9781422280393

Muscular Dystrophy and Other Neuromuscular Disorders

Genes and How You Became You Human beings inherit traits, such as height, hair color, eye color, and the shape of facial features, from their parents. These traits are determined at the time of conception, when a sperm cell is united with an egg cell, creating a new human being. Information about these traits is carried in genes. Some traits are determined by a single pair of genes, one from each parent; other, more complex, traits involve many genes. Every cell in the body contains a nucleus, and within each nucleus are 46 microscopic, threadlike chromosomes , 23 from each parent. These 23 pairs of chromosomes carry the genes inherited from each parent. Each pair of chromosomes carries tens of thousands of paired genes. Traits resulting from all but one of these 23 pairs of chromosomes are unrelated to whether they came from the male or female parent. In that one pair of chromosomes, called the X and Y chromosomes, the source may matter. For example, the gene that determines whether a child will be male or female is found on the male chromosome of that 23rd pair. The male parent has one X - chromosome and one Y - chromosome. The female parent has two X-chromosomes. Therefore a female will always pass on a gene from an X chromosome, but the male will pass on genes from his X chromosome about half the time, and from his Y chromosome about half the time. If the male passes on a gene from the Y-chromosome, the child will then have one X and one Y gene, and will therefore be male. If the male passes on a gene from the X-chromosome, the child will have two X genes, one from each parent, and will therefore be female.

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