Daily Newsletter

November 5, 2013 -

Mendel's First Law of Inheritence

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.
There can be more than 2 alleles though for the entire human population; each individual can only have a maximum of 2.

A haploid individual has only 1 allele for each gene.

Homozygous - A diploid organism that has the same allele for a given gene.
Heterozygous - A diploid organism that has different alleles for a given gene.
Genotype - The specific alleles an individual possesses for a given trait.

The following picture will help you visualize the concept of Alleles, Homozygous, and Heterozygous. Alleles

Mendel's work with the garden pea, Pisum sativum, resulted in two laws of inheritance. The focus of his work was to determine the inheritance pattern of specific traits. For example, if you have a pure breeding strain that produces white flowers, and a pure breeding strain that produces purple flower, what is the percentage of offspring which will possess purple flowers? 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 (F₀ generation with homozygous purple and homozygous white individuals). 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, this contradicted the idea of blending. F1 generation

Mendel decided to self-cross (self-pollinate) this generation of purple flowers (F₁ heterozygous individuals). 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. F2 Generation

Mendel inferred the following from his mathematical calculations:

Each individual possesses two "factors" which determined the specific trait, e.g., Flower Color. (Mendel's Factors = Genes).
When an ovum or pollen is produced, it holds only one Factor (Gene).
When an ovum and pollen join, the new individual will carry one factor (gene) from the ovum (mother) and one factor (gene) from the pollen (father).
There is a 50% chance (probability) that the mother will donante one factor over another, and a 50% chance (probability) that the father will donate one factor over another.
You thus have four possible outcomes.

An easy way to view these possible outcomes is to use a Punnett square, a simple visual probability tool. The square shown to the right demonstrates the inheritence (genotype and phenotype) probability of pea flower color. On the left hand side of the square you will see the female (pistil) allelic contribution, and on top the male (pollen) allelic contribuition. The mother can donate either B or b; the father can donate either B or b.
If the offspring came from the joining of B from the mother and B from the father, then it will be Homozygous B (dominant) and Purple. An offspring with Bb will be heterozygous and Purple, while an offspring with bb will be homozygous (recessive) and white. There is a 1/4 probability of BB (25%), a 1/2 probability of Bb (50%), and a 1/4 probability of bb (25%). The most common is the heterozygous condition (remember this).
We now understand more regarding the mechanism which Mendel inferred, and have molecular data to support his initial findings. Mendel's factors are genes, and alleles describe the differences between factors. Cells undergo meiosis to produce gametes (sperm/ovum, pollen/ovum, etc...), and it is this process that causes the seperation of genes. We will talk more about meiosis next week.
But why did the offspring of the pure-breeding plants produce only purple flowers?
The traits he picked had variations based on a mutation of a single gene. Today we would call this type of mutation a knock-out mutation, because a function was knocked out. 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). Mendelian 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

How does the first law of inheritance affect our understanding of how genes are passed from generation to generation, and how does it affect our understanding of evolution. Consider the consequences of a single mutation, such as the one seen in Sickle Cell Anemia. How does that move from generation to generation, and how does natural selection play a role once we understand inheritance?