How do we know that monozygotic twins are not identical genotypically as adults?

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Understand that monozygotic twins originate from a single fertilized egg, so they start with identical genotypes at conception.

Recognize that as twins develop and age, various genetic and epigenetic changes can occur, such as mutations and DNA methylation, which can alter gene expression without changing the DNA sequence.

Consider that environmental factors and random cellular events can lead to differences in gene regulation and expression between the twins, contributing to phenotypic differences.

Learn about studies that compare the DNA sequences of adult monozygotic twins, which have found small genetic differences due to somatic mutations accumulated over time.

Conclude that these genetic and epigenetic changes explain why monozygotic twins are not completely identical genotypically as adults, despite originating from the same zygote.

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Monozygotic Twins and Genetic Identity

Monozygotic twins originate from a single fertilized egg that splits, resulting in two individuals with nearly identical DNA at conception. However, despite their initial genetic identity, differences can arise over time due to various biological processes.

Somatic Mutations and Genetic Variation

As monozygotic twins develop and age, somatic mutations—random changes in DNA occurring in body cells—can accumulate differently in each twin. These mutations contribute to genetic differences that make their genomes not perfectly identical in adulthood.

Epigenetic Modifications

Epigenetic changes, such as DNA methylation and histone modification, alter gene expression without changing the DNA sequence. Environmental factors and life experiences cause distinct epigenetic patterns in each twin, leading to phenotypic and functional differences despite similar genotypes.