2022英语周报八下YY答案

写作Paragraph 1: Jatin looked up and saw the other boys rushing ahead Refusing togive up at this point, he jumped to his feet and ran as fast as he could. Hedashed across the finishing line and won the fourth prize. Thinking he would lether sister down, Jatin couldn t help sobbing. Just then he suddenly heard theannouncement from the broadcast that he won the thirdprize as the runner whocollided with Jatin was disqualified for breaking the rule. holding the shoes closeto his chest during the award ceremony Jatin felt overjoyed and relieved as heeventually won the shoes for his sister.(要点1 Jatin获奖)Paragraph 2Filled with pleasure, Jatin walked home quickly.(高分句型一) He could notwait to see his sister and share the good news. On his arrival, Jatin found Zahrawas waiting outside. Nervous and anxious, Zahra wondered whether herbrother had brought her what she was expectingZahra, see what i have gotfor you. Jatin said excitedly taking the prize out of his bag with his tremblinghands. The instant Zahra saw the shoes, her face lit up. So thrilled was shethat she threw herself at Jatin.(高分句型二)" Thank you!ltin." Tears of joywelled up in their eyes as the brother and the sister hugged tightly together (4 R2把奖品给妹妹)

20.【答案】(除注明外,每空2分,共12分)(1)常染色体显性遗传(1分)子代雌雄个体的表现型及其比例均是黑身:灰身=3:1(2)减数分裂形成配子的过程中,等位基因会随着同源染色体的分开而分离,分别进入两个配子中,独立地随配子遗传给后代让亲本雌果蝇与灰身雄果蝇(或亲本雄果蝇与灰身雌果蝇)交配,观察子代的表现型及比例(3)X(1分)X染色体上的B基因纯合致死(或不含b基因的个体致死)4)17【解析】(1)根据子代雌雄个体的表现型及其比例均是黑身:灰身=3:1,可确定黑身是显性性状,相应基因位于常染色体上。(2)基因分离定律的实质是减数分裂形成配子的过程中,等位基因会随着同源染色体的分开而分离,分别进入两个配子中,独立地随配子遗传给后代;验证基因分离定律可以进行测交实验,即利用亲本雌果蝇与灰身雄果蝇(或亲本雄果蝇与灰身雌果蝇)交配,观察子代的表现型及比例。(3)长翅雌蝇与短翅雄蝇交配,子代雌蝇中长翅:短翅=1:1,雄蝇中只有短翅,可确定基因位于X染色体,且亲木的基因型为X"x×xY,子代雄蝇的基因型应该有2种,但实际只有一种,说明X"Y个体没有存活,即B基因纯合致死(或不含b基因的个体致死)。(4)子代中的黑身长翅雌果蝇的基因型为1/3AAXx、2/3Aaxx2,黑身短翅雄果蝇的基因型分别为1/3AAXY、23AaXY,二者交配子代出现灰身长翅雌果蝇(aaX"x)的概率为2/3×2/3×1/4×1/31/27

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