Genetics vocabulary | Inheritance and variation | Middle school biology | Khan Academy

Summary

This video explains the basics of genetics, focusing on how sexually reproducing organisms inherit DNA from both parents. It covers the structure of chromosomes and how they relate to genes and alleles. The video differentiates between genotype (the genetic makeup) and phenotype (the expressed traits), and explains dominant and recessive alleles. It also introduces the concepts of homozygous (same alleles) and heterozygous (different alleles) genotypes, providing a foundational understanding of how genetic traits are inherited and expressed.

Takeaways

• 🧬 Human beings have 23 pairs of chromosomes, with each pair consisting of one chromosome from the male parent and one from the female parent.
• 🌟 Chromosomes are made up of DNA strands and other packaging materials, which contain genes that code for various traits.
• 🔍 Genes are segments of DNA that code for specific proteins, which can influence traits such as height, hair color, and eye color.
• 🔄 Each chromosome in a pair generally codes for the same genes, meaning there are regions on each that code for the same traits.
• 🧬 Alleles are different versions of a gene, which can result in different expressions of a trait, such as different hair colors.
• 🔑 Knowing the alleles an organism has allows us to determine its genotype, which is the genetic makeup for a particular trait.
• 👀 The phenotype is the observable expression of a trait, such as hair color, which can be influenced by the underlying genotype.
• 🏳️‍🌈 Dominant alleles can mask the expression of recessive alleles, meaning that just one dominant allele can result in the dominant phenotype.
• 🎲 There are various genotypes that can result in the same phenotype, depending on whether alleles are dominant or recessive.
• 🔄 Homozygous refers to having two identical alleles for a gene, while heterozygous means having two different alleles.
• 📚 Understanding these genetic concepts allows for a deeper discussion and analysis of genetics and inheritance patterns.

Q & A

How do sexually reproducing organisms, including humans, receive their DNA?

Sexually reproducing organisms, including humans, receive DNA from both their male and female parents. Humans have 23 pairs of chromosomes, with one chromosome of each pair coming from each parent.

What are chromosomes and what do they consist of?

Chromosomes are structures that contain DNA tightly coiled around proteins. They are composed of DNA and other molecules that help package and protect the genetic information.

What is the relationship between DNA, genes, and traits like height or hair color?

DNA contains segments that code for different genes. These genes can be translated into proteins, which in turn influence traits such as height, hair color, eye color, and many other physical and functional characteristics.

How can the same gene have different effects on an organism's traits?

The same gene can have different versions called alleles, which can result in different expressions of traits. For example, different alleles for hair color can result in blonde or brown hair.

What is an allele and how does it relate to genes?

An allele is a version of a gene. Each gene can have multiple alleles, which may code for different characteristics of the same trait, such as different hair colors.

What is meant by an organism's genotype?

An organism's genotype refers to the specific set of alleles it has for a particular gene or set of genes. Knowing the alleles an organism has allows us to determine its genotype.

How is an organism's phenotype different from its genotype?

The phenotype is the observable expression of an organism's traits, such as hair color, while the genotype is the genetic makeup that determines these traits. The phenotype is what you can see, while the genotype is what you can infer from the organism's genetic code.

What is the difference between a dominant and a recessive allele?

A dominant allele is one that will express its trait even if only one copy is present, while a recessive allele will only express its trait if two copies are present. In the context of the script, the brown hair allele is considered dominant over the blonde hair allele.

Can you explain the terms homozygous and heterozygous in relation to genotypes?

Homozygous refers to a genotype where both alleles for a gene are the same, while heterozygous refers to a genotype where the alleles are different. For example, having two dominant alleles (BB) or two recessive alleles (bb) makes an organism homozygous for that gene, whereas having one dominant and one recessive allele (Bb) makes it heterozygous.

How can different genotypes result in the same phenotype?

Different genotypes can result in the same phenotype if the dominant allele masks the expression of the recessive allele. For instance, both BB and Bb genotypes could result in a brown hair phenotype if the brown allele is dominant.

What is the simplest way to get a recessive phenotype, such as blonde hair in the given scenario?

The simplest way to get a recessive phenotype, like blonde hair, is to have two copies of the recessive allele (bb), which would then be expressed because there is no dominant allele to mask it.

Outlines

00:00

🧬 Understanding DNA and Chromosomes

This paragraph introduces the fundamental concept of sexual reproduction in organisms, including humans, where DNA is inherited from both parents. It explains the structure of chromosomes, which are tightly packed strands of DNA, and how they contain genes that code for different traits. The segment discusses the concept of alleles, which are different versions of a gene that determine traits like hair color. The instructor also introduces the terms 'genotype' and 'phenotype', explaining that genotype refers to the specific alleles an organism has, while phenotype is the observable expression of those alleles. The dominance of certain alleles is also touched upon, with the example of brown hair being dominant over blonde.

05:01

🔬 Homozygous and Heterozygous Genotypes

The second paragraph delves deeper into genetic terminology, focusing on homozygous and heterozygous genotypes. Homozygous refers to having two identical alleles for a gene, while heterozygous indicates the presence of two different alleles. The instructor uses the example of hair color alleles to illustrate these concepts, explaining that an organism with two dominant brown hair alleles would have a homozygous dominant genotype, whereas one with a dominant and a recessive allele would be heterozygous. The paragraph concludes by reinforcing the understanding of these genetic terms, equipping the viewer with the knowledge to discuss genetics more confidently.

Keywords

💡Sexually reproducing organism

A sexually reproducing organism is a living being that produces offspring through the combination of genetic material from two parents. In the context of the video, this is used to explain the origin of genetic diversity in humans, highlighting that each individual receives DNA from both a male and a female parent, contributing to the unique genetic makeup of each person.

💡Chromosomes

Chromosomes are thread-like structures within the nucleus of a cell, composed of DNA and proteins. They carry genetic information in the form of genes. In the video, the instructor uses the example of human beings having 23 pairs of chromosomes to illustrate how genetic traits are inherited, with each pair consisting of one chromosome from each parent.

💡DNA

DNA, or deoxyribonucleic acid, is the molecule that carries the genetic instructions for the development, functioning, growth, and reproduction of all known living organisms. The script explains that chromosomes are essentially bunched up strands of DNA, emphasizing the role of DNA in coding for various genes that determine traits.

💡Genes

Genes are segments of DNA that code for specific proteins and are responsible for hereditary traits. The video script uses genes to explain how different physical characteristics, such as height and hair color, are determined by the genetic code present on DNA strands.

💡Alleles

Alleles are different versions of a gene that can exist at the same locus on homologous chromosomes. In the script, alleles are used to explain the concept of genetic variation, with examples given for different hair color alleles, such as 'b' for blonde and 'B' for brown.

💡Genotype

Genotype refers to the genetic makeup of an individual in terms of the specific alleles of genes they carry. The video script explains genotype by describing the combination of alleles an individual has inherited from their parents, such as having a 'B' allele from one parent and a 'b' allele from another.

💡Phenotype

Phenotype is the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment. The instructor in the video uses the term to discuss how the combination of alleles (genotype) influences the physical appearance or traits (phenotype), such as hair color.

💡Dominant allele

A dominant allele is a version of a gene that will mask the effect of another allele (recessive) when present. The script simplifies this concept by stating that if an individual has at least one dominant allele for a trait, such as brown hair, that trait will be expressed in their phenotype.

💡Recessive allele

A recessive allele is a gene variant that is only expressed phenotypically when an individual has two copies of it. The video script illustrates this with the example that an individual would have to inherit two 'b' alleles to express the phenotype of blonde hair.

💡Homozygous

Homozygous refers to the genetic condition where an individual has two identical alleles for a particular gene. The video script uses this term to describe a genotype where both alleles for a gene are the same, such as having two dominant 'B' alleles for hair color.

💡Heterozygous

Heterozygous is the genetic condition where an individual has two different alleles for a particular gene. The instructor explains this in the context of having one dominant and one recessive allele, such as 'Bb' for hair color, which would result in a heterozygous genotype.

Highlights

Sexual reproduction in organisms involves inheriting DNA from both male and female parents, including humans.

Humans possess 23 pairs of chromosomes, each from a different parent.

Chromosomes are composed of DNA strands and other packaging materials.

DNA contains segments that code for different genes, influencing traits like height and hair color.

Each chromosome in a pair generally codes for the same set of genes.

Genes can be translated into proteins, which manifest as physical or observable traits.

Different versions of a gene are called alleles, which determine traits like hair color.

An organism's genotype refers to the specific alleles it has inherited for a gene.

The phenotype is the observable expression of an organism's genotype.

Dominant alleles can mask the expression of recessive alleles in determining an organism's phenotype.

Multiple genotypes can result in the same phenotype, such as brown hair.

Blonde hair phenotype would only occur with two recessive alleles.

Genotypes can be homozygous, where alleles are the same, or heterozygous, where they differ.

Homozygous dominant genotypes have two copies of the same dominant allele.

Homozygous recessive genotypes have two copies of the same recessive allele.

Heterozygous genotypes contain different alleles for a gene.

Genetics involves complex interactions beyond simple dominant and recessive patterns.

Transcripts

- [Instructor] We know that

any sexually reproducing organism

is getting DNA from both its male parent

and its female parent

and that's true also for human beings.

You might know we have 23 pairs of chromosomes

but let's zoom in on one of those pairs.

So let's say this is one of the chromosomes in a pair

and let's say that this is the other chromosome in the pair.

So one came from one parent.

The other came from the other parent.

Chromosomes are nothing more than bunched up strands of DNA

plus other things that help package it.

So let's unravel a little bit

to understand how these relate to genes.

And then, we'll dive

into a little bit more terminology around genetics.

So you have this big, long strand of DNA

and segments of this code for different genes.

Maybe this part right over here

is related to how tall you become.

So let's say that part right over there,

that might be related to hair color.

And in other videos,

we talk about how these genes

can essentially be translated eventually

into proteins which are expressed in forms

like height or hair color or eye color,

or many, many, many, many other ways.

But each of these chromosomes

actually code, generally speaking,

for the same genes.

So if this one has a region that codes for that gene,

this one will as well

and this one will also.

The other chromosome in the pair will also have a region

that codes for this other gene.

Now, as you can imagine,

the genes that get translated into the proteins

that eventually are show up in your hair color,

they can come in different versions.

So what we call the different versions of a gene

is an allele.

So for example, from this parent,

you might have gotten the allele here

that would describe,

let's say, blonde hair,

I'll denote that with lowercase b,

while from this parent,

you might have gotten the allele

that's associated with say, brown hair.

So they're both associated or they're both coding

for the same type of protein

that might play a role in hair color

but they're different versions of it.

So you could view an allele

as versions or a version,

I'll say, alleles are versions of a gene.

Now, if you know which versions you have,

then you're able to say, "I know my or I know

the organism's genotype."

So in this situation, we know we have the capital B allele

which is for the brown hair

and we have the lowercase b allele

which is for the blonde hair,

and so the genotype in this situation,

where you actually know what alleles you have,

you would say that that is, you have a capital B

from one parent

and you have the lowercase b

from another parent.

Now the thing you might be asking is,

but what does this organism look like?

How does this genotype actually get expressed?

And if you're asking that question

where you're asking a question of phenotype.

Phenotype is what is expressed

and this is usually observable in some way.

So the phenotype in this situation

might be, and it's seldom this simple in genetics,

but we're gonna make it simple

in this scenario,

that if you have a brown allele and blonde allele,

that maybe the brown allele is dominant

which is that if you have even one brown allele,

at the end of the day, you're going to see brown.

So I'll just write that down for this scenario

although that's not actually going to be the case.

Genetics gets a lot more complicated than that

but certain genes, certain alleles,

they are that simply dominant or recessive.

Now you can imagine there's many different genotypes

based on what I just described

that could result in this phenotype.

You could have this genotype that we just showed.

You could get a brown allele from both parents.

Either of these genotypes would express themselves

as the phenotype brown, if we assume,

and that is a big if,

if we assume that this is the dominant version of the gene

or the dominant allele.

If that is, then in either case,

it will express itself as the phenotype of brown.

The only way you would get blonde hair in this scenario

is if you had two of the recessive gene

or the recessive version of the gene

or the recessive allele.

This would lead to a blonde phenotype

right over here.

We've talked about dominant alleles.

We've talked about recessive alleles

and another piece of terminology

that you will hear when you talk about genetics

are notions of homozygous, homozygous

and/or versus heterozygous genotypes.

And when people are talking

about homozygous or heterozygous,

they're talking about the genotype.

Homozygous, you might know the prefix homo refers to same.

So this is a situation

where both the alleles are the same, same alleles,

and heterozygous is a situation where hetero,

that's referring to different, different alleles.

So what would we call this scenario right over here?

We would call this a heterozygous genotype for this gene.

This would be homozygous dominant.

We have two of the same dominant allele

and this would be homozygous recessive.

I'll leave you there.

You're now armed to talk a lot about genetics.