the solid colored figure represents an individual affected by a disease
the empty figure represents a healthy individual
Let us assign the symbol X⁺ to represent the dominant allele linked to the X-chromosome and expressing healthiness, and X⁻ to represent the recessive allele expressing the dissease.
According to this pedigree
I1 is a man affected by the disease, YX⁻
I2 is a healthy woman X⁺X⁻
we can see that among the progeny (generation II) there are two individuals affected (a boy and a girl) and one healthy girl. This means that the mother I2 is heterozygous for the trait.
So, having their genotypes we can know what are the probabilities of getting a son with DMD
Parentals) YX⁻ x X⁺X⁻
Gametes) Y X⁻ X⁺ X⁻
Punnett square)
X⁺ X⁻
X⁻ X⁺X⁻ X⁻X⁻
Y X⁺Y X⁻Y
F1)
The probabilities of getting a healthy daughter X⁺X⁻ are 25%
The probabilities of getting a healthy son X⁺Y are 25%
The probabilities of getting a daughter with DMD X⁻X⁻ are 25%
The probabilities of getting a son with DMD X⁻Y are 25%
