Test Cross Worksheet (A



Test Cross Classwork7th Grade PSI Science Name _______________________________________In plants and animals, organisms exhibiting the dominant trait may be either hybrid (heterozygous) or homozygous dominant (purebred). Since we cannot see an organism’s genes from the outside, and in breeding selectively it may be important to know the exact genotype; a test cross may be helpful. To make a test cross, the organism whose genotype is being tested is crossed with an organism that is homozygous recessive for that trait. Every offspring from such a mating would receive one recessive allele from the homozygous recessive parent. If an offspring received a recessive allele from the other parent as well, then it would show the recessive phenotype. From these results we would know that the other parent was hybrid and not pure for the dominant characteristic. If the offspring does not receive a recessive allele from the other parent, then it will show the dominant phenotype.TEST-CROSS RULES:A. Always cross the organism with the unknown genotype with an organism that is Homozygous Recessive (two lowercase alleles)B. If possible, examine large numbers of offspring to ensure accuracy in determining the unknown genotype.In fruit flies, red eyes are dominant over brown eyes. You manage to capture a red-eyed fruit fly and desire to learn more about its genotype.Give the phenotypes (physical eye color) of the flies involved in your test-cross:___________X ___________If the two flies have 400 offspring that all turn out to be red-eyed, what would be the genotypes of the flies used in your test-cross? (Use “R” and “r”)___________ X__________ Complete the cross on the right:What percentage is heterozygous? = ______________%What percentage shows the dominant trait? =______________%If you performed the same experiment but this time 200 of the 400 flies had brown eyes, what would be the genotype of the red-eyed fly? _______________Let’s say you decide to become a mink farmer. In mink, black fur is dominant to white fur. Since black mink coats cost more than white mink, you decide to only raise black mink. Everything is going well but the guy you bought your mink from seemed a little crooked! You want to make sure they are pure breeds so you run a test-cross.Give the phenotypes of the mink in your test-cross:___________ X ___________In your first test-cross, 30 out of 60 offspring are black and the rest are white! What are the genotypes of the mink used in your test-cross? (Use “B” and “b”)___________ X __________ Complete the cross on the right:What percentage is heterozygous? = ______________%What percentage shows the dominant trait? =______________%Since you have many mink and there may only be one bad one in the bunch, you decide to do a second test-cross on a different mink. This time out of 55 offspring, everyone is black! What are the genotypes of the mink used in this test-cross?___________ X___________ Complete the cross on the right:What percentage is heterozygous? = ______________%What percentage shows the dominant trait? =______________%Answer Key1.Red eyes x Brown eyesRR x rrrrRrRrRrRrRRWhat percentage is heterozygous? = _____100_____%What percentage shows the dominant trait? =_____100_____%If you performed the same experiment but this time 200 of the 400 flies had brown eyes, what would be the genotype of the red-eyed fly? _________Rr___2.Black Fur x White Fur Bb x bbBbBbbbBbbbbbWhat percentage is heterozygous? = ______50______%What percentage shows the dominant trait? =_____50______%3. a. BB x bbBBBbBbBbBbbbWhat percentage is heterozygous? = ______100______%What percentage shows the dominant trait? =_____100______% ................
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