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Zorluk: OrtaMendel's First Law and Monohybrid Inheritance

In rabbits, the allele for black coat color (BB) is completely dominant over the allele for brown coat color (bb). A black-furred rabbit mated with a brown-furred rabbit and produced 28 black-furred offspring and 26 brown-furred offspring over several litters. Which of the following represents the most likely genotypes of the parent rabbits?

  1. BbBb and bbbbCevap
  2. B
    BBBB and bbbb
  3. C
    BbBb and BbBb
  4. D
    BBBB and BbBb

Cevap

The black-furred parent is heterozygous (BbBb) and the brown-furred parent is homozygous recessive (bbbb).
The observed offspring ratio of 28 black to 26 brown is approximately a 1:1 ratio. In monohybrid inheritance, a 1:1 phenotypic outcome occurs when a heterozygous dominant individual (BbBb) is crossed with a homozygous recessive individual (bbbb). The brown parent must contribute a recessive allele (bb) to every offspring, so the presence of brown offspring (bbbb) proves that the black parent must also carry a hidden recessive allele (BbBb).

Adım Adım Çözüm

1
Determine the phenotype and genotype of the recessive parent
Since brown coat color is recessive (bb), the brown parent must be homozygous recessive (bbbb).
Recessive traits are only expressed phenotypically when two copies of the recessive allele are present.
2
Analyze the ratio of the offspring phenotypes
The offspring ratio is 28 black : 26 brown, which approximates a 1:1 ratio (1/21/2 black : 1/21/2 brown).
A 1:1 offspring ratio in a monohybrid cross indicates a test cross between a heterozygous dominant individual and a homozygous recessive individual.
3
Verify parental genotypes using a Punnett square
Crossing Bb×bbBb \times bb produces gametes BB and bb from the black parent, and bb from the brown parent, yielding 50% Bb50\%\ Bb (black) and 50% bb50\%\ bb (brown).
According to Mendel's First Law of Segregation, alleles segregate during gamete formation so each gamete carries only one allele for each gene.

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Mendel's First Law and Monohybrid Test Cross Ratios
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Mendel's First Law and Monohybrid Inheritance

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