(1) At the late stage of the first meiosis, two chromosomes X and Y were assigned to two secondary spermatocytes respectively. After replication, the two chromatids on the X chromosome each have a W gene, and the two W genes are separated during centromere division in the late stage of the second meiosis.

(2) When the B gene on a chromosome is copied, an A on a deoxynucleotide chain is replaced by T, which can only lead to the mutation of a DNA molecule. But after meiosis, one spermatogonia can produce four sperm.

(3) There are two Y chromosomes in the sperm cells in Figure 2, which means that the centromere on the Y chromosome did not divide at the late stage of the second meiosis, resulting in the separation of a pair of sister chromatids.

(4) If the gene of winged Drosophila is located on chromosome 4, homozygous winged Drosophila without chromosome 4 will produce two gametes, one containing chromosome 4 (with winged gene) and the other containing no chromosome 4; Drosophila only produces one kind of gamete with residual wing gene, so they mate, and Drosophila and its offspring appear in the ratio of 1: 1. If the gene of Drosophila is not on chromosome 4, then the offspring should all have wings.

So the answer is: (1)X or y? After the second meiosis? (2) 1/4(3) A pair of sister chromatids (or Y chromosomes) did not separate during the second meiosis (4) Residual wings? 1: 1 all have wings.