The causes of color blindness are 100% genetic. What simplifies understanding is that the gene in question is unique. It is carried largely by the X chromosome.
The gene, implicated in daltonism, is "recessive", which means that it will not express itself if it is carried by a single chromosome. In this case, the daltonism will be "hidden" by the so-called "healthy" gene. The gene becomes active only if it is present on both X chromosomes. Children inherit a maternal chromosome and a paternal chromosome.
For years, confusion has led to the belief that girls can not be color-blind. And yet, this is possible, even if it is rare since only 0.5% of women are affected by color blindness, against 8% of men.
Here are the different possible cases of transmission:
Mother (X chromosome carrying the gene) + Father (normal X chromosome) => Non-color-blind girl
Mother (normal X chromosome) + Father (chromosome X carrying the gene) => Non-colorblind girl
Mother (X chromosome carrying the gene) + Father (normal Y chromosome) => Colorblind boy
Mother (chromosome X carrying the gene) + Father (chromosome X carrying the gene) => Colorblind girl
In summary, the causes of daltonism are reduced to the influence of a single gene. The latter is active when it is present on the X chromosome of a boy, or on both X chromosomes of a girl.
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