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Magazine Chapter 5: Problem 45
Long ears in some dogs are an autosomal dominant trait. Two dogs mate and produce a litter in which \(75 \%\) of the puppies have long ears. Of the dogs with long ears in this litter, \(1 / 3\) are known to be phenocopies. What are the most likely genotypes of the two parents of this litter?
Short Answer
Expert verified
The most likely genotypes of the two parents are Ll and Ll.
Step by step solution
01
Understand the Dominant Trait
Long ears in dogs are considered an autosomal dominant trait. This means that if a dog has at least one allele for long ears (L), it will show the phenotype of having long ears. A dog with short ears would have a recessive genotype (ll).
02
Analyze the Given Data
We know that 75% of the puppies in the litter have long ears. Furthermore, 1/3 of these are phenocopies, meaning they appear to have the trait but genetically they do not have the dominant allele (L).
03
Calculate True Long-Eared Puppies
Since 1/3 of the long-ear phenotype puppies are phenocopies, this means only 2/3 of the long-ear phenotype are genuinely carrying the L allele. Let鈥檚 calculate: if 75% of the litter has long ears, then 2/3 of these are true long-ear genotype puppies, giving us 50% (since 2/3 of 75% is 50%).
04
Determine Genotypes of Puppies
The true long-eared puppies must have at least one L allele. The observed phenotype frequencies suggest that 50% are either LL or Ll. With 75% showing as long-eared but only 50% genetically L, we need to consider the genetic distribution from the parents.
05
Determine Possible Parent Genotypes
For 50% of the offspring to be genuinely long-eared (genotype L_), one parent must be Ll, as they are contributing to the phenocopies by not having only L alleles. The other parent must also have an L allele to maintain the occurrence of long ears in 75% of the puppies. Therefore, the other parent could be either LL or Ll.
06
Evaluate Parent Combinations
The likely combination for parents that results in 75% long-eared puppies, where 1/3 are phenocopies, is that both parents are Ll. This combination allows for a mix of Ll, LL, and ll phenotypes, explaining both the presence of phenocopies and the genetic long-ear puppies.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Autosomal Dominant Trait
In genetics, an autosomal dominant trait is one where only a single copy of a dominant allele is necessary for the trait to manifest in an individual's phenotype. Consider long ears in dogs, which are an example of an autosomal dominant trait. This means if a dog inherits the dominant allele (L) from one or both parents, the dog will exhibit long ears. The dominance of the trait ensures that it can be prevalent even if only one parent contributes the allele.
- A dominant allele will mask the presence of a recessive allele.
- If a dominant allele is present, the phenotype associated with it will be expressed.
- To not express the trait, an individual must inherit two recessive alleles (ll in this case).
Phenocopy
Phenocopies can be quite fascinating in the study of genetics. A phenocopy occurs when an individual displays a trait that seems to be genetically determined, but actually, it arises from environmental factors or other non-genetic influences. In the given problem, 1/3 of the long-eared puppies are phenocopies.
- These puppies appear to have long ears, not due to having the dominant allele but due to some other cause.
- The presence of phenocopies can complicate genetic analysis because the phenotype does not match the expected genotype.
Genotype
The term genotype refers to the actual genetic makeup of an organism鈥攕pecifically, the alleles it carries. In the case of our exercise, the genotypes involved are combinations of the alleles for ear length (L for long ears and l for short ears).
- An individual with genotype LL carries two dominant alleles, leading to a long-eared phenotype.
- A dog with genotype Ll also has long ears but has a recessive allele as well.
- The genotype ll will result in a short-eared phenotype.
Allele
Alleles are different forms of a gene found at the same place on a chromosome. They can be dominant or recessive. This variety in alleles can bring about different traits in organisms. In long-eared dogs, we primarily consider the alleles "L" (dominant) and "l" (recessive).
- Dominant alleles (L) will determine the phenotype when present.
- Recessive alleles (l) will only express their phenotype if the dominant allele is absent (ll).
- Puppies inherit one allele from each parent, giving a combination that determines ear length.
Phenotype
The phenotype of an organism is the observable expression of its genotype. It includes physical appearance, like the length of a dog's ears. The phenotypes we see result from the interaction of an organism鈥檚 genetic make-up and environmental factors.
- Puppies with an L in their genotype will have a long-eared phenotype.
- Those with a genotype of ll will have short ears, illustrating a recessive phenotype.
- Despite its genotype, a phenocopy may show a phenotype similar to long ears without actually having the dominant allele (L).
