Daily Newsletter March 13, 2012
Today's Topic: Mendel's First Law of Genetics
To understand Mendel, we must first understand some terms and put them into context.
- Gene - A segment of DNA that holds a sequence of nucleotides that provide the instructions on how to create either RNA or a Protein. These are known as gene products.
- Chromosome - This is a molecule of DNA and associated proteins. The chromosome is only visible during specific phases of nuclear division. The word chromosome is used loosely (weak sense) as a synonym of DNA.
- Haploid - A cell that has only one copy of each DNA molecule (chromosome).
- NOTE: each organism has a known number of DNA molecules (chromosomes).
- Humans have 23 different DNA molecules (23 different chromosomes).
- Diploid - A cell that has two copies of each DNA molecule.
- Humans have 46 chromosomes total: 2 copies of each of the 23 different DNA molecules.
- Mutation - A change in the nucleotide sequence of DNA.
- Genetic Variation - Different versions of the same gene generated by mutation. These could be subtle or pronounced alterations.
- Phenotype - The specific physical form an individual's genetics produces. Generally we start by looking at one well defined physical trait, such as flower or seed color.
- Allele - Genetic Variation in a single gene that results in different phenotypic expression.
- There can be many different genetic variations in a population.
- An individual can only possess a number of allele equal to the # of chromosomal copies.
- So, a human is diploid, possessing 2 copies of each gene.
- Humans can therefore have at most 2 alleles for each gene.
- A haploid individual has only 1 allele for each gene.
- Genotype - The specific alleles an individual possesses for a given trait.
His work was based on probability mathematics, and as we have discussed previously, mathematical certainty is needed in the establishment of laws.
Mendel's First Law is known as the Law of Segregation. Remember, Mendel did not know about genes or even DNA. He was working solely with gross physical characteristics that could be observed with the naked eye.
Going back to flower color, Mendel first wanted to see what would happen if you took pure-breeding white flowered peas and crossed (mated) them with pure-breeding purple flowered peas. Many of Mendel's contemporaries held the view that the offspring were produced by a blending of characteristic. What Mendel saw directly contradicted this view. He saw only purple flowers.
Mendel decided to self-cross (self-pollinate) this generation of purple flowers. The next generation held both purple and white flowered individuals, but in a very specific ratio - 3:1. He repeated his experiment, and even used different characteristics. The same thing happened: pure-breeding parents produced offspring with a specific trait, and when self-crossed, these produced offspring in which the original parental traits appeared in a 3:1 ratio.
Mendel inferred the following from his mathematical calculations:
- Each individual possesses two "factors" which determined the specific trait, e.g., Flower Color.
- When an ovum or pollen is produced, it holds only one Factor.
- When an ovum and pollen join, the new individual will carry one factor from the ovum (mother) and one factor from the pollen (father).
But why did the offspring of the pure-breeding plants produce only purple flowers?
Mendel was lucky. The traits he picked had variations based on a mutation of a single gene. Today we would also call the mutations here as knock-out mutations. The purple color is produced by a fully functional gene. It produces a functional pigment. The white color is produced because the gene that codes for the pigment is flawed; it can't produce the pigment. What you have is one functional gene product (dominant) that masks a non-functional gene product (recessive).
Genetics become much more murky when you have multiple genes coding for a trait or when you don't have a complete knockout
Daily Challenge: Using one of Mendel's genetic models, other than flower color, describe his experiment and the law of segregation.
Administrative Note:
Administrative Note: The Website for Milestone 2 will be open later today. You will receive a special notice regarding the new website when it is ready to accept papers.
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