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Basic Concepts of Heredity
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Basic Concepts of Heredity

Author: Amanda Soderlind

Identify the components of heredity

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Source: Video and Images Created by Amanda Soderlind

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Welcome to this lesson today on basic concepts of heredity. Today we are going to be examining some basic concepts associated with heredity and the terms or vocabulary that go along with that.

So, heredity is the passing of traits from parents to offspring. And Gregor Mendel is actually considered the father of genetics because he did a lot of work with heredity and examining how information is passed through generations from parent to offspring. So, Mendelian inheritance is actually a term that was coined due to his research on pea plants. So basically, it describes how genes are passed from parents to offspring. And some different types of inheritance include autosomal dominant, autosomal recessive, x-linked inheritance, x-lined dominant. So, these are some different ways in which information or genetic information is passed from parent to offspring, some different methods.

So, genes are found on chromosomes and genes are passed from parents to offspring and contain information about specific traits. So, I'm going to draw a little picture over here as we talk about some of these terms that will hopefully help clarify.

OK. So what I have here is a picture of homologous chromosomes.

Now, homologous chromosomes are chromosomes found within your cells that contain variations of the same information. They're the same size, the same shape, and contain variations of the same information. So, you have one chromosome of this type from your mom and one from your dad. So you have 46 total chromosomes in your body, 23 from each parent. So, homologous chromosomes are the chromosomes from your mom and your dad that contain variations of the same information.

So, a locus is a location on a chromosome where a trait will be located. So, our trait that we're going to use in our examples today is going to be hairline. So, a person can have a straight hairline or they can have a widow's peak. So, let's say that this area right here that I've highlighted is the locus for hairline.

So, these are actually alleles as well. And alleles are different versions of a gene. And you have two alleles from each trait, as I mentioned-- one from your mom and one from your dad. One on each homologous chromosome.

So, the combination of alleles that you inherit will determine the outcome of that trait. So, right here we have a pair of alleles at this specific locus and each allele is a form of the gene for hairline. OK? So, let's take a look at some more terms down here. An example of a little bit more about inheritance.

So, we're going to say that hairline, the hairline gene, as I mentioned, we have alleles for each gene. OK. So, the hairline gene has two alleles, as we mentioned here. And the hairline gene you can either have a straight hairline or a widow's peak. OK.

So, we actually in genetics will use letters in order to represent alleles. So, when we use a capital letter to represent an allele, we say that is dominant. And when we use a lowercase letter to represent an allele, we say that that's a recessive allele. OK. So capital letters represent dominant alleles, lower case letters represent recessive alleles.

Now, recessive traits are only expressed when a dominant allele is not present. So in this case right here when we have, as I mentioned, we have two alleles for each trait, one from mom and one from dad. If a recess allele is in the presence of a dominant allele, the dominant allele will always rule. So whatever trait is ruled by the dominant allele is a trait that will show up in that individual. OK? So when we have two dominant alleles together, both parents have donated a dominant allele, we call this homozygous dominant. Oops. OK. Homozygous dominant.

So, widow's peak is actually a dominant trait. So if you have at least one dominant allele, you will inherit a widow's peak. So your hairline will look something like this. You'll have the widow's peak. OK. So, this person, because they're homozygous dominant, they have two dominant alleles, will end up with a widow's peak.

So, we call this here the genotype, and this the phenotype. So, the genotype are the genes that you inherit that represent an allele. And the phenotype is actually the physical manifestation of those genes. So, our genotype is big W big W, our phenotype is widow's peak. So the big W big W represents a widow's peak. Genotype. Phenotype.

If you have a big W, little w, the phenotype is still going to be a widow's peak. Because remember, as I mentioned, if you have at least one dominant allele, that's all it takes for that trait to show up in the individual. So this person, this is their genotype-- big W, little w --and their phenotype is widow's peak.

Now, if you end up with two small alleles or two lowercase alleles we call this homozygous recessive. I'm sorry, I'm going to skip back to this one really quick. This is called heterozygous. OK? The prefix hetero means different. So you have one big and one little, they are different. Homo, the prefix homo, means the same. So homozygous recessive they're both the same and they're both recessive. Homozygous dominant they're both the same and they're both dominant. So that's how you can remember that.

So if both alleles are recessive, this will result in a straight hairline.

OK. So, I know there's a lot of vocab words that go along with this and can get a little bit confusing, but hopefully these diagrams will help you out a little bit.

So, the genotype in this case is little w little w, and then the phenotype would be a straight hairline. So the genotype is the alleles that we used to represent the trait, the phenotype is the physical manifestation of those genes.

OK. So, this lesson has been an overview on the basic concepts of heredity.

Terms to Know

Variation in a gene. An example would be eye color; there is a gene for eye color, but there are different versions of genes that allow for different eye colors.

Dominant Alleles

When one allele masks the expression of another allele of the same gene. This is seen at the phenotype level.


A genetic unit of heredity; a specific section of DNA that codes for a specific protein.


A trait or characteristic expressed at the genetic level, that is, the genetic makeup of an organism.

Gregor Mendel

Dubbed the "father of modern genetics" and arguably modern biology, Mendel was an Austrian monk who studied heredity and inheritance in plants.


The passing of traits from parents to offspring.


An organism with two different alleles of a gene.


An organism with two identical alleles of a gene.


Receiving genetic characteristics from parents, the manner in which genes are passed down to offspring.


The term used to describe the specific location of a gene/DNA sequence on a chromosome; variations of these genes are referred to as alleles.


An observable characteristic of someone’s traits, examples: eye color, skin color, height, gender etc.

Recessive Allele

An allele that is masked by a dominant allele, recessive alleles are only expressed when they are found in homozygous pairs.