Question

Huntington's chorea is dominant lethal allele in humans. What are the F1 genotypic and phenotypic ratios of parents who are homozygous dominant and heterozygous?

Answer 1

50% will be homozygous and 50% heterozygous, all will display the Huntington phenotype.

Explanation:

Lets call the dominant allele for Huntington's disease `color(red)"H"` and the recessive allele `color(green)"h"`.

Parent 1 is homozygous = `color(red)"HH"`
Parent 2 is heterozygous = `color(red)"H"color(green)"h"`

Knowing this, we can make a cross table for F1, the first generation offspring:

This shows that the chance to get homozygous offspring (`color(red)"HH"`) is 50% and 50% chance to get heterozygous offspring (`color(red)"H"color(green)"h"`).

The allele for a Huntington phenotype is dominant, so you require only 1 dominant allele to display the phenotype. In this case 100% of the F1 offspring will show the phenotype.