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What are Genotypes? - Definition & Examples

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  • 0:02 What Is a Genotype
  • 0:26 How Do Geotypes Come About?
  • 1:18 Alleles & Genotypes
  • 3:09 How Genotypes Are Passed
  • 4:17 Lesson Summary
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Lesson Transcript
Instructor: Marta Toran

Marta has taught high school and middle school Science and has a Master's degree in Science Education.

In this lesson, you will learn about what genotypes are, where they come from and how they play a major role in determining how your body works and what you look like.

What is a Genotype?

As the name suggests, a person's genotype refers to the types of genes he or she has for a particular inheritable trait. Genotypes determine which characteristics an individual will express, for example: whether they have freckles or not, if they are lactose intolerant, if they have hair on their knuckles or if their eyes will be blue, brown or another color.

How do Genotypes Come About?

Genes are found on chromosomes, those tightly-packed DNA structures in the cell nucleus. In sexually-reproducing organisms, chromosomes come in pairs, one from the mother and one from the father. For example, each person will have two 'Chromosome 1s' and two 'Chromosome 2s'.

Chromosomes in a pair (except sex chromosomes) are called homologous chromosomes because they contain the same genes. For example, both chromosome 8s contain, among many others, the gene that determines whether or not a hairline forms a widow's peak.'

There are always two copies of each gene, one from each parent. A gene, however, can have different versions, called alleles. Alleles are various versions of a gene. The combination of alleles inherited from the parents is what gives rise to genotypes.

Examples of Alleles and Genotypes

To better understand how alleles and genotypes work, let's take a look at the gene that determines whether a person is lactose intolerant, which is found on chromosome 2. This gene contains the recipe for the body to make lactase, the enzyme that breaks down lactose sugar in milk.

Alleles for Lactase Production

Notice that the allele for lactase enzyme is represented by a capital letter because it is dominant over faulty lactase production. The 'weaker' allele is referred to as recessive and uses a 'lowercase letter. So, whenever the person has both versions of the gene, in this case Ll, the ability to produce lactase, being the dominant allele, will be the one expressed. For someone to be lactose intolerant, they have to have inherited two recessive 'l' alleles, one from each parent.

So, there are three possible genotypes for lactase production:

  • LL means a person is able to make lactase and therefore can digest milk just fine.
  • Ll means a person is able to make lactase and therefore can digest milk just fine.
  • ll means a person is not able to digest milk properly because they can't make lactase, and they are said to be lactose intolerant.

When a person has two dominant alleles for a gene (e.g. LL), they are referred to as being homozygous dominant for that trait. If they have two recessive alleles (ll), they are homozygous recessive. If they inherited one of each allele (Ll), they are heterozygous for that trait.

How Genotypes Are Passed Down

You may be wondering, can children have different genotypes than their parents for a specific trait? The answer is yes. Look at this example showing how alleles are distributed to eggs and sperm and how they recombine during fertilization. In this case, the dad is homozygous dominant, and the mom is homozygous recessive. When their alleles join up, the only possible resulting genotype in their children will be heterozygous (Ll), which is different from either parent.

Graphic Showing Allele Recombination

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