(20 12? Some of the X and Y chromosomes of Drosophila melanogaster are homologous fragments (such as fragment I in IN). The other part is a non-homologous fragment (in).

(1) During meiosis, the X and Y chromosomes can be recombined by exchanging their homologous fragments, that is, fragment I in the figure.

(2)① The cross combination 1: setae (♀ )× setae (♂)→F 1 are all setae, indicating that setae are relative to setae.

② Cross combination 2: shearing (♀) × mane (♂ )→ shearing (♀): mane (♂) = 1: 1, indicating that the inheritance of the relative characters of mane and shearing is related to sex, so it can be judged in this way.

③ According to ① and ②, the gene controlling this trait is located on the sex chromosome, and bristles are dominant traits relative to bristles (denoted by A and A). If the gene controlling this trait is located in II-2 fragment, then the cross combination II: bristle (♀XaXa)× bristle (♂XAY)→F 1 should be cut (♀). If the gene controlling this trait is located in Ⅱ-1fragment, the offspring of female Drosophila should not have this trait.

So the answer is:

( 1)Ⅰ

② ① A bristle? 2 sex? ③Ⅰ

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