Complete the following practice problems. There are six steps to completing a dihybrid cross.

Source: Gray and White rabbit, Common Places.com

1. In a certain breed of rabbits, gray fur is dominant over white fur, and black eyes are dominant over red eyes. Cross a rabbit that is homozygous dominant for fur and eye color with a rabbit that is homozygous recessive for fur and eye color.

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   Fur Color:
   Dominant allele: G=gray
   Recessive allele: g=white
   
   Eye Color:
   Dominant allele: B=black
   Recessive allele: b=red
   
   Cross: GGBB x ggbb
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   Check Your Answer

   Fur Color: Dominant allele: G=gray Recessive allele: g=white Eye Color: Dominant allele: B=black Recessive allele: b=red Cross: GGBB x ggbb Genotypes: 100% GgBb Phenotypes: 100% gray fur and black eyes

   


Source: Guinea Pigs, Pets 4 homes and Pet Mania

2. A guinea pig breeder crosses a Texel guinea pig with heterozygous dark eyes and a homozygous dominant straight fur guinea pig with red eyes. Texel guinea pigs have curly fur, and this is a recessive trait. Dark eyes are a dominant allele in guinea pigs. What are the possible genotypes and phenotypes of the offspring?

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   Fur Texture:
   Dominant allele: S=straight
   Recessive allele: s=curly
   
   Eye Color:
   Dominant allele: D=dark
   Recessive allele: d=red
   
   Cross: ssDd x SSdd
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   Check Your Answer

   Fur Texture: Dominant allele: S=straight Recessive allele: s=curly Eye Color: Dominant allele: D=dark Recessive allele: d=red Cross: ssDd x SSdd Genotypes: 50% SsDd 50% Ssdd Phenotypes: 50% straight fur and dark eyes 50% straight fur and red eyes

   


Source: Hairline shape, Groovy Post


3. In humans, a widow's peak allele is dominant, and a straight hairline allele is recessive, and the allele for attached earlobes is recessive, while having free earlobes is dominant. Cross two individuals that are heterozygous for each trait.

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   Hairline:
   Dominant allele: W=widow's peak
   Recessive allele: w=straight
   
   Earlobes:
   Dominant allele: F=free earlobes
   Recessive allele: f=attached
   
   Cross: WwFf x WwFf
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   Check Your Answer

   Hairline: Dominant allele: W=widow's peak Recessive allele: w=straight Earlobes: Dominant allele: F=free earlobes Recessive allele: f=attached Cross: WwFf x WwFf Genotypes: Genotypes: 1 over 16 1 16 WWFF 2 over 16 2 16 WWFf 2 over 16 2 16 WwFF 4 over 16 4 16 WwFf 1 over 16 1 16 WWff 2 over 16 2 16 Wwff 1 over 16 1 16 wwFF 2 over 16 2 16 wwFf 1 over 16 1 16 wwff Phenotypes: 9 over 16 9 16 Widow's peak and free earlobes 3 over 16 3 16 Widow's peak and attached earlobes 3 over 16 3 16 Straight hairline and free earlobes 1 over 16 1 16 Straight hairline and attached earlobes

   



Source: Fun Experiments, Michigan Reach Out

4. In humans, the allele for having dimples is dominant, while the allele for no dimples is recessive; the ability to roll your tongue is dominant, while not being able to roll your tongue is recessive. Cross a male who is homozygous dominant for dimples and cannot roll his tongue with a female who has no dimples and is homozygous dominant for tongue rolling.

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   Dimples:
   Dominant allele: D=dimples
   Recessive allele: d=no dimples
   
   Tongue Rolling:
   Dominant allele: R=roll tongue
   Recessive allele: r=cannot roll tongue
   
   The male’s genotype would be DD for dimples and rr for tongue rolling since he cannot roll his tongue. The female has no dimples, so her genotype for no dimples must be dd. She has the homozygous dominant genotype or RR for tongue rolling.
   
   Cross: DDrr x ddRR
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   Check Your Answer

   Dimples: Dominant allele: D=dimples Recessive allele: d=no dimples Tongue Rolling: Dominant allele: R=roll tongue Recessive allele: r=cannot roll tongue Cross: DDrr x ddRR Genotypes: 100% DdRr Phenotypes: 100% Dimples and ability to roll tongue