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WonderGenetics fact

Tortoiseshell cats are almost always female. Early in development, each embryo cell randomly switches off one of its two X chromosomes, and all its descendants, pigment cells included, keep that choice. One X carries an orange coat-colour gene, the other a non-orange version, so the switched-off X sets each patch's colour.

X-inactivation

BiologyScienceEvolution

The story behind the card

The process is called X-inactivation, or lyonisation, after the British geneticist Mary Lyon, who proposed it in 1961 from her studies of coat colour in mice. Female mammals have two X chromosomes and males one; switching one X off in each female cell keeps the dose of X-chromosome genes the same in both sexes. Because the choice is made while the embryo is still a small ball of cells and is then copied to every descendant cell, the colour appears in patches rather than a fine speckle. A male cat can be tortoiseshell only if he carries an extra X chromosome (XXY), a rare condition that usually leaves him sterile. Calico cats show the same mechanism, plus a separate gene that adds white patches.

Türkçe oku
Kaplumbağa kabuğu desenli kediler neredeyse her zaman dişidir. Gelişimin başında embriyodaki her hücre iki X kromozomundan birini rastgele kapatır; ondan türeyen hücreler bu seçimi korur. X'lerden biri turuncu renk genini, öteki turuncu olmayan çeşidini taşır; hangisinin kapandığı lekenin rengini belirler.

Bu sürecin adı X inaktivasyonudur. Fikri 1961'de farelerde tüy rengini inceleyen İngiliz genetikçi Mary Lyon ortaya attığı için lyonizasyon da denir. Memelilerde dişilerin iki, erkeklerin tek X kromozomu vardır; dişi hücrelerde X'lerden birinin kapanması, bu kromozomdaki genlerin payını iki cinsiyette eşitler. Seçim, embriyo henüz küçük bir hücre topağıyken yapılıp sonraki bütün hücrelere aktarıldığı için renk ince benekler yerine lekeler hâlinde görünür. Erkek bir kedinin kaplumbağa kabuğu desenli olabilmesi için fazladan bir X kromozomu (XXY) taşıması gerekir; bu ender durum genellikle kısırlığa yol açar. Üç renkli (calico) kedilerde de aynı düzenek işler; beyaz lekeleri ise ayrı bir gen ekler.

Sources and further reading

Background reading: X-inactivation
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