Science: An Elementary Teacher’s Guide/Genetics and Inheritance

The History in Genetics

In the 1860s, when an Austrian monk by the name of Gregor Mendel began experimenting with peas. Mr. Mendel wanted to find out how lifeforms pass physical characteristics, also known as traits, from one generation to the next.

The traits that Gregor Mendel focused his study on were the height of a pea plant, the color of pea seeds, and the shape of pea seeds. By cross pollinating the pea plants, he carefully controlled which plants reproduced, and tracked how each of these traits was passed on from generation to generation. Cross pollination means that Gregor Mendel took pollination from a pea plant which he selected and put it on another pea plant he selected.

The Genetics of a pea

In his early experiments, Mendel took pollen from short stemmed pea plants and put it on other short stemmed pea plants. The result, as you might expect, is that all the offspring were all short stemmed pea plants. Mendel called these true breeders, because all the offspring were the same as the parents.

He then took pollen from long stemmed pea plants and put it on other long stemmed pea plants. What do you thing the result was? If you guessed that all the offspring would also be long stemmed, then you made the same guess as Gregor Mendel. Interestingly, this was not the result. Some of the long stemmed parent plants produced only tall offspring, as you would expect. However, some of the long stemmed pea plants also produced short stemmed offspring. How can this be? Gregor Mendel discovered that some of the tall pea plants were true breeders, meaning that they only produced other tall pea plants, while others of the tall plants were not true breeders, because they produced a mixed offspring of both tall and short plants.

Next, Mendel took the pollen from true breeding tall pea plants and put it on true breeding short plants. He called these true breeding parent plants the P1 generation. He wanted to find out which trait would be passed to the offspring, the trait for being tall, or the trait for being short. What do you think happened? To his surprise, all of the offspring were tall. Mendel called this second generation of plants the F1 generation. The trait for short stemmed plants seemed to have simply disappeared. Finally, Gregor Mendel took pollen from this F1 generation of pea plants, and put it on other pea plants in the same F1 generation. What do you think happened?

Amazingly, the short stemmed trait reappeared. Some of the offspring were short. Gregor Mendel realized something very important. He realized that every plant had not one, but two genes for each trait. This meant that every pea plant could have two tall genes, two short genes, or a tall and a short gene. If a plant inherited two tall genes, how do you think it would grow? Well, there is no short gene, so the plant will grow to become a tall plant. What if a plant has two short genes? Because there is no tall gene, the plant will be short. But what will happen if a plant has both a tall gene and a short gene?

Mendel discovered that some genes are dominant, or stronger than other genes. When there are two different kinds of genes (a tall and a short) the dominant gene will determine how the plant will grow. In pea plants, the tall gene is dominant, while the short gene is recessive. Recessive is a big word that means “not dominant.” This means that if a pea plant has both a tall gene, and a short gene, the tall gene, which is dominate will make the plant grow tall. The short gene is still there, and can still be passed on to future generations of plants.

Gregor Mendel developed a hypothesis about how genes are passed from parents to their offspring. This hypothesis, which scientists still use today, says that a pea plant (And all other life forms) have two genes, for tallness, making a gene pair. They also have a gene pair for every other trait, including seed shape, seed color, or in humans, eye color, hair color skin color, etc. When reproducing, each parent can pass only one gene to their offspring, from each gene pair. This means that the offspring will inherit one gene from each parent, making a new gene pair

The Genes and DNA

The modern working definition of a gene is a portion (or sequence) of DNA that codes for a known cellular function or process ("making melanin molecules"). A nucleotide contains: a Phosphate, a Sugar, and a Base. The Four Nitrogenous Bases are: Adenine, Guanine, Thymine, and Cytosine. A single gene may have a small number of nucleotides or a large number of nucleotides, in the same way that a word may be small or large. A single gene often interacts with neighboring genes to produce a cellular function and can even be ineffectual without those neighboring genes. A series of nucleotides can be put together without forming a gene (non coding regions of DNA), like a string of letters can be put together without forming a word. Nonetheless, all words have letters(AGCT), like all genes must have nucleotides(PBS).

Mutations

During the process of DNA replication, errors occasionally occur in the polymerization of the second strand. These errors, called mutations, can have an impact on the phenotype of an organism, especially if they occur within the protein coding sequence of a gene. Error rates are usually very low—1 error in every 10–100 million bases—due to the "proofreading" ability of DNA polymerases. Processes that increase the rate of changes in DNA are called mutagenic: mutagenic chemicals promote errors in DNA replication, often by interfering with the structure of base-pairing, while UV radiation induces mutations by causing damage to the DNA structure. Chemical damage to DNA occurs naturally as well and cells use DNA repair mechanisms to repair mismatches and breaks. The repair does not, however, always restore the original sequence.

In organisms that use chromosomal crossover to exchange DNA and recombine genes, errors in alignment during meiosis(cell division that reduces the number of chromosomes in the parent cell by half and produces four gamete cells; also, exchanging, crossing over information with the chromosomes) can also cause mutations. Errors in crossover are especially likely when similar sequences cause partner chromosomes to adopt a mistaken alignment; this makes some regions in genomes more prone to mutating in this way. These errors create large structural changes in DNA sequence – duplications, inversions, deletions of entire regions – or the accidental exchange of whole parts of sequences between different chromosomes (chromosomal translocation).

Asexual and Sexual Reproduction

When cells divide, their full genome is copied and each daughter cell inherits one copy. This process, called mitosis, is the simplest form of reproduction and is the basis for asexual reproduction. Asexual reproduction does not require a Mate. Asexual reproduction can occur in multicellular organisms, producing offspring that inherit their genome from a single parent. Offspring that are genetically identical to their parents are called clones.

Eukaryotic organisms often use sexual reproduction to generate offspring that contain a mixture of genetic material inherited from two different parents. The process of sexual reproduction alternates between forms that contain single copies of the genome (haploid) and double copies (diploid).Haploid cells fuse and combine genetic material to create a diploid cell with paired chromosomes. Diploid organisms form haploids by dividing, without replicating their DNA, to create daughter cells that randomly inherit one of each pair of chromosomes. Most animals and many plants are diploid for most of their lifespan, with the haploid form reduced to single cell gametes such as sperm or eggs.

Although they do not use the haploid/diploid method of sexual reproduction, bacteria have many methods of acquiring new genetic information. Some bacteria can undergo conjugation, transferring a small circular piece of DNA to another bacterium. Bacteria can also take up raw DNA fragments found in the environment and integrate them into their genomes, a phenomenon known as transformation. These processes result in horizontal gene transfer, transmitting fragments of genetic information between organisms that would be otherwise unrelated.