People with MS: What is your birth order?

grew up as the "only child" in my home, but I was second child for my father. How do you rank that?

Your father's first child would still be your sibling--albeit your "step(brother/sister)." So, you would basically be the second child since you share DNA. Even though you both would have different mothers. Also, the propensity to develop MS, whether being a dominant or recessive trait would also have bearing on whether you would be considered first or second.

Dominant trait refers to a genetic feature that hides the recessive trait in the phenotype of an individual. Many traits are determined by pairs of complementary genes, each inherited from a single parent. Often when these are paired and compared, one gene (the dominant) will be found to effectively shut out the instructions from the other, recessive gene. For example, if a person has one gene for blue eyes and one for brown, that person will always have brown eyes because they are the dominant trait. For a person to have blue eyes, both their genes must be blue (recessive). When a person has two dominant alleles, they are referred to as homozygous dominant. If they have one dominant allele and one recessive allele, they are referred to as heterozygous.

The term "recessive gene" refers to an allele that causes a phenotype (visible or detectable characteristic) that is only seen in homozygous genotype (an organism that has two copies of the same allele) and never in a heterozygous genotype. Every person has two copies of every gene on autosomal chromosomes, one from mother and one from father. If a genetic trait is recessive, a person needs to inherit two copies of the gene for the trait to be expressed. Thus, both parents have to be carriers of a recessive trait in order for a child to express that trait. If both parents are carriers, there is a 25% chance with each child to show the recessive trait.

Recessive genetic disorders occur when both parents are carriers and each contributes an allele to the embryo, meaning these are not dominant genes. As both parents are heterozygous for the disorder, the chance of two disease alleles landing in one of their offspring is 25% (in autosomal dominant traits this is higher). 50% of the children (or 2/3 of the remaining ones) are carriers. When one of the parents is homozygous, the trait will only show in his/her offspring if the other parent is also a carrier. In that case, the chance of disease in the offspring is 50%.

I'm my mother's 1st (but after a miscarriage), and my father's 3rd (but he gave his technical 1st up for adoption). There's 1 brother after me. Make sense?

This poll is nice, but I think it leaves out some other major criteria. I am third born, but I am the only girl. My mother also has MS. Leaving out gender and family history definitely gives a narrow and jaded view.

This spreadsheet from the census shows number of births per 1000 broken down by birth order. We can use this data to get a sense of what the distribution should be in the general population. For the record, it looks like:

• First (only and not only): 40%
• Second: 32%
• Third: 17%
• Fourth+: 11%

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Art Mellor, Accelerated Cure Project for MS, art-msnews -at- acceleratedcure.com