Exercises: Other patterns of inheritance

Explain what codominance is and give a human example in which two alleles are expressed simultaneously.

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.

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.

Explain the difference between codominance and incomplete dominance, and give an example of each one.

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.

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.

In the four o'clock plant, flower colour 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.

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.

In four o'clock plants, flower colour 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 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.

In Andalusian hens, plumage colour shows incomplete dominance: BB is black, Bb is blue and bb is white. Cross two blue hens and state the expected genotypic and phenotypic ratios in the chicks.

A heterozygous blood-group-A woman ($I^A i$) and a heterozygous blood-group-B man ($I^B i$) have a child. State all the blood groups the child can have and the probability of each one.

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 colour-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 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 couple attends a genetic counseling consultation: the man has normal vision and so does the woman, but she is concerned because her maternal grandfather was color-blind (her maternal grandmother and her father had normal vision and had no family history). The man claims there is nothing to worry about, stating: "Since I am not color-blind, it is impossible for us to have a color-blind daughter or son."

1. a) Reasonably deduce the probability that the woman is a carrier based on the data from her maternal family.
2. b) Refute or validate the man's claim using genetic arguments for daughters and sons separately, showing the possible crosses.
3. c) Calculate the final probability that the first child born to this couple is a color-blind male.

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."

1. 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.
2. b) If this same couple has more offspring, predict the expected phenotypic percentages for each blood type.
3. c) Justify whether any of the future siblings could donate red blood cells to the type 0 patient.

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.

1. 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.
2. 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.
3. c) Justify at the chromosomal level why males cannot be healthy carriers of an X-linked recessive disease.

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.

1. 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.
2. 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?

In humans, color blindness is determined by an X-linked recessive allele ($X^d$), while the Rh factor is inherited by an autosomal gene where the positive allele ($Rh^+$) is dominant over the negative allele ($Rh^-$). A woman with normal vision and Rh-positive blood type, whose father was color-blind and Rh-negative, has children with a man with normal vision who is heterozygous for the Rh factor.

1. a) Deduce the parents' genotypes, justifying the woman's based on information about her father.
2. b) Carry out the necessary genetic analysis to calculate the probability that they have a son who is both color-blind and Rh-negative. Express the result as a fraction and as a percentage (with a decimal comma).
3. c) Explain, without needing to develop the entire Punnett square for daughters, why the probability of having a color-blind daughter in this couple is exactly 0%.

Seán and Niamh have normal blood clotting and have had a first son diagnosed with hemophilia (an X-linked recessive disorder). Now Niamh is pregnant with twins: a boy and a girl. During a family gathering, two comments are made:

1. "Since the first son was already born hemophilic, by probability the next boy will be born completely healthy."

2. "The future girl has a 50% chance of suffering from hemophilia, just like her brother."

1. a) Deduce Seán and Niamh's genotypes and clarify from which parent the first son inherited the allele responsible for the disease.
2. b) Refute each of the two family members' statements using genetic reasoning and probability calculations. State the actual probability that the new son will be healthy and that the girl will suffer from the disease.
3. c) Calculate the probability that the future girl will be a carrier of the hemophilia-causing allele without showing symptoms.

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.

1. a) Predict the expected genotypic and phenotypic ratios in the offspring using the corresponding Punnett square.
2. 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.
3. 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.

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."

1. 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.
2. 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.
3. 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.

Unit challenge. How are traits transmitted in your own family? Become a genetic counsellor and prepare a reasoned biological report on your inheritance.

1. Data collection: Design a table and survey $4$ or more family members (or close acquaintances), recording a dominant and recessive trait (tongue rolling or detached earlobe, treated here with a simplified Mendelian model, although their real inheritance is more complex) and their ABO blood type.

2. Demographic comparison: Consult the Irish Blood Transfusion Service (IBTS) website for ABO system frequencies in Ireland (approx. $45.0\,\%$ group A, $42.0\,\%$ O, $9.0\,\%$ B, and $4.0\,\%$ AB) and compare your sample's distribution with the national average.

3. Genetic analysis: Deduce the possible genotypes of each participant (stating the certain one when the phenotype or the pedigree allows it) using standard nomenclature ($A$, $a$, $I^A$, $I^B$, $i$). Justify how alleles interact in the Mendelian trait versus codominance and multiple allelism in the ABO group.

4. Final report: Write a one-page report that includes your data table, a basic reasoned pedigree chart, and the explanation of both inheritance patterns.

Assessment criteria:

• Thorough and justified deduction of genotypes using formal genetic symbols.

• Clear conceptual distinction between complete dominance and other patterns (codominance and multiple alleles).

• Accurate comparative analysis between your own sample and reference population data.