Monthly Archives: March 2012

Consanguinity and the coefficient of relationship

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Or What percentage of your genetic code do you share with your second cousin?

Two people who share some of their genetic code (their DNA) are said to be consanguineous (“co” meaning shared and “sanguis” being Latin for blood) and the extent to which their DNA overlaps is known as the coefficient of relationship and is measured as a percentage.

Bearing in mind that you get 50% of your genes from each of your parents, it’s relatively easy to calculate the percentage of your DNA that you share with any relative you choose.

The closer to white a relation is, the lower the amount of DNA shared with that person.

To explain where these figures come from let’s take an example: How much of my DNA do I share with my sister Caroline?

Every one of your genes is split into two parts called alleles; you get one of these two alleles from each of your parents to make up each of your genes. If we take as an example a random gene, OCA2 (a gene that controls for eye colour) it will make it easier to understand the calculation process.

I got half of my OCA2 gene from half of my Dad’s OCA2 gene and the other half of my OCA2 gene from my Mum’s OCA2 gene; the same is true for my sister. I’ll call my Dad’s OCA2 alleles ED1 and ED2 and my Mum’s OCA2 alleles EM1 and EM2. There are therefore four possibilities for my and my sister’s OCA2 genes:

  • ED1EM1
  • ED1EM2
  • ED2EM1
  • ED2EM2

With four possible choices for the OCA2 gene there is a 25% chance of us sharing 100% of our DNA, a 50% chance (25%+25%) of sharing 50% of our DNA and a 25% chance of us sharing 0%. If you sum these percentages across all of our genes you get:
(25% × 100%) + (50% × 50%) + (25% × 0%) = 50%.

To quickly calculate the percentage of DNA you share with a relative simply count the number of (vertical) steps you have to take to get to them, and raise one-half to that power. For example, to find the percentage of DNA shared with a second cousin once removed you have to go three steps up and three steps down (the step between grandparents and great-grandparents is not counted twice) giving six steps in total. One half to the power of six is 0.015625 or 1.5625%; this is shown on the diagram below.

All of these calculations assume that none of your distant ancestors interbred with each other: i.e. that you have four unique grandparents, eight unique great-grandparents, sixteen unique great-great-grandparents and so on. Any half-relationships (e.g. half-brother, step-mother) cannot be included and of course I’m ignoring the fact that we are all related if you go back far enough.

A rectangular galaxy

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Most galaxies are either spiral or elliptical (“lenticular” galaxies fall somewhere inbetween).

A typical spiral galaxy, the Pinwheel Galaxy (M101, NGC 5457).

A dwarf elliptical galaxy, M110 (NGC 205).

A new arXiv preprint describes the discovery of LEDA 074886, which has a unique rectangular shape.

LEDA 074886 is thought to have formed as a result of a collision between two disc-shaped elliptical galxies that met “face-on”. The galaxy, 21 megaparsecs (68 million lightyears) away from Earth, is described by the authors of the paper as being “Emerald Cut”, which I think it pretty accurate.

Source: Alister W. Graham et al, “LEDA 074886: A remarkable rectangular-looking galaxy”, arXiv:1203.3608v1.

 

Smiths

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Most people are familiar with the term “blacksmith” and think that it refers to someone who works with metal, but this isn’t entirely correct. The term “blacksmith” only refers to someone who works with the “black” metals such as iron and steel.

Redsmiths* work with copper, whitesmiths work with light-coloured metals such as pewter or tin and brightsmiths work with silver. Many smiths are known by the type of object they produce, for example a gunsmith makes guns and and a bladesmith makes knives and swords; others are known by the metal they work with: goldsmiths and zincsmiths work with gold and zinc respectively.

* Redsmiths are also known as brownsmiths.

Water in Bermuda

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Fresh water is very hard to come by on the Islands of Bermuda. There are no lakes, and only a small number of water-collecting “lenses” underground. Rainwater is not salty, as the salt from seawater does not evaporate with the water, so by law every home in Bermuda must collect 80% of the rain that falls on its roof.

Bermudan rooves are made of local limestone and channel rainwater into large underground tanks where the water is treated so that it can be used in homes. On average each Bermudan home can store about 50000 litres of water per bedroom.

Because Bermuda does not have a centralised water distribution system it also does not have a centralised sewage system and therefore has the highest density of private cesspits per square kilometre of anywhere in the world.