Background Information

While developing his theory of evolution by natural selection, Charles Darwin was unaware of the molecular basis for evolutionary change and inheritance. Around the same time that Darwin was making his landmark observations that led to his publication, The Origin of Species, the Augustinian monk Gregor Johann Mendel was performing experiments with garden peas that dramatically influenced the field of biology. Mendel's interpretation of his experimental results served as the foundation for the discipline known as genetics. Genetics is the discipline of biology concerned with the study of heredity and variation. In 1866, Mendel published a classic paper in which he described phenomena in garden peas that laid down the principles for genetic inheritance in living organisms.

Our understanding of genetics in higher organisms such as humans has advanced considerably since Mendel's work. This has occurred, in part, because of significant advances in our understanding of the molecular biology of living cells. The molecular biology revolution that led to our current understanding of genetics was greatly accelerated in 1953 by James Watson and Francis Crick, who revealed the structure of DNA as a double-helical molecule. Considering that a detailed understanding of the structure of DNA, chromosomes and genes, and the events of meiosis were not known during Mendel's time, Mendel's work was a particularly incredible accomplishment. Approximately 40 years passed before the significance of Mendel's insight was realized. Mendel's work gained acceptance as his experiments were replicated and publicized by scientists who performed genetic experiments several years after Mendel died. Mendel's landmark principles on inheritance continue to form the basis for our current understanding of genetics in living organisms. As a modern-day student studying biology, having the benefit of hindsight by studying gene and chromosome structure, and understanding how gametes form by meiosis, is a great advantage in developing an understanding of inheritance.

The experimental model for Mendel's work involved performing cross-pollination experiments with a strain of garden peas called Pisum sativum. Mendel was interested in studying the inheritance of a number of different characters, or heritable features, of Pisum. He considered seven different characters including flower color, flower position, seed color, seed shape, pod color, pod shape, and stem length. Variations of a given character are known as traits. For example, when studying flower color as a character, Mendel traced the inheritance of two traits for flower color, purple flowers and white flowers. Many of the basic genetic principles established by Mendel arose from his observations of the results produced by a simple cross-pollination experiment called a monohybrid cross. In a monohybrid cross, individuals with one pair of contrasting traits for a given character are mated. For example, a plant with purple flowers is mated with a plant that has white flowers.

Mendel proposed several basic principles to explain inheritance. One of his first principles was that characters are determined by what he described as "heritable factors" or "units of inheritance." The factors that Mendel described are what we now call genes. Mendel noted that alternative forms of a gene, what we now call alleles, are responsible for variations in inherited characters. For example, in Pisum sativum there are two alleles for flower color, a white flower allele and a purple flower allele. Mendel also proposed that certain alleles, called dominant alleles, are always expressed in the appearance of an organism and that the expression or appearance of other alleles, called recessive alleles, was sometimes hidden or masked. Recall that modern-day geneticists frequently use capital letters to indicate dominant genes and lower case letters to indicate recessive genes. The appearance of a particular trait is referred to as the phenotype of an organism while the genetic composition of an organism is known as the organism's genotype. Mendel established that an organism typically inherits two alleles for a given character, one from each parent. Using modern genetic terminology, we say that an organism that contains a pair of the same alleles is homozygous for a particular trait while an organism that contains two different alleles for a trait is heterozygous for that trait. Mendel also postulated that when an organism contains a pair of units, the units separate, or segregate, during gamete formation so that each individual gamete produced receives only one unit of the pair. This postulate became known as Mendel's law of segregation of alleles.

One example of a monohybrid cross performed by Mendel involved the inheritance of pea flower color. Mendel began by crossing true-breeding homozygous parents called the P generation. He crossed a plant with purple flowers with a plant showing white flowers. The first offspring produced from such a cross, called the F1 generation, all showed purple flowers. The results of this cross were explained when Mendel determined that the F1 plants were heterozygotes and that these plants showed purple flowers because the purple flower allele was dominant and the white flower allele was recessive. Self-pollination of F1 plants produced a second generation of plants called the F2 generation, in which the phenotypic ratio of purple-flowered plants to white- flowered plants was approximately 3:1. As he performed monohybrid crosses for several other characters, Mendel discovered that this ratio was characteristic of a cross between heterozygotes. The genotypes and phenotypes that may result from a genetic cross can be predicted by using a Punnett square. You should be familiar with constructing Punnett squares to predict the results of monohybrid and dihybrid crosses. Mendel used dihybrid crosses to explain his law of independent assortment, in which he postulated that alleles for different characters (for example, pea color and pea shape) segregate into gametes independently of each other. A dihybrid cross between F1 heterozygotes reveals a phenotypic ratio of approximately 9:3:3:1 in the F2 generation.

The 3:1 ratio predicted for Mendel's monohybrid cross and the 9:3:3:1 phenotypic ratio predicted for a dihybrid cross are hypothetical expected ratios. When performing a real genetic cross based on Mendelian principles, however, such a cross is subject to random changes and experimental errors that produce chance deviation in the actual phenotypic ratios that one may observe. To accurately evaluate genetic inheritance, it is essential that observed deviation in a phenotypic ratio be determined and compared with predicted ratios.

Chi-square analysis is a statistical method that can be used to evaluate how observed ratios for a given cross compare with predicted ratios. Chi-square analysis considers the chance deviation for an observed ratio, and the sample size of the offspring, and expresses these data as a single value. Based on this value, data are converted into a single probability value (p=value), which is an index of the probability that the observed deviation occurred by random chance alone. Biologists generally agree on a p=value of 0.05 as a standard cutoff value, known as the level of significance, for determining if observed ratios differ significantly from expected ratios. A p=value below 0.05 indicates that it is unlikely that an observed ratio is the result of chance alone. When we predict that data for a particular cross will fit a certain ratio­for example, expecting a 3:1 phenotypic ratio for a monohybrid cross between heterozygotes­this assumption is called a null hypothesis. Chi-square analysis is important for determining whether a null hypothesis is an accurate prediction of the results of a cross. Based on a p=value generated by chi-square analysis, a null hypothesis may either be rejected or fail to be rejected. If the level of significance is small (p < 0.05), it is unlikely (low probability) that the observed deviation from the expected ratio can be attributed to chance events alone. This means that your hypothesis is probably incorrect and that you need to determine a new ratio based on a different hypothesis. If, however, the level of significance is high (p > 0.05), then there is a high probability that the observed deviation from the expected ratio is simply due to random error and chi-square analysis would fail to reject your hypothesis.

It is important to realize that the principles established by Mendel can be easily explained by understanding the chromosomal basis of inheritance. For example, in humans, somatic cells contain the diploid number (2n) of chromosomes, which consists of 46 chromosomes organized as 23 pairs of homologous chromosomes (homologues). Chromosomes 1 through 22 are called autosomes and the twenty-third pair of chromosomes are called sex chromosomes. During meiosis, gamete formation leads to the formation of sex cells that contain a single set of 23 chromosomes­the haploid number (n) of chromosomes. Because chromosomes are present as pairs in human cells, genes located on each chromosome are also typically present as pairs. Mendel¹s law of segregation of alleles is explained by the separation of each pair of homologous chromosomes that occurs during meiosis resulting in each individual gamete receiving only one copy of each chromosome. This law applies to, and is explained by, the separation of unlinked genes because these genes are located on different chromosomes. As a result, during meiosis the chromosomes align at the metaphase plate and separate in a random, independent fashion.

An expansion in our knowledge of Mendelian genetics has led to a detailed understanding of different types of inheritance events such as codominance, incomplete dominance, linked inheritance, inheritance of multiple alleles, lethal mutations, and the genetic transmission of a number of different genetic disorders. For many reasons, Mendel's pea plants served as an ideal model organism for him to study. In fact, pea plants allowed Mendel to succeed where others failed. Peas are easy to grow, they cross-fertilize and self-fertilize easily, and they mature quickly. More recently, a number of other organisms have served similar roles as model organisms for scientists who study genetics. Some of these include the flowering plant Arabidopsis thaliana, a common inhabitant of home aquariums called the striped zebrafish (Brachydanio rerio), many species of mice, and the common fruit fly, Drosophila melanogaster.

In particular, Drosophila has been one of the most well studied model organisms for learning about genetics and embryo development. These small flies are hardy to grow under lab conditions, and they reproduce easily with a relatively short life cycle compared with vertebrate organisms; hence, crosses can be performed and offspring counted over reasonable intervals of time. Another advantage of Drosophila is that the loci for many genes on the four chromosomes in the fly's genome have been determined, and a very large number of mutations of the wild-type fly have been developed that affect different phenotypes in Drosophila. Because of some of these characteristics, Drosophila has an important historical place in the field of genetics and continues to be an important model organism for studying genetic inheritance that universally applies to most organisms. Using FlyLab, you will design and carry out experimental crosses using Drosophila melanogaster. In the future, you will have the opportunity to use PedigreeLab to study inheritance of different genetic disorders through several generations of humans.

References

  1. Klug, W. S., and Cummings, M. R. Essentials of Genetics, 2nd ed. Upper Saddle River, NJ: Prentice Hall, 1996.

  2. Lindsley, D. and Zimm, G. The Genome of Drosophila melanogaster. San Diego: Academic Press, 1992.

  3. Orel, V. Mendel. New York: Oxford University Press, 1984.

  4. Soudek, D. "Gregor Mendel and the People Around Him." Am. J. Hum. Genet., May 1984.

  5. Stern, C., and Sherwood, E. R. The Origin of Genetics: A Mendel Source Book. San Francisco: W. H. Freeman, 1966.