Genetics is the study of heredity. Heredity is a biological process where a parent passes certain genes onto their children or offspring. Every child inherits genes from both of their biological parents and these genes in turn express specific traits. Some of these traits may be physical for example hair and eye color and skin color etc. On the other hand some genes may also carry the risk of certain diseases and disorders that may pass on from parents to their offspring. Bateson was the first person used the genetics word.
Genetic variation: Genetic variation is a fact that a biological system, individual and population is different over space. It is the base of the genetic variability of different biological systems in space.
Eugenics: Eugenics is a movement that is aimed at improving the genetic composition of the human race. Historically, eugenicists advocated selective breeding to achieve these goals. Today we have technologies that make it possible to more directly alter the genetic composition of an individual. However, people differ in their views on how to best (and ethically) use this technology.
Concepts:
a. Phenotype: An individual's phenotype consists of the traits we can observe. These can include features of appearance, behavior, metabolism, or anything else we can detect. On the other hand, an individual's genotype is what we call the genes that help to create that phenotype. There may be one gene at work, or more than one. And genes don't always tell the whole story; sometimes your environment also affects what phenotype you end up with.
Examples:
Appearance: Traits related to appearance are sometimes the easiest to observe. When Gregor Mendel was doing his famous experiments with pea plants, he observed the plants' appearance: the peas might be green or yellow, smooth or wrinkly. The plants could be regular height or dwarfed.
Humans have appearance phenotypes, too: for example, your height and your eye color are both phenotypes controlled, at least partly, by your genes.
Behavior: Behavior can be a phenotype, too. Border collies were bred to herd sheep, so even if they have never seen a sheep in their life, they will display herding behaviors - like running around your house collecting all your pillows.
Metabolism: Most genes don't do anything as flashy as changing our eye color; instead, they make enzymes deep inside our cells. These enzymes do different little jobs that are important to keeping us alive, like running chemical reactions to help us digest our food or burn energy. These chemical reactions are called our metabolism.
One phenotype related to metabolism is lactose intolerance. If you have a gene that makes the enzyme lactase, you can easily digest the sugar (lactose) in milk. But if you are lactose intolerant, you don't have that enzyme, so you can't digest lactose and will feel sick when you drink milk.
b. Genotypes: As the name suggests, a person's genotype refers to the types of genes he or she has for a particular inheritable trait. Genotypes determine which characteristics an individual will express, for example: whether they have freckles or not, if they are lactose intolerant, if they have hair on their knuckles or if their eyes will be blue, brown or another color.
c. Alleles: An allele is an alternative form of a gene (one member of a pair) that is located at a specific position on a specific chromosome. These DNA codings determine distinct traits that can be passed on from parents to offspring through sexual reproduction. The process by which alleles are transmitted was discovered by Gregor Mendel and formulated in what is known as Mendel's law of segregation.
Mendel’s Genetic law: Once upon a time (1860's), in an Austrian monastery, there lived a monk named Mendel, Gregor Mendel. Monks had a lot of time on there hands and Mendel spent his time crossing pea plants. As he did this over & over & over & over & over again, he noticed some patterns to the inheritance of traits from one set of pea plants to the next. By carefully analyzing his pea plant numbers (he was really good at mathematics), he discovered three laws of inheritance.
Mendel's Laws are as follows:
* the Law of Dominance
* the Law of Segregation
* the Law of Independent Assortment
Now, notice in that very brief description of his work that the words "chromosomes" or "genes" are nowhere to be found. That is because the role of these things in relation to inheritance & heredity had not been discovered yet. What makes Mendel's contributions so impressive is that he described the basic patterns of inheritance before the mechanism for inheritance (namely genes) was even discovered. Mendel is father of genetics.
Thomas Hunt Morgan (1866 - 1945) established the chromosomal theory of heredity: Thomas Hunt Morgan, an embryologist who had turned to research in heredity, in 1907 began to extensively breed the common fruit fly, Drosophila melanogaster. He hoped to discover large-scale mutations that would represent the emergence of new species. As it turned out, Morgan confirmed Mendelian laws of inheritance and the hypothesis that genes are located on chromosomes. He thereby inaugurated classical experimental genetics.
After breeding millions of Drosophila in his laboratory at Columbia University, in 1910 Morgan noticed one fruit fly with distinctive characteristic, white eyes instead of red. He isolated this specimen and mated it to an ordinary red-eyed fly. Although the first generation of 1,237 offspring was all red-eyed but for three, white-eyed flies appeared in larger numbers in the second generation. Surprisingly, all white-eyed flies were male.
These results were suggestive for hypotheses of which Morgan himself was skeptical. He was at the time critical of the Mendelian theory of inheritance, mistrusted aspects of chromosomal theory, and did not believe that Darwin's concept of natural selection could account for the emergence of new species. But Morgan's discoveries with white- and red-eyed flies led him to reconsider each of these hypotheses.
In particular, Morgan began to entertain the possibility that association of eye color and sex in fruit flies had a physical and mechanistic basis in the chromosomes. The shape of one of Drosophila's four chromosome pairs was thought to be distinctive for sex determination. Males invariably possess the XY chromosome pair (Morgan used a more cumbersome notation) while flies with the XX chromosome are female. If the factor for eye color was located exclusively on the X chromosome, Morgan realized, Mendelian rules for inheritance of dominant and recessive traits could apply.
In brief, Morgan had discovered that eye color in Drosophila expressed a sex-linked trait. All first-generation offspring of a mutant white-eyed male and a normal red-eyed female would have red eyes because every chromosome pair would contain at least one copy of the X chromosome with the dominant trait. But half the females from this union would now possess a copy of the white-eyed male's recessive X chromosome. This chromosome would be transmitted, on average, to one-half of second-generation offspring one-half of which would be male. Thus, second-generation offspring would include one-quarter with white eyes and all of these would be male.
Intensive work led Morgan to discover more mutant traits some two dozen between 1911 and 1914. With evidence drawn from cytology he was able to refine Mendelian laws and combine them with the theory first suggested by Theodor Boveri and Walter Sutton that the chromosomes carry hereditary information. In 1915, Morgan and his colleagues published The Mechanism of Mendelian Heredity. Its major tenets -
* Discrete pairs of factors located on chromosomes like beads on a string bear hereditary information. These factors—Morgan would soon call them genes—segregate in germ cells and combine during reproduction, essentially as predicted by Mendelian laws. However
* Certain characteristics are sex-linked—that is, occur together because they arise on the same chromosome that determines gender. More generally
* Other characteristics are also sometimes associated because, as paired chromosomes separate during germ cell development, genes proximate to one another tend to remain together. But sometimes, as a mechanistic consequence of reproduction, this linkage between genes is broken, allowing for new combinations of traits.
Morgan's experimental and theoretical work inaugurated research in genetics and promoted a revolution in biology. Evidence he adduced from embryology and cell theory pointed the way toward a synthesis of genetics with evolutionary theory. Morgan himself explored aspects of these developments in later work, including Evolution and Genetics published in 1925, and The Theory of the Gene in 1926. He received the Nobel Prize in Physiology or Medicine in 1933.
Charles Darwin's Theory of Evolution: Darwin's theory of evolution is based on five key observations and inferences drawn from them. These observations and inferences have been summarized by the great biologist Ernst Mayr as follows:
1) Species have great fertility. They make more offspring than can grow to adulthood.
2) Populations remain roughly the same size, with modest fluctuations.
3) Food resources are limited, but are relatively constant most of the time.
From these three observations it may be inferred that in such an environment there will be a struggle for survival among individuals.
4) In sexually reproducing species, generally no two individuals are identical. Variation is rampant.
5) Much of this variation is heritable.
From this it may be inferred: In a world of stable populations where each individual must struggle to survive, those with the "best" characteristics will be more likely to survive, and those desirable traits will be passed to their offspring. These advantageous characteristics are inherited by following generations, becoming dominant among the population through time. This is natural selection. It may be further inferred that natural selection, if carried far enough, makes changes in a population, eventually leading to new species.
These observations have been amply demonstrated in biology, and even fossils demonstrate the veracity of these observations.
Variation: There is Variation in Every Population.
Competition: Organisms Compete for limited resources.
Offspring: Organisms produce more Offspring than can survive.
Genetics: Organisms pass Genetic traits on to their offspring.
Natural Selection: Those organisms with the Most Beneficial Traits are more likely to survive and Reproduce.
Jean-Baptiste Lamarck: The inheritance of acquired characteristics is a hypothesis that physiological changes acquired over the life of an organism (such as the enlargement of a muscle through repeated use) may be transmitted to offspring. It is also commonly referred to as the theory of adaptation equated with the evolutionary theory of French naturalist Jean-Baptiste Lamarck known as Lamarckism.
