12.
In the following pedigree, what will be the genotype of the parent indicated by the dark
square? Dark symbols indicate individuals with a specific phenotype, controlled by the
locus with alleles A or a.
a. AA
b. Aa
c. aa
d. It could be either AA or aa
Direct answer:
The genotype of the parent indicated by the dark square is Aa (heterozygous) for the trait locus.
Understanding the pedigree question
The question shows a pedigree in which dark (filled) symbols represent individuals with a specific phenotype controlled by alleles A and a. The parent labelled above the F1 generation is a dark square (affected male) married to a white circle whose genotype is explicitly given as aa (unaffected recessive).
Because the unmapped parent is affected while the spouse is aa, the task is to decide whether this dark square must be AA, Aa, aa, or could be either AA or aa. The pattern of affected and unaffected offspring in F1 and F2 helps determine whether the dark phenotype behaves as a dominant or recessive trait.
Step‑by‑step genetic reasoning
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Dark phenotype is dominant (A)
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Two unaffected parents can have affected offspring only if the trait is recessive (aa); here, however, an affected individual appears in every generation and unaffected parents never produce a dark child, which is more consistent with a dominant trait.
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Therefore, dark symbols are taken as phenotype A– (either AA or Aa), and white symbols are aa.
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Spouse genotype is fixed as aa
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The white circle spouse is given as aa, so all gametes from this parent carry only the recessive allele a.
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Any affected child must therefore have received the dominant allele A from the dark‑square parent.
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Using the offspring to infer the dark parent’s genotype
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From the cross “dark square × aa spouse,” the dark parent would produce:
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All affected children if he were AA, because all offspring would be Aa.
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A 1:1 mix of affected (Aa) and unaffected (aa) children if he were Aa.
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In the pedigree, this couple produces both dark and white children in F1, so the affected parent cannot be AA; he must be Aa.
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F2 generation consistency check
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The F1 heterozygous children (dark) married to aa partners again give 1:1 affected to unaffected offspring, matching the pattern shown.
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White F1 children (aa) never have dark offspring unless partnering with an affected individual, confirming that the dominant allele originates from heterozygous dark parents.
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Thus, all evidence in the pedigree supports the conclusion that the dark square parent is heterozygous Aa.
Detailed explanation of each option
Option a. AA
If the dark square parent were AA, the cross would be AA × aa, producing only Aa (all affected) offspring. The pedigree clearly shows both affected (dark) and unaffected (white) children in the F1 generation, so AA is incompatible with the observed segregation and is therefore incorrect.
Option b. Aa
With Aa × aa, each child has a 50% chance of being Aa (affected) and a 50% chance of being aa (unaffected). The presence of a mix of dark and white offspring from this couple exactly matches this expectation, making Aa the only genotype consistent with the pedigree.
Option c. aa
A genotype of aa would give a white (unaffected) phenotype if the dark trait is dominant, contrary to the diagram where the parent is clearly dark. Additionally, an aa × aa cross can never produce dark offspring, yet dark children are present, so aa is impossible and this option is incorrect.
Option d. It could be either AA or aa
This option suggests ambiguity, but the segregation pattern actually removes all ambiguity. The coexistence of affected and unaffected offspring from an affected × aa mating excludes AA, while the affected phenotype excludes aa, leaving only Aa; therefore this option is incorrect.
Short SEO‑friendly introduction (for article body)
Pedigree analysis questions are common in genetics exams and often ask students to infer unknown genotypes from a family tree. In this article, the focus is on determining the genotype of the parent indicated by the dark square in the pedigree, using Mendelian rules and the distribution of affected and unaffected descendants across two generations.
