During an IMSS blood donation drive, Ximena (group AB) wonders whether her future child could have the same blood group as her partner, who is group O. Mendel's complete dominance model is not enough to answer: there are flowers with an intermediate color, people in whom two alleles show up at the same time, and diseases that almost only affect men because their gene travels on the X chromosome.
Codominance
If you cross a pure red-haired cow with a pure white-haired one, some breeds produce "roan" calves, with well-defined red hairs and white hairs mixed all over the body, and not pink calves. How can two traits be expressed at once without one dominating or being diluted in the other?
The clearest human example is the AB blood group: the red blood cell of an AB person carries both antigen A and antigen B on its membrane, each one produced by its own allele.
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Explain what codominance is and give a human example in which two alleles are expressed simultaneously.
Explain what codominance is and give a human example in which two alleles are expressed simultaneously.
If a plant of the four o'clock flower or marvel of Peru (Mirabilis jalapa, native to Mexico) with pure red flowers is crossed with another with pure white flowers, all the offspring have flowers of a uniform pink. Why does no color dominate, as happened with Mendel's peas?
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Explain what incomplete dominance is and describe which phenotype appears when crossing a red-flowered plant with a white-flowered plant if the trait follows this…
Explain what incomplete dominance is and describe which phenotype appears when crossing a red-flowered plant with a white-flowered plant if the trait follows this pattern.
In four o'clock plants, flower color shows incomplete dominance: red (RR), pink (Rr) and white (rr). Complete the Punnett square in the diagram, where two pink-flowered…
In four o'clock plants, flower color shows incomplete dominance: red (RR), pink (Rr) and white (rr). Complete the Punnett square in the diagram, where two pink-flowered plants are crossed, and state the genotypic and phenotypic ratios of the offspring.
In a blood bank, donations are classified into four groups (A, B, AB, and O, which is also written 0). With only two alleles, four phenotypes could not be explained. How many alleles determine the blood group?
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The alleles $I^A$, $I^B$ and $i$ are involved in blood groups. State which genotypes correspond to blood groups A, B, AB and O.
The alleles $I^A$, $I^B$ and $i$ are involved in blood groups. State which genotypes correspond to blood groups A, B, AB and O.
A person with blood group AB and a person with blood group O have a child. State the possible blood groups of the child and their probabilities, excluding each parent's…
A person with blood group AB and a person with blood group O have a child. State the possible blood groups of the child and their probabilities, excluding each parent's blood group from the possibilities.
A medical emergency occurs in a hospital and a patient with blood type 0 urgently needs a transfusion. When testing their biological parents, it is found that the father…
A medical emergency occurs in a hospital and a patient with blood type 0 urgently needs a transfusion. When testing their biological parents, it is found that the father is type A and the mother is type B. A student intern claims that there must be an error in parentage, arguing that "two parents with dominant A and B antigens can never produce a type 0 child, who lacks both antigens."
a) Refute the student's statement by deducing the exact genotypes that the parents must have, and represent the cross that demonstrates the genetic viability of the patient's birth.
b) If this same couple has more offspring, predict the expected phenotypic percentages for each blood type.
c) Justify whether any of the future siblings could donate red blood cells to the type 0 patient.
In humans, sex is determined chromosomally: females are XX and males are XY. Explain, using the corresponding cross, why the probability of having a son or a daughter is…
In humans, sex is determined chromosomally: females are XX and males are XY. Explain, using the corresponding cross, why the probability of having a son or a daughter is equal (1:1 ratio) and which parent provides the chromosome that determines the sex of the offspring.
Color blindness and hemophilia: "criss-cross" inheritance
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A woman who is a carrier of the color-blindness allele ($X^D X^d$) has children with a man with normal vision. What is the probability of each son being color-blind? And…
A woman who is a carrier of the color-blindness allele ($X^D X^d$) has children with a man with normal vision. What is the probability of each son being color-blind? And each daughter? Reason both answers using a genetic cross.
Incomplete dominance, codominance, multiple alleles, and sex-linked inheritance increase the possible combinations of alleles in a population. This variability, together with mutations, is the raw material on which natural selection acts: if individuals with different traits survive and reproduce differently, species change over the generations.
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Queen Victoria of Great Britain was a heterozygous carrier of hemophilia ($X^H X^h$) and her consort, Prince Albert, had normal blood clotting ($X^H Y$). Among their…
Queen Victoria of Great Britain was a heterozygous carrier of hemophilia ($X^H X^h$) and her consort, Prince Albert, had normal blood clotting ($X^H Y$). Among their descendant European dynasties, several affected males were observed, but no female with hemophilia.
a) A rumor at the time claimed that if Prince Albert had been hemophilic, 100% of the couple's daughters would have been born with the disease. Demonstrate through a genetic cross whether this statement is true or false, indicating the expected phenotypic and genotypic percentages for daughters in that scenario.
b) Deduce what two simultaneous genetic and phenotypic conditions had to coincide in both parents for a hemophilic girl to be born in that era.
c) Justify at the chromosomal level why males cannot be healthy carriers of an X-linked recessive disease.