Textbook Question
Organisms with the genotypes AABbCcDd and AaBbCcDd are crossed. What are the expected proportions of the following progeny?
A–B–ccdd
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3. Extensions to Mendelian Inheritance
Understanding Independent Assortment
1:10 minutesProblem 27b
Textbook Question
The wild-type (normal) fruit fly, Drosophila melanogaster, has straight wings and long bristles. Mutant strains have been isolated that have either curled wings or short bristles. The genes representing these two mutant traits are located on separate chromosomes. Carefully examine the data from the following five crosses.
Assign gene symbols and, for each cross, determine the genotypes of the parents.
Verified step by step guidance1
Step 1: Assign gene symbols to the traits. Since the wild-type fruit fly has straight wings and long bristles, assign dominant alleles for these traits, for example, let 'W' represent the dominant allele for straight wings and 'w' for curled wings; similarly, let 'B' represent the dominant allele for long bristles and 'b' for short bristles.
Step 2: Understand that the two genes are on separate chromosomes, so they assort independently according to Mendel's law of independent assortment. This means you can analyze each gene separately and then combine the results.
Step 3: For each cross, analyze the phenotypes of the offspring to infer the genotypes of the parents. Use Punnett squares to predict the expected offspring genotypes and phenotypes based on different possible parental genotypes.
Step 4: Compare the observed offspring phenotypes with the expected ratios from your Punnett squares to determine the most likely genotypes of the parents for each cross. Remember that dominant phenotypes can be either homozygous dominant or heterozygous, while recessive phenotypes are homozygous recessive.
Step 5: Summarize the genotypes of the parents for each cross using the gene symbols assigned, indicating whether each parent is homozygous or heterozygous for each gene based on the offspring data.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Wild-type and Mutant Alleles
Wild-type alleles represent the normal, most common form of a gene in a population, while mutant alleles are variants that cause altered traits. In genetics, wild-type traits are often denoted by uppercase letters and mutants by lowercase. Understanding these distinctions helps in assigning gene symbols and predicting phenotypes.
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Mendelian Inheritance and Genotype Determination
Mendelian inheritance principles explain how alleles segregate and assort independently during gamete formation. By analyzing offspring phenotypes from specific crosses, one can deduce the genotypes of the parents using Punnett squares and segregation ratios, especially when genes are on separate chromosomes and assort independently.
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Independent Assortment and Chromosomal Location
Genes located on separate chromosomes assort independently according to Mendel’s second law. This means the inheritance of one gene does not affect the inheritance of another. Recognizing this allows for the prediction of offspring phenotypes and genotypes in dihybrid crosses involving traits like wing shape and bristle length.
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