A pedigree diagram is a genetic lineage that traces the inheritance of characteristics over multiple generations, using circles to represent females, squares for males, and shaded symbols to indicate affected individuals; to interpret these diagrams, start by identifying individuals with homozygous recessive alleles (unaffected by dominant disorders), then work backwards to determine genotypes by understanding that each offspring inherits one allele from each parent, and apply this method to autosomal, blood group, and sex-linked inheritance patterns.
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Grade 12 - Life Sciences - Pedigree DiagramAdded:
[music] >> Hello viewers. My name is Miss Taba from Ndameni Secondary School in Joe Gabi District.
Welcome to the Eastern Cape Department of Education Broadcast Studio.
Today's lesson is about pedigree diagram which is under the topic genetics and inheritance.
Now, what we are going to look for, firstly we need to look at our exam guideline.
What is it that is expected from us because that is what the exam guideline is supposed to to to guide us about.
What is it that is expected from us?
So, there's our exam guideline.
Uh it is explaining uh what is a pedigree diagram for.
Uh a genetic lineage or a pedigree traces the inheritance of characteristics over many generations.
And another thing that we need to know about the pedigree diagram is that is the interpretation of the pedigree diagram.
Then, let us use uh our first example because the best way to teach pedigree diagrams is to use examples, activity-based teaching.
So, our first example that says the pedigree diagram shows inheritance of eye color in humans over three generations in a family. We have brown eye color which is represented by capital letter B which is dominant over blue eye color, which is represented by a small letter b, meaning that it is represented by a recessive allele. Now, here's our pedigree diagram.
The first thing to look for, or the first thing to understand about the the the pedigree diagram is that there are circles, squares, and these circles and squares, they represent individuals. They represent people.
Hence, you see the name Joshua, Ronnel, Sarah, Peter, Veronica, because they are people. So, let us first um explain. Let me first explain about the the square and the the the the the the circle.
What it represents.
A square represents a male.
Even if the key is not given in the question, just know that a square will always represent a male. And a circle will always represent a female.
Then, the shaded ones, it simply means that they are affected with the disorder that we are we are talking about.
Then, there are lines between these squares and circles.
This line, that is the the horizontal line, this line, it means that Joshua and Ronnel have mated. Then, the vertical line, this line, it links the the the the the parents to their offsprings, meaning that Joshua and Ronnel mated and their offsprings are Sarah and Peter.
Okay.
Now, so here we are talking about um as a eye color, brown eye color and blue eye color. We already said that >> [clears throat] >> the brown color is dominant over the the the blue the blue eye color. So, another thing that you have to do when you see a pedigree diagram is to fill in the genotypes of the individuals.
You start with the ones that have recessive alleles.
Homozygous recessive alleles.
And in most cases, it's those that who are not affected by the disorder that we are talking about.
Then you work backwards because each and every allele that is in an offspring, that offspring inherited it from both parents.
Let us look at Malena. Let us start with Malena.
Malena, the phenotype of Malena, it is a female with blue eyes.
Malena is a female with blue eyes. Now, let us put now the let us fill in the genotype of Malena.
Malena it her genotype has to be small letter B and small letter B because has not affected she is not affected by the the the the the brown eye color.
And the brown eye color, we all know that it is uh dominant over blue eye color. Now, let us go to Jack. I said we start with all those with uh homozygous recessive Also, Jack has small letter b small letter b.
And remember, each allele each allele is inherited from both parents. And John, small letter b small letter b.
Now, let us go to their parents. We can see Sorry, [clears throat] Veronica is also not affected. Is also a female with with the blue eyes. So, meaning that her genotype it is small letter b and small letter b.
Now, it simply means that Veronica, sorry, John, Jack, and Malina, they have inherited one recessive allele from their their their their their mother, who is Veronica. All these children, Malina, Jack, Frank, all of them, and Gail included, she also inherited the recessive allele from uh Veronica, their mother.
Then, let us go to Peter.
Peter is a male with the brown eyes, meaning that he has a dominant allele because he has brown eyes.
But, the fact that Peter has Malina, Jack, and John, who has uh two recessive alleles, it means that one of their recessive allele, they inherited it from their father, Peter.
So, Peter also has a small letter b.
This is how we we we we we we we we we we interpret our pedigree diagram.
So, now, we can go to our examples.
Activity number one.
Activity number one, it is an auto- autosomal we're going to talk about the autosomal disorder.
We are going to talk about the autosomal disorder.
This is last year's question paper DBE November 2025.
A life sciences paper two. Now, the question says CADASIL is an autosomal genetic disorder.
Autosomal genetic disorder. The reason why I'm emphasizing this part, the autosomal genetic disorder is that um when we say autosomal, when we say the disorder it is autosomal, we mean that it is nothing to do with the sex chromosomes.
If you remember in meiosis, we talked about autosomes. We talked about gonosomes. Now, when the disorder is autosomal, it means it is it has nothing to do with the XX chromosomes or XY chromosomes.
So, CADASIL is an autosomal genetic disorder caused by a dominant allele, which is represented by a capital letter D.
It results in the thickening of the walls of blood vessels.
The diagram below shows the inheritance of CADASIL in a family.
Remember, it is controlled by a dominant allele, capital letter D.
Here are our uh questions.
The first question says, "A name 3.3 2.3.1 Name the type of diagram represented above, which is the pedigree diagram."
Then the second one, "How many offsprings How many offspring do parents three and four have?"
Let us look at parent three and four.
There's parent three and four.
As I've already said, here's parent three.
Here's parent three and four. It means that they mated.
Then the vertical line I said, the vertical line links the offspring to to their parents. Now, we only have one offspring in this case.
We only have one offspring.
Now, let us go to the next question.
The next question says, "Give the genotype of individual number 10."
Another thing that I wanted to emphasize about this question is that when you are asked about the phenotype of an individual, please make sure that you go straight to the key. Use what is the written in the key. Make sure that you use what is written in the key. Don't come up with your your your your your your your own words.
Then, the genotype of individual number 10. If we look at individual number 10, individual number 10 is a female with a female with keratosis. Meaning that he or she has Let us first look at the parents. The parents of individual number 10, it's individual number seven and individual number eight. Meaning that as much as individual number 10 is affected with keratosis or is a female with keratosis, she also has a recessive allele because she inherited a recessive allele from individual number eight.
So, the the the the the genotype of individual number 10, it has to be capital letter D and small letter D. As much as individual number 10 is affected, but she is heterozygous.
Hetero means different. 2.3.3A wanted only the phenotype. So, I already gave the genotype the phenotype and the genotype, but it doesn't matter. Let us go to question 2.3.3. Give Give three Give the genotype of individual number five. Let us go to individual number five.
Individual number five.
Here is individual number five. We want the genotype. The genotype of individual number five. Let us look at the parents of individual number five. Individual number five is Let us first look at the phenotype. Is a female without keratosis. A female without keratosis.
So, it is very very simple to to to to to write her genotype because her is she is a female without keratosis. So, the genotype is going to be small letter D small letter D. We don't even have to look at her parents. Then, let us move to the next question. The next question says, "Using evidence from the diagram, explain why individuals one and two are both heterozygous.
Why individual one and two are both heterozygous?" Let us go to individual one and two. Individual one and two, they both have keratosis. Individual one and two, they both have keratosis. That is the the first mark because if you look at the question, the question has four marks. It has four marks. Yes, it has four marks. Now, the first mark that you're going to get is for stating that individual one and two, they have their their phenotype. By stating their phenotype, that they are they have they both have keratosis. And their genotype is capital letter D and small letter D.
The reason why we are saying that it's capital letter D and small letter D, we are looking at their children. They have children without keratosis. So, this is the the second mark. Now, for you to get the third mark, you have to tell us about their offsprings. So, as much as but they they have keratosis, but they have children without keratosis. They have children who does not have keratosis. Meaning that Oh, that's the third mark. The phenotype of their children. They have children without keratosis. Then the fourth mark, you're going to get when you you you you you you say meaning that they are offsprings inherited one recessive allele from each parent.
Individual number five, individual number six inherited one recessive allele from each parent. Then you get all your your four marks. Let us move to the next question. The next question says, "What is the percentage chance of parents seven and eight having another child with a kidusel?"
What is the percentage chance of parents number seven and eight having another child with a kidusel?
So, let us go to parents seven and eight. Parents seven and eight.
There's seven.
There's eight. Let me Parents seven and eight. So, we want the percentage chance of of them having another child with with a kidusel. So, let us look first at their genotype. So, that we can be able to come up with a conclusion. Individual number seven, we can conclude that as much as he's a male with kidusel, he does have a recessive allele because of that individual number nine. That individual number nine, remember, for individual number nine to be to have two recessive alleles, she inherited one recessive allele from individual number seven from here. And inherited another recessive allele from individual number eight.
It simply means that individual number seven, the genotype of individual number seven is capital letter D, small letter D.
Then, he mated with individual number eight.
Individual number eight is homozygous recessive, meaning that she has small letter D and small letter D. Then, what you need to do is a quick uh small genetic cross where you are only crossing these uh genotypes.
What is the question one? What is the question one? What is the percentage chance of parents seven and eight having another child with a kidusel? Meaning that those who are affected with the cattle is this one and this one. Now, how do we calculate the percentage chance?
We are saying these are the affected ones.
We are saying we have two children who have cattle out of how many children? Out of four children.
Then we multiply by we multiply by 100. Not 100% but 100. Then that is going to give us 50%.
Please be careful. We do not multiply by 100%. We multiply by 100. Then you get your mark. Let us move to the next question. Then our next question is on blood groups. Blood groups. So this one it is from Eastern Cape September 2025. It was written last year in September. So this one is based on blood groups. The pedigree diagram below shows the inheritance of blood groups in a family.
The pedigree diagram below shows the inheritance of blood groups in a family.
The phenotypes of the individuals are indicated in the different shapes.
Here are the the the phenotypes we already know uh which one is the male or which one is the female.
Now they also have their blood groups.
Please remember this guys. Blood group A. If I'm saying blood group A, that is the phenotype. Then the genotype has to be for blood group A this is the genotype. This is the genotype or this is the genotype for blood group A.
If I am writing blood group A this one. This is the phenotype.
This is the phenotype.
Now, these are the blood groups we have how many generations?
We have the first generation, generation of James and Anga. Then we have the second generation, the generation of Aneeka, Jade, Lisa, Tando, Jesse. Then the third generation, uh we have the generation of these individuals and Penny, Henry, and Martha. So, we have three generations to trace the the the the the blood groups. Let us go to the questions.
2.3.1 State the phenotype of Anga.
Let us go to Anga.
Anga, we don't I said, if you remember, I said when you are asked to about to to give the phenotype on the pedigree diagram, you go straight to your key.
What is the key? Let us go to Anga.
Where is Anga? It's this one.
How are we going to know Anga's um phenotype?
Anga, let us go to her parent offsprings.
Anga is the mother to Bella, Jade, and Tando. Bella's blood group is AB, meaning that Anga has this allele.
Because Bella, it simply means that Bella inherited allele for blood group A from James and blood group allele for blood group B from Anga. Remember, we inherit alleles. We do not inherit the blood group.
We inherit alleles, not the blood groups.
I am saying Bella inherited an allele for blood group A from James.
As you can see that. And he inherited an allele for blood group B from Anga. Now, we need to do know the second allele now of Anga.
Let us go to Jade. Jade is not going to help us. Let us go to Tando. Tando is blood group O, blood group O, meaning that uh uh Tando we all know that the the the genotype for Tando is that one, small letter I, small letter I. Now, where did the Tando get the the those alleles?
Tando inherited one recessive allele from James and another recessive allele from Anga, meaning that Anga's genotype is is capital letter I raised to B and small letter I, meaning that the genotype of Anga is a female with blood group B, female with blood group B. Please, it is very important to to to indicate the gender when you are dealing with the blood groups. It is very important to indicate the gender because those circles and squares, they indicate the gender. So, you also have to indicate the gender.
Let us go to the next question. The next question says Give the number of individuals with homozygous recessive alleles in this pedigree diagram. Give the number of individuals with homozygous recessive alleles in this pedigree diagram.
Remember this.
In blood groups, we said blood groups are controlled by how many alleles?
Three alleles.
Which are this one, this one, and this one. These are the alleles that controls the that control the blood groups.
Now, when they say they say they want the individuals with homozygous recessive alleles, they're talking about the people with the blood group O.
Homozygous recessive. Homo means same.
So, we are looking for people with blood group O.
How many people have blood group O in this pedigree diagram? It is Oneka and Tando. So, we only have two people with blood group O.
With the with the homozygous recessive alleles. Then, question number 2.3.3.
Explain why the future children of Jade and Alyssa could display different blood groups compared to their current children. Why the future children of Jade and Alyssa? Let us look at the Jade and Alyssa. Here is Jade and Alyssa.
Jade is a blood group B. Alyssa is a blood group A.
Now, they want to know why the future children of them could display a different blood groups compared to their children. They already have a child with a blood group A and they also have a child with blood group B. Now, what they want to know here is that Jade and Lisa they they they they they Okay, their their children their their next children they will inherit one of them is going to inherit an allele for blood group A from Lisa and a an allele for blood group B from a Jade.
That is This this is what I'm talking about.
Sorry.
The first child is going to inherit allele for blood group A from Lisa and another allele the second allele from a Jade.
Then the other one is going to inherit a recessive allele from each parent.
This is the second child. So, these ones they have different blood groups because this one is going to be blood group AB and this one is going to be blood group O.
That is what they wanted in this one.
Let us go to the next question.
Now, using a genetic cross to show the percentage probability of Bella and Onika having a child with a blood group O.
Use a genetic cross.
Use a genetic cross to show the percentage probability of Bella and Onika. Let us look at Bela and Onika. Here's Bela.
Here's Onika. Now, we want to know the percentage chance of this couple having a baby having a child with a blood group O.
Using a genetic cross.
Now, let us go back to our monohybrid cross.
When you do the monohybrid cross, the first thing to do is to write P1.
P1, which stands for the parents.
Then, you write the phenotype.
The phenotype. What is the phenotype of these parents?
Blood group AB.
Blood group A B crossed with blood group Blood group O.
Then, this is the phenotype. I already emphasized this. This is the phenotype.
Then, let us go to the genotype.
The genotype.
Now, we go to the genotype.
Crossed with Sorry.
Small letter I.
Small letter I.
Then, now we have our genotypes.
The next thing that has to happen now, meiosis.
Meio meiosis.
Then, there's Mendel's law of segregation, which states that during meiosis, these alleles, they have to separate.
The law of segregation. They have to separate. Now, they're separating, we will have capital letter I raised to A and capital letter I raised to B. I usually circle these.
I'm going to explain later why.
Then, here we have small letter I and another small letter I.
This was law of segregation during what?
During meiosis.
Now, we no longer call these the the the the the the genotypes now, but they are gametes. That is why I usually circle them because I want I I want us to see them as the cells now, the gametes, the sex cells now, because now, we no longer call them the the the the the the genotype, but the gametes.
This is This are our our our our our sperm cells and these are our egg cells.
Now, after the gametes, now that we have the gametes, it means fertilization can take place.
Let us do fertilization.
Fertili fertilization.
I usually recommend what we call a Punnett square. That is a square with the two vertical lines and the two horizontal lines.
Here [snorts] we'll have the gametes.
Our gametes is a capital letter I raised to A and the capital letter I raised to B then small letter I and small letter I.
Now let us do the genetic let let the fertilization now.
This capital letter I has to be here and small letter I.
Then this one will have capital letter I raised to B and small letter I.
Then this side we'll have this this side.
Then let us move to the next step.
Now our next step is that we do have the the the the um the gametes sorry the offsprings now. We need to write F1.
F1 the genotype of F1.
The genotype of F1.
I usually say to my learners they must write I know that they do have the the the the the offsprings here. These are the genotype of our offsprings. But I usually uh say to them they must write again so that they cannot be confused.
Here are our uh genotype for our offsprings. Or you can write them as two This is this are two.
And two which means these ones.
Then let us go to the phenotype.
The phenotype now.
The phenotype. Okay.
We have two blood group A. Remember that recessive allele is being masked by the the dominant one, by this capital letter I raised to capital letter A. I I I hope you understand that. That is why I'm saying this one is blood group A. And we do have two blood group.
What is this one? Blood group B. Because the recessive allele will always be masked by the dominant allele in a heterozygous condition.
Now we are done with our phenotype.
When you do a genetic cross I usually say after doing a the the the the the genetic cross, go back to the question and check that have you answered the question fully?
Because the question says, I'm done with the genetic cross now, but the question says the question says using a genetic cross to show the percentage probability of Bella and Onika having a child with a blood group O. They wanted the probability of this couple having a child with a blood group O. Let us look at this.
How many children do have How many children having blood group O? None. We don't have. Zero divide by how many children? Four. Times 100 which is equals to 0%.
Remember, we multiply by 100, not 100%.
So, now you have answered the question.
This is what we call the compulsory mark. Even if if I did not do this part, the the the the percentage part, I was going to get 5 over 6.
Even though I did everything correctly, but the fact that I did not answer the question fully, I was going to get 5 out of 6 marks.
Now, let us move to the next question.
We are done with the blood groups.
Let us move to the I believe it's the last one.
Sex-linked disorders. Sex-linked disorders.
Question 2.5. Oh, this question is from DBE May-June 2025.
Life Sciences Paper 2.
Question 2.5, it says, "Hypophosphatemia is a sex-linked inherited inherited condition that is caused by a dominant allele.
There's a genotype on the X chromosomes.
It results in abnormally low levels of phosphate in the blood, which can cause rickets. Recessive allele codes for normal phosphate levels. The pedigree diagram below shows the inheritance of hypophosphatemia in one family.
This is a family of how many generations?
One.
One.
This is the second one.
Then the third one. This family is have is having three um generations. We have the the parents generation, the F1 generation, and the F2 generation. Now, let us go to the questions. Please be careful. When when it comes to sex-linked disorders, it is simply means that our alleles will be on the X chromosomes or X chromosome. We are dealing with the sex chromosomes now. You are not going to just write the alleles, but you have to write the alleles on top of the the the the sex chromosomes. Let us go to the questions. There is the key. Um The question now says We're not going to do 2.2 2.5.1.
Um Let us go straight to 2.5.2.
2.5.2 says State two effects of hypophosphatemia in humans. Two Sorry, no. It's not important. How many How many individuals in the F1 generation have hypophosphatemia?
phosphatemia? How many individuals in the F1 generation have hypophosphatemia?
in F1 generation. Okay.
Let us go. This is the F1 generation. As I've already said.
This is the the P1 generation. That is the parents.
So, the F1 generation, people who are affected Sorry. [clears throat] People who are affected by this disorder, they are one two three.
They are affected with this with there are three people. Three. Then Give all the possible genotypes of individual number three.
This is individual number three that we're talking about.
We want the possible genotypes.
Remember, this is a sex-linked disorder.
We'll have to write first Let let let let let me say individual number three. We'll have to write first X and Y. Is he affected? Is it a male with all type of thalassemia or a male with hypophosphatemia?
So, individual number three is a male without hypophosphatemia.
Now, it means that the alleles should this this this guy has a recessive allele because he is not affected.
Then, let us move to the next question, B.
Give all the possible genotypes of individual number four. The genotype of individual number four.
Individual number four.
Let us look at individual number four.
There's individual number four.
Individual number four her phenotype is female with hypophosphatemia.
Meaning that individual number four has a dominant allele.
Let us write the dominant allele because we are sure that she has a dominant allele. Let us look at the offsprings now.
Offsprings of individual number four.
Individual number 11 and number 12.
They are not affected. It means that they are without hypophosphatemia.
Now, what does that mean? It simply means that individual number 11 and individual number 12, they inherited recessive alleles from both parents.
Meaning that individual number four has a recessive allele.
He is She is heterozygous for this disorder.
Let us move to the next question.
Explain why all the daughters of individuals one and two will have hyper- hypophosphatemia.
Hypophosphatemia. Hypophosphatemia.
Children of individual one and two. The daughters, only the daughters. Why will they have the disorder?
It's six marks. Now, let us look at individual number number one and two.
This is individual number one and number two. Individual number one is affected and individual number two is a female without hypophosphatemia.
Then, this is how we answer this question.
Point number one, you need to give us the phenotype of these. Let us start with individual number one. Individual number one, we have to give his phenotype.
Individual number number one is a male with hypophosphatemia.
That's the mark the first mark. Then, point number two, you need to give us his genotype. His genotype is going to be Yes, because he's a male. That's the genotype then the second mark. Then the third mark, you you explain to us how are these daughters now going to have this from individual number one? So, the daughters will always inherit this allele from their father. They will always inherit this allele. That is why they are going to be affected. Then let us go to point number four. Point number four, we go to parent number two, which is the the the individual number two.
What is the phenotype? We need to tell the phenotype of individual number two, which is a female without hypophosphatemia.
So, that is the the first mark. Then the second mark, you are going to get it by writing her genotype, which is um X raised to small letter h and capital letter X raised to small letter h. Then the sixth mark, you you you you you go to the how part.
Then it means that these daughters will inherit one of this.
That is why they will be affected because if they have let let let let let let let us say this is their their their their their their their their genotype.
Their genotype will be like this.
Remember this, they got this one from the father. Then the second allele, this one they got it from from the mother.
So, this is how they are going to be affected.
We have come to the end of our lesson.
You can leave your questions on the comments. Don't forget to leave the name of your school so we can give you shout-outs. Please share, recommend, and subscribe to our YouTube channel. Thank you for watching.
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