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Chapter 5: Problem 17

When a Chinese hamster with white spots is crossed with another hamster that has no spots, approximately \(1 / 2\) of the offspring have white spots and \(1 / 2\) have no spots. When two hamsters with white spots are crossed, \(2 / 3\) of the offspring possess white spots and \(1 / 3\) have no spots. a. What is the genetic basis of white spotting in Chinese hamsters? b. How might you go about producing Chinese hamsters that breed true for white spotting?

Short Answer

Expert verified
White spotting is likely due to a lethal homozygous \( WW \). Breeding \( Ww \) with \( ww \) maintains consistent white spotting.

Step by step solution

01

Determine the Possible Genotypes

To find the genetic basis of the white spotting trait, let's represent the alleles. Let the allele for white spotting be denoted as \( W \), and the allele for no spotting as \( w \). Based on the information given, white spotting is not present in all offspring, suggesting it is not a dominant simple Mendelian trait.
02

Analyze the Cross of White Spot and Non-spot

When a hamster with white spots (\( Ww \)) is crossed with a hamster with no spots (assumed to be \( ww \)), the phenotypic ratio according to the problem is approximately \( \frac{1}{2} \) with white spots and \( \frac{1}{2} \) with no spots. This suggests a simple Mendelian 1:1 ratio, indicating the hamster with white spots is heterozygous \( Ww \).
03

Evaluate the Cross between Two White Spot Hamsters

When two hamsters with white spots are crossed, the ratio of offspring is \( \frac{2}{3} \) with white spots and \( \frac{1}{3} \) with no spots. If both parents are \( Ww \), according to Mendelian genetics, the expected genotypic ratio would be \( \frac{1}{4} WW \), \( \frac{1}{2} Ww \), and \( \frac{1}{4} ww \). The phenotypic ratio of white spotted to non-spotted would be 3:1. However, because \( WW \) might result in not viable (lethal), the actual viable offspring are \( \frac{1}{3} Ww \) with spots, and \( \frac{1}{3} ww \) no spots, confirming the given phenotypic ratio \( \frac{2}{3} \) with white spots (only \( Ww \) is viable with spots).
04

Propose Genetic Basis

The genetic basis for white spotting in Chinese hamsters is likely due to incomplete dominance or a lethal homozygous condition \( WW \), which results in \( W \) being non-viable. Hence, \( Ww \) shows white spots and \( ww \) shows no spots.
05

Producing Hamsters that Breed True for White Spotting

To create a line of hamsters that will always produce offspring with white spots, focus on the heterozygous \( Ww \) genotype, but given the lethal homozygosity, true breeding isn鈥檛 possible. Instead, perpetuate \( Ww \) by continued careful breeding of \( Ww \) with \( ww \) to get a consistent pattern of 1:1 observed.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Mendelian Genetics
Mendelian Genetics is named after Gregor Mendel, a monk who conducted experiments on pea plants to understand how traits are passed down through generations.
His research forms the basis of classical genetics and explains how traits are inherited through distinct units known as genes.
In essence, Mendelian genetics revolves around the idea that:
  • Traits are inherited as discrete units.
  • Each trait is governed by alleles, one from each parent.
  • The combination of alleles determines the trait observed in the offspring.
In simpler terms, each parent contributes one allele (a version of a gene) for every trait to the offspring. The way these alleles interact explains why certain traits are either expressed or hidden. For example, the white spotting in the Chinese hamsters follows these principles, where the observed trait (spots or no spots) is determined by the alleles inherited from each parent. Understanding these fundamental concepts allows us to predict the outcomes of genetic crosses more effectively.
Alleles
An allele is a form of a gene that is responsible for the variations in inherited characteristics. Typically, organisms possess two alleles for each gene, one inherited from each parent.
These alleles can be classified as:
  • Dominant: An allele that expresses its trait even if only one copy is present. It's usually represented with a capital letter, such as "W" for white spotting.
  • Recessive: An allele that only expresses its trait when two copies are present. It's represented with a lowercase letter, such as "w" for no spotting in hamsters.
In the context of the white spotting condition in hamsters, alleles play a crucial role. When hamsters with different spotting traits mate, their offspring get a combination of spotting alleles. For example, a hamster with "Ww" genotype has both a dominant (W) allele and a recessive (w) allele, leading to visible white spots.
Moreover, when both alleles are the same, such as "ww", the resulting phenotype reflects the recessive trait 鈥� in this instance, no white spots.
Heterozygous
A heterozygous genotype occurs when an organism has two different alleles for a particular gene. This condition is essential for genetic diversity because it allows for mixing different traits from both parents.
To better understand how it functions, consider:
  • A heterozygous organism will have one dominant and one recessive allele, like "Ww" in Chinese hamsters.
  • This combination often results in the phenotype controlled by the dominant allele, although exceptions exist鈥攕uch as incomplete dominance or co-dominance.
  • Being heterozygous increases variability in a population, as it creates diversity in the offspring.
In the Chinese hamsters' example, when a heterozygous hamster ( $Ww$ ) is crossed with a homozygous recessive ( $ww$ ), the offspring's probabilities were seen to follow the classic Mendelian 1:1 ratio for showing traits.
The concept of heterozygosity is crucial for understanding how breeders develop certain traits within species, as it clearly influences the potential phenotypic traits in the offspring.

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Most popular questions from this chapter

The \(L^{\mathrm{M}}\) and \(L^{\mathrm{N}}\) alleles at the MN blood-group locus exhibit codominance. Give the expected genotypes and phenotypes and their ratios in progeny resulting from the following crosses. a. \(L^{\mathrm{M}} L^{\mathrm{M}} \times L^{\mathrm{M}} L^{\mathrm{N}}\) b. \(L^{\mathrm{N}} L^{\mathrm{N}} \times L^{\mathrm{N}} L^{\mathrm{N}}\) c. \(L^{\mathrm{M}} L^{\mathrm{N}} \times L^{\mathrm{M}} L^{\mathrm{N}}\) d. \(L^{\mathrm{M}} L^{\mathrm{N}} \times L^{\mathrm{N}} L^{\mathrm{N}}\) e. \(L^{\mathrm{M}} L^{\mathrm{M}} \times L^{\mathrm{N}} L^{\mathrm{N}}\)

Match each of the following terms with its correct definition (parts \(a\) through \(i\) ) Phenocopy __________ Pleiotropy __________ Polygenic trait ____________ Penetrance _________ Sex-limited trait _________ Genetic maternal effect __________ Genomic imprinting __________ Sex-influenced trait _________ Anticipation __________ a. The percentage of individuals with a particular genotype that express the expected phenotype. b. A trait determined by an autosomal gene that is more easily expressed in one sex. c. A trait determined by an autosomal gene that is expressed in only one sex. d. A trait that is determined by an environmental effect and that has the same phenotype as a genetically determined trait. e. A trait determined by genes at many loci. f. The expression of a trait is affected by the sex of the parent that transmits the gene to the offspring. g. The trait appears earlier or is more severe in succeeding generations. h. A gene affects more than one phenotype. i. The genotype of the maternal parent influences the phenotype of the offspring.

Turkeys have black, bronze, or black-bronze plumage. Examine the results of the following crosses: $$ \begin{array}{ll} {\text { Parents }} & {\text { Offspring }} \\ \hline \text { Cross 1: black and bronze } & \text { all black } \\ \text { Cross 2: black and black } & 3 / 4 \text { black, } 1 / 4 \text { bronze } \\ \text { Cross 3: black-bronze and } & \text { all black-bronze } \\ \text { black-bronze } & \\ \text { Cross 4: black and bronze } & 1 / 2 \text { black, } 1 / 4 \text { bronze, } 1 / 4 \\ & \text { black-bronze } \\ \text { Cross 5: bronze and black- } & 1 / 2 \text { bronze, } 1 / 2 \text { black-bronze } \\ \text { bronze } & \\ \text { Cross 6: bronze and bronze } &\\\ &\text {4 bronze,} \text {1 / 4 black-bronze} \end{array} $$ Do you think these differences in plumage arise from incomplete dominance between two alleles at a single locus? If yes, support your conclusion by assigning symbols to each allele and providing genotypes for all turkeys in the crosses. If your answer is no, provide an alternative explanation and assign genotypes to all turkeys in the crosses.

In unicorns, two autosomal loci interact to determine the type of tail. One locus controls whether a tail is present at all; the allele for a tail \((T)\) is dominant to the allele for tailless \((t)\). If a unicorn has a tail, then alleles at a second locus determine whether the tail is curly or straight. Farmer Baldridge has two unicorns with curly tails: when he crosses them, \(1 / 2\) of the progeny have curly tails, \(1 / 4\) have straight tails, and \(1 / 4\) do not have a tail. Give the genotypes of the parents and progeny in Farmer Baldridge's cross. Explain how he obtained the 2: 1: 1 phenotypic ratio in his cross.

Palomino horses have a golden yellow coat, chestnut horses have a brown coat, and cremello horses have a coat that is almost white. A series of crosses between the three different types of horses produced the following offspring: $$ \begin{array}{ll} {\text { Cross }} & {\text { Offspring }} \\ \hline \text { palomino } \times & 13 \text { palomino, } 6 \text { chestnut, } 5 \\ \text { palomino } & \text { cremello } \\ \text { chestnut } \times \text { chestnut } & 16 \text { chestnut }\\\ \text { cremello } \times \text { cremello } & 13 \text { cremello }\\\ \text { palomino } \times \text { chestnut } & 8 \text { palomino, } 9 \text { chestnut }\\\ \text { palomino } \times \text { cremello } & 11 \text { palomino, } 11 \text { cremello }\\\ \text { chestnut } \times \text { cremello } & 23 \text { palomino }\\\ \end{array} $$ a. Explain the inheritance of the palomino, chestnut, and cremello phenotypes in horses. b. Assign symbols for the alleles that determine these phenotypes, and list the genotypes of all parents and offspring given in the preceding table.
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