3.1The History of Genetics
Mendelian Genetics: Transmission of Traits to the Next Generation
Fig. 3-1. Overview of Mendel's Laws Using the Concept of Genes
Each individual organism receives one gene each from its father and mother for each trait. We can call the dominant gene "A" and the recessive gene "a." When the same form of a gene is passed on from both the father and mother, the combination AA or aa is formed. This state of having two of the same kind of genes is called "homozygous." If offspring are produced by a cross between AA and aa parents, the offspring's genetic type, called "genotype," will be Aa, and the phenotypes of all offspring will be dominant. An unmatched combination of a dominant and a recessive gene, such as Aa, is called "heterozygous." If offspring are produced by both parents having an Aa genotype, the offspring will have one of the three phenotypes: AA, Aa, or aa. Counting the frequency of expression shows that the ratio of the dominant phenotype to the recessive phenotype is 3:1. Genes that express mutually different traits such as A and a are referred to as "alleles"*2.
The story goes back to the 19th century. A monk named Gregor Mendel was growing peas in the backyard of a monastery. Normally, when seeds taken from flowers are sown without any special modifications, the peas will naturally grow with flowers in various colors and seeds in different colors and shapes during the following year. However, Mendel carefully self-pollinated the pea flowers by artificially pollinating their pistils with their own pollen, and was able to select peas from the subsequent generation that all expressed the same phenotype*1. As a result of these experiments, Mendel discovered the following three laws about the traits of succeeding generations: the Law of Dominance, the Law of Segregation, and the Law of Independent Assortment.
*1 Such peas correspond to the modern idea of a pure line.
■The Law of Dominance
Pea phenotypes have mutually opposite traits such as stem length (long or short), seed color (green or yellow), and shape (round or wrinkled). When peas with these opposite traits are crossed, only peas with long stems and round, green seeds are obtained in the next generation. This result shows that two types of traits are expressed as phenotypes: dominant traits, which are easily expressed, and recessive traits, which are not easily expressed. When these two kinds of traits compete with each other, the dominant one is expressed. These experimental results are called the "law of dominance." It is worth mentioning that crossing the peas having different traits will not cause intermediate traits to be expressed.
■The Law of Segregation
When progeny peas with dominant traits were self-pollinated, two phenotypes-one showing dominant traits and one showing recessive traits-were obtained in the subsequent generation.
Furthermore, when Mendel counted the frequency of the obtained dominant and recessive traits, he obtained a ratio of dominant to recessive traits as 3:1 (Fig. 3-1). This result can be explained by the idea that even when the phenotype is expressed as a dominant trait, the recessive trait does not completely disappear but is only covered up. The transmission of each gene of a gene pair (see explanation in Fig. 3-1) into a separate gamete in a 1:1 ratio is called the "law of segregation."
■The Law of Independent Assortment
Opposite pairs of traits related to stem length, seed color, and seed shape can be dominant or recessive. Stem length does not affect seed color. Moreover, there is no relationship between the color and shape of seeds. Thus, the fact that each trait is passed on without affecting other traits is referred to as the "law of independent assortment."
To explain Mendel's laws, it is reasonable to introduce the concept "genes" as what are passed down from parents to their offspring. One set of genes from the mother and another from the father are passed on to their offspring. If one dominant gene is transmitted from either of the parents, the dominant phenotype will be expressed. If corresponding recessive genes are passed on from both parents, the recessive phenotype will be expressed. If we call the dominant trait "A" and the recessive trait "a," crossing A and a with each other will yield a ratio of 3:1 (Fig. 3-1).
Later research showed that some types of particles were being equally distributed into the new cells*3 during cell division (Fig. 3-2). The law of independent assortment could easily explain the expression of these particles called "chromosomes." If the genes that determine each trait are on separate chromosomes, then each trait will follow the law of independent assortment.
When Mendel's three laws were published in 1865, no one could comprehend them. After the beginning of the 20th century, however, Mendel's laws were rediscovered, and his results then finally gained acceptance. Although Mendel's laws have required some revision, they are now widely accepted as important laws of genetics. The re-discovery of Mendel's laws also gave rise to a primitive concept of genes as things that pass on heritable traits.
*2 Referred to as "daughter cells."
*3 This term describes not only genes but also variations in DNA sequences in matching positions on matching chromosomes.
Watson and Crick's Discovery
The rediscovery of Mendel's laws during the 20th century sparked a renewed effort to search for the true nature of genes. Results of experiments to determine the actual substance of genes proved that a chemical substance called "deoxyribonucleic acid (DNA)" somehow performed an important function as the actual substance of genes. Under these circumstances, Watson and Crick published the double-helix model of DNA in 1953. DNA is formed of two intertwined strands (chains) (see Chapter 2, Fig. 2-5).
Four bases form the DNA strands: A, G, C, and T. Like pieces of a puzzle, C binds to G and T binds to A. Based on this orderly G-C and A-T bonding, two DNA strands take on the structure of a precise helix and intertwine. When this orderly spatial structure was discovered, it was easily speculated that the sequence of bases in the DNA structure carried genetic information.
Mechanism for Accurate Replication of Genes
The accurate transmission of genes to subsequent generations can be explained by the double-helix model of DNA. DNA is formed from A, G, C, and T, which line up in a row similar to a chain to form a sequence. Furthermore, the double-helix structure of DNA is formed of two strands. One strand forms a sequence that complements the base sequence on the other strand according to the AT/GC rule. When DNA replicates, first, the structure of these two strands unwinds to expose both the base sequences. Being complementary to the exposed DNA sequence, a new strand is then formed with C complementing G and T complementing A. The two DNA strands that have been obtained now have exactly the same sequence as the original DNA double strand. This process of one strand of DNA becoming the template of a newly-synthesized DNA sequence is called "semiconservative replication" (Fig. 3-3).
Mendel's Laws as Observed in Humans
In modern life sciences, many genes are usually thought to be involved in any one biological phenomenon, as explained in Chapter 4. In fact, there are almost no cases of one trait (phenotype) being determined by one gene. In humans, characteristics such as double eyelids, blood type, head shape (Widow's peak), and dimples are well-known examples to which Mendel's laws apply. In case of blood type, however, some researchers claim that in the strict sense, Mendel's laws do not apply because this trait is controlled by multiple genes. Genetic diseases are defined as diseases caused by gene mutations, and the genes that cause many of these diseases have been discovered. Considered at the genetic level, most of these genetic diseases follow Mendel's laws. Muscular dystrophy is one such disease.