Variation, genetic differences essential to the survival of a species, are passed on from parent to offspring, generation to generation. Genetics, the study of this inheritance, is an expanding and ever-changing branch of science. New discoveries are being made daily as our knowledge and understanding of the molecule of inheritance, DNA, grows. From its beginnings with the work of Gregor Mendel to the modern-day face of genetics, patterns of inheritance have emerged, been refined, and even been rejected in the light of new information.
Patterns of inheritance first became evident through the work of Gregor Mendel. Mendel bred and experimented with pea plants to examine why variation in traits such as pod color and pod shape existed and how they were passed from one generation to the next. He realized that each trait was governed by factors inherited from each parent. An individual had two copies of these factors, today called genes, where one was paternal and one was maternal. From his extensive data he was able to determine that each trait seems to have two forms, one that is dominant and outwardly expressed if present. The other form is recessive and can be masked by the dominant form. For example, pea plants that were tall could be bred with those that were short. The resulting offspring were all tall. If these tall plants were bred, the next generation showed a three to one ratio of tall to short plants. This led Mendel to a law of inheritance called the Law of Dominance. Mendel did hundreds of crosses that followed one trait, a monohybrid cross, through several generations and this data trend repeated itself. He also examined pairs of traits, dihybrid crosses, through several generations. In this type of cross, he found that the factors that govern each of the two traits are inherited independently of each other. This was due to the presence of offspring that had new combinations of traits than the parental generation. For example, he crossed plants with yellow, round seeds with plants with green, wrinkled seeds. All of the offspring of this cross had yellow, round seeds. However, when he allowed the yellow, round plants to self-pollinate he found offspring with yellow, round seeds only 9/16th of the time. The green, wrinkled combination only showed up 1/16th of the time.New combinations of yellow, wrinkled and green, round each showed up 3/16th of the time. This evidence led to the Law of Independent Assortment.
These basic patterns of inheritance hold true in many situations, but anomalies kept showing up that did not follow what is now called Mendelian Inheritance or simple inheritance patterns. These anomalies have been shown to fall into one of the many complex inheritance patterns: Incomplete Dominance, Co-dominance, Multiple Allelism, Linkage or Sex-linkage, Polygenic Inheritance, and Epistasis.Incomplete dominance involves individuals that show an intermediate phenotype or physical appearance when compared to the parent generation. In snap dragons for instance, a red and a white flower when bred create all pink offspring. Two pinks produce 25% red, 50% pink, and 25% white. This does not meet the 3:1 ratio expected when following Mendelian patterns. Co-dominance leads to individuals that show both parental phenotypes.For example, a black feathered chicken mated with a white feathered chicken produces offspring with black and white banded feathers. Some traits are governed by more than two forms, today called alleles. A common example is coat color in rabbits. There are four alleles with varying degrees of dominance. The presence of multiple alleles definitely complicates predicting the outcome of genetic crosses.
Another complex pattern of inheritance is called linkage. This is seen when traits are located on the same chromosome. This was discovered while studying dihybrid crosses. These crosses should have produced a 9:3:3:1 phenotypic ratio based on the Law of Independent Assortment. The data from these crosses which were carried out in Drosophila melanogaster, the common fruit fly, did not fit that which was predicted. Instead, a high number of offspring were produced that matched the physical appearance of the parental generation. Through further study, Alfred Sturtevant working under the supervision of Thomas Hunt Morgan, determined that the factors or genes for the two traits had to be located on the same chromosome and were therefore linked. A consequential discovery from this work was a understanding of crossing over, a form of genetic recombination that occurs during the process of meiosis, the cell division process that creates sex cells. Sturtevant reasoned that since the genes were linked only parental phenotypes should be seen in offspring, but for some reason new combinations of phenotypes were seen which he dubbed recombinant types. His data showed an exception to one of Mendel's laws and showed that genetic recombination could occur during the formation of sex cells, a refinement of our understanding of meiosis at that time.
Another discovery made in the "fly lab" of Morgan, was the inheritance pattern known as sex-linkage. This discovery was also made using Drosophila. Students noticed fruit flies with white eyes when the normal eye color was red. After doing breeding experiments, it was determined that the gene for the white eye trait was located on the female X-chromosome and was recessive to the red eye condition. As a result, the white eye trait was rarely seen in females, but occurred more often in males. This is due to the fact that males possess only a single X-chromosome. For a female to inherit white eyes, her father had to have white eyes and her mother had to a least be a carrier of one white-eye gene. A consequence of this discovery was the fact that genes were carried on specific chromosomes. This discovery led modern biology to an understanding of the chromosomal basis of inheritance. Some human traits that follow sex-linked patterns of inheritance include hemophilia, color blindness, and disorders such as Duchenne Muscular Dystrophy and ALD, adrenoleukodystrophy.
Sometimes traits may be governed by more than one gene.This is true in two complex patterns of inheritance. In polygenic inheritance several genes may control the phenotype of a given trait. Human examples of polygenic inheritance include eye color, skin color, and height. In this pattern, there tends to be a broad range of phenotypes which would resemble of bell-shaped curve. For example, with human height, few people are at the extremes while more individuals are of "average" height. A second pattern including more than one gene is epistasis. Epistasis involves two genes governing the expression of a trait. An example would be mouse coat color. In mice, black is is dominant to brown, while a second gene controls pigment production. Pigments are produced if a dominant copy of the gene is present while no pigment being produced is recessive. Instead of a 9:3:3:1 ratio as seen with Mendel, a 9:3:4 ratio results wherein nine are black, three are brown, and the remaining four are white (no pigment).
Since 1953, with the ground-breaking discovery of James Watson and Francis Crick of DNA structure, our understanding of genetics and inheritance have continued to change. Today, we realize that not only do genes control the outward expression of traits, but also patterns of DNA methylation can also impact phenotype. This discovery known as epigenetic inheritance is one of the newest and most shocking. DNA, alone, does not control inheritance. Epigenetics involves looking at factors outside of the genome that effect gene expression in offspring. Genomic imprinting involves DNA methylation patterns of the maternal and paternal DNA that are passed on in the egg and sperm, respectively. DNA methylation can silence a gene and therefore it is never expressed in the offspring. This seems to be important in certain aspects of embryonic development and is still being studied.
Overall, inheritance is never simple and though patterns exist, understanding the complexity of genetic variation and its importance to species survival still proves challenging today. From simple curiosity about why some peas are round and some are wrinkled to the understanding that DNA alone does not control heredity, genetics as a science has grown into one of the most fascinating and important branches of biology. Many questions still need answers and we are waiting for the next big discovery.