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MONOHYBRID CROSSES Dominant- Recessive Steps in solving problems: 1. Determine symbols for each allele. 2. Write the genotype for each parent. 3. Determine the gametes that each parent can produce. 4. Make a punette square and combine gametes. 5. Answer the question! 1. A homozygous plant for tall is crossed with one homozygous for dwarf. Tall is dominant over dwarf. a. What will be the genotypic and phenotypic proportions of the F1 b. Cross to F1 to find the F2 c. A cross between the F1 and its tall parent d. A cross between the F1 and its dwarf parent? 2. A tall plant crossed with a dwarf one produces offspring of which about 1/2 are tall and 1/2 are dwarf. What are the genotypes of the parents? 3. 4. In poultry, rose comb is dominant over single comb. A farmer believes that some of his rosecombed chickens may carry a factor for single comb. Can you suggest a method for finding out which chickens are heterozygous? 5. Lack of pigment in the human body is an abnormal recessive trait called "albinism". Using "N" and "n" to represent the dominant (normal) allele and the recessive (albino) allele respectively, three genotypes and two phenotypes are possible, list them. 6. A man and a woman plan to marry and wish to know the probability of their having any albino children. What would you tell them if: a. both the man and the woman are normally pigmented, but each has one albino parent. b. the man is an albino; the woman is normal but her father is an albino c. the man is an albino, and there has never been any albinism in the woman's family. 7. An extra finger in humans (polydactylism) is rare but is due to a dominant gene. When one parent is normal and the other parent has an extra finger but is heterozygous for the trait, what is the probability that their first child will be normal? Their second child? Their third child? 8. A dominant gene "G" is responsible for the gray body color of fruit flies: its recessive allele "g" produces black body color. A cross of a gray female with a black male produces 52 black and 58 gray in the F1. If the gray F1 females are crossed to their black F1 brothers, what genotypic and phenotypic ratios would be expected in the F2? Blood Typing 9. Write all the possible genotypes for these blood types: A B AB O 10. Explain why it is impossible for a man with AB blood type to have a child with O blood type. 11. A woman with A blood has 2 children. One with O and the other with type B blood. What type blood must the father of these 2 children have? Also, give the genotypes that the mother and the children have. 12. Suppose 2 newborn babies were mixed up in the hospital and there was a question as to what parents the babies belonged to. From the following blood types, determine which baby belongs to which parents. Also determine the genotype of each of the 6 people. Baby 1 Baby 2 Ms. Brown Mr. Brown Ms. Smith Mr. Smith type O type A type B type AB type B type B Sex Linked (X-linked) Problems 13. In humans the gene for hemophilia is sex linked and recessive to the gene for normal clotting. Diagram the genotypes of the parents of the following crosses and then determine the expected phenotypic ratios for the crosses. a. b. c. d. hemophilic woman X hemophilic man normal heterozygous woman X hemophilic man normal homozygous woman X hemophilic man normal heterozygous woman X normal man 14. Colorblindness is a sex-linked recessive trait. A family has two colorblind and on normal vision sons. What are the genotypes of the boys and their parents?