Other Inheritance Patterns
At the blood bank of the hospital in Santiago, donations are sorted into four groups: A, B, AB, and O. If each gene had just one dominant allele and one recessive allele, there would not be four groups. In addition, some flowers have an intermediate color, and some diseases mostly affect men. How do we explain all of this?
Codominance
When a pure white cow is crossed with a pure black bull, some calves are born with sharply defined black and white spots, not gray. How can two traits be expressed at once without one dominating or blending into the other?
A human example is blood group AB: the red blood cells carry both antigen A and antigen B.
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.
Explain the difference between codominance and incomplete dominance, and give an example of each one.
Explain the difference between codominance and incomplete dominance, and give an example of each one.
Incomplete Dominance
If a plant of the four-o'clock flower with pure red flowers is crossed with another with pure white flowers, all the offspring have pink flowers. Why doesn't either color dominate, as happened with Mendel's peas?
To solve a cross, you use a Punnett square, just as in complete dominance. The difference is that each genotype has its own phenotype.
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 the four o'clock plant, flower color shows incomplete dominance: RR is red, Rr is pink and rr is white. Cross a pink-flowered plant with a white-flowered one and…
In the four o'clock plant, flower color shows incomplete dominance: RR is red, Rr is pink and rr is white. Cross a pink-flowered plant with a white-flowered one and state the genotypic and phenotypic ratios of the offspring.
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.
Multiple Alleles: The ABO Blood Groups
At the blood bank, people talk about four groups (A, B, AB, and O; sometimes O is written as 0). With only two alleles they could not be explained. How many alleles determine the blood group?
| Group | Possible genotypes |
|---|---|
| A | $I^AI^A$, $I^Ai$ |
| B | $I^BI^B$, $I^Bi$ |
| AB | $I^AI^B$ |
| O | $ii$ |
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 has blood group O. State their genotype and reason whether they can have any $I^A$ or $I^B$ allele.
A person has blood group O. State their genotype and reason whether they can have any $I^A$ or $I^B$ allele.
For the ABO blood groups there are three different alleles, but each person only has two of them. Explain what multiple allelism is and why this happens.
For the ABO blood groups there are three different alleles, but each person only has two of them. Explain what multiple allelism is and why this happens.
Crosses with Blood Groups
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 person with blood group AB and another with heterozygous blood group A ($I^A i$) have a child. Calculate the probability of each possible blood group in the offspring…
A person with blood group AB and another with heterozygous blood group A ($I^A i$) have a child. Calculate the probability of each possible blood group in the offspring and reason why the child cannot be group O.
A couple consisting of a woman with blood type A and a man with blood type B, both with heterozygous genotypes for the ABO system ($I^A i$ and $I^B i$), is planning to…
A couple consisting of a woman with blood type A and a man with blood type B, both with heterozygous genotypes for the ABO system ($I^A i$ and $I^B i$), is planning to have children.
- a) Predict the expected genotypic and phenotypic ratios in the offspring using the corresponding Punnett square.
- b) In the event of an accident, the mother needs an urgent red blood cell transfusion. Taking ABO system compatibility into account, calculate the probability that a child of this couple, chosen at random, will be a compatible donor for her.
- c) Calculate the combined probability that their first two children will have blood type 0 and the third will belong to blood type AB. Express the result as a simplified fraction and as a percentage with a decimal comma.
Deducing Genotypes and Assigning Parentage
Sometimes you know the child's blood group and have to work out the parents' genotypes. The trick: each of the child's alleles comes from one parent, and an O child ($ii$) received an $i$ from each.
A man with blood group AB and a woman with blood group A have a child with blood group B. Deduce the mother's genotype and explain your reasoning.
A man with blood group AB and a woman with blood group A have a child with blood group B. Deduce the mother's genotype and explain your reasoning.
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 a hospital maternity ward, a mix-up with three newborns occurred following a technical incident. The blood types of the three babies and the three couples have been…
In a hospital maternity ward, a mix-up with three newborns occurred following a technical incident. The blood types of the three babies and the three couples have been determined:
-
Baby 1: Type 0
-
Baby 2: Type AB
-
Baby 3: Type B
-
Couple X: Both parents are type AB.
-
Couple Y: The mother is type A and the father is type B; both know that their respective fathers (the child's grandfathers) were type 0.
-
Couple Z: The mother is type 0 and the father is type AB.
- a) Reasonably assign each baby to their corresponding couple. To do this, deduce the parents' genotypes and explain why you rule out the other combinations.
- b) If Couple Y decided to have another child, what would be the probability (in percentage) that they are born with the same blood type as Baby 2?
Sex-Linked Inheritance
Sex Is Inherited Too
Sex is determined by the chromosomes: a woman is XX and a man is XY. The mother produces only gametes with an X; the father produces half with an X and half with a Y. So the father provides the chromosome that decides the sex, with probability 1/2 of a girl (XX) and 1/2 of a boy (XY): a 1:1 ratio.
| X (mother) | X (mother) | |
|---|---|---|
| X (father) | XX girl | XX girl |
| Y (father) | XY boy | XY boy |
Explain what it means for a gene to be linked to the X chromosome and reason why men (XY) have only one allele for those genes, while women (XX) have two.
Explain what it means for a gene to be linked to the X chromosome and reason why men (XY) have only one allele for those genes, while women (XX) have two.
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
Color blindness (difficulty telling certain colors apart, such as red and green) and hemophilia (blood clots poorly) are recessive hereditary conditions linked to the X chromosome. We use $X^D$ (normal) and $X^d$ (color blindness); for hemophilia, $X^H$ and $X^h$.
| Genotype | Phenotype |
|---|---|
| $X^DX^D$ | woman, normal vision |
| $X^DX^d$ | carrier woman (normal vision) |
| $X^dX^d$ | color-blind woman |
| $X^DY$ | man, normal vision |
| $X^dY$ | color-blind man |
This is called "criss-cross" inheritance because an affected man passes the allele to his daughters (carriers), and they can pass it to their sons: from grandfather to grandson, skipping a generation.
Back to the beginning: the four blood groups are explained by three alleles (multiple alleles and codominance), the intermediate color by incomplete dominance, and the diseases that mostly affect men by genes on the X chromosome.
A color-blind man ($X^d Y$) has children with a woman with normal vision who is not a carrier ($X^D X^D$). State the genotypes and phenotypes of the sons and of the…
A color-blind man ($X^d Y$) has children with a woman with normal vision who is not a carrier ($X^D X^D$). State the genotypes and phenotypes of the sons and of the daughters, and explain why this type of inheritance is said to go "criss-cross" from fathers to daughters.
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.
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.
Human Nutrition: A Brief Review
Genes don't act alone: for an organism to function, its organ systems work together. One example is nutrition, which is also related to health: hemophilia, for instance, affects the blood that carries what the body needs.
Four systems work together in it:
| System | Role in nutrition |
|---|---|
| Digestive | Breaks down food and absorbs nutrients (for example, those from mangú or sancocho). |
| Respiratory | Takes oxygen from the air and expels carbon dioxide. |
| Circulatory | The blood carries nutrients and oxygen to all the cells and picks up waste. |
| Excretory | Filters the blood and eliminates waste, such as urea, in the urine. |
A man with hemophilia ($X^h Y$) and a homozygous non-carrier woman ($X^H X^H$) have offspring: a son and a daughter. Upon reaching adulthood, both children have…
A man with hemophilia ($X^h Y$) and a homozygous non-carrier woman ($X^H X^H$) have offspring: a son and a daughter. Upon reaching adulthood, both children have offspring with healthy partners who have no history of the disease.
The son fears transmitting hemophilia to his children, while the daughter is convinced that her offspring can never suffer from it "because her own mother did not carry the disease allele."
- a) Using the F1 and F2 crosses, evaluate the fears and beliefs of both siblings and determine which of the two is making an error in their reasoning.
- b) Calculate the probability that a grandson (son of the daughter) will have hemophilia and the probability that a granddaughter (daughter of the daughter) will be hemophilic.
- c) Explain at a cytogenetic level why this type of sex-linked recessive traits exhibits a pattern of inheritance with phenotypic generation skipping in the male lineage.