The key pieces of information from the question are:
- Huntingtons is autosomal dominant
- Stargardt is autosomal recessive
- Father's family has a lot of both diseases
- Mother's family has none and she is homozygous for both
From this, we can deduce that
- Mother's alleles for huntingtons gene is hh and her alleles for the stargardt gene is RR
- Father's alleles for the huntingtons gene is either HH or Hh (He must have had at least one dominant H allele to be able to pass it down). His alleles for the stargardt gene must be either rr or Rr (for the same reason as before; he must have a recessive allele to be able to pass it down since the mother only has dominant alleles).
This allows us to answer part A. The patient must have inherited one recesssive huntingtons gene (h)
from mother and one dominant stargard gene from mother (R)
because their mother only has those alleles to pass down.
In order for the patient to be heterozygous for huntingtons, he must have inherited one dominant allele from father (H).
In order for patient to be heterozygous for stargardt, he must have inherited one recessive allelle from father (r)
Therefore, alleles from mother are hR and alleles from father are Hr
For part B, we just need to model meiosis and include one example of crossing over
For the first row, you label your paternal and maternal chromosomes from part A. Recall that homologous chromosomes are the pair of chromosomes where one is from the father and the other is from the mother. Also recall that at this point the sister chromatid has already replicated.
The second row shows what happens after crosssing over (which is the exchange of genetic material between homologous chromosomes at the chiasma). The suggested answer shows crossing over occuring at the point where the lower right of the paternal chromosome touches the lower left of the maternal chromosome. That's why the letters representing the alleles have exchanged at that point.
In the third row, you show the resulting gametes at the end of meiosis. Each sister chromatid of each chromosome separate into separate gametes. This results in four total gametes in the end.
Hope this helps!