"Runs of homozygosity" explained: Jo Davidson's family relationship to Tommy Hunter in Line of Duty

The latest example of this came last night when Steve Arnott (Martin Compston) sombrely informed Kate Fleming (Vicky McClure) that an examination of DNA from Jo Davidson (Kelly Macdonald) returned "unusually high" runs of homozygosity.

The relevance of the remark is easy to miss if you haven't brushed up on your biology recently, but this finding actually exposes a shocking fact about Jo's past which is likely to be important in the remaining two episodes.

Read on for everything you need to know about homozygosity, including the dark link between Tommy Hunter and Jo Davidson.

Line of Duty: What is homozygosity?

Homozygosity is a term used when describing the genetic make-up of a person.

While human beings are all made up of the same genes, there are variations in each of us which cause differences in our physical appearance and some health conditions (i.e. hair colour, eye colour, cystic fibrosis).

Every gene is comprised of two alleles, one from each biological parent. If they are different (heterozygous), then whichever allele is dominant will be inherited (for example, brown eye colour overrides blue eye colour).

However, if both alleles are the same (homozygous), then the corresponding trait will be inherited whether it is dominant or recessive (for example, two matching alleles are required for a person to have ginger hair).

While it isn't uncommon for a person to feature homozygous genotypes, the key thing to remember is that Steve describes Jo Davidson's run of homozygosity as "unusually high", which can be a result of inbreeding.

Line of Duty: Jo Davidson and Tommy Hunter link explained

Jo Davidson is subtly confirmed to be the child of an incestuous relationship later in the episode by a sticky note on the wall in one of AC-12's meeting rooms.

It reads: "Jo Davidson's DNA has been identified at Farida Jatri's house. A match has been made to Tommy Hunter, stating she is both his niece and daughter."

Tommy Hunter is a violent criminal, a former boss of the mysterious OCG that has been hounding AC-12 for the last decade or so, as well as a pedophile known to have abused the children at Sands View boys home.

For this reason, it's possible that Jo Davidson's mother – Tommy Hunter's sister – could have been a victim of sexual assault, but it's not entirely clear why Jo feels so angry towards her (throwing a glass of wine at her photo earlier in the series).

Line of Duty continues on BBC One at 9pm on Sunday 25th April. You can discover all the other Line of Duty acronyms, abbreviations and key phrases in our terminology guide. Catch up now on BBC iPlayer. Take a look at more of our Drama coverage or visit our TV Guide to see what's on tonight.