Mendel's first rule?

Moin,

Phenotype: The (external) manifestation of a feature. Here: black fur or white fur.

Genotype: The genetic equipment of the living organism (with respect to certain characteristics). Here: The living being can be pure (homozygot) in relation to the feature fur color; then in the diploid state it has twice the same allele. Or it can be mishbig (heterozygot); then it has two different alleles.

allele definition
A: black coat
a: white fur

Genotype (diploids) Parent body cells
AA (pure black coat) x aa (pure white coat)

(haploids)
A and A x a and a

genotype (diploid) body cells of the F1 generation
all Aa (mixed black coat)

A parent has black fur. The text says that parents are purebred. Therefore, this parent (here the mother) has the genetic equipment AA (two times the same allele for the dominant black-haired coat).
The other parent (here the father) has white fur. He is also pure and therefore has twice the same allele, but this time the recessive allele a for white fur.

Since both parents are purebred, they can only give an allele into their germ cells (eggs or sperms). The mother’s egg cells always have the allele A (for black fur), while the father’s sperm cells are always equipped with the allele a (for white fur).

When the eggs of this mother come together with the sperms of this father and merge into a cygote (fertilization), then the alleles A (black fur from the mother) and a (white fur from the father) come together in the cygote. This means that all descendants of the F1 generation of these two parents are always merciful Aa. And because A (black fur) is dominant over a (white fur), all descendants of these parents also have black fur…

LG from the Waterkant