Mendel and Genetic Terms

Alleles - Versions of a Gene

Reviewing Genes

Before we dive into the depths of basic genetics, it is important that we orient ourselves as to what the subject entails. Recall that a gene is a section of DNA that codes for a protein. Proteins, as you will remember, are the "doers" of all cells - if something needs doing, a protein will do it. So genes code for proteins, and proteins allow you to do things, look the way you do, etc.

What are Alleles?

Alleles are those things that you used to write in those Punnett squares that everyone remembers and loves (because they were easy to fill in!) from middle school. But we will worry about crossing in Punnett squares after we truly understand what alleles are. For a given gene, alleles represent different options. So, if there was a protein for flower color, as shown, each different allele codes for a different color. Each allele is represented by a letter, in this case B and b. B is the purple allele, so it codes for purple petals. b is the white allele - it codes for white petals.

Genotypes vs. Phenotypes

Now recall that organisms we will work with are diploid. So they have two copies of every chromosome. So there is a chromosome, say chromosome number 1, for this plant that has a gene that codes for petal color. Simple enough, right? Well, remember that being diploid means that it has two copies of chromosome 1... meaning it has two alleles - one on each chromosome copy. In this example, there are two different alleles - A or a. So, if you have two copies and there are two different options, you can have 2 A's, 2 a's, or 1 of each. Now the math behind this will come in the next section, so for now I just want you to be comfortable with the fact that organisms will have two alleles for each gene.

Your combination of alleles is referred to as your genotype. So if an individual has 2 A alleles, then their genotype would be AA. Other genotype options in this example are Aa and aa. Your genotype determines your phenotype, or the physical attributes for that gene (so purple petals or white petals).

Homozygous vs. Heterozygous

Your genotype can be classified as either homozygous or heterozygous. Recall that 'homo' means same and 'hetero' means different. So, if your alleles are the same (i.e. AA or aa), then you are considered to be homozygous. If your alleles are different (i.e. Aa), then you are considered to be heterozygous. Getting comfortable using this terminology is going to be crucial in this unit, so make sure you practice now.

Heterozygotes are also referred to as carriers because they carry the recessive allele (a), but do not show it in their phenotype. So they can pass it on to their children and they might not even know that they have the allele!

Dominant vs. Recessive Alleles

Not all alleles are created equal - some are 'stronger' than others. When you are a heterozygote (meaning you are Aa in our example), it is pretty easy to tell which allele is 'stronger'. Just take a look at the phenotype of the heterozygote. In this case, if you have a genotype of Aa, your phenotype shows purple petals. So even though this individual has a white allele (a), it is not expressed, or shown, in its phenotype.

So in a heterozygote for a gene such as this petal color, one allele is expressed (purple, or A) and the other is masked, or covered up (white, or a). You wouldn't know that individual had a white allele unless you knew its genotype somehow. So we would call this individual (and any heterozygote) a carrier for the hidden allele. The allele for purple color, A, is the dominant allele because it covers up the other allele and is expressed even if the individual only has one copy of it. The allele for white color, a, is the recessive allele because you only express that phenotype if you have all recessive alleles. So in order to have white petals, you'd have to have a genotype of aa. If there is even a single dominant allele (A), you will not have white petals.

Mendel crossed thousands of pea plants and made meticulous observations that formed the foundation of modern genetics. While he did not understand everything about genetics (we still do not, of course), without his discoveries genetics would have evolved very differently.

His key discoveries led to the establishment of two 'laws'. Now, like all of biology, laws are rarely resolute - there are exceptions to these laws as we delve deeper into more complex questions, but these are absolutely necessary for you to know and understand moving forward.

These laws are known as Mendel's law of segregation and Mendel's law of independent assortment.

A. Mendel's Law of Segregation

Recall that meiosis results in haploid gametes. Each gamete contains half of the chromosomes (each with one allele of each gene). When two gametes fuse via fertilization, they form a zygote which will grow and develop into an adult organism.

When those gametes were formed, the two alleles (from the parent) for each gene segregate into different daughter nuclei. This means that the combination of alleles across all chromosomes in a gamete is random. This increases the genetic variation of the gametes each parent makes. Track each allele within the diagram below as the gametes are formed.