This study investigates the gut microbiome in obese and lean female monozygotic and dizygotic twin pairs and their mothers to understand how host genotype, environmental exposures, and adiposity influence the gut microbiome. The researchers collected fecal samples from 154 individuals, including 31 monozygotic (MZ) twin pairs, 23 dizygotic (DZ) twin pairs, and their mothers, and characterized the microbial communities using 16S rRNA sequencing and shotgun sequencing. Key findings include: 1. **Familial Similarity**: Individuals from the same family had more similar bacterial community structures than unrelated individuals, suggesting that family genetics and early environmental exposures play a significant role in shaping the gut microbiome. 2. **Core Microbiome**: Despite the diversity of microbial lineages, a core set of functional groups was identified across all sampled individuals, indicating a shared functional core in the gut microbiome. 3. **Obesity-Associated Changes**: Obese individuals showed reduced bacterial diversity and altered representation of bacterial genes and metabolic pathways compared to lean individuals. Specifically, obesity was associated with an increase in Actinobacteria and a decrease in Bacteroidetes. 4. **Core Functional Groups**: A core set of functional groups was identified, representing a high level of functional redundancy in the gut microbiome. These core groups included pathways involved in carbohydrate and amino acid metabolism. 5. **Meta-Analyses**: The findings were consistent with meta-analyses of previously published gut microbiome datasets, suggesting a robust and reproducible core microbiome across different studies. The study highlights the importance of both genetic and environmental factors in shaping the gut microbiome and provides insights into the functional core of the human gut microbiome.This study investigates the gut microbiome in obese and lean female monozygotic and dizygotic twin pairs and their mothers to understand how host genotype, environmental exposures, and adiposity influence the gut microbiome. The researchers collected fecal samples from 154 individuals, including 31 monozygotic (MZ) twin pairs, 23 dizygotic (DZ) twin pairs, and their mothers, and characterized the microbial communities using 16S rRNA sequencing and shotgun sequencing. Key findings include: 1. **Familial Similarity**: Individuals from the same family had more similar bacterial community structures than unrelated individuals, suggesting that family genetics and early environmental exposures play a significant role in shaping the gut microbiome. 2. **Core Microbiome**: Despite the diversity of microbial lineages, a core set of functional groups was identified across all sampled individuals, indicating a shared functional core in the gut microbiome. 3. **Obesity-Associated Changes**: Obese individuals showed reduced bacterial diversity and altered representation of bacterial genes and metabolic pathways compared to lean individuals. Specifically, obesity was associated with an increase in Actinobacteria and a decrease in Bacteroidetes. 4. **Core Functional Groups**: A core set of functional groups was identified, representing a high level of functional redundancy in the gut microbiome. These core groups included pathways involved in carbohydrate and amino acid metabolism. 5. **Meta-Analyses**: The findings were consistent with meta-analyses of previously published gut microbiome datasets, suggesting a robust and reproducible core microbiome across different studies. The study highlights the importance of both genetic and environmental factors in shaping the gut microbiome and provides insights into the functional core of the human gut microbiome.