Exercises: Non-Mendelian inheritance

Explain what codominance is and how it differs from incomplete dominance, giving an example of each type of inheritance.

In certain breeds of cattle, crossing a red-coated bull with a white-coated cow produces roan-coated calves (a mixture of red and white hairs). State the genotypes of the three phenotypes and explain why the two alleles are said to be codominant.

A person with blood group A has a father with group 0. Deduce that person's genotype for the AB0 system and justify your answer.

State the only possible genotype of a person with blood group 0 and all the possible genotypes of a person with group AB. Explain what dominance relationships exist between the alleles of the AB0 system.

What is an allelic series, or series of multiple alleles? Explain how many different alleles of the AB0 system exist in the human population and how many each individual can possess at most, reasoning your answer.

A man with blood group A and a woman with group B have a child with group 0. a) Deduce the genotypes of both parents. b) Calculate the probability that their next child will be group AB. c) State all the blood groups that this couple's descendants can have and the probability with which each one will appear.

A couple in which both spouses have blood group A have a child with group 0. a) Explain how this result is possible. b) Deduce the genotypes of the parents and of the child. c) State which blood groups the rest of their children can have and which one cannot appear in any case.

In mice, the allele $A^y$ produces a yellow coat and is dominant over the agouti coat allele (A), but homozygous $A^y A^y$ mice die in the early stages of development. Two yellow mice are crossed with each other. a) State the phenotypic ratios expected among the mice born alive. b) Explain why the 3:1 ratio characteristic of the monohybrid cross is not obtained. c) State what alleles such as $A^y$ are called.

In roan-coated cattle, a roan bull and a roan cow are crossed with each other. a) State the phenotypic and genotypic ratios expected among the calves. b) Reason whether a pair consisting of a red bull and a white cow can have a red calf, justifying your answer with the genotypes.

A woman with blood group AB and a man with group 0 have children. a) State the genotypes of both. b) Calculate which blood groups their children can have and with what probability. c) Reason why no child of this couple can be group 0 or group AB.

In rabbits, coat colour depends on a series of four alleles: $c^+$ (wild type), which is dominant over all the others; cch (chinchilla), which is dominant over the next two; ch (Himalayan), which is dominant only over the last one; and c (albino), recessive to all of them. A chinchilla male of genotype cch ch is crossed with a Himalayan female of genotype ch c. a) State the phenotypic ratios expected in the offspring. b) State which phenotypes of the series cannot appear in that offspring. c) Calculate how many different genotypes are possible in the population for this trait.

In the sweet pea (Lathyrus odoratus), the development of purple petal color depends on two independent genes (C/c and P/p) involved in a sequential metabolic pathway:

Colorless precursor --(Enzyme 1, gene C)--> Colorless intermediate substance --(Enzyme 2, gene P)--> Purple pigment.

Alleles c and p encode defective, non-functional enzymes. The presence of at least one dominant allele at both loci is essential to complete the pathway; any blockage produces white flowers.

1. a) Two different pure-breeding lines with white flowers (Line A and Line B) are crossed. Surprisingly, all of the resulting F1 (100 %) displays purple flowers. Deduce with justification the genotypes of Line A, Line B, and the F1.
2. b) Self-fertilizing the F1 yields an F2. If a white-flowered plant from this F2 is selected at random, calculate the probability that it carries at least one functional dominant allele. Detail the genotypic distribution of the group of white plants to justify your calculation.
3. c) In the laboratory, white flower petals from Line A and Line B are submerged in a solution containing the "Colorless intermediate substance". After 2 hours, the petals of Line A turn purple, while those of Line B remain colorless. Interpret this biochemical assay by deducing which defective gene each of the two lines had, and confirm whether your genotypic assignment from section a) is consistent or must be reassigned.

A cat breeder claims to have achieved, after years of selection, a purebred line of Manx cats (tailless) that, when crossed with each other, only produces tailless offspring. An audit analyses the records of 40 litters from these crosses and confirms three facts: 1) the average litter size is 3.1 live kittens compared to the usual 4.2 in domestic cats; 2) among the 124 live-born kittens, 82 are tailless and 42 have normal tails; and 3) in autopsies of deceased breeding females, resorbed embryos are detected in the early stages of gestation.

1. a) Refute the breeder's claim on a genetic basis and determine the actual genotype of the breeding cats.
2. b) Justify through the corresponding cross whether the observed empirical data (82 tailless : 42 tailed) fit the model of a lethal allele and calculate the theoretical phenotypic ratios expected among live births.
3. c) Design a breeding strategy that allows the breeder to obtain the highest possible proportion of Manx kittens among live births without embryonic mortality occurring. Justify your proposal by indicating genotypes and ratios.

In a haematological study prior to a bone marrow transplant, the ABO blood group system is typed for a family. Both parents have blood group AB; however, one of their biological children (confirmed by DNA microsatellite markers) phenotypically has group 0. A trainee student claims that the results necessarily indicate a laboratory error or a switch of newborns, as "it is biologically impossible for two AB parents to have a group 0 child". Note that, in addition to the $I$ gene, an $H$ gene is involved: its dominant allele $H$ encodes the enzyme that synthesises the H substance, the precursor on which the transferases encoded by $I^A$ and $I^B$ act, whereas the recessive allele $h$ is non-functional.

1. a) Refute the student's statement by explaining the genetic phenomenon (epistasis and Bombay phenotype) that makes this result compatible. Indicate the complete genotypes of the parents and the child for the two genes involved ($I$ and $H$).
2. b) If both parents are heterozygous for the $H$ gene, calculate the probability that they will have a second child who phenotypically displays blood group AB.
3. c) If this child with the Bombay phenotype were to suffer severe haemorrhage and require an urgent packed red blood cell transfusion, explain in a reasoned manner whether they could receive blood from their parents (group AB) or from a standard group 0 donor.

In mice, coat colour synthesis follows a sequential metabolic pathway dependent on two independent genes:

Colourless precursor --(Enzyme 1, gene C)--> Black pigment --(Enzyme 2, gene A)--> Agouti pigment

The mutated recessive alleles ($c$ and $a$) produce non-functional enzymes. The $C$ allele is indispensable for synthesising any pigment (the homozygous $cc$ is albino). The $A$ allele converts the black pigment into agouti, while the homozygous $aa$ retains the uniform black colour.

An albino mouse is crossed with a black female, yielding an F1 offspring in which all individuals are agouti. When the F1 individuals are crossed with each other, the following are recorded in the F2: 62 agouti mice, 22 black mice, and 28 albino mice.

1. a) Deduce the genotypes of the initial parents (P) and the F1 individuals, justifying whether the F2 counts agree with the theoretical ratio for this pathway.
2. b) Explain the type of gene interaction (epistasis) established and justify why a mouse with genotype $ccAA$ is completely albino and does not manifest an agouti phenotype.
3. c) If an albino mouse from the F2 is chosen at random and crossed with a black mouse heterozygous for the pigmentation gene ($Ccaa$), calculate the overall probability of obtaining agouti-coated offspring, taking into account the frequencies of the different possible genotypes among the F2 albinos.

In a colony of mice, the $A^y$ allele is studied, which is dominant over the wild-type agouti allele ($A$) regarding yellow coat colour, but recessive lethal in early embryonic development. A research team collects the following experimental data:

• Experiment 1: When crossing yellow mice with each other, the average litter size at birth is 6 live pups (4 yellow and 2 agouti).

• Experiment 2: When crossing yellow females with agouti males, the average litter size at birth is 8 live pups.

1. a) Interpret and justify the difference in average litter size between both experiments, and calculate the theoretical percentage of embryonic loss in Experiment 1.
2. b) A breeder claims that, by continuously crossing yellow mice with agouti mice and selecting only yellow offspring for ten generations, they will achieve a pure line of homozygous yellow mice free of lethality. Reasonably refute the breeder's proposal.
3. c) If, in a biotechnological study, 300 early blastocysts obtained in vitro from the yellow $\times$ yellow cross are analysed before implantation and embryonic death occur, reasonably predict the number of embryos expected to be found for each of the possible genotypes.

In a hospital maternity ward, a mix-up occurs involving two newborn babies following an emergency evacuation. The blood of both parental couples and both babies is tested for the AB0 and MN systems (the latter determined by a gene with two codominant alleles, $M$ and $N$):

• Couple 1: Mother (Group A, phenotype M); Father (Group B, phenotype MN).

• Couple 2: Mother (Group 0, phenotype N); Father (Group AB, phenotype N).

• Baby X: Group A, phenotype MN.

• Baby Y: Group B, phenotype N.

1. a) Unequivocally assign each baby to their corresponding couple, justifying in detail the compatible and incompatible genotypes of each family for both traits.
2. b) Determine the possible complete genotypes of the parents in Couple 1 and, assuming the mother is heterozygous for the AB0 system, calculate the probability that their next child will be group 0 and phenotype MN.
3. c) Compare at the molecular and functional levels the codominance of the MN system with the dominance/codominance relationship of the AB0 system, explaining why in the MN system there are never hidden recessive phenotypes.