Genetics practice problems involving pedigree tables for colorblindness inheritance.

Let’s go step by step to figure out the genotypes and answer each question. We’re dealing with colorblindness, which is a recessive X-linked trait. That means:

- The gene is on the X chromosome.
- Males (XY) only have one X, so if they get the bad allele (let’s call it X^c), they are colorblind.
- Females (XX) need two bad alleles (X^c X^c) to be colorblind. If they have one good and one bad (X^C X^c), they are carriers but not colorblind.
- Normal vision allele = X^C
- Colorblind allele = X^c

We’ll assign genotypes to people 1–15 based on who is shaded (colorblind) and their relationships.

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Step 1: Assign known phenotypes from the pedigree

Shaded = colorblind → genotype must include at least one X^c (and for males, just one; for females, two).

Unshaded = normal vision → could be homozygous dominant or carrier (for females).

Let’s list them:

Generation I:
- 1: female, unshaded → ?
- 2: male, unshaded → X^C Y
- 3: female, unshaded → ?
- 4: male, shaded → X^c Y

Generation II:
- 5: female, unshaded → ?
- 6: male, shaded → X^c Y
- 7: female, unshaded → ?
- 8: male, shaded → X^c Y
- 9: female, unshaded → ?
- 10: female, unshaded → ?
- 11: male, unshaded → X^C Y

Generation III:
- 12: male, shaded → X^c Y
- 13: female, shaded → X^c X^c
- 14: male, unshaded → X^C Y
- 15: female, unshaded → ?

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Step 2: Work backwards to find parents’ genotypes

#### Look at person 6 (male, colorblind): X^c Y
His father is #2 → unshaded male → X^C Y → he gave Y to son.
So the X^c must come from mother #1.
→ So #1 must be a carrier: X^C X^c

#### Look at person 8 (male, colorblind): X^c Y
Father is #4 → shaded male → X^c Y → he gave Y to son.
Mother is #3 → unshaded female → she must have given the X^c to son.
So #3 must be a carrier: X^C X^c

(That answers the first question!)

#### Now look at generation III: children of #7 and #8

#8 is male, colorblind → X^c Y → gives either X^c or Y to kids.

Children:
- 12: male, shaded → X^c Y → got Y from dad, X^c from mom → so mom (#7) must have given X^c
- 13: female, shaded → X^c X^c → got X^c from dad (he only has X^c to give) and X^c from mom → so mom (#7) must have given X^c
- 14: male, unshaded → X^C Y → got Y from dad, X^C from mom → so mom gave X^C
- 15: female, unshaded → must be X^C X^c (since dad gave X^c, and she’s not colorblind → so mom gave X^C)

Wait — this is a problem. Mom (#7) gave:
- To 12: X^c
- To 13: X^c
- To 14: X^C
- To 15: X^C

So #7 must be X^C X^c (carrier) — that works! She can pass either.

But wait — daughter #13 is colorblind → X^c X^c → she got X^c from dad (#8) and X^c from mom (#7). So yes, #7 must carry X^c.

Also, #7’s parents are #1 and #2.

#1 we already said is X^C X^c (because she had colorblind son #6)
#2 is X^C Y

So #7 (female, unshaded) could be X^C X^C or X^C X^c — but since she passed X^c to her kids, she must be X^C X^c

Now let’s fill in all genotypes:

I:
1: X^C X^c
2: X^C Y
3: X^C X^c
4: X^c Y

II:
5: ? (female, unshaded) — parents are 1 (X^C X^c) and 2 (X^C Y) → possible: X^C X^C or X^C X^c → we don’t know for sure, but since no info, maybe leave as ? or assume most likely? But actually, we can’t determine exactly without more data. However, since she’s not shaded, and no kids shown, we might not need her. Let’s skip unless needed.

Actually, let’s do all:

5: female, unshaded → from parents 1 (X^C X^c) and 2 (X^C Y) → possible genotypes:
- From mom: X^C or X^c
- From dad: X^C
→ So 5 is either X^C X^C or X^C X^c → we can’t tell which → so probable genotype: X^C X^? → but since question asks for “probable”, and she’s not affected, and no evidence she’s carrier, but actually in pedigrees, if not specified, we often say “could be” — but let’s see what the questions ask.

Actually, the main questions don’t involve #5, so maybe we don’t need to pin her down.

Similarly, #9, #10, #11 — children of #3 and #4.

#3: X^C X^c
#4: X^c Y

Their children:

Daughters get X from dad → dad is X^c Y → so daughters get X^c from dad.

Sons get Y from dad.

So:

#9: female, unshaded → got X^c from dad → to be unshaded, must have gotten X^C from mom → so genotype: X^C X^c

#10: same → female, unshaded → also X^C X^c

#11: male, unshaded → got Y from dad, and X from mom → mom is X^C X^c → he got X^C → so X^C Y

Now back to #7: we said X^C X^c

#8: X^c Y

Their children:

#12: male, shaded → X^c Y → got Y from dad, X^c from mom → consistent

#13: female, shaded → X^c X^c → got X^c from dad, X^c from mom → consistent

#14: male, unshaded → X^C Y → got Y from dad, X^C from mom → consistent

#15: female, unshaded → got X^c from dad, and must have gotten X^C from mom → so X^C X^c

Perfect.

So now we have:

Final Genotypes:

1: X^C X^c
2: X^C Y
3: X^C X^c
4: X^c Y
5: X^C X^? (probably X^C X^C or X^C X^c — but not determinable for sure; however, since she’s not shaded and no affected kids, maybe assume X^C X^C? But actually, in strict terms, we can’t know. But for the purpose of this worksheet, perhaps they expect us to say she’s likely X^C X^C? Wait — no, because her mother is carrier, so 50% chance she’s carrier. But since no info, maybe leave as unknown? Actually, looking at standard practice, if not specified and no reason to think otherwise, we might say "X^C X^C" but that’s not accurate. Hmm.)

Actually, let’s check: the question says “determine the probable genotype”. For #5, since she’s female, unshaded, and her mother is carrier, father normal, then probability she’s carrier is 50%. But “probable” might mean most likely? Or just possible? In many textbooks, they would write “X^C X^C or X^C X^c” but since it’s “probable”, and she has no affected sons, perhaps they want X^C X^C? But that’s not right — she could still be carrier.

Actually, I think for this level, they might accept:

5: X^C X^C or X^C X^c — but since the question doesn’t ask about her, and the follow-up questions don’t involve her, maybe we can skip detailed assignment for her. Similarly for others not involved.

But let’s complete as best as possible.

For #5: since she’s not shaded, and no children shown, and parents are 1 (carrier) and 2 (normal), her genotype is uncertain. Probable? Well, 50% chance carrier, 50% not. But “probable” might imply the most likely single genotype — but there isn’t one. Perhaps in context, we can say “X^C X^C” assuming she inherited normal from mom, but that’s arbitrary.

Looking at similar problems, often they leave it as “X^C X^-” or specify if possible. But here, since the questions focus on others, let’s proceed.

Actually, let’s list all with what we know:

1: X^C X^c
2: X^C Y
3: X^C X^c
4: X^c Y
5: X^C X^C or X^C X^c (cannot determine)
6: X^c Y
7: X^C X^c
8: X^c Y
9: X^C X^c
10: X^C X^c
11: X^C Y
12: X^c Y
13: X^c X^c
14: X^C Y
15: X^C X^c

Okay, now answer the questions.

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Question 1: How did you determine the genotype of the mother at 3?

Person 3 is the mother of person 8. Person 8 is a colorblind male (X^c Y). He got his Y chromosome from his father (person 4), so he must have gotten his X^c chromosome from his mother (person 3). Since person 3 is not colorblind (unshaded), she must have one normal allele (X^C) and one colorblind allele (X^c). So her genotype is X^C X^c — she’s a carrier.

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Question 2: Number 8 was colorblind just like his father. Where did the son at 8 get his allele for colorblindness?

Even though his father (person 4) is colorblind, fathers pass their Y chromosome to sons, not their X. So person 8 got his Y from his father, and his X chromosome from his mother (person 3). Therefore, he got the colorblind allele (X^c) from his mother, not his father.

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Question 3: Neither numbers 1 nor 2 were colorblind. How did they have a colorblind son (6)?

Person 6 is a colorblind male (X^c Y). He got his Y from his father (person 2, who is normal: X^C Y). He got his X from his mother (person 1). Since person 1 is not colorblind but passed on X^c, she must be a carrier (X^C X^c). So even though she looks normal, she carried the colorblind allele and passed it to her son.

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Question 4: What must be the genotypes of the parents of a colorblind daughter? Explain.

A colorblind daughter has genotype X^c X^c. She needs one X^c from each parent.

- Her father must be colorblind (X^c Y) because he only has one X to give, and it must be X^c.
- Her mother must have at least one X^c. Since the daughter got X^c from mom, mom could be either colorblind (X^c X^c) or a carrier (X^C X^c). But to guarantee the daughter gets X^c, mom must contribute X^c — so mom must have at least one X^c. However, if mom is carrier (X^C X^c), there’s only 50% chance she passes X^c. But the question says “must be” — meaning necessary conditions.

Actually, for the daughter to be colorblind, she MUST receive X^c from both parents.

So:
- Father: must be X^c Y (colorblind)
- Mother: must have at least one X^c — so she could be X^c X^c or X^C X^c. But if she is X^C X^c, she might pass X^C, so the daughter wouldn’t be colorblind. But the question is: what MUST be true for the parents if they HAVE a colorblind daughter.

So, necessarily:
- Father is colorblind (X^c Y)
- Mother has at least one X^c allele — so she is either colorblind or a carrier.

But to be precise: the mother must contribute an X^c, so she cannot be X^C X^C. She must be X^C X^c or X^c X^c.

In the pedigree, person 13 is a colorblind daughter. Her parents are #7 and #8.

#8 is colorblind male: X^c Y → he gave X^c to daughter.

#7 is unshaded female: we determined she is X^C X^c → she gave X^c to daughter.

So yes, mother was carrier, father was colorblind.

So answer: The father must be colorblind (genotype X^c Y), and the mother must have at least one colorblind allele — so she is either colorblind (X^c X^c) or a carrier (X^C X^c).

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Question 5: If number 13 marries a normal man, what is the probability that their sons will be colorblind?

Person 13 is a colorblind female → genotype X^c X^c.

She marries a normal man → normal male is X^C Y.

Their children:

- Sons get Y from father, and X from mother.
- Mother is X^c X^c → so she always gives X^c to sons.
- Father gives Y to sons.

So all sons will be X^c Y → colorblind.

Probability = 100% or 1.

Daughters would get X^c from mom and X^C from dad → so X^C X^c → carriers, not colorblind.

But question asks for sons.

So probability that their sons will be colorblind is 1 (or 100%).

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Now, compiling final answers to the questions.

Note: The initial part asked to determine genotypes of persons 1-15, but since the follow-up questions are the main task, and the user didn't specify to list all, but the instruction says "solve the problem", and the problem includes determining genotypes and answering questions.

However, looking at the response format, it seems the Final Answer should contain the answers to the questions posed.

The questions are:

1. How did you determine the genotype of the mother at 3?
2. Where did the son at 8 get his allele for colorblindness?
3. How did they have a colorblind son (6)?
4. What must be the genotypes of the parents of a colorblind daughter? Explain.
5. If number 13 marries a normal man, what is the probability that their sons will be colorblind?

So I'll answer these clearly.

Also, for the genotype determination, since it's part of the task, but the questions build on it, I've used it in reasoning.

Now, for Final Answer, I'll provide concise answers to each question.

Let me write them neatly.