Pedigrees | Classical genetics | High school biology | Khan Academy

Summary

This educational script explains the concept of a pedigree, a tool used to analyze inheritance patterns of traits within families. It uses the example of freckles, an autosomal dominant trait, to illustrate how traits are passed down through generations. The script clarifies the difference between phenotype (observable traits) and genotype (genetic makeup), and demonstrates how to deduce the genetic status of individuals based on the presence or absence of the trait in their offspring.

Takeaways

• 👨‍👩‍👧‍👦 A pedigree is a tool used to analyze inheritance patterns of traits within families.
• 🔍 It helps in understanding the genetics of a trait and provides insights into past occurrences and future probabilities.
• 📊 In a pedigree, a circle represents a female and a square represents a male in the family.
• 🎨 A filled-in circle or square indicates that the individual exhibits the trait, such as having freckles in this example.
• 🔄 The phenotype is the observed trait, like freckles, but it doesn't reveal the exact genotype.
• 🤝 A horizontal line connecting a male and a female signifies a marriage or coupling, indicating reproduction.
• 🌱 A vertical line descending from the marriage line represents the line of descent, showing the children of the couple.
• 👫 Sibling lines connect brothers and sisters, showing the family's offspring.
• 🧬 Freckles are an autosomal dominant trait, associated with non-sex chromosomes and only one dominant allele is needed to exhibit the trait.
• 🧬‍🤝 The genotype can be inferred from the phenotype patterns observed across generations.
• 🤔 By analyzing the presence or absence of freckles in offspring, one can deduce whether a parent is homozygous dominant or heterozygous.

Q & A

What is a pedigree used for?

A pedigree is used for analyzing the inheritance patterns of a trait within a family, which can help understand the genetics of the trait and provide insights into what might happen in the future.

What do circles and squares represent in a pedigree?

In a pedigree, circles represent females and squares represent males in the family.

What does it mean if a circle or square is colored in a pedigree?

If a circle or square is colored in, it indicates that the person exhibits the trait being analyzed, such as having freckles in the example provided.

What is the term for the observed trait in a pedigree?

The observed trait in a pedigree is called a phenotype, which is determined by looking at the person and noting whether they have the trait or not.

How is the relationship between parents and children represented in a pedigree?

A horizontal line connecting a male and a female represents a marriage line, indicating they are reproducing. A vertical line descending from this line is the line of descent, connecting parents to their children.

What is the term for individuals connected by a vertical line at the bottom?

Individuals connected by a vertical line at the bottom are siblings, indicating they are brothers and sisters.

What does 'autosomal' mean in the context of genetics?

Autosomal refers to a trait associated with one of the non-sex chromosomes, which are the 22 pairs of chromosomes that do not determine an individual's sex.

What is the significance of a dominant trait in genetics?

A dominant trait means that an individual will exhibit the trait if they have at least one dominant allele for that trait, regardless of the other allele being dominant or recessive.

How can you infer the genotype of an individual with freckles from the pedigree?

If an individual exhibits freckles, their genotype could be either homozygous dominant (FF) or heterozygous (Ff). However, if they have children without the trait, they must be heterozygous.

What is the genotype of an individual who does not exhibit the trait and has children with the trait?

If an individual does not exhibit the trait but has children with the trait, their genotype must be heterozygous (Ff), carrying one dominant and one recessive allele.

How can you determine if an individual is homozygous dominant or heterozygous based on their offspring?

If an individual is homozygous dominant, all their offspring would exhibit the trait if it is autosomal dominant. If some offspring do not exhibit the trait, the parent must be heterozygous.

Outlines

00:00

👨‍🏫 Understanding Pedigrees and Inheritance Patterns

This paragraph introduces the concept of a pedigree, a tool used to analyze inheritance patterns of traits within families. It explains the significance of pedigrees in understanding genetic traits and predicting future occurrences. The paragraph details the visual representation of a pedigree, using circles for females, squares for males, and color to indicate the presence of a trait, such as freckles. It also discusses the importance of phenotypes and genotypes, and how they can be inferred from the patterns observed across generations. The basics of autosomal dominant traits are introduced, explaining that these are traits linked to non-sex chromosomes and that only one dominant allele is needed to express the trait.

05:04

🔍 Deciphering Genotypes from Phenotypes

Building upon the understanding of pedigrees, this paragraph delves into the process of inferring genotypes from observed phenotypes. It uses the example of freckles to illustrate how the presence or absence of the trait in offspring can help determine the genotype of the parents. The paragraph clarifies that if a parent exhibits the trait (freckles) but not all offspring do, the parent must be heterozygous (having one dominant and one recessive allele). Conversely, a parent without the trait must be homozygous recessive. The explanation provides a foundation for further genetic analysis and understanding inheritance patterns within families.

Keywords

💡Pedigree

A pedigree is a diagram that represents the genetic relationships and patterns of inheritance within a family. In the context of the video, it is used to analyze the inheritance patterns of a trait, such as freckles. The script explains that a pedigree can help us understand past genetic occurrences within a family and predict future probabilities of trait expression.

💡Inheritance Patterns

Inheritance patterns refer to the ways in which traits are passed down from parents to offspring. The video script discusses how a pedigree can be used to analyze these patterns, particularly focusing on the autosomal dominant inheritance pattern of freckles, where the trait is expressed even if only one parent passes on the dominant allele.

💡Phenotype

Phenotype is the observable characteristic or trait of an individual, such as the presence of freckles. The script uses the term to describe what can be seen or measured, like the coloration of a person's skin, and how it can be used to infer possible genotypes based on the observed expression of the trait.

💡Genotype

Genotype refers to the genetic makeup of an individual, specifically the combination of alleles they possess for a particular trait. The video script explains that while the phenotype is visible, the genotype can be inferred from the patterns of trait expression in a pedigree, such as the presence of freckles.

💡Autosomal

Autosomal describes genes that are located on non-sex chromosomes. In the script, the term is used to explain that the trait for freckles is associated with one of the 22 pairs of non-sex chromosomes, as opposed to being linked to the sex chromosomes.

💡Dominant Trait

A dominant trait is one that is expressed when at least one copy of the dominant allele is present. The video script clarifies that in the case of freckles, having just one dominant allele (represented as 'F') results in the expression of the trait, regardless of the second allele being dominant or recessive.

💡Allele

An allele is a variant form of a gene that occurs at a specific position on a chromosome. The script uses the term to describe the different forms of the gene for freckles, where 'F' is the dominant allele for having freckles and 'f' is the recessive allele for not having freckles.

💡Homozygous Dominant

Homozygous dominant refers to an individual having two copies of the dominant allele for a trait. The script uses this term to discuss one possible genotype for the individual with freckles ('FF'), but then rules it out based on the observed phenotypes in their offspring.

💡Heterozygote

A heterozygote is an individual with two different alleles for a trait, one dominant and one recessive. The script explains that the individual with freckles must be a heterozygote ('Ff') because not all of their children exhibit the trait, indicating that they must have passed on a recessive allele as well.

💡Marriage Line

In a pedigree, a marriage line is a horizontal line connecting two individuals, indicating that they are in a relationship that results in offspring. The script uses this term to describe the connection between a male and a female who have reproduced, leading to the next generation.

💡Line of Descent

The line of descent in a pedigree is a vertical line that connects parents to their children, showing the direct path of inheritance. The script uses this term to illustrate how traits are passed down from one generation to the next, such as from the first generation to the second.

Highlights

A pedigree is a method for analyzing inheritance patterns of traits within a family.

Pedigrees can provide insights into the genetics of a trait and predict potential future occurrences.

In a pedigree, a circle represents a female and a square represents a male.

Colored shapes indicate the presence of the trait, such as freckles in this example.

Phenotype is the observed trait, such as the presence or absence of freckles.

Genotype cannot be directly observed from the phenotype but can be inferred.

A horizontal line between shapes signifies a marital or reproductive relationship.

A vertical line descending from a horizontal line represents the line of descent, indicating children.

Sibling lines connect brothers and sisters in the pedigree.

Freckles are identified as an autosomal dominant trait.

Autosomal refers to traits associated with non-sex chromosomes.

Dominant traits manifest with just one dominant allele.

The genotype for freckles can be deduced from the observed phenotypes across generations.

Individual I-2's genotype must be heterozygous (Ff) based on the phenotypes of their children.

If I-2 were homozygous dominant (FF), all children would exhibit freckles, which is not observed.

The partner of I-2 must have a genotype of ff, as they do not exhibit freckles.

Understanding the genotype of family members allows for the prediction of inheritance patterns.

Pedigree analysis is a valuable tool for genetic counseling and understanding hereditary traits.

Transcripts

- [Instructor] What you see depicted

here is known as a pedigree.

And a pedigree is a way of analyzing

the inheritance patterns of a trait within a family.

And it can be useful to understand more about that trait,

maybe to make some insights about

the genetics of that trait, and it's a way

to think about what's happened in the past in a family,

and then maybe we can help get some probabilities

or get some understanding of what

might happen in the future.

But let's just break it down, so we understand

all of the different parts of a pedigree.

So big picture, a circle represents a female in the family.

And a square represents a male in the family.

If either the circle or the square is colored in,

like we see right over here, or we see right over here,

that means that that person exhibits the trait.

And the trait of this particular pedigree

is for is the trait of freckles.

And whether or not we're colored in, that is a phenotype.

That is an observed trait.

You just look at the person and say

do they have freckles or not.

This male right over here does not have freckles.

This female right over here does have freckles.

This male right over here does have freckles.

But once again, this phenotype,

that's the observed characteristic,

we don't know exactly the genotype just from this.

Although we might be able to infer some ideas

about the genotype by seeing how the phenotype patterns

spread from generation to generation.

But just going back to understanding the pedigree itself.

When you have a horizontal line

connecting a male and a female, it's called a marriage line.

And it means that they are coupling up

and they are reproducing.

They don't necessarily, I guess, have to be married.

It's more that they reproduce.

So this male and this female have reproduced.

And then this vertical line that goes

from that horizontal line,

that's known as the line of descent.

So it's going from that first generation

to the second generation.

And so all of the people connected to that vertical line

at the bottom of the vertical line,

these are their children.

All the people who are directly connected to this.

And this is known as a sibling line.

So, this person, this person and that person,

they are all brothers and sisters.

You can see that generation one,

they had two daughters, one of whom exhibited freckles,

and one son, who had freckles.

Now these other people in generation two,

these aren't children of the first generation.

These are people, you could say,

who are brought into the family.

They either married into the family,

or you could just say they reproduced with the children.

But that's what's happening with generation two.

And so you can see, you have another

what's known as a marriage line,

but I guess it could be a coupling line, a line of descent,

and then they have two daughters in this case.

So it turns out that freckles

is an autosomal dominant trait.

And I will explain what that means in a second.

Autosomal

dominant,

dominant trait.

Autosomal means that it is a trait associated

with one of the non-sex chromosomes.

We have 23 pairs of chromosomes as humans.

One of those pairs determines our sex,

the other ones don't determine our sex.

So this means that freckles is associated

with one of the 22 pairs of chromosomes

that do not determine your sex.

And dominant means that in general

you get two versions of a gene.

One from your mom, and one from your father.

And each of those versions of the gene for say

whether or not you have freckles is known as an allele.

And if a trait is autosomal dominant,

that means that if you have even just one allele

for that trait, that you are going to exhibit the trait.

And so, for example, if the alleles for freckles,

if capital F is you have freckles,

it's dominant, so you write it in capital,

and lowercase F means that you don't have freckles,

if someone has a genotype of capital F, capital F,

or capital F, lowercase F, or, I guess we could say

lowercase F, capital F if we wanted to think about order.

Which one you're getting from which parent.

Well then in all of these situations,

they would exhibit the phenotype of freckles.

And we talk about genotype and phenotype in other videos.

And of course, the only genotype in which

they would not exhibit freckles

would be having both of the recessive traits.

So with this out of the way, what can we infer

about the genotype of this individual right over here?

And this individual will sometimes be

referred to as individual I-2.

They're in generation one,

and they are individual two in generation one.

Well, we know that they exhibit freckles.

So we know for sure that since the phenotype is freckles,

they're in one of these scenarios.

And these two, these two are frankly equivalent.

So this person, we know that they're either

capital F, capital F, so that would be homozygous dominant.

Or they're capital F, lowercase F.

Which are, they are a heterozygote.

Well, let's first think about the situation

if they are homozygous dominant.

If they are homozygous dominant,

that means to every one of her children,

she would give one of these alleles.

And either of those are dominant.

And so that would mean if this were the case,

that all of her children would get

at least on dominant allele.

Well, if they all get at least one dominant allele,

then all of her children would have freckles.

But we're seeing a situation where

all of her children do not have freckles.

Two of her three children had freckles.

So this cannot be the case.

So this individual right over here

must be heterozygous for freckles.

So they must be capital F, lowercase F.

And what do we know about this individual right over here,

the person that she coupled up with?

Well, they don't exhibit the phenotype for freckles.

So they must be lowercase F, lowercase F.

And then from that, you might be able

to intuit a lot more about that.

And we'll do that in future worked examples.