mtDNA and Y-STRs

Basics

• Each cell only has one nucleus

• Many mitochondria per cell, organelle

• Responsible for energy production

• The mitochondria have DNA but no nucleus

• Cell nucleus only 2 copies of DNA

• Multiple copies of DNA in mitochondria

• Proteins involved in the electron transport chain, which generate ATP through oxidative phosphorylation

• rRNA and tRNA molecules necessary for protein synthesis within the mitochondria

Mitochondrial DNA

• Closed circular dsDNA

• About 16,569bp**

• Heavy vs. light strand

  • Heavy and G- More A

  • Light- and T

mtDNA

• D loop, displacement loop or non-coding region

• About 1,122bp

• HV1, HV2 and HV3

  • Hyper Variable Regions

• High mutation rate

• Contains the Origin of Replication

mtDNA Inheritance

• Maternally inherited

• Mothers pass on the mtDNA to the next generation

• Female – mitochondria located in the cytoplasm of the egg cell

• Male – mitochondrial sheath located on the tail of sperm, does not enter the cell

mtDNA Analysis

• Base by base sequence

  • SNPs!

• Compare your sequence to a reference sequence:

  • the Revised Cambridge Reference Sequence or Anderson Sequence

• Report out the differences seen in the Case Report

Revised Cambridge Reference Sequence

• Based on the first sequencing of mtDNA done

• European woman

• Not the “perfect” genome, just a genome that everyone will compare against for consistency

RCRS Analysis

• Polymorphisms – differences from the Anderson sequence

• Point Heteroplasmy – 2 bases found at the same base position

  • Person has 2 different mitotypes within their body

  • Rare to have more than point heteroplasmic positions

RCRS Common Changes

• Transitions:

  • Most common

  • Purine to purine (G/A)

  • Pyrimidine to pyrimidine (C/T)

• Transversions:

  • Less common

  • Purine to pyrimidine or vice versa

• Insertions - added base

  • More common

• Deletions - deleted base

  • Less common

• Changes the 1,122bp length

Y-Chromosomes

• Second smallest human chromosome (21 is the smallest)

• Found in males (XY)

• Encodes for 76 genes

Y-Chromosome Inheritance

• Paternally inherited

• Fathers will pass the Y chromosome to their sons

Y-Chromosome Analysis

• Can target SNPs or STRs

• Still looking at non-coding DNA

• Beneficial because we only target male DNA

Issues with Y-STR Testing

• All related males will have the same profile

• You cannot say if it was the son, father, uncle, grandfather, etc.

• Think about sperm donor issues

Y-STR in Sexual Assaults

If we have a rape kit and do an extraction, what happens?

Male Specific DNA Identification: Y-STR analysis targets markers on the Y chromosome, making it useful for identifying male DNA in mixed samples

Detection of Multiple Male Contributors: Y-STR profiling can differentiate between DNA from multiple male individuals in a single sample

Persistence of Male DNA: Y-STR testing can detect low levels of male DNA, even when sperm is absent, enhancing its utility in cases with degraded or limited biological evidence

Benefits of mtDNA

Maternal Lineage Tracing: mtDNA is maternally inherited, making it useful for identifying individuals or remains when only maternal relatives are available for comparison

Persistence in Degraded Samples: mtDNA is more abundant and resilient than nuclear DNA, making it ideal for analyzing highly degraded or old forensic samples, including bones, teeth, and hair shafts

Species and Population Studies: mtDNA analysis helps in population studies and distinguishing between human and animal remains in forensic contexts

Issues with mtDNA

Lower Discriminatory Power: mtDNA lacks the variability of nuclear DNA and cannot distinguish between individuals with the same maternal lineage

Potential for Contamination: The high copy number of mtDNA increases the risk of contamination during analysis, complicating results

No Paternal Contribution: mtDNA analysis does not provide information about paternal lineage, limiting its use for comprehensive genetic profiling

• "Seven Daughters of Eve" refers to a theory proposed by geneticist Bryan Sykes – all can trace back to early origin