The recessive gene for color blindness is located on the X chromosome. Three daughters with normal vision and two sons with color blindness were born from the marriage of a woman with normal vision, whose relatives suffered from color blindness, and a man with normal vision, whose father was color blind. What are the genotypes of parents and offspring? From which parent did the boys get the color blindness gene?
Let’s designate the chromosome with which the gene responsible for the development of color blindness is linked as “Xd”, then the sex chromosome responsible for the development of normal color discrimination will look like “XD”.
A man who can distinguish colors normally will have a set of XDY chromosomes and produce XD and Y spermatozoa.
Both sons of this couple suffer from color blindness – XdY.
Since all sons carrying the maternal chromosome X suffer from color blindness and this woman’s ancestors were also color blind, it can be argued that this woman with normal color discrimination is a carrier of the color blindness gene and is heterozygous – XDXd.
Three daughters have normal vision – XDXd or XDXD. Both options are possible.
The gene responsible for the development of color blindness, the boys received from their mother, because their Y chromosome from the father does not carry information about distinguishing colors.
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