Textbook Question
Write a short essay that explains why multiple and lethal alleles often result in a modification of the classic Mendelian monohybrid and dihybrid ratios.
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Ch. 4 - Extensions of Mendelian Genetics
Klug12th EditionConcepts of Genetics ISBN: 9780135564776
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Table of contents
- Ch. 1 - Introduction to Genetics17
- Ch. 2 - Mitosis and Meiosis36
- Ch. 3 - Mendelian Genetics51
- Ch. 4 - Extensions of Mendelian Genetics74
- Ch. 5 - Chromosome Mapping in Eukaryotes51
- Ch. 6 - Genetic Analysis and Mapping in Bacteria and Bacteriophages46
- Ch. 7 - Sex Determination and Sex Chromosomes33
- Ch. 8 - Chromosome Mutations: Variation in Number and Arrangement40
- Ch. 9 - Extranuclear Inheritance31
- Ch. 10 - DNA Structure and Analysis43
- Ch. 11 - DNA Replication and Recombination44
- Ch. 12 - DNA Organization in Chromosomes30
- Ch. 13 - The Genetic Code and Transcription54
- Ch. 14 - Translation and Proteins47
- Ch. 15 - Gene Mutation, DNA Repair, and Transposition41
- Ch. 16 - Regulation of Gene Expression in Bacteria29
- Ch. 17 - Transcriptional Regulation in Eukaryotes41
- Ch. 18 - Post-transcriptional Regulation in Eukaryotes33
- Ch. 19 - Epigenetics33
- Ch. 20 - Recombinant DNA Technology44
- Ch. 21 - Genomic Analysis36
- Ch. 22 - Applications of Genetic Engineering and Biotechnology36
- Ch. 23 - Developmental Genetics37
- Ch. 24 - Cancer Genetics34
- Ch. 25 - Quantitative Genetics and Multifactorial Traits54
- Ch. 26 - Population and Evolutionary Genetics45
Chapter 4, Problem 3
In shorthorn cattle, coat color may be red, white, or roan. Roan is an intermediate phenotype expressed as a mixture of red and white hairs. The following data were obtained from various crosses:
How is coat color inherited? What are the genotypes of parents and offspring for each cross?
Verified step by step guidance1
Step 1: Identify the inheritance pattern. The data shows that crossing red with red produces all red offspring, white with white produces all white offspring, and red with white produces all roan offspring. Roan crossed with roan produces a 1:2:1 ratio of red, roan, and white. This suggests incomplete dominance, where neither allele is completely dominant, and the heterozygote shows an intermediate phenotype.
Step 2: Assign genotypes to the coat colors. Let the allele for red be \( R \) and the allele for white be \( W \). Since red crossed with white produces all roan, roan must be the heterozygous genotype \( RW \). Thus, red is \( RR \), white is \( WW \), and roan is \( RW \).
Step 3: Analyze each cross using genotypes. For red x red (\( RR \times RR \)), all offspring are \( RR \) (red). For white x white (\( WW \times WW \)), all offspring are \( WW \) (white). For red x white (\( RR \times WW \)), all offspring are \( RW \) (roan).
Step 4: For roan x roan (\( RW \times RW \)), use a Punnett square to determine offspring genotypes. The possible genotypes are \( RR \), \( RW \), and \( WW \) in a 1:2:1 ratio, corresponding to red, roan, and white phenotypes respectively.
Step 5: Summarize the inheritance: coat color in shorthorn cattle is controlled by incomplete dominance with two alleles, \( R \) and \( W \). The genotypes and phenotypes are: \( RR \) = red, \( WW \) = white, and \( RW \) = roan.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Incomplete Dominance
Incomplete dominance occurs when neither allele is completely dominant over the other, resulting in a heterozygous phenotype that is intermediate between the two homozygous phenotypes. In shorthorn cattle, red and white coat colors show incomplete dominance, producing roan offspring with a mix of red and white hairs.
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Variations on Dominance
Genotype and Phenotype Relationships
Genotypes represent the genetic makeup of an organism, while phenotypes are the observable traits. Understanding how different allele combinations (e.g., RR, RW, WW) correspond to coat colors (red, roan, white) is essential to predict offspring outcomes from various crosses.
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Punnett Square and Mendelian Ratios
Punnett squares are tools used to predict the genotypic and phenotypic ratios of offspring from parental crosses. The observed ratios in roan x roan crosses (1/4 red, 1/2 roan, 1/4 white) reflect Mendelian inheritance patterns consistent with incomplete dominance.
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Related Practice
Textbook Question
In mice, a short-tailed mutant was discovered. When it was crossed to a normal long-tailed mouse, 4 offspring were short-tailed and 3 were long-tailed. Two short-tailed mice from the F1 generation were selected and crossed. They produced 6 short-tailed and 3 long-tailed mice. These genetic experiments were repeated three times with approximately the same results. What genetic ratios are illustrated? Hypothesize the mode of inheritance and diagram the crosses.
Textbook Question
In foxes, two alleles of a single gene, P and p, may result in lethality (PP), platinum coat (Pp), or silver coat (pp). What ratio is obtained when platinum foxes are interbred? Is the P allele behaving dominantly or recessively in causing (a) lethality; (b) platinum coat color?
Textbook Question
The discussion centers on extensions and modifications of Mendelian principles and ratios. In the process, we encountered many opportunities to consider how this information was acquired. On the basis of these discussions, what answers would you propose to the following fundamental questions?
For genes whose expression seems to be tied to the sex of individuals, how do we know whether a gene is X-linked in contrast to exhibiting sex-limited or sex-influenced inheritance?
Textbook Question
List all possible genotypes for the A, B, AB, and O phenotypes. Is the mode of inheritance of the ABO blood types representative of dominance, recessiveness, or codominance?
Textbook Question
The discussion centers on extensions and modifications of Mendelian principles and ratios. In the process, we encountered many opportunities to consider how this information was acquired. On the basis of these discussions, what answers would you propose to the following fundamental questions?
How do we know that specific genes are located on the sex-determining chromosomes rather than on autosomes?
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