Gregor Mendel, Alfred Day Hershey, Barbara McClintock


Genetics are the study of heredity and the variation of inherited characteristics. The people that study genetics are called geneticists.
  • The genetic properties or features of an organism, characteristics, etc.

Heredity- The passing of traits from parents to child is the basis of heredity.

Our genes encode the instruction that define our traits. Each of us has thousands of genes, which are made of DNA and reside in our chromosomes. The environment we grow up and live in also helps define our traits. Humans have two complete sets of 23 chromosomes. When parents conceive a child, they each contribute one complete set to the child. In this way, parents pass genes to the child. As a result, every child will have a unique combination of traits. Some will resemble the mother, and some will resemble the father. Still others will be unique, a product of the new combination of chromosomes.

Traits- A trait is a notable feature or quality in a person. Each of us has a different combination of traits that make us unique. Traits are passed from generation to generation. We inherit traits from our parents, and we pass them to our children. Physical Traits are characteristics of one's physical makeup, including hair color, eye color, and height. Behavioral Traits are characteristics of the way one acts. An increased risk of getting a certain disease is also a trait that can be passed from parent to child. Environmental influences can also give us certain traits.

DNA- DNA stands for DeoxyriboNucleic Acid. Encodes a detailed set of plans, like a blueprint, for building different parts of the cell. The DNA molecule comes in the form of a twisted ladder shape scientists call a "double helix". The ladder's rungs are built with the four letter DNA alphabet: A, C, T, and G. These alphabet pieces join together according to special rules, A always pairs with T, and C always pairs with G. The DNA strand is made of letters, the letters make words, and the words make sentences. The "sentences" are called genes. Genes tell the cell to make other molecules called proteins. Proteins enable a cell to perform special functions, such as working with other groups of cells to make hearing possible.

Genes- They are instruction manuals for our bodies. They are the directions for building all the proteins that make our bodies function. Genes are made of DNA. One strand of our DNA contains many genes. All of these genes are needed to give instructions for how to make and operate all parts of our body.

Proteins- They are the things that make all living things function. Every cell contains thousands of different proteins, which work together as tiny machines to run the cell. For proteins, each part looks different and has its own job in helping run things. Receptor proteins are responsible for picking up the signal and passing it to the next cell. Proteins are very very small, even with the most powerful electron microscope, you would have trouble seeing them.

Chromosomes- DNA is packaged into compact units called chromosomes. The packaging of DNA into a chromosome is done in several steps, starting with the double helix of DNA. Then the DNA is wrapped around some proteins. These proteins are packed tightly together until they form a chromosome. Chromosomes are efficient storage units for DNA. Each human cell has 46 chromosomes. They are organized into two sets of 23 chromosomes. Each set of living things has a different number of chromosomes.

Sources:,. 'Human Genetics Research Methods: Pedigrees And Population Genetics - Video & Lesson Transcript | Study.Com'. N. p., 2015. Web. 2 Sept. 2015.,. 'Tour Of Basic Genetics'. N. p., 2015. Web. 7 Sept. 2015.

Gregor Mendel

In 1866, Gregor Mendel published the results of years of experimentation in breeding pea plants. He showed that both parents must pass discrete physical factors that transmit information about their traits to their offspring at conception. An individual inherits one such unit for a trait from each parent. Mendel's principle of dominance explained that most traits are not a blend of the father's traits and those of the mother as was commonly thought. Instead, when an offspring inherits a factor for opposing forms of the same trait, the dominant from of that trait will be apparent int that individual. The factor for the recessive trait, while not apparent, is still part of the individual's genetic makeup and may be passed to offspring. Mendel's experiment demonstrated that when sex cells are formed, the factors for each trait that an individual inherits from its parents are separated into different sex cells. When sex cells unite at conception the resulting offspring will have at least two factors (alleles) for each trait. One inherited factor from the mother and one from the father. Mendel used the laws of probability to demonstrate that when sex cells are formed, it is a matter of chance as to which factor for a given trait is incorporated into a particular sperm or egg.

Sources:,. 'Genetics: The Study Of Heredity'. N. p., 2013. Web. 3 Sept. 2015.


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Alfred Day Hershey

During this time, there weren't many people working on bacteriophage. Two other scientists who read Hershey's papers, Max Delbrück and Salvador Luria, were collaborating on experiments using bacteriophage. In 1943, Delbrück invited Hershey to Nashville to visit his lab. In 1946, working with Delbruck, Hershey discovered that phage can recombine when co-infected into a bacteria host. This led to a new area of phage genetics. At the Carnegie Institution of Washington's Department of Genetics' at Cold Spring Harbor, he and Martha Chase did the Hershey-Chase blender experiment that proved that phage DNA, and not protein, was the genetic material. For this, and his body of work on bacteriophage, Hershey shared the 1969 Nobel Prize for Physiology or Medicine with Max Delbrück and Salvador Luria.

Sources:,. 'Alfred Day Hershey :: DNA From The Beginning'. N. p., 2015. Web. 7 Sept. 2015.


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Barbara McClintock

Barbara McClintock began her scientific career at Cornell University, where she pioneered the study of cytogenetics-a new field in the 1930s-using maize as a model organism. Indeed, the marriage of cytology and genetics became official in 1931, when McClintock and graduate student Harriet Creighton provided the first experimental proof that genes were physically positioned on chromosomes by describing the crossing-over phenomenon and genetic recombination. Although Thomas Hunt Morgan was the first person to suggest the link between genetic traits and the exchange of genetic material by chromosomes, 20 years elapsed before his ideas were scientifically proven, largely due to limitations in cytological and experimental techniques (Coe & Kass, 2005). McClintock's own innovative cytogenetic techniques allowed her to confirm Morgan's ideas, and these techniques are numbered among her greatest contributions to science. McClintock worked with what is known as the Ac/Ds system in maize, which she discovered by conducting standard genetic breeding experiments with an unusual phenotype. Through these experiments, McClintock recognized that breakage occurred at specific sites on maize chromosomes. Indeed, the first transposable element she discovered was a site of chromosome breakage, aptly named "dissociation" (Ds). Although McClintock eventually found that some TEs can "jump" autonomously, she initially noted that the movements of Ds are regulated by an autonomous element called "activator" (Ac), which can also promote its own transposition.

Of course, these discoveries were preceded by extensive breeding experimentation. It was known at the time from previous work by Rollins A. Emerson, another American maize geneticist and the "rediscoverer" of Mendel's laws of inheritance that maize had genes encoding variegated, or multicolored, kernels; these kernels were described as colorless (although they were actually white or yellow), except for spots or streaks of purple or brown (Figure 2). Emerson had proposed that the variegated streaking was due to an "unstable mutation," or a mutation for the colorless phenotype that would sometimes revert back to its wild-type variant and result in an area of color. However, he couldn't explain why or how this occurred. As McClintock discovered, the unstable mutation Emerson puzzled over was actually a four-gene system. In her experiments, McClintock bred females that were homozygous for C and bz and that lacked Ds (denoted CCbzbz--, where the dashes indicate the absence of Ds alleles) with males that were homozygous for C', Bz, and Ds (denoted C'C'BzBzDsDs) to yield heterozygotes with an aleurone layer that had the genotype C'CCBzbzbz--Ds. (Remember, in double fertilization, the sperm provides one set of alleles, and the egg provides two.) Because of the presence of the dominant inhibitor allele C', the offspring kernels were expected to be colorless, no matter what their genetic makeup at the Bz/bz locus. In fact, upon crossbreeding, many of these kernels were indeed colorless. However, McClintock also observed many kernels with colorless backgrounds and varying amounts of dark brown spots or streaks, and she concluded that individual cells in those kernels had lost their C' and Bz alleles because of a chromosomal break at the Ds locus. Without either the C' allele (to prevent color expression) or the Bz (purple) allele, the cells that had experienced a breakage at the Ds locus ended up with some brown coloring.


Feschotte, Cédric, Ning Jiang, and Susan R. Wessler. 'PLANT TRANSPOSABLE ELEMENTS: WHERE GENETICS MEETS GENOMICS'. Nature Reviews Genetics 3.5 (2002): 329-341. Web. 7 Sept. 2015.


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