INTRODUCTION TO MONOHYBRID CROSSESS (Single trait Inheritance)
Problem: In Humans, the color of the skin is determined by a dominant gene which controls the production of the pigment melanin (brown in color). Individuals who are lacking the dominant gene are not capable of producing an enzyme called tyrosinase which is needed to produce melanin. This genetic disorder is referred to as ALBINISM and occurs in about 1 in every 20,000 human births in the United States.
1. What TRAIT is being considered in this problem? ______
2. List the 2 possible PHENOTYPES for the trait under study:
A. ______B. ______
3. Use a capital "A" to represent the dominant ALLELE for the trait and a lower-case letter "a" to represent the recessive ALLELE. (ALLELE refers to a pair of genes, found on homologous chromosomes, which both control the same trait.) List the 3 possible GENOTYPES, their correct genetic descriptions and their corresponding PHENOTYPES for the trait under study:
Genotypes / Genetic Descriptions / PhenotypesA.
B.
C.
The inheritance of albinism in fruit flies is the same as that in humans. We will now demonstrate the CROSS or MATING between a handsome, homozygous dominant male fruit fly and his lovely albino bride.
4. List the PHENOTYPES of our parent flies: P1 = ______x ______
5. List the GENOTYPES of our parents: P1 = ______x ______
During MEIOSIS, the homologous chromosomes that carry the alleles for pigmentation will separate or segregate into the developing monoploid gametes (sperm and egg).
6. List the possible alleles each sperm could have: _____ or ______
7. List the possible alleles each egg could have: _____ or ______
Using a PUNNETT SQUARE in the space below, diagram the cross between our parent flies from above. Remember, the mating of our P1 flies will result in the first generation of offspring or the F1 generation (F= filial or family).
8. List all possible PHENOTYPES of the F1 (offspring): ______
9. List all possible GENOTYPES of the F1: ______
10. Give the PHENOTYPIC RATIO: ______Give the GENOTYPIC RATIO: ______
11. Give the PROBABILITES or CHANCES of obtaining:
A. / A pigmented fly in the F1: / B. / An albino fly in the F1:C. / A homozygous dominant fly: / D. / A heterozygous fly:
E. / A homozygous recessive fly:
12. If 500 offspring result from this mating, how many would you expect to be?
A. Pigmented: ______B. Albino: ______C. Hybrid: ______
Problem: In order to better understand the inheritance of ALBINISM in fruit flies, and therefore humans, we will now mate two of the offspring from the F1 generation. We will have better results if you choose a male fly and a female. These two flies will now be referred to as our P2 or second parental cross.
13. List the PHENOTYPE of our P2: ______x ______
14. List the GENOTYPE of our P2: ______x ______
15. List the GENOTYPES of the Male's sperm cells: ______and ______
16. List the GENOTYPES of the female's egg cells: ______and ______
17. Give the PHENOTYPIC RATIO: ______Give the GENOTYPIC RATIO: ______
18. Give the PROBABILITIES or CHANCES of obtaining:
A. / A pigmented fly in the F2: / B. / An albino fly in the F2:C. / A homozygous dominant fly: / D. / A heterozygous fly:
E. / A homozygous recessive fly:
19. If 200 offspring result from this mating, how many would you expect to be?
A. Pigmented: ______B. Albino: ______C. Hybrid: ______