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Genes vs Alleles – What is the Difference?

Genes vs Alleles – What is the Difference?

In this post we will see what are genes and alleles and the difference between them. Genes and Alleles are two terms we use so frequently in genetics. To understand these two terms we would have to start with our DNA. DNA is our genetic material and it is polymer of nucleotides which codes for all our physical characteristics, proteins etc. Here we are saying that DNA codes for all these but what exactly does that mean?

Say for example, a patch of DNA containing a particular nucleotide sequence codes for eye color. This patch of DNA is called GENE (figure a). Likewise there can be many genes in DNA which codes for various characteristics, proteins or enzymes etc.

a. gene

  • Therefore, GENE = a patch of DNA containing specific nucleotide sequence that codes for something.

Now let us go to chromosome and see the same gene on chromosome. We talked about a gene coding for eye color, just for an example assume that this gene is present on chromosome number 1. We have 23 pairs of chromosomes meaning every chromosome has a pair so here chromosome 1 also has a homologous pair. In this homologous pair also gene coding for eye color is present in the same locus as shown in figure b. This point is very important to understand that the gene location is always specific on chromosomes meaning at whichever locus the gene for eye color is present at in chromosome 1 is same for the homologous chromosomes also. This means we contain two copies of the same gene, isn’t it! And each copy in this gene is called an ALLELE. So we have two alleles of the same gene.

b. Alleles

Now let us understand one more point. See both the copies of genes are same meaning it has function of making eye pigmentation but in both the copies of the genes there can be minute difference in the nucleotide sequence which makes it code for different pigmentation or variation. For example, we saw chromosome 1 has two alleles for eye color gene, one of these alleles code for black pigmentation and the other alleles on homologous chromosome codes for brown pigmentation. And you will see phenotype of whatever alleles is dominant.

  • Therefore, ALLELE = variation of the same gene.

In a population there can be multiple alleles. That is how we have different eye colors such as black, light brown, dark brown, green, blue etc. same for different hair colors and all other physical traits that we see. But in an individual we can have only two alleles present for any particular given gene and that is because we carry homologous pairs of every chromosome. Each chromosome in that pair carries that particular allele; since we have two chromosomes in each pair we can have only two alleles of any particular gene.

Hope I am able to make it clear to you guys 🙂

 
Watch this video for better understanding.

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Chromosome, Chromatin, Chromatid – What is the Difference?

Chromosome, Chromatin, Chromatid – What is the Difference?

To be honest with you guys, I use to get really confused between the terms – chromosome, chromatin and chromatid. So one day I decided no matter what, I am going to clear them up. And guess what? When I understood the difference, I realised that I already knew these things but didn’t actually knew how to differentiate them. Funny ha!! So I thought why not share it with you all. Hope it will be helpful.

These terms sound very similar and that is why it can be confusing sometimes. So let’s start from chromatin and then will move to chromosome and then to chromatid. Why this particular order? You will know once you read it. It becomes very easy to understand in this order.

We know that DNA is our genetic material. But DNA is too long (about 2m) to be fit inside the small nucleus (5μm – 10μm in diameter). So in order to fit inside the nucleus, DNA has a packaging strategy. And this strategy involves wrapping of DNA around the histone proteins. So our double stranded DNA would wrap around histone proteins and it looks like show in figure a. This assembly where we have DNA and histone (DNA + Histone) looks like thread is called CHROMATIN. And this DNA in chromatin phase is not condensed yet.

  • Chromatin = DNA Wrap around Histone Proteins OR DNA + Histone Proteins

Chromatin

a. Chromatin

Now when cell undergoes cell division, the DNA will start getting condensed (remember in chromatin phase as we just saw DNA is not condensed). So this condensed form of DNA would appear like in figure b and this form is called CHROMOSOME.

  • Chromosome = Condensed form of DNA 

 

                                          b. Chromosome                                                             

Now we know that when the cell is dividing, each chromosome needs to replicate and make its identical copy. Because the newly divided cell should have the complete set of chromosomes. To achieve this each chromosome duplicates itself and they will be attached at the point of centromere which looks like shown in figure c. This assembly where we are observing DUPLICATED chromosome initially attached at the point of centromere, each strand in that or copy in that is called CHROMATID. We sometimes refer to it as sister chromatids when we talk about recombination right!! So the chromatid is nothing but duplicated chromosome which has not yet separated. Now when cell will divide each of the new cell will get one copy of chromatid and this chromatid will act as chromosome itself in that new cell. (The same chromatid would decondenses into a chromatin strand)

  • Chromatids = Duplicated Chromosome Attached at the Point of Centromere

 c. Chromatids

Time for a true story now. For a really long time while I was studying, in mind chromosome was like shown in figure c whereas actually it’s the duplicated chromosome or two chromatids but that is how I thought it looked like. Reason was, that is how we use to observe in karyotyping metaphase stage where the chromosomes have duplicated but yet not separated. Lack of my understanding of course  🙁

Watch this video for better understanding.