# Dogs have 78 chromosomes in their diploid cells. How many chromosomes are in their haploid cells?

There are $39$ chromosomes in their haploid cells.
During meiosis I, the $78$ chromosomes in a diploid cell line up at the equator of the cell. Note that the chromosomes have already replicated.
$39$ chromosomes will then move to each pole of the cell. The difference between anaphase in meiosis I and mitosis is that in meiosis I, the centromere does not split, so the sister chromatids are still held together.
The result is $39$ chromosomes at the poles of the cell. The chromosome number is reduced from $2 n$ (diploid) to $n$ (haploid). Then the cell will undergo meiosis II, where the cell will divide, producing $4$ haploid daughter cells with half the number of chromosomes of the parent cell.