Genetic problems// Monohybrid Crosses

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I. Monohybrid Crosses


1. In sweet peas the gene for the red color in flowers (R) is dominant over the gene for the white flowers (r). If a homozygous red-flowered plant is crossed with a white­-flowered plant, what will be the genotype of the offspring? What will be the result of the next generation if the first offspring (F1) are allowed to self-pollinate?


2. A blue-eyed man, both of whose parents are brown-eyed, marries a brown-eyed woman whose father was brown-eyed and whose mother was blue-eyed. They had one child who is blue-eyed. What are the genotypes of all the members mentioned?


3. The ability to taste phenylthiocarbamide (PTC) in humans appears to be due to a dominant factor. These persons are called tasters. A man who is a taster, and who has a non-taster mother, marries a woman who is a taster. She has 3 brothers and 2 sisters, three of whom are non-tasters. What chances will the children of this marriage have of being non-tasters?


4. Twenty-five percent of the offspring of a certain pair of white sheep are black. What was the genotype of the parents?


5. A woman has a rare abnormality of the eyelids called ptosis, which makes it impossible for her to open her eyes completely. The condition is due to a dominant gene (P). The woman's father had ptosis, but her mother had normal eyelids. What are the genotypes of the woman, her father, and her mother? If she married a man with normal eyelids, what proportion of the children would be expected to have ptosis and what proportion would be normal?


6. Two dark-haired parents have a red-haired child. What genotype must the two parents have to permit this?


 7. Assume that the dimple is inherited as a simple dominant gene. A dimpled man whose mother had no dimple married a woman with no dimple. What are the genotypes of the father, mother, F1 generation, and F2 generation?


8. In mice, yellow coat color (Z) is a dominant factor and black (z) is its recessive. The homozygous condition (ZZ) is lethal and kills the embryo within a few days after conception. If two yellow mice bred, what genotypes would you expect and in what ratio? What will be the phenotypes and in what ratio?


9. One variety of snapdragon with dominant yellow leaves always produce normal green leaf plants as well as yellow leaf heterozygous plants. The homozygous yellow leaf lack the ability to make chlorophyll and die as seedlings. Show the result of the cross of two yellow leaf plants.


10. Can wavy haired parents have a curly haired child?


11. Yellow guinea pigs crossed with white ones always produce cream-colored off­spring. Matings between the two cream colored guinea pigs produced, in one case, 22 yellow, 47cream, and 19 white. What type of inheritance is this?


II. Sex-linked Characteristics


12. In man, colorblindness, an inability to distinguish between red and green, is inherited as a sex-linked character on the X chromosome. It is recessive. If a colorblind man marries a homozygous normal woman, what chance is there for the children of being colorblind? If a normal man marries a colorblind woman, where will colorblindness be found in the family? Why is it so commonly believed that women cannot be colorblind?

13. Let us assume that a colorblind man has a colorblind sister and his mother has normal vision. What is the genotype, as to color vision, of this man's mother and of his father? What kind of color vision will his sons and his daughters have, and in what ratios, if he marries a woman identical in genotype to his mother?


14. In man, hemophilia (failure of blood to clot) is a recessive and sex-linked character. A man and a woman, both normal in respect to blood-clotting, have (a) a hemophilic son who has a daughter who is normal, (b) a daughter who has normal blood and who has one hemophilic and one normal son, and (c) another daughter with normal blood who has five sons, all normal. What are the probable genotypes of grandpar­ents, children, and grandchildren?


15. In the fruit fly, red eye is dominant to white eye, which is sex-linked. Specify the distribution of eye color (a) among the males and (b) among the females, derived from each of the following crosses:

  1. white-eyed female x red-eyed male
  2. heterozygous red-eyed female x white-eyed male
  3. heterozygous red-eyed female x red-eyed male


III. Multiple Alleles


16. In man, blood groups are inherited on the basis of multiple alleles. Determine the genotypes of the following parents.

  1. Father group A, mother group B. All four groups are represented among the children.
  2. Father group AB, mother group B, 1/4 of children A, 1/4 AB, and 1/2 B.
  3. Both parents are A, 3/4 of the children are A, 1/4 type O.


 17. Suppose a father and a mother claim that they have been given the wrong baby at the hospital. The mother belongs to group B, the child to AB, one possible father to B, the other to A. Which man is the most probable father? Who would be the most probable father if the mother belongs to group B, the child to 0, the possible father to A, and the other to AB?


18. A man with normal vision and blood type B marries a woman with normal vision and blood type A. They have two children, a color-blind boy with blood type A and a girl with normal vision and blood type O. What are the genotypes of the parents?


19. An Rh+ man, type A blood, marries an Rh- woman, type B blood. Their first child is Rh+ type A. Could the second child be Rh-, type O?


IV. Dihybrid Crosses


20. If a pure tall, red-flowered plant is crossed with a dwarf white-flowered plant, what will be the appearance of the members of the F1 generation? What will the F2 generation be like in appearance?


21. A man with short eyelashes and a Roman nose marries a woman with long eyelashes and a Roman nose. They have two children, one with long eyelashes and a straight nose and the other with short eyelashes and a Roman nose. By a later marriage with another woman, who also has long eyelashes and a Roman nose, the man has 9 children, all with long eyelashes and Roman noses. What are the genotypes of the man and his two wives?


22. It has been reported that the purple flower color in one species of the Japanese morning glory may be caused by dominant alleles at either one of two separate gene pairs, e.g., A-bb or aaB-. When dominant alleles are present at both gene pairs (A-B-), the color of the flower is blue, and when both are homozygous recessive (aabb), the color is scarlet.


If AAbb was crossed with aaBB the flower color would be blue. If these blue flowers (F1) were crossed, what would be the phenotype and genotype of the F2? If the F1 were crossed to either of the parents, what would be the phenotype and genotype of the F2?

23. If a male white coffee fruit fly were crossed with a female with scalloped wings, what would be the phenotype and genotype of the F1 and F2?


V. Trihybrid Crosses


24. A woman has migraine headaches (homozygous condition), is left-handed, and carries the trait for Tay-Sachs disease. She marries a man who has no headaches, has no trait for Tay-Sachs disease, and is right-handed (however, his mother is left-handed). What are the possible phenotypes of their children?

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