It looks like you're working with a genetic scenario where "b" represents the recessive allele for blonde hair, and "B" would typically represent the dominant allele for brown hair. Based on the information provided, the family tree you described consists of:
- Father's mother: bb (blonde hair)
- Mother's mother: bb (blonde hair)
- Mother's father: bb (blonde hair)
- Child: bb (blonde hair)
Since all individuals in this scenario have the "bb" genotype, they express the blonde hair phenotype. Here’s a summary of the allele combinations you’ve described:
- Father's mother (bb): Only has the blonde hair allele
- Mother's mother (bb): Only has the blonde hair allele
- Mother's father (bb): Only has the blonde hair allele
- Child (bb): Only has the blonde hair allele
Considering these combinations, it appears that all individuals in this family tree are homozygous recessive for blonde hair, and therefore, they could only pass on the "b" allele.
Possible Allele Combinations for Brown/Blonde Hair:
-
For Brown Hair (genotype possibilities):
- BB: Homozygous dominant for brown hair
- Bb: Heterozygous, with brown hair phenotype
-
For Blonde Hair (genotype possibilities):
- bb: Homozygous recessive, expressing blonde hair
Missing Combinations:
The chart you've described only includes bb combinations, which means the following combinations are missing from your family's genetic makeup:
- BB (both parents contribute B): Not present in this lineage because both grandparents of the child were bb.
- Bb (one parent contributes B, the other b): Also not present in the ancestry provided, signifying one brown-haired parent could pass on the B allele.
Conclusion:
Since all individual ancestors are bb, this indicates a lack of any dominant alleles (B) for brown hair in this family lineage, which is why the child also has the genotype bb. The missing combinations (BB and Bb) are not represented in this family genealogy.
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