Non-Mendelian inheritance

In the four o'clock plant (Mirabilis jalapa), a red-flowered plant is crossed with a white-flowered one and the whole F1 has pink flowers. State the genotypes of the parental generation and of the F1, and explain why this result does not contradict Mendel's laws even though a new phenotype appears in the F1.

In the snapdragon (Antirrhinum majus), flower colour shows incomplete dominance: red, pink or white. a) State the phenotypic ratios expected in the F2 obtained by self-fertilising pink plants. b) Calculate the ratios expected when a pink plant is crossed with a red one. c) Explain why, in incomplete dominance, the phenotypic and genotypic ratios of the F2 coincide.

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.

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.

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.

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.

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.

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 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 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.

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 laboratory rodent species, coat color exhibits three possible phenotypes (black, grey, and white), and pigment distribution can be uniform or show white spots. Two controlled crosses are carried out:

• Cross 1: Two grey individuals with spots are crossed with each other and produce offspring with 6 distinct phenotypes distributed in the ratio 6:3:3:2:1:1.

• Cross 2: A grey female with spots from the previous offspring is taken and crossed with a white, uniform male. The resulting litter exhibits an identical phenotypic segregation of 1/4 grey with spots, 1/4 uniform grey, 1/4 white with spots, and 1/4 uniform white.

1. a) Determine the pattern of inheritance of both traits, define the allelic nomenclature, and deduce with justification the genotypes of the parents in Cross 1.
2. b) Demonstrate mathematically how the 6:3:3:2:1:1 ratio originates in Cross 1 by combining the individual segregations of each locus.
3. c) Deduce the genotype of the female used in Cross 2 and justify whether this second cross constitutes a strict testcross for both genes.