Pedigrees

Summary

This video script from the Amoeba Sisters dives into the concept of pedigrees, which are like family trees for inherited traits. It explains the basics of autosomal recessive traits using the example of attached earlobes, illustrating how to deduce genotypes from phenotypes in a family. The script also contrasts autosomal with sex-linked traits, emphasizing the importance of understanding these patterns for genetic disorders. The engaging narrative encourages viewers to stay curious about genetics.

Takeaways

• 🌳 A pedigree is a visual representation of a family tree that shows inherited traits across generations.
• 🔠 In a pedigree, circles represent females and squares represent males, with the alphabetic order hinting at the choice of shapes.
• 👨‍👩‍👧‍👦 Roman numerals in a pedigree denote generations, and a marriage line connects parents, with lines to children indicating the number of offspring.
• 🖌️ Shaded shapes in a pedigree indicate the presence of a specific recessive trait being tracked.
• 🧬 The script discusses an autosomal recessive trait, meaning it is not linked to sex chromosomes and is inherited independently of gender.
• 🧬.1 Humans have 46 chromosomes, with the first 44 being autosomes and the last two being sex chromosomes.
• 👂 The example used in the script is attached earlobes, which are considered a recessive trait in this hypothetical scenario.
• 🔠 Genotypes are represented with 'E' for the dominant allele associated with free earlobes and 'e' for the recessive allele associated with attached earlobes.
• 🔍 Heterozygous individuals (Ee) are carriers of the recessive trait but do not express it due to the presence of the dominant allele.
• 🤔 The script encourages viewers to consider all possible genotypes for individuals in a pedigree, as multiple combinations can result in the observed phenotypes.
• 🧬.2 The script also discusses sex-linked recessive traits, which are carried on the X chromosome and typically affect males more frequently than females.
• 🔄 The importance of understanding pedigrees is highlighted for the study of genetic disorders and inheritance patterns.

Q & A

What is a pedigree and how is it represented in the script?

A pedigree is a diagram that shows the inheritance pattern of a particular trait across generations in a family. In the script, circles represent females and squares represent males. Shaded shapes indicate individuals with the recessive trait being tracked, which in this case is attached earlobes.

Why are circles used to represent females and squares for males in a pedigree?

Circles and squares are used to represent females and males, respectively, because the letter 'C' (for circle) comes before 'S' (for square) in the alphabet, mirroring the alphabetical order of 'F' (female) before 'M' (male).

What does the shading in the pedigree represent?

In the pedigree, shading represents the presence of the recessive trait being tracked. In this script, shaded shapes indicate individuals with attached earlobes.

What are the two key facts about the trait being tracked in the script?

The two key facts are: 1) The trait is recessive, meaning it is only expressed when no dominant allele is present. 2) It is an autosomal recessive trait, meaning it is not linked to the sex chromosomes.

How is the genotype of an individual with attached earlobes represented in the script?

Individuals with attached earlobes, a recessive trait, are represented with the genotype 'ee', where 'e' is the lowercase letter indicating the recessive allele.

What is the significance of the marriage line and the line connecting parents to children in a pedigree?

The marriage line in a pedigree connects two individuals who are married, indicating a potential for offspring. The line connecting parents to children shows the genetic relationship and helps trace the inheritance of traits.

Why can't the father in the first generation be homozygous dominant (EE)?

The father cannot be homozygous dominant (EE) because he has a child with the recessive trait (attached earlobes). Each child must inherit one allele from each parent, so if he were EE, he would not be able to pass on the recessive 'e' allele.

What is the genotype of the paternal grandmother in the script, and why?

The paternal grandmother could be either EE or Ee. This is because her offspring include a child with the recessive trait, indicating she must carry the 'e' allele, but her own phenotype is not shaded, suggesting she could also have a dominant 'E' allele.

How does the script explain the inheritance of a sex-linked recessive trait?

The script explains that for a sex-linked recessive trait, females can be carriers (X^RX^) or affected (X^rX^r), while males are either normal (XY) or affected (X^rY). The trait is passed from a carrier mother to her sons, who will be affected.

What does it mean when a pedigree is half-shaded, and why is it used?

A half-shaded pedigree indicates that the individuals are carriers of the recessive trait. This is used to show that they have one dominant and one recessive allele, and can pass on the recessive trait to their offspring.

Why is understanding pedigrees important in genetics?

Understanding pedigrees is important in genetics because it helps in tracing the inheritance of traits and disorders, particularly in the study of genetic disorders and their patterns of transmission in families.

Outlines

00:00

👨‍👧‍👦 Understanding Pedigrees and Recessive Traits

The first paragraph introduces the concept of a pedigree as a family tree that tracks inherited traits across generations. It uses a visual example of a small family's pedigree to explain the representation of males and females with squares and circles, respectively, and how generations are denoted by roman numerals. The script delves into autosomal recessive traits, using attached earlobes as an example, explaining that these traits require two recessive alleles to be expressed. It clarifies that autosomal refers to non-sex chromosomes and that the trait in question is not linked to sex chromosomes. The paragraph also discusses how to deduce genotypes from the phenotypes displayed in the pedigree, emphasizing the importance of each child receiving one allele from each parent.

05:01

🧬 Analyzing Genotypes in Pedigrees and Sex-Linked Traits

The second paragraph expands on the analysis of pedigrees by considering the possibility of sex-linked recessive traits, using color-blindness and male pattern baldness as examples. It instructs on how to label a sex-linked pedigree, indicating XX for females and XY for males to represent their sex chromosomes. The script explains the inheritance patterns of sex-linked traits, noting that only females with two recessive alleles will express the trait, while males need only one. The paragraph challenges viewers to deduce genotypes for various family members based on the phenotypes of their offspring, emphasizing the necessity of each child receiving a recessive allele from each parent. It concludes by highlighting the importance of understanding pedigrees in the context of genetic disorders and encourages viewers to explore further with provided handouts.

Keywords

💡Pedigree

A pedigree is a diagram that represents multiple generations of a family, typically used to track the inheritance of certain traits or conditions. In the context of the video, the pedigree is used to illustrate the inheritance pattern of earlobe attachment, which is a key theme of the video. The script explains that the pedigree includes symbols for different generations and relationships, such as marriage lines and connecting lines between parents and children.

💡Inherited Trait

An inherited trait refers to a characteristic that is passed down from parents to their offspring through genes. The video uses the example of attached earlobes as an inherited trait, which is the focus of the pedigree analysis. The script clarifies that the trait being tracked in the pedigree is recessive, meaning it only manifests when there is no dominant allele present.

💡Recessive Trait

A recessive trait is a characteristic that only appears in an individual's phenotype if they have two copies of the recessive allele. In the script, attached earlobes are described as a recessive trait, which means that a person would only exhibit this trait if they inherited two 'e' alleles, one from each parent.

💡Autosomal Recessive Trait

An autosomal recessive trait is a genetic trait that is located on one of the non-sex chromosomes (autosomes) and is only expressed when an individual has two copies of the recessive allele. The video explains that the trait being tracked, attached earlobes, is autosomal, not linked to the sex chromosomes, and therefore the inheritance pattern is different from sex-linked traits.

💡Phenotype

Phenotype refers to the observable characteristics or traits of an individual, which result from the interaction of their genetic makeup (genotype) with the environment. In the video, the phenotypes discussed are free earlobes (dominant) and attached earlobes (recessive). The script uses the terms to describe the physical appearance of individuals in the pedigree.

💡Genotype

Genotype is the genetic composition of an individual, describing the specific set of genes inherited from the parents. The script explains genotypes in the context of the earlobe trait, using 'E' for the dominant allele and 'e' for the recessive allele. It illustrates how genotypes can be deduced from the observed phenotypes in a pedigree.

💡Carrier

A carrier in genetics is an individual who has a recessive allele for a particular trait but does not express the trait due to the presence of a dominant allele. In the script, the father is identified as a carrier of the recessive allele for attached earlobes, as he has the genotype Ee and shows the dominant phenotype of free earlobes.

💡Sex Chromosomes

Sex chromosomes are the chromosomes that determine the sex of an individual. Humans typically have two sex chromosomes: XX for females and XY for males. The video script contrasts autosomal traits with sex-linked traits, explaining that autosomal traits are not located on the sex chromosomes, unlike sex-linked traits.

💡Sex-Linked Trait

A sex-linked trait is a genetic trait that is located on the sex chromosomes and therefore exhibits different inheritance patterns, particularly in males and females. The script explores the concept of sex-linked recessive traits, using a hypothetical scenario to illustrate how such traits would be represented in a pedigree.

💡Allele

An allele is a variant form of a gene that arises by mutation and is found at the same place on a chromosome. The script uses the terms 'E' and 'e' to represent different alleles for the earlobe trait, with 'E' being dominant and 'e' being recessive. Understanding alleles is crucial for interpreting the inheritance patterns shown in the pedigree.

💡Mendelian Inheritance

Mendelian inheritance refers to the patterns of inheritance for traits that follow the laws of segregation and independent assortment, as first described by Gregor Mendel. The video script explains that in Mendelian inheritance, dominant traits are expressed when at least one dominant allele is present, which is key to understanding the inheritance of the earlobe trait.

Highlights

Introduction of a unique picture representing a pedigree, a family tree for inherited traits.

Explanation of how to represent family members in a pedigree with circles for females and squares for males.

Mnemonic for remembering the representation: Alphabetical order of 'C' for circle and 'S' for square.

Description of how to denote generations and marriage lines in a pedigree.

Significance of shaded shapes in a pedigree indicating the presence of a tracked recessive trait.

Clarification of autosomal recessive traits and their distinction from sex-linked traits.

Introduction of the specific recessive trait being tracked: attached earlobes.

Assumption of a single gene for the trait of attached earlobes and its inheritance pattern.

Genotype analysis of individuals in the pedigree, identifying carriers and those with the recessive trait.

Determination of genotypes based on offspring inheritance and the presence of the recessive trait.

The concept of heterozygosity and its role in the expression of dominant and recessive traits.

Exploration of an imaginary large family pedigree to illustrate genetic inheritance patterns.

Challenge for viewers to solve the genotypes of individuals in the imaginary family pedigree.

Discussion on the probability of genotypes and the inclusion of possible options in pedigree analysis.

Transition to the concept of sex-linked recessive traits with examples like color-blindness.

Instructions on how to represent sex chromosomes in a sex-linked pedigree.

Explanation of carrier status in sex-linked recessive traits and its implications for offspring.

The importance of ensuring each child in a pedigree receives alleles from both parents.

Consideration of dominant traits in pedigree analysis and the impact on inheritance patterns.

Mention of half-shading in pedigrees to indicate carrier status for certain traits.

Emphasis on the importance of understanding pedigrees in the study of genetic disorders.

Transcripts

So I have a really awesome  picture for you. Here it is! 

Oh it may look like circles and squares to  you, but make no mistake, this is no ordinary 

picture. This is a pedigree. A  pedigree is like a family tree- it can 

show information about an inherited trait passed  across generations. And this one is actually 

a small pedigree of us! Well, our  human forms. See, that's me. My sister. 

My mom. My dad. I’m not arbitrarily picking 

random shapes to represent us either.  In a pedigree, the circles represent 

females. Squares represent males. One  way you can remember that is that the 

letter “C” (for circle) comes before the  letter “S” (for square) in the alphabet. 

Alphabetically, the letter “F” (for female)  comes before the letter “M” (for male). 

I’ve got a thing about letters of the alphabet  though- I realize that may not work for 

everyone. So if we take a look at this  pedigree, these roman numerals represent 

generations. There's 2 generations here. This  between my parents is called a marriage line. 

This line here connects parents to children  so you can see there are two children from 

this marriage. Now you may wonder, why are some  of these shapes shaded? What does that mean? 

Well, the shaded shapes represent a trait  that is being tracked in the pedigree. 

Here are 2 important facts about the  particular trait I am choosing to track. 

Fact #1 about this trait being  tracked is that it’s recessive. 

Recall that in typical Mendelian inheritance,  dominant alleles---if present---will express 

dominant traits. Recessive  alleles only are expressed 

when the dominant allele is not present.  Fact #2 about this trait is that it is 

an autosomal recessive trait.  Just a reminder that autosomal 

means a chromosome that is not a sex chromosome.  In human body cells, there are 46 chromosomes. 

The first 44 (22 pairs) are autosomes.  The last 2 (1 pair) are sex chromosomes. 

So this trait is not sex-linked since it is  autosomal and that means it does not need 

to be written as coefficients (correction:  EXPONENTS) on the sex chromosomes. 

So what is the trait we’re  tracking? Attached earlobes! 

Yes, you may not realize it, but look around  and you'll see that humans may have free or 

attached earlobes. Although we  want to point out that there 

may be more than just these two categories for  ear lobes, and while this example is used often 

in basic genetics, there’s probably  more to this than just one simple gene. 

For our example, let’s assume a one gene  trait and that free earlobes is dominant, 

meaning at least one dominant  allele must be around. 

Attached earlobes is recessive,  showing no dominant allele is present. 

So if we were to put the genotypes next to  each of these shapes, what would they be? 

Well the shaded ones would be easy. Because we  just mentioned that attached earlobes is the 

trait we’re tracking and it is  an autosomal recessive trait. 

So if we use the letter “e” then these shaded  shapes must be lowercase e, lowercase e. 

Any capital (dominant) letter and  the individual would have to have 

free earlobes and not be shaded.  So let’s look at individual #2 

in the first generation. That’s our father. He’s 

not shaded so he can’t be little e little e.  What about big E, big E? Well there’s a problem. 

See his children? Us, ha. Each  child must get an allele from EACH 

parent. So if I received a little “e” from my  mom, then I had to get my other “e” from my dad. 

Therefore he can’t be big E big E  or he’d have no little “e” to give! 

His genotype must be the  heterozygote genotype, Ee. 

He’s what we call a carrier though he still  has a phenotype of free earlobes because of 

that one capital. But he carries the lowercase  allele. Now that’s just one tiny pedigree. 

Let’s look at a big family reunion! Um, well  an imaginary one, because I have to confess 

I don’t really know whether our  relatives have free or attached earlobes. 

I thought about sending a survey out to all  of them, but...it felt a little awkward. 

So here we go, big giant  imaginary family of relatives! 

Ok just to make sure you understand  this---how many siblings does my dad have? 

Well look, here’s my dad in generation 2 (#4).  He has three siblings, all brothers, right here. 

What is the phenotype of my paternal  grandfather? Well look, here’s my dad. 

Here is my dad’s dad---that  would be my paternal grandfather. 

And because his square is shaded---that  means his phenotype is attached earlobes. 

Ok, so let’s go ahead and label all these shaded  shapes with the genotype little e little e 

since we know that’s the trait we’re tracking.  Now take a look at generation 1, individual 1. 

That would be my paternal  grandmother. What’s her genotype? 

We know it’s not little e little  e or her shape would be shaded. 

But if we went with EE could that still work?  Yes, all the offspring could get a big E from 

her and a little e from my  grandfather. But what about 

Ee. Would that work too? Yes! 

Because the children could still get a big E  from her and a little e from my grandfather. 

It may be less of a probability, but it’s  possible and therefore we must list both 

that she is EE OR Ee, because we don’t know.  All the offspring of my paternal grandparents 

though are going to have to be Ee.  Remember they have to get an allele from 

each parent and that means they're going  to have to pick up that little “e” from my 

grandfather. They will be heterozygotes  and that’s the only option here. Pause 

this video and try to solve the  right side of this pedigree now! 

Ta Da! Imaginary family  done! So how’d you do? Well, 

here are some of the tricky ones.  Did you see that generation 1, 

individual 4 has to be a carrier only (Ee)?  Because if not, then the shaded individual 

children would not be able to get the “ee”  that that they have, because they have to get a 

little “e” from both parents.  How about individual 9? This 

female married in, but that’s not the reason that she can be either EE or Ee. If you  

look at the children, they aren’t shaded. So while they will have to get a little “e”  

from number 8 as that’s all #8 can give…the other capital letter can be obtained from  

#9 regardless of whether she’s EE or Ee. Remember, one option may be more likely,  

but if it’s possible, you need to include both. Now remember we had made a big deal  

about how this was autosomal pedigree? Well, what if you are dealing with a sex-linked  

trait and therefore a sex-linked pedigree? There are a lot of sex-linked recessive  

traits. Color-blindness and some male patterns of baldness can be sex-linked. Let’s pretend now, we 

are told this is a sex-linked recessive trait.  I’m going to keep the old labeled pedigree here 

that showed an autosomal recessive  trait just for comparison, but now here 

is a brand new sex-linked pedigree. First  of all, all the females (the circles) should 

have an XX to indicate two  X sex chromosomes by them. 

Remember females have two X chromosomes.  Males should have an XY to indicate 

an X and a Y sex chromosome. That's  always a good thing to do first. 

Now remember that we were told this pedigree  is tracking sex-linked recessive traits. 

So the shaded one here has the  sex-linked recessive trait. 

A reminder from our sex-linked  video about how this works. 

Let's use the letter "R" for an allele.  Now recall that females that do not 

have the trait can be either this or this. And  the heterozygote genotype (this) is a carrier. 

She doesn’t have the trait herself because  of the dominant allele but she’s carrying it. 

Only a female that is this will  have the sex-linked recessive trait. 

So if I’m looking at this pedigree, what would  the genotype for individual 1 in generation 1 be? 

Well notice here she has 3 children  and one of her sons here is shaded. 

Where does her son get his Y  sex chromosome from? The father. 

Where does he get his X sex  chromosome from? His mother! 

Individual 1 doesn’t have the trait or she  would be shaded, but she must be a carrier 

if her son received a X chromosome  with a recessive allele on it. 

Now what about individual 2 in generation  2? Well she can be this like her mother. 

But look, she could also be this because it’s  possible to get one of those from each parent. 

If it’s possible, you must include it. Now pause  the video and try to solve the last female. 

How’d you do? Remember the key  here is to always check and 

make sure that when you look at a child---they  have to be able to get one of their 

alleles from EACH parent.  Now remember that both of 

these examples were recessive.  It doesn’t have to be that way. 

On our handout, you can try one out that  follows an autosomal dominant trait. 

If it’s a dominant allele that you are tracking,  remember it would only take ONE dominant allele 

for a person to have that trait. Another  quick thing to point is sometimes 

you will see pedigrees that are half shaded.  Well that’s just awesome because they're 

basically letting you know that  the half-shaded ones are carriers. 

If I wanted to turn our first one into 

that - that half-shading -  then it would look like this. 

Mapping and understanding pedigrees is important,  especially as we continue to make advancements 

in understanding how genetic  disorders are inherited. 

Well that’s it for the Amoeba Sisters,  and we remind you to stay curious!