Problem 13 - Biological Science

Question

The smooth feathers on the back of the neck in pigeons can be reversed by a mutation to produce a "crested" appearance in which feathers form a distinctive spike at the back of the head. A pigeon breeder examined offspring produced by a single pair of non-crested birds and recorded the following: 22 non-crested and 7 crested. She then made a series of crosses using offspring from the first cross. When she crossed two of the crested birds, all 20 of the offspring were crested. When she crossed a non-crested bird with a crested bird, 7 offspring were non-crested and 6 were crested. \(\cdot\)For these three crosses, provide genotypes for parents and offspring that are consistent with these results. \(\cdot\)Which allele is dominant?

Step-by-Step Solution

Answer

In the initial cross, the non-crested parent pigeons have the genotype "Cc" (heterozygous). Their offspring show approximately a 3:1 ratio of non-crested ("Cc") to crested ("cc") birds. When two crested birds are crossed, all offspring are crested with the genotype "cc". In the cross between a non-crested and a crested bird, there is approximately a 1:1 ratio of non-crested ("Cc") to crested ("cc") birds. The dominant allele is the non-crested appearance ("C"), while the recessive allele is the crested appearance ("c").

Step 1: Since the single pair of non-crested birds produced both non-crested and crested offspring, it is evident that they were heterozygous for the trait. This means that both parent pigeons had the genotype "Cc". When two birds with the genotype "Cc" are crossed, it is expected that 3 out of 4 offspring will carry at least one dominant allele "C" (non-crested), while 1 out of 4 will carry two recessive alleles "cc" (crested). In this case, 22 out of 29 (about 75%) offspring were non-crested ("Cc") and 7 out of 29 (about 25%) were crested ("cc"). #Step 2: Analyzing the Cross Between Two Crested Birds#

The breeder crossed two crested birds and observed that all the 20 offspring were crested. Since crested birds have the genotype "cc", this cross can be represented as "cc" x "cc", which will always produce offspring with the "cc" (crested) genotype. This observation is consistent with the recessive nature of the crested phenotype. #Step 3: Analyzing the Cross Between Non-Crested and Crested Birds#

Step 2: The breeder crossed a non-crested bird with a crested bird and observed 7 non-crested and 6 crested offspring. Since the non-crested bird is likely heterozygous ("Cc") and the crested bird has the genotype "cc", this cross can be represented as "Cc" x "cc". When these birds are crossed, 1 out of 2 offspring are expected to be non-crested ("Cc") and 1 out of 2 offspring are expected to be crested ("cc"). In this case, 7 out of 13 (about 50%) offspring were non-crested ("Cc") and 6 out of 13 (about 50%) were crested ("cc"). #Step 4: Identifying the Dominant Allele#

The dominant allele is identified as the one that is expressed in the presence of the recessive allele when they are both present in the genotype. In this case, the non-crested appearance is dominant over the crested appearance, which means the allele "C" (non-crested) is dominant and the allele "c" (crested) is recessive.

Key Concepts

Alleles

Alleles are different forms of a gene that are located on the same spot, or locus, on each of a pair of chromosomes. They are responsible for the variation in traits that we observe in organisms. In the case of pigeon breeding, alleles play a crucial role in determining the appearance of the pigeon’s feathers. Specifically, there are two alleles involved in determining whether a pigeon will have a crested or non-crested appearance.
The non-crested appearance is determined by the presence of at least one dominant allele "C." On the other hand, the crested appearance is associated with two recessive alleles "cc." Understanding alleles is essential because they combine in different ways to produce various genetic outcomes, allowing for diverse traits within a species.

Dominant and Recessive Traits

When it comes to genetics, traits are often described as either dominant or recessive. Dominant traits are those that are expressed even when only one copy of the dominant allele is present in the genotype. Recessive traits, however, only manifest when both alleles in the genotype are recessive.
In the exercise, the relationship between crested and non-crested pigeons demonstrates this principle. The non-crested trait is dominant, represented by the allele "C," while the crested trait is recessive, represented by the allele "c." This means that a pigeon will only have a crested appearance when it possesses the genotype "cc." Conversely, as long as at least one "C" allele is present, the pigeon will show the non-crested phenotype.

• Dominant allele: "C" (non-crested)
• Recessive allele: "c" (crested)

Pigeon Breeding

Pigeon breeding is a fascinating process that allows breeders to selectively mate birds to achieve desired traits. In this scenario, a breeder has crossed non-crested and crested pigeons to observe the inheritance patterns of these traits.
Initially, two non-crested birds with heterozygous genotypes (Cc) were crossed, producing a mix of non-crested and crested offspring, indicating the presence of recessive alleles. Another cross involved two crested pigeons (cc), resulting in all offspring also being crested, showing that crested is a recessive trait. Finally, a cross between non-crested (Cc) and crested (cc) pigeons produced both types of offspring in nearly equal numbers, illustrating the inheritance of traits through genotype combinations.
By understanding these principles, breeders can predict the outcomes of future breeding efforts.

Genotype and Phenotype

In genetics, the terms "genotype" and "phenotype" are essential for understanding how traits are expressed in organisms. The genotype refers to the genetic makeup of an organism, particularly with reference to its allelic composition. It dictates which traits can be passed on to the next generation.
In the case of the pigeons, we have different genotypes that result in two distinct phenotypes, or physical appearances: non-crested (CC or Cc) and crested (cc). Thus, the phenotype is the observable expression of the genotype.
It's important to note that two organisms with different genotypes can have the same phenotype if one of them has a dominant trait. For the pigeons, both CC and Cc genotypes lead to the same non-crested phenotype, emphasizing how dominant alleles mask the effects of recessive ones in heterozygotes.

• Genotype: The genetic arrangement (e.g., CC, Cc, or cc)
• Phenotype: The observable characteristics (e.g., crested or non-crested)

Understanding these concepts helps in predicting the visible traits of offspring from particular genetic pairings.