Difficulty: Introductory to Intermediate | Prerequisites: Punnett squares, codominance basics, understanding of chromosomes (XX/XY)
Big picture: Blood typing is one of the most practical applications of codominance you will encounter: the ABO system has three alleles rather than two, and two of them are codominant. Sex-linked genetics introduces a twist on inheritance by placing genes on the X chromosome, which means males and females inherit these traits differently. You should be comfortable with codominance and basic Punnett squares before working through this material.
Human blood type is determined by three alleles (I^A, I^B, and i) following a codominance pattern: I^A and I^B are codominant with each other, and both are dominant over i. The Rh factor (+ or -) is inherited separately as a simple dominant/recessive trait. Sex-linked traits like hemophilia and colorblindness are carried on the X chromosome, which means males (XY) need only one recessive allele to be affected, while females (XX) need two.
Multiple alleles
A gene that has more than two allelic forms in the population. The ABO blood group has three alleles: I^A, I^B, and i. Any individual still carries only two of these (one from each parent).
Codominance (in blood typing)
I^A and I^B are codominant: a person with genotype I^A I^B expresses both A and B antigens on their red blood cells, producing type AB blood. Think of it as "both alleles contribute equally."
Rh factor
A separate gene that determines whether a person has the Rh antigen on their red blood cells. Rh-positive (+) is dominant over Rh-negative (-). In simple terms, if you have at least one + allele, your blood is Rh-positive.
Sex-linked trait
A trait whose gene is located on a sex chromosome, usually the X chromosome. Because males have only one X, a single recessive allele on that X is enough to express the trait. Females need two copies.
Carrier
A female who is heterozygous for a sex-linked recessive trait. She does not show the trait herself but can pass the recessive allele to her offspring. Think of it as "carrying the allele silently."
Hemizygous
The state of having only one copy of a gene (rather than two). Males are hemizygous for X-linked genes because the Y chromosome does not carry a corresponding allele.
The ABO blood group is governed by one gene with three alleles:
I^A (dominant, codes for A antigen)
I^B (dominant, codes for B antigen)
i (recessive, codes for no antigen)
I^A and I^B are codominant with each other. Both are dominant over i.
Blood Type | Possible Genotypes |
|---|---|
Type A | I^A I^A (homozygous) or I^A i (heterozygous) |
Type B | I^B I^B (homozygous) or I^B i (heterozygous) |
Type AB | I^A I^B (codominant) |
Type O | ii (homozygous recessive) |
Cross 1: Dad homozygous B (I^B I^B) x Mom type O (ii)
All offspring are I^B i. Result: 0% A, 100% B, 0% AB, 0% O.
Cross 2: Both parents type AB (I^A I^B x I^A I^B)
Offspring: 25% I^A I^A (type A), 50% I^A I^B (type AB), 25% I^B I^B (type B). Result: 25% A, 25% B, 50% AB, 0% O. Two AB parents cannot produce a type O child.
Cross 3: Mom heterozygous A (I^A i) x Dad type O (ii)
Offspring: 50% I^A i (type A), 50% ii (type O). Result: 50% A, 0% B, 0% AB, 50% O.
The Rh factor is inherited independently from the ABO blood type. It follows simple Mendelian dominance:
Rh-positive (+) is dominant
Rh-negative (-) is recessive
Rh Status | Possible Genotypes |
|---|---|
Positive | ++ (homozygous) or +- (heterozygous) |
Negative | -- (homozygous recessive) |
Cross 1: Both parents heterozygous (+- x +-)
Offspring: 25% ++, 50% +-, 25% --. Result: 75% positive, 25% negative.
Cross 2: Mom homozygous positive (++) x Dad homozygous negative (--)
All offspring are +-. Result: 100% positive, 0% negative. All are carriers of the negative allele.
Cross 3: Both parents homozygous positive (++ x ++)
All offspring are ++. Result: 100% positive, 0% negative.
Exam questions may ask you to work both systems at once. Treat them as two separate Punnett squares. For example, if a man is heterozygous A (I^A i) and homozygous Rh-positive (++), and a woman is heterozygous B (I^B i) and homozygous Rh-negative (--), you do one Punnett square for ABO (I^A i x I^B i, giving 25% AB, 25% A, 25% B, 25% O) and one for Rh (++ x --, giving 100% +-positive). The blood type and Rh results are then reported together.
Hemophilia is a disorder where a defective allele on the X chromosome prevents normal blood clotting. The allele is recessive.
X^H = normal clotting (dominant)
X^h = hemophilia (recessive)
Y = no corresponding gene
Individual | Genotype |
|---|---|
Unaffected female | X^H X^H |
Carrier female | X^H X^h |
Female with hemophilia | X^h X^h |
Unaffected male | X^H Y |
Male with hemophilia | X^h Y |
Males need only one recessive allele (they are hemizygous). Females need two.
Cross 1: Carrier female x unaffected male (X^H X^h x X^H Y)
Offspring: 25% X^H X^H (unaffected female), 25% X^H X^h (carrier female), 25% X^H Y (unaffected male), 25% X^h Y (hemophiliac male). Overall probability of hemophilia: 25% (only among males). Probability of a male child having hemophilia: 50%. Probability of a female child having hemophilia: 0%.
Cross 2: Carrier female x hemophiliac male (X^H X^h x X^h Y)
Offspring: 25% X^H X^h (carrier female), 25% X^h X^h (hemophiliac female), 25% X^H Y (unaffected male), 25% X^h Y (hemophiliac male). Overall probability of hemophilia: 50%. Male child: 50%. Female child: 50%.
Cross 3: Hemophiliac female x unaffected male (X^h X^h x X^H Y)
Offspring: 50% X^H X^h (carrier females), 50% X^h Y (hemophiliac males). Overall: 50%. All sons have hemophilia. No daughters have hemophilia, but all are carriers.
Colorblindness follows the same inheritance pattern as hemophilia: recessive, X-linked.
X^B = normal vision (dominant)
X^b = colorblind (recessive)
Y = no corresponding gene
Individual | Genotype |
|---|---|
Unaffected female | X^B X^B |
Carrier female | X^B X^b |
Colorblind female | X^b X^b |
Unaffected male | X^B Y |
Colorblind male | X^b Y |
Cross 1: Carrier female x unaffected male (X^B X^b x X^B Y)
Offspring: 25% X^B X^B, 25% X^B X^b, 25% X^B Y, 25% X^b Y. Overall probability of colorblindness: 25%. Male child: 50% chance. Female child: 0%.
Cross 2: Carrier female x colorblind male (X^B X^b x X^b Y)
Offspring: 25% X^B X^b (carrier female), 25% X^b X^b (colorblind female), 25% X^B Y (unaffected male), 25% X^b Y (colorblind male). Overall: 50%. Male child: 50%. Female child: 50%.
Cross 3: Unaffected female (homozygous) x unaffected male (X^B X^B x X^B Y)
All offspring are unaffected. 0% colorblind across the board. No recessive allele is present in either parent.
Students often think type O blood means "no blood type." Type O means the person carries two recessive i alleles and produces no A or B antigens, but it is still a blood type with its own clinical significance.
A common error is thinking two parents with type A blood cannot have a type O child. They can, if both are heterozygous (I^A i x I^A i gives a 25% chance of ii).
Students frequently confuse carrier status with being affected. A carrier female (X^H X^h) does not have hemophilia. She has normal clotting but can pass the recessive allele to her children.
Many students assume that if a father has hemophilia, all his children will too. His daughters will all be carriers (at minimum), but none of his sons will inherit the X-linked allele from him, because sons receive the Y chromosome from their father.
⚠️ Blood typing questions often combine ABO and Rh factor. Be prepared to run two separate Punnett squares and report the results together.
⚠️ Know the difference between homozygous and heterozygous blood types. "Type A blood" could be I^A I^A or I^A i. The question will specify (e.g. "heterozygous for A").
⚠️ For sex-linked problems, always write out the full X and Y notation. Do not use simple uppercase/lowercase letters without the chromosome. X^H Y is correct; HY is not.
⚠️ Questions may ask for the probability of "any offspring" having a condition vs. the probability of "a male offspring" or "a female offspring" having it. These are different numbers. Read carefully.
True or false: A person with type AB blood can have a child with type O blood if the other parent is type O.
False. A type AB parent (I^A I^B) must pass either I^A or I^B to each child. None of their children can be ii.
Fill in the blank: A carrier female for hemophilia has the genotype ____.
X^H X^h.
True or false: A colorblind father will pass colorblindness to all of his sons.
False. Fathers pass their Y chromosome to sons, not their X. Sons inherit their X from their mother.
Fill in the blank: If both parents are heterozygous for the Rh factor (+/-), the probability of a Rh-negative child is ____.
25%.
True or false: Type O blood is determined by two recessive alleles.
True. The genotype for type O is ii.
Q: Both parents have type AB blood. What blood types are possible in their children?
A: Type A (25%), type AB (50%), and type B (25%). Type O is not possible because neither parent carries an i allele.
Q: A man is heterozygous for type A blood and homozygous Rh-positive. A woman is heterozygous for type B blood and homozygous Rh-negative. What are the chances of a type O, Rh-negative child?
A: ABO cross (I^A i x I^B i) gives 25% type O. Rh cross (++ x --) gives 100% +- (all positive). The chance of a type O, Rh-negative child is 0%, because all children will be Rh-positive.
Q: A woman who is a carrier for hemophilia marries a hemophiliac man. What is the probability that a female child will have hemophilia?
A: 50%. The cross is X^H X^h x X^h Y. The female offspring are 50% X^H X^h (carrier) and 50% X^h X^h (hemophiliac).
Q: Why is colorblindness more common in males than females?
A: Males have only one X chromosome (XY), so a single recessive colorblind allele on that X is enough to express the trait. Females need two copies (one on each X), which is far less likely.
Q: Can a colorblind mother and a normal-vision father have a daughter with normal vision?
A: Yes. The cross is X^b X^b x X^B Y. All daughters are X^B X^b (carriers with normal vision). All sons are X^b Y (colorblind).
Blood typing builds directly on the codominance concepts from the previous set of notes, adding the wrinkle of three alleles instead of two. The Rh factor is a clean example of simple Mendelian dominance running alongside a codominant system, which is why exam questions often combine both.
Sex-linked inheritance connects to chromosome biology and meiosis: understanding why sons get their X from their mother (and their Y from their father) requires knowing how sex chromosomes segregate during gamete formation. These concepts also tie into pedigree analysis, where you trace inheritance patterns across generations.
Related Terms / Search Tags
blood typing, ABO blood group, multiple alleles, codominance in blood type, I^A I^B alleles, type O blood, Rh factor, Rh positive, Rh negative, sex-linked inheritance, X-linked recessive, hemophilia genetics, colorblindness genetics, carrier female, hemizygous, Punnett square blood type, sex chromosomes XX XY, non-Mendelian genetics, CP biology