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19. In humans, brown eyes (H) are dominant over blue (b). A brown-eyed man marries a blue eyed woman and they have three children, two are brown-eyed and one is blue-eyed. Draw the Punnett square showing this marriage. What is the man's genotype? What are the genotypes of the children?

20. In mice, the trait for brown fur is dominant to black fur. The F1 cross crosses a homozygous dominant brown mouse with a homozygous recessive black mouse. What will be the results of the F2 cross?

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19. Punnett Square for Brown-Eyed Man and Blue-Eyed Woman

Genotypes:

  • Brown-eyed man: Since he has brown eyes, he can either be homozygous (HH) or heterozygous (Hb). However, since he has blue-eyed children, he must be heterozygous (Hb).
  • Blue-eyed woman: She is homozygous recessive (bb).

Punnett Square Setup for Hb x bb:

      b      b
   ----------------
H |   Hb    Hb  |
   |                |
h |   bb    bb  |

Punnett Square Results:

  • Hb (Brown) = 2 (for brown-eyed children)
  • bb (Blue) = 2 (for blue-eyed children)

Children's Genotypes:

  • 2 children are Hb (brown-eyed)
  • 1 child is bb (blue-eyed)

Conclusion:

  • The man's genotype is Hb (heterozygous).
  • The genotypes of the children are: 2 Hb (brown-eyed) and 1 bb (blue-eyed).

20. F1 and F2 Cross Results in Mice

Initial Cross:

  • Brown fur (dominant, B) x Black fur (recessive, b)
  • Homozygous dominant brown mouse: BB
  • Homozygous recessive black mouse: bb

F1 Generation:

  • All offspring will be Bb (brown fur). Since brown is dominant, all F1 offspring will have brown fur.

F2 Cross: To find the results of the F2 cross, we take two F1 offspring (Bb x Bb) and perform the cross:

Punnett Square Setup for Bb x Bb:

      B      b
   ----------------
B |   BB    Bb  |
   |                |
b |   Bb    bb  |

Punnett Square Results:

  • BB (Homozygous brown) = 1
  • Bb (Heterozygous brown) = 2
  • bb (Homozygous black) = 1

Final F2 Generation Ratio:

  • 1 BB : 2 Bb : 1 bb
  • This translates to:
    • 3 (brown fur) : 1 (black fur)

So, the phenotypic ratio in the F2 generation will be 3 brown fur mice to 1 black fur mouse.