THE CHALLENGE
I would like to hire your services. I'm Alexa and I'm 30. You should know that I'm colour blindness, my eyes are blue and I have an autosomal recessive disease at the 7th chromosome, I have cystic fibrosis. I would like my daughter not to suffer neither colour blindness nor cystic fibrosis. Could you tell me how should be the other biological parent genetically to make these possible?
Firstly, the technique with which the sex of the baby can be chosen is through an in vitro fertilization cycle in which an ovarian hormonal stimulus is carried out to obtain oocytes and these are inseminated with the male's sperm to obtain embryos. Once you have the embryos, you have to perform a biopsy to obtain a cell and analyze it. This analysis is what we call Preimplantation Genetic Diagnosis, that is, obtaining genetic information from that embryo and then introducing it into the maternal uterus. In the case of a sex selection, what we are interested in knowing is whether the embryo is XX or XY, that is, a girl or a boy, respectively. Once we know it's a girl (XX) the fertilized egg is transferred to a uterus.
Secondly, cystic fibrosis is an autosomal recessive disease due to a mutation in the CFTR gene on chromosome 7. For a person to be affected both alleles must be recessive, that is, they must have inherited two alleles with the mutated gene from their two parents. But what you have asked us is your daughter not to be affected, that is why the other parent wouldn’t either suffer from the disease or be a carrier. In this case, the girl would inherit the affected allele from you but she would inherit the healthy one from the other parent, therefore, the girl would be a carrier, which means that she can transmit the disease to her possible future children but she will not present symptoms.
On the other hand, color blindness is a recessive and sex-linked disease, specifically linked to chromosome X. Having this disease means that you have both alleles affected and that is why you need the other parent to be healthy, that him X chromosome does not have the same affected gene. This will turn your future daughter into a carrier and, as in the previous case, she will be able to transmit the disease but not suffer from it.
Finally, the green color of the eyes. Light eye colors are due to recessive genes found on both chromosomes 15 and 19. For starters, eye color is regulated by about 15 genes. Among them we find the OCA2 gene, on chromosome 15, and it is responsible for producing tyrosine, which is the precursor of melanin. On this same chromosome we also find the HERC2 gene that regulates the expression of the previous gene. On chromosome 15 are the alleles that determine the color brown or green, the first being dominant and the second recessive, therefore, both alleles must be recessive for the eyes to be green. In addition, on chromosome 19, we find the EYCL1 gene, which is responsible for determining the colors green and blue. If both alleles are recessive, the color will be blue, on the other hand, if one or both alleles are dominant, the eyes will be green. In conclusion, you have the two alleles of chromosome 15 that determine between recessive brown and green recessive, and the other parent will have to be the same. Instead, on chromosome 19 you have both recessive alleles, therefore your eye color is blue, so the other parent should have them dominant for the eyes of your daughter to be green.
When we have the donor sperm, through artificial insemination taking into account the first technique to choose the sex of the baby, the fertilized egg will be introduced into your uterus. Later, when the fetus is developing, we will perform a karyotype, in this way we can see the set of chromosomes that each of your daughter's cells has. With the karyotype it will be possible to know if your future daughter meets all the characteristics that you had asked for. For this test, amniocentesis is done to obtain a sample of the amniotic fluid, a fluid that surrounds and protects the baby during pregnancy and contains fetal cells. Once the sample is obtained, we will carry out a cell culture in the appropriate medium. From these cell cultures, dividing cells are obtained, where the chromosomes are visible.