Example punnet square for sex-linked recessive trait | High school biology | Khan Academy

Summary

This instructional video script explains hemophilia, an X-linked recessive trait affecting blood clotting. It details the genetic inheritance, using a carrier woman and a hemophiliac man as an example. The script guides viewers through understanding genotypes and Punnett squares to calculate the probability of their daughter having hemophilia, revealing a 50% chance if born female. It also addresses related inheritance probabilities, such as for hemophiliac sons and non-hemophiliac offspring.

Takeaways

• 🩸 Hemophilia is an X-linked recessive trait that affects blood clotting.
• 🧬 Individuals with hemophilia have a genetic condition where their blood has difficulty clotting.
• 👩‍👧 A carrier woman has one X chromosome with the hemophilia allele and one without, but does not exhibit the trait.
• 👨‍👦 A hemophiliac man has the hemophilia allele on his single X chromosome, leading to the expression of the trait.
• 🧬 The mother's genotype is represented as X^HX^h, indicating one dominant allele (H) and one recessive allele (h).
• 🧬 The father's genotype is represented as X^hY, indicating the hemophilia allele on the X chromosome and a Y chromosome.
• 🔍 To determine the offspring's genotype, a Punnett square is used to visualize the possible genetic combinations.
• 👧 Daughters are XX, so the scenarios with two X chromosomes are considered for determining if a daughter has hemophilia.
• 🤔 The chance of having a daughter with hemophilia, given a carrier mother and a hemophiliac father, is 50%.
• 👦 The chance of having a hemophiliac son is 25%, as only one scenario out of four results in a son with the condition.
• 👶 The overall chance of having a hemophiliac offspring is 50%, considering both sons and daughters.
• 👦 Given a son, there is a 50% chance that he does not have hemophilia, as only one scenario out of the two son scenarios results in a non-hemophiliac son.

Q & A

What is hemophilia?

Hemophilia is an X-linked recessive trait that affects blood clotting, causing individuals with the condition to have difficulty with their blood clotting properly.

How does hemophilia inheritance work?

Hemophilia is inherited in an X-linked recessive pattern, meaning it is more common in males and can be carried by females without them showing symptoms.

What does it mean to be a carrier for hemophilia?

A carrier for hemophilia is a female who has one X chromosome with the hemophilia allele (recessive gene) but does not exhibit symptoms because the other X chromosome has the dominant allele.

What are the genotypes of a carrier woman and a hemophiliac man?

A carrier woman has the genotype X^HX^h, where X^H is the normal allele and X^h is the hemophilia allele. A hemophiliac man has the genotype X^hY, with the Y chromosome being the male sex chromosome.

How can you determine the likelihood of a daughter having hemophilia using a Punnett square?

By using a Punnett square, you can visualize the possible combinations of alleles that a child can inherit from both parents. In the case of a carrier woman and a hemophiliac man, the Punnett square will show the possible genotypes of their offspring.

What is the percent chance that a daughter of a carrier woman and a hemophiliac man will have hemophilia?

There is a 50% chance that a daughter will have hemophilia, as she can either inherit the X chromosome with the hemophilia allele from both parents or only from her father.

What are the different scenarios for the offspring of a carrier woman and a hemophiliac man?

The scenarios include a daughter with hemophilia (X^hX^h), a daughter who is a carrier (X^HX^h), a son with hemophilia (X^hY), and a son without hemophilia (X^HY).

What is the percent chance of having a son with hemophilia?

There is a 25% chance of having a son with hemophilia, as only one of the four Punnett square outcomes will result in a son with the X^hY genotype.

What is the percent chance of having a non-hemophiliac son?

There is also a 25% chance of having a non-hemophiliac son, represented by the X^HY genotype in the Punnett square.

What is the overall percent chance of having a hemophiliac offspring?

The overall percent chance of having a hemophiliac offspring is 50%, as two out of the four Punnett square outcomes result in hemophilia.

Given that they had a son, what is the percent chance that the son does not have hemophilia?

Given that they had a son, there is a 50% chance that the son does not have hemophilia, as one out of the two male scenarios results in a son without the condition.

Outlines

00:00

🧬 Hemophilia Genetics and Inheritance

This paragraph discusses the genetic basis of hemophilia, an X-linked recessive trait that affects blood clotting. It explains the genotypes of a carrier woman and a hemophiliac man and how they can affect their offspring. The mother, being a carrier, has one normal (H) and one hemophilia allele (h) on her X chromosomes, while the father has hemophilia and thus carries the hemophilia allele on his single X chromosome and a Y chromosome. The use of a Punnett square is introduced to visualize the possible genetic outcomes for their children, specifically focusing on the chances of their daughter having hemophilia, which is 50% if she inherits the hemophilia allele from both parents.

Keywords

💡Hemophilia

Hemophilia is a genetic disorder that impairs the body's ability to make blood clots, a process necessary to stop bleeding. It is the main theme of the video as it explains how this condition is inherited. In the script, hemophilia is described as an X-linked recessive trait that affects blood clotting, with the example given that a hemophiliac man's blood has trouble clotting.

💡X-linked recessive trait

This term refers to a pattern of inheritance where a gene causing a particular trait is located on the X chromosome and is not expressed unless it is present in two copies (homozygous). The video script uses this concept to explain that hemophilia is inherited in this manner, affecting males more frequently than females due to the difference in sex chromosome composition.

💡Blood clotting

Blood clotting, also known as coagulation, is the process by which blood transforms from a liquid to a gel-like state to prevent excessive bleeding from injuries. In the context of the video, the difficulty in blood clotting is central to understanding the effects of hemophilia.

💡Carrier

A carrier in genetics is an individual who carries a recessive allele for a trait but does not express the trait themselves. The script explains that a carrier woman has one X chromosome with the hemophilia allele and another without it, but she does not have hemophilia because the dominant allele masks the recessive one.

💡Genotype

Genotype refers to the genetic makeup of an individual regarding a particular trait. The script discusses the genotypes of the mother and father to explain the possible genetic outcomes for their offspring, focusing on the sex chromosomes and the presence of the hemophilia allele.

💡Sex chromosomes

Sex chromosomes are the chromosomes that determine the biological sex of an individual. Humans typically have two sex chromosomes: XX for females and XY for males. The video script uses the concept of sex chromosomes to illustrate how hemophilia can be inherited differently in sons and daughters.

💡Dominant allele

A dominant allele is a version of a gene that is expressed in the phenotype of an organism even when only one copy is present, masking the effect of a recessive allele. In the script, 'H' is used to denote the dominant allele for the absence of hemophilia, contrasting with the recessive 'h' allele that causes the condition.

💡Recessive allele

A recessive allele is a version of a gene that is only expressed in the phenotype when two copies are present. The script explains that the lowercase 'h' represents the recessive allele for hemophilia, which is only expressed when there is no dominant allele to mask it, as in the case of a hemophiliac man.

💡Punnett square

A Punnett square is a diagram used to predict the genotypes of offspring in a genetic cross. It is central to the script's explanation of how to determine the likelihood of a daughter inheriting hemophilia, by showing the possible combinations of alleles from each parent.

💡Offspring

Offspring refers to the progeny or descendants produced by a particular set of parents. The script uses the term to discuss the potential genetic outcomes for the children of a carrier woman and a hemophiliac man, including the chances of having a hemophiliac son or daughter.

💡Probability

Probability is the measure of the likelihood that a particular event will occur. The script concludes by calculating the probabilities of different genetic outcomes for the offspring, such as the 50% chance of having a daughter with hemophilia, using the Punnett square.

Highlights

Hemophilia is an X-linked recessive trait affecting blood clotting.

Individuals with hemophilia have blood that clots with difficulty.

A carrier woman has one X chromosome with the hemophilia allele and one without.

A hemophiliac man has the hemophilia allele on his single X chromosome.

The Punnett square is used to determine the offspring's genotypes.

The father can contribute either an X with hemophilia or a Y chromosome.

The mother can contribute either an X without hemophilia or an X with hemophilia.

Daughters are represented by the XX scenarios in the Punnett square.

There is a 50% chance for a daughter to have hemophilia if born to a carrier woman and a hemophiliac man.

Sons are represented by the XY scenarios and have different probabilities for hemophilia.

A hemophiliac son has a 25% chance of occurring.

A non-hemophiliac son also has a 25% chance of occurring.

The overall chance of having a hemophiliac offspring is 50%.

Given a son, there is a 50% chance that he does not have hemophilia.

The Punnett square is a useful tool for answering various genetic probability questions.

The video provides a step-by-step guide on using the Punnett square for X-linked recessive traits.

The transcript explains the genetic inheritance of hemophilia in a clear and educational manner.

Transcripts

- [Instructor] Hemophilia is an X-linked recessive trait

that affects blood clotting.

If someone has hemophilia, their blood has trouble clotting.

If a carrier woman and a hemophiliac man have a daughter,

what is the percent chance that she, the daughter,

will have hemophilia?

So, if you're so inspired, pause this video,

and try to work through this on your own.

All right, now let's work through this together.

And to do this, let's think about the genotypes

of both the mother and the father.

So, let's start with the mother.

And since it is an X-linked recessive trait,

we're going to be dealing with the sex chromosomes.

So, let's just remind ourselves that the mother,

because she's female, is going to be XX,

have two X chromosomes.

The father,

the father is going to have an X chromosome

and a Y chromosome.

But now let's think about the hemophilia.

They say it's a carrier woman.

So, a carrier woman means that one of her two X chromosomes

would have the hemophilia allele,

and the other one wouldn't.

Carrier implies that she doesn't show the trait.

She doesn't have hemophilia, but she's carrying one of these

X-linked recessive hemophilia alleles.

So, we could say that one of her chromosomes,

one of her X chromosomes does not have hemophilia.

I use capital H for the dominant allele,

which you would say is no hemophilia.

We tend to use the capital for the dominant.

And then the lowercase h,

well, this would be the X chromosome

that has the hemophilia allele.

And I did it lowercase h because it is a recessive trait.

And so this is

hemophilia.

And now, what about the father?

Well, it's X-linked,

and the father only has one X chromosome.

And they say it's a hemophiliac man.

And so that one X chromosome

is going to have the hemophilia allele.

And so the reason why he exhibits hemophilia

is that there is no other X chromosome

with the dominant allele to be dominant

over the hemophilia allele.

But now let's cross these two with a Punnett square.

So, the father to the offspring could contribute

the X chromosome that has the hemophilia allele

or a Y chromosome.

And the mother can contribute the X chromosome

that doesn't have the hemophilia allele

or the X chromosome that does have the hemophilia allele.

And now I'll draw my little Punnett square here.

So, there you go.

And so what are the different scenarios?

Well, this top-left scenario would be I get the X chromosome

from the mother that does not have the hemophilia allele,

and I get the X chromosome from the father

that does have the hemophilia allele.

This scenario is the X chromosome from the mother

with the hemophilia allele and the X chromosome

from the father with the hemophilia allele.

This scenario is the X chromosome from the mother

without the hemophilia allele, so no hemophilia,

or with the non-hemophilia allele,

and the Y chromosome from the father.

And this is the X chromosome from the mother

that has the hemophilia allele

and the Y chromosome from the father.

Now, what is the percent chance that she,

that a daughter would have hemophilia?

So, which of these scenarios show a daughter?

Well, the daughters are going to be the XX scenarios.

So, these are the two scenarios

in which they have a daughter.

These two, because there's a Y chromosome,

these would be them having a son.

So, out of the two scenarios where they have a daughter,

one of those scenarios

you have two of the recessive alleles.

And so this is a situation where you have a daughter,

daughter with hemophilia,

hemophilia,

right over here.

And so, if we know they have a daughter,

what is the percent chance that she will be a hemophiliac?

Well, one out of two,

so there is a 50% chance.

Now, what's useful about this Punnett square

is you could answer all sorts of questions.

You could say what is the percent chance

that they have a hemophiliac son?

Well, one out of these four scenarios is a hemophiliac son.

Then you would say there's a 25% chance

that they have a hemophiliac son.

Similarly, there is a 25% chance

that they have a non-hemophiliac son.

If someone said what is the percent chance

that they have a hemophiliac offspring?

Well, that would be these two scenarios right over here.

So, two out of the four scenarios are hemophiliac offspring,

so that would be 50%.

If you said, given that they had a son,

what is the percent chance that they don't have hemophilia?

Well, you would say, given that they have a son,

so that would be those two scenarios,

one out of those two scenarios,

the son does not have hemophilia.

So, that would again be a 50%.

So anyway, hopefully this was useful.