + 12.50%  →  18.75%

non-tongue roller, free-hanging ear lobes  →  rrEE + rrEe  →  6.25% + 12.50%  →  18.75%

non-tongue roller, attached ear lobes  →  rree  →  6.25%


ASSIGNMENT

The assignment will be completed during class time on Thursday, February 7.  Students will work in groups of three. A calculator and a pencil are needed to complete the lab.

A series of problems like the two examples given above will be distributed to each group during class. For each problem, students will calculate the genotypes, genotype proportions, phenotypes, and phenotype proportions for possible offspring of a given mating. Be sure the phenotype descriptions are complete. All work must be shown on the answer sheet.

All proportions must be expressed as percentages. For percentages that are not whole numbers, answers must be rounded to hundredths place.


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Last updated on January 21, 2008
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