We chromosomes So I think we have genotype

We view manipulation of genes in our crops and livestock as a recent development.

Yet, man has been manipulating the genetic makeup of his food for thousands of years through cultivation and breeding. This lesson will begin to help you understand how genetics works.

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Genetic Selection

The principles of genetics have existed for thousands of years – we just never called it ‘genetics.’ Think about farmers trying to cultivate heartier, more delicious crops, or horse breeders who want to develop faster, better racing horses. They would do this by only selecting the best horses in each generation for breeding.

Without realizing it, early farmers and breeders practiced genetic selection before we really knew what genetics was. Thousands of years later, Gregor Mendel’s studies of pea plants formed the foundation upon which modern genetics was built. But rather than read about pea plants though, let’s go visit our modern-day scientist, Adrian, to see these principles in action.

Genotype and Phenotype

Adrian is excited because he’s just discovered a brand new species of hamster that can fly. He thinks this is going to be a scientific gold mine so he decides to drop everything that he’s been working on and study this new species.Let’s take a minute to talk about terminology so we can understand Adrian’s work.

Genetic mutations in the DNA code cause changes in phenotype
genotype, but it’ll be difficult to track the genetic changes occurring in our flying hamsters. It’d be a lot easier to monitor observable traits.Recall that the gene products are what provide functionality in the cells and for the organism. So in performing these various functions, gene products are producing observable traits. And these observable traits are referred to as phenotype.

So what this means then is if I was to make a change in the genotype – let’s say we change this ‘T’ and mutate it to an ‘A’ – a change in the genotype will in turn cause a change in the phenotype, which then can be observed by us as scientists.Geneticists are interested in heritable traits. A mutation in the DNA can be passed on to offspring and in that way, this change in the DNA code has become a permanent change in the genome of this organism. However, not all traits are heritable.

Heritable Traits

For instance, if one of our experimental hamsters happens to break a leg, that hamster won’t pass on a broken leg to its offspring; this is not a ‘heritable trait’. Because of the link between genotype and phenotype, you can see how farmers would select for genetic improvements in their crops and their livestock by selecting for phenotypic changes occurring over time. The different types of corn that we see in the supermarket today – yellow, blue, white , etc.

– are good examples of genetic manipulation by cultivation.

Chromosomes and the Genome

A diploid has two versions of each chromosome called homologs or homologous chromosomes
Homologous chromosome

So I think we have genotype and phenotype pretty square, but we need a few more terms to be able to really understand the full extent of genotype.Now, genes are located on chromosomes, and this makes sense because we just got through saying how a gene is made up of DNA. A chromosome is just a discrete structure of DNA that’s found in the genome.By observing the chromosomes under a microscope, Adrian determines that the flying hamster, like most animals, is a diploid, and furthermore, he determines that flying hamsters have a total of six chromosomes.

So hopefully all those words ring a bell from our previous lessons, but here’s another way of looking at it:A chromosome is defined by a specific sequence of DNA, which in turn means it’s defined by a specific set of genes; however, because the organism is diploid, it has two version of each chromosome. Each of these versions is called a homologous chromosome or a homolog.So to summarize, our flying hamsters have three different chromosomes – we have chromosome one here, chromosome two and chromosome three.

Because the flying hamster is a diploid, that totals up to three pairs of homologous chromosomes in each hamster cell.We’ll refer to a specific location on a chromosome as a locus. In general, a locus is going to refer to a specific gene on that chromosome, although it could be just be referring to a specific location on the chromosome, not necessarily a gene.

Genetic Differences

Different versions of a gene account for different traits in organisms
Genetic differences

So we have a pretty good grasp on traits and chromosomes now, but we need a better way of talking about differences in DNA.

Adrian has collected hamsters from a bunch of different geographical areas and he notices that hamsters from one area are brown and those from this other area are white. He hypothesizes that the same gene encodes coat color in each animal but different versions of the gene account for the different colors.So we call each version of the gene an allele. In this case, we would have two different alleles; we’d have a brown allele and we’d have a white allele, and the differences in these alleles is what encodes the different coat color in the two different animals.So if Adrian’s hypothesis is correct, a single gene determines the coat color and one allele encodes a brown coat and a second allele encodes a white coat.

Lesson Summary

So to summarize, we’ve learned that genotype describes the DNA sequence associated with a locus.

A locus is a specific location on a chromosome and it’s often used in reference to the location of a gene, although it also can refer to a specific physical location on the chromosome that’s not associated with the gene.The genotype of an organism determines its phenotype, or an observable characteristic of the organism. Different traits, such as a difference in coat color in our flying hamsters for example, are determined by different versions of the same gene, which are called alleles.So stay tuned and see if Adrian’s hypothesis about flying hamster coat color is correct.


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