Genetic crosses with sex linked traits are examined.
In J.R.R Tolkien's The Lord of the Rings, the Black Riders of Mordor ride steeds with eyes of fire. As a geneticist, you are very interested in the inheritance of the fire-red eye colour. A discovery is made that the eyes contains two types of pigments, brown and red, that are usually bound to core granules in the eye. In wild-type steeds, precursors are converted by these granules to the above pigments, but in steeds homozygous for the recessive X-linked gene w (white eye), the granules remain unconverted and a white eye results. The metabolic pathways for the synthesis of the two pigments are shown in the attached file. Each step of the pathway is controlled by a gene: mutation v results in vermillion eyes; cn results in cinnabar eyes; st results in scarlet eyes; bw results in brown eyes; and se results in black eyes. All these mutations are recessive to their wild-type alleles, and all are unlinked. For the following genotypes, show the proportions of steed eye phenotypes that would be obtained in the F1 of the given matings.
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...with white eyes. We will deal with this gene differently than the others. In the crosses, if they do not mention a certain gene, assume that both are normal and don't consider them in your calculations.
For the first cross, we have the following genotypes:
w/w ; bw+/bw+ ; st/st // w+/Y ; bw/bw ; st+/st+
It's not necessary but it may be useful to look at what are the phenotypes of the parents. The mom (on the left) has white eyes as she's double recessive for w, even though she produces a functional red pigment. She is double recessive for the scarlet eyes, but because she is w/w, they will appear white.
The dad has brown eyes. He is homozygous for the bw trait. He is normal for everything else. Notice that he only has one w+. This is the sex-linked trait and the one thing to keep in mind. Sex linked traits are found on the X chromosome only. What makes them special is that in females, they act like normal genes (ie, they have two alleles, because they have two X chromosomes) but in males, who only have one X chromosome, have only one allele.
So back to the question....
There are only two possible genotypes that ...