Dihybrid Crosses 1. In man, assume that spotted skin (S) is dominant over non-spotted skin (s) and that wooly hair (W) is dominant over non-wooly hair (w). Cross a marriage between a heterozygous spotted, non-wooly man with a heterozygous wooly-haired, non-spotted woman. Give genotypic and phenotypic ratios of offspring. 2. In horses, black is dependent upon a dominant gene, B, and chestnut upon its recessive allele, b. The trotting gait is due to a dominant gene, T, the pacing gait to its recessive allele, t. If a homozygous black pacer is mated to a homozygous chestnut trotter, what will be the appearance of the F1 generation? 3. In mice, the ability to run normally is a dominant trait. Mice with this trait are called running mice (R). The recessive trait causes mice to run in circles only. Mice with this trait are called waltzing mice (r). Hair color is also inherited in mice. Black hair (B) is dominant over brown hair (b). For each of the following problems, determine the parent genotypes, determine possible gametes then construct a Punnet square to solve. a. Cross a heterozygous running, heterozygous black mouse with a homozygous running, homozygous black mouse. List all of the possible phenotypes from this cross. Pedigrees Shaded individuals have Huntington’s Disease 1. Write the generation on the pedigree numbers (roman numerals). 2. Which members of the family above are afflicted with Huntington’s Disease? 3. There are no carriers for Huntington’s Disease- you either have it or you don’t. With this in mind, is Huntington’s disease caused by a dominant or recessive trait?
The Correct Answer and Explanation is:
Dihybrid Crosses
1. Spotted Skin and Wooly Hair in Humans
- Traits:
- Spotted skin (S) is dominant to non-spotted skin (s)
- Wooly hair (W) is dominant to non-wooly hair (w)
- Man’s genotype: Heterozygous spotted, non-wooly → Ssww
- Woman’s genotype: Heterozygous wooly-haired, non-spotted → ssWw
- Gametes from Man (Ssww): Sw, sw
- Gametes from Woman (ssWw): sW, sw
| sW | sw | |
|---|---|---|
| Sw | SsWw | Ssww |
| sw | ssWw | ssww |
Genotypic outcomes:
- SsWw: 1
- Ssww: 1
- ssWw: 1
- ssww: 1
Genotypic ratio: 1:1:1:1
Phenotypic outcomes:
- SsWw → Spotted, Wooly
- Ssww → Spotted, Non-wooly
- ssWw → Non-spotted, Wooly
- ssww → Non-spotted, Non-wooly
Phenotypic ratio: 1 Spotted Wooly : 1 Spotted Non-wooly : 1 Non-spotted Wooly : 1 Non-spotted Non-wooly
2. Horses: Coat Color and Gait
- B = Black, b = Chestnut
- T = Trotting, t = Pacing
- Parents:
- Homozygous black pacer → BBtt
- Homozygous chestnut trotter → bbTT
- F1 Genotype: All offspring = BbTt
- Phenotype:
- All offspring are Black (due to B) and Trotting (due to T)
Answer:
- F1 appearance: All Black Trotters
3. Mice: Running & Hair Color
- Running (R) is dominant over Waltzing (r)
- Black hair (B) is dominant over Brown hair (b)
- Cross:
- Heterozygous running, heterozygous black → RrBb
- Homozygous running, homozygous black → RRBB
- Gametes:
- RrBb → RB, Rb, rB, rb
- RRBB → All RB
- Punnett Square:
| RB (from RRBB) | |
|---|---|
| RB | RRBB |
| Rb | RRBb |
| rB | RrBB |
| rb | RrBb |
Genotypes:
- RRBB, RRBb, RrBB, RrBb
All phenotypes: Running, Black
Answer:
- All offspring will be Running with Black hair
Pedigrees – Huntington’s Disease
1. Generations:
- Use Roman numerals for each generation:
- Top row: I
- Second row: II
- Third row: III, and so on.
2. Afflicted Members:
- Shaded individuals are affected by Huntington’s Disease.
3. Is Huntington’s Disease Dominant or Recessive?
Answer: Huntington’s Disease is caused by a dominant trait.
Explanation:
Huntington’s Disease is a neurodegenerative genetic disorder that is inherited in an autosomal dominant pattern. This means that a person only needs one copy of the mutant gene to develop the disease. In a pedigree, this pattern is often easily recognizable because the disease appears in every generation and affects both males and females equally.
There are no carriers of Huntington’s Disease. If someone carries the gene, they will eventually develop the condition, typically in middle adulthood. This is different from recessive disorders, where individuals can carry a defective gene without showing symptoms because they also possess one normal gene.
In dominant inheritance, when an individual with Huntington’s (Hh) has children with someone who does not have the gene (hh), there is a 50% chance that each child will inherit the disease. If a person in the pedigree has Huntington’s Disease and at least one of their parents also had it, this supports dominant inheritance.
Recessive conditions, by contrast, often skip generations, and two unaffected parents can have an affected child if both are carriers. In Huntington’s pedigrees, affected individuals do not skip generations, confirming its dominant nature.
Furthermore, since Huntington’s is linked to a mutation in the HTT gene on chromosome 4, which is autosomal, the trait is not sex-linked, again supporting autosomal dominant inheritance. Genetic testing and family history are the main tools for diagnosis and confirmation.
