Wednesday, January 6, 2010

Answering Creationist Claims (Part 8 - The Fluid Nature of Chromosomal Count)

Our genes lie within our chromosomes , which plays an extremely important part in heredity. A common creationist claim would be the “fixity of species”, and from this comes Bible Life Ministries’ and the Creation-Evolution Encyclopaedia's claim that the chromosomes counts are fixed (or if it changes, it’s deleterious). Such a claim can only come from ignorance on the topic of genetics, and it shall be my topic today.

What’s a Chromosome? (Brief)

A chromosome is a structure in our cells that is made up of DNA and proteins. It contains almost all of our genetic information, and comes with gene regulation proteins. Depending on the species, the chromosome can contain from 10,000 to 10,000,000 base pairs.

Chromosomes of Eukaryotes

Eukaryotes are organisms which cells contain nuclei (plants, animals, etc.). The nucleus of the eukaryotic cells houses the chromosomes, which are linear and rod shaped. Near the centre of the chromosome is the centromere, which is the point where 2 homologous chromatids and microtubules fuse. At the the tips of the chromosome are the telomeres, which contain repetitive DNA (with 2 chromatids present, that makes it 4 telomeres). It is shown that these repetitive DNA are responsible for maintaining the integrity of our genes, as they protect our genes by “capping” them. Eukaryotic chromosomes replicate through cell division, either by mitosis; and meiosis (in which the chromosomal count of the cell is halved), which is responsible for producing gametes.

Chromosome Sketch
(1) Chromatid. One of the two identical parts of the chromosome after S phase. (2) Centromere. (3) Short arm (4) Long arm.

Chromosomes of Prokaryotes

Prokaryotes (bacteria & archaea) chromosomes are single and circular, and vary greatly in size, from 160,000 to 12,200,000 base pairs in the bacterium Candidatus Carsonella ruddii and Sorangium cellulosum, respectively. Prokaryotic chromosomes are less sequential than those of eukaryotes, and replicate from a (or multiple) point(s) of origin. As they do not have a nucleus, prokaryotic chromosomes are organised into a structure known as the nucleoid instead.

Best Case of Chromosome Number Change – Chromosome 2

Most members of the family Hominidae have 48 chromosomes. Humans have 46. Huh? How could this be? After all, if evolution was true, we should have same chromosome counts, right? Well, at least that’s what claimed by creationists. To find the answer to this, let’s take a look at Chromosome 2 of humans.

Chromosome 2

Chromosome 2: Strong Evidence for Common Ancestry of Humans and Modern Apes

What’s so special about Chromosome 2? Because it’s damn similar (near-identical) to Chromsome 2a and 2b of chimps combined. Moreover, it’s a telomere-telomere fusion at region 2q13. And we have scientific evidence for that.

Firstly, there is is a inverted head-head arrangment of the TTAGGG array and the adjacent sequences at the predicted fusion site, which are surprisingly similar to the telomere points found in human (and ape) chromosomes. In other words, after the repeated TTAGGG sequence, the sequence inverts, becoming CCCTAA (it’s not GGGATT as CCCTAA is the reverse sequence that TTAGG maps to {A <> T, G <> C}).

Secondly, since Chromosome 2 is said to be a fusion of Chromosome 2a and 2b, there must be 2 vestigial centromeres, right? Well, that’s exactly the case. When scanning with DNA probes, signals for the presence of a centromere was detected somewhere around q21.3-q22.1, in the long arm of Chromosome 2. The other centromere is actually used (so it's not vestigial after all), and it lines up with 2p chromosome of chimps.

These 2 chromosomes strongly point to the fact that after diverging from chimps around 6-7 million years ago, Chromosome 2a and 2b underwent a telomere-telomere fusion, and thus is evidence for evolution.

Other Cases in Which Individuals of a “Kind” Have Different Chromosome Counts

Since chromosomes counts are supposedly “fixed” and all chromosomal changes are deleterious, let’s see the diploid chromosome count of “kinds”.

Fox “Kinds”

Arctic Fox
Alopex lagopus
Bat-eared Fox
Otocyon megalotis
Bengal Fox
Vulpes bengalensis
Fennec Fox
Vulpes zerda
Gray Fox
Urocyon cinereoargenteus
Kit Fox
Vulpes macrotis
Red Fox
Vulpes vulpes
Tibetan fox
Vulpes ferrilata

Horse “Kinds”

Equus ferus caballus
Przewalski's Horse
Equus przewalski poliakov
Equus africanus asinus
Moutain Zebra
Equus zebra
Plains Zebra
Equus quagga
Grévy's Zebra
Equus grevyi
Burchell's Zebra
Equus quagga burchellii

On a side note, a normal horse (64 chromosomes) can breed with a Przewalski's Horse (66 chromosomes) and produce a completely fertile hybrid offspring (65 chromosomes).


Thus, as it have been shown, the chromosome count is in fact variable. Not only can we see evidence for it in Chromosome 2, but even different species of the same "kind" (which means nothing in science) have different chromosome counts. Thus the creationist claim that all chromosome changes are harmful and that there is a "fixity of species" is completely wrong.

And since the most of Bible Life Ministries following claims are completely unrelated to evolution, I will only address the one that is. (Tip: something to do with the age of the Earth).


IJdo et al. (1991). "Origin of human chromosome 2: an ancestral telomere-telomere fusion". Proceedings of the National Academy of Sciences 88: 9051–5. doi:10.1073/pnas.88.20.9051. PMID 1924367

Previous: Answering Creationist Claims (Part 7 – The 2nd Law of Thermodynamics Truthfully Explained)

Next: Answering Creationist Claims (Part 8 – Deny The Age of the Earth and You Deny Science Itself)