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Year 11 Genetics Monohybrid Crosses Answers 1. In maize starchy is dominant to sugary. Two purebreeding plants were crossed, one grown from a startchy seed and one from a sugary seed. a. What is the phenotype of the F1 seeds? Starchy b. These F1 seeds grew into mature plants and were allowed to self fertilise. What is the expected ratio of starchy to sugary seeds in the cobs on the F2 plants. 3:1 2. Mendel crossed pure breeding plants having green unripe pods with pure breeding plants having yellow unripe pods. The F1 plants had green pods. He allowed these to self fertilise and collected 580 F2 plants. How many of these would be expected to have green pods and how many yellow pods? 435 green, 145 yellow 3. The allele causing cystic fibrosis in humans is recessive to the normal allele. A man is heterozygous for the cystic fibrosis. Does he suffer from the disease? No, but he is a carrier. 4. In guinea pigs, short hair is dominant to angora (or long hair). A heterozygous animal was mated to a homozygous recessive. a. What are the phenotypes of these animals? Short hair (heterozygous) and Angora (homozygous recessive) b. Using suitable symbols give their genotypes. H = short hair, h = long hair. Hh is heterozygous, hh is homozygous recessive. c. Draw out the genetic cross and state the proportion of their litter are expected to be angora. 50% 5. ‘White forelock’, a patch of white hair at the front of the head, is a dominant condition in humans. A woman with a white forelock married a normal man and had a daughter with a white forelock. The daughter also married a normal man. Using genetic crosses find the likelihood that her first child will have a white forelock. W = white forelock, w = no white forelock 6. Red Poll cattle are chestnut red. When crossed to White Shorthorn, the calves have a mixture of red and white hairs and are called roan. Give the possible phenotypes of the offspring of the following crosses and the probability with which they will occur. a. roan X roan , red (25%) white(25%) and roan (50%) b. roan X white, roan (50%) and white (50%) c. roan X red, red (50%) and roan (50%) 7. The ability to roll the tongue is a dominant trait in humans. A man and a woman, both able to roll their tongues had four children. Some children were able to roll their tongues and some were not. Draw out the cross to show the genotypes of the parents and the offspring. How many children would you expect to be able to roll their tongues? (75%) T = tongue roller, t = non tongue roller 8. A budgerigar breeder bred together homozygous dark blue and homozygous pale blue birds and obtained all mid-blue progeny. In an attempt to obtain more mid-blue birds he bred the mid-blue birds with each other. To his surprise the outcome was not as he expected. What phenotypes will he have got by crossing the mid-blue birds and in what proportions? (25% dark blue, 25% pale blue, 50% mid-blue) What cross should he carry out to be sure that the offspring will all be midblue? dark blue X mid blue