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Difficulty: HardBasic Genetics Terminology and Concepts

In a genetic analysis of a plant species, a researcher observes that crossing a true-breeding round-seeded plant with a true-breeding wrinkled-seeded plant yields offspring that all produce round seeds. When these offspring self-pollinate, the resulting progeny exhibit both round and wrinkled seeds. Which statement correctly identifies the genetic terms for the hereditary units involved and their physiological state in the first-generation offspring?

  1. The hereditary units are alleles of a single gene occupying the same chromosome locus, and the first-generation offspring are heterozygous.Answer
  2. B
    The hereditary units are non-allelic genes on separate chromosomes, and the first-generation offspring are homozygous dominant.
  3. C
    The hereditary units are sister chromatids expressing codominance, and the first-generation offspring are homozygous recessive.
  4. D
    The hereditary units are homologous chromosomes governing continuous variation, and the first-generation offspring are hemizygous.

Answer

The hereditary units are alleles of a single gene occupying the same chromosome locus, and the first-generation offspring are heterozygous.
Alternative versions of a gene located at the exact same locus on homologous chromosomes are called alleles. Crossing two true-breeding parents with contrasting phenotypes (RR×rrRR \times rr) yields F1F_1 offspring that inherit one allele from each parent (RrRr). Because these offspring possess two different alleles for the gene, their genetic condition is heterozygous, and because the round trait completely hides the wrinkled trait, it shows complete dominance.

Step-by-Step Solution

1
Identify the genetic term for alternative forms of a gene controlling seed shape.
Alternative forms of a gene that occupy identical positions (loci) on homologous chromosomes are called alleles.
The round and wrinkled traits represent variations of a single gene sequence governing seed texture.
2
Determine the genetic state of the first-generation (F1F_1) offspring.
The F1F_1 offspring inherit one dominant allele (RR) from the round parent and one recessive allele (rr) from the wrinkled parent, forming genotype RrRr (heterozygous).
True-breeding parental lines are homozygous (RRRR and rrrr), so their offspring must receive two distinct alleles for that gene locus.
3
Evaluate the mode of gene expression displayed.
Since all F1F_1 plants express only the round seed phenotype, the round allele exhibits complete dominance over the wrinkled allele.
Complete dominance occurs when the presence of one dominant allele completely masks the physical expression of a recessive allele in a heterozygous state.

Key Concept

Basic Genetics Terminology: Alleles, Locus, Heterozygosity, and Dominance
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Basic Genetics Terminology and Concepts

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