Long before the advent of genetic testing or even complete understanding of DNA and RNA, astute observers noticed that genetic traits, including many disorders, were passed from one generation to another in somewhat predictable patterns. These came to be known as autosomal dominant, autosomal recessive, X-linked recessive and X-linked dominant patterns of inheritance.
To understand heredity, you have to know a little about human chromosomes and how they work.
Chromosomes come in pairs in the cell’s nucleus. Humans have 46 chromosomes in each cell nucleus, which are actually 23 pairs of chromosomes. For 22 of these pairs, numbered chromosome 1 through chromosome 22, the chromosomes are the same; that is, they carry genes for the same traits. One chromosome comes from a person’s mother, the other from his father.
The 23rd pair is an exception and determines gender. The 23rd chromosomal pair differs according to whether you’re a male or a female. Males have an X and a Y chromosome, while females have two Xs for this 23rd pair of chromosomes. Every female gets one X chromosome from her mother and one X from her father. Every male gets an X chromosome from his mother and a Y from his father.
|Diseases inherited in an autosomal dominant pattern require only one genetic mutation to cause symptoms.|
|Diseases inherited in an autosomal recessive pattern require two genetic mutations (one from each parent) to cause symptoms.|
|Diseases inherited in an X-linked recessive pattern mostly affect males, because a second X chromosome usually protects females from showing symptoms.|
Y chromosomes are unique to males and, in fact, determine maleness. If a man passes to his child an X chromosome from this 23rd pair, it will be a girl; if he donates a Y, it will be a boy.
Autosomal dominant conditions require only one mutation to show themselves. When specialists use the term autosomal dominant, they mean that the genetic mutation is on an autosome, one of the chromosomes that’s not an X or a Y. They also mean that the condition caused by the mutation can occur even if only one of the two paired autosomes carries the mutation. It’s a way of saying that the mutated gene is dominant over the normal gene.
In autosomal dominant disorders, the chance of having an affected child is 50 percent with each conception.
Autosomal recessive conditions require two mutations to show themselves. When they use the term autosomal recessive, they mean that the disorder is again located on chromosomes that aren’t Xs or Ys. However, when a disorder is recessive, it takes two mutated genes to cause a visible disorder in a person.
The word “recessive” comes from the idea that, when only one gene mutation exists, it may remain undetected (“recede” into the background) for several generations in a family — until someone has a child with another person who also has a mutation in that same autosomal gene. Then, the two recessive genes can come together in a child and produce the signs and symptoms of a genetic disorder.
You can think of recessive genes as “weaker” than “dominant” genes, in that it takes two of them to cause a problem.
People with one gene mutation for disorders that require two to produce the disorder are said to be carriers of the disorder. Carriers are usually protected from showing symptoms of a genetic disease by the presence of a normal corresponding gene on the other chromosome of each chromosome pair.
Sometimes, biochemical or other electrical testing, or certain conditions (for example, vigorous exercise or fasting) will reveal subtle cellular abnormalities in carriers of various genetic conditions.
In autosomal recessive disorders, the chance of having an affected child is 25 percent with each conception.
X-linked disorders affect males and females differently. Another important inheritance pattern is the X-linked pattern. X-linked disorders come from mutations in genes on the X chromosome.
X-linked disorders affect males more severely than they do females. The reason is that females have two X chromosomes, while males have only one. If there’s a mutation in an X-chromosome gene, the female has a second, “backup” X chromosome that almost always carries a normal version of the gene and can usually compensate for the mutated gene. The male, on the other hand, has no such backup; he has a Y chromosome paired with his sole X.
In reality, females sometimes have disease symptoms in X-linked conditions despite the presence of a backup X chromosome. In some X-linked disorders, females routinely show symptoms of the disease, although they’re rarely as serious (or lethal) as those in the males.
Some experts prefer the term X-linked recessive for the type of X-linked disorder in which females rarely show symptoms and X-linked dominant for the type in which females routinely show at least some disease symptoms.
Females with mild or no disease symptoms who have one mutated gene on an X chromosome and a normal version of the gene on the other X chromosome are called carriers of an X-linked disorder.
In X-linked recessive disorders, when the mother is a carrier, the chance of having an affected child is 50 percent for each male child. If the father has the mutation and is able to sire children, boys won’t be affected because they receive only a Y chromosome from him. Girls receive his X chromosome and will be carriers.
No. Many of us have seen diagrams like those above during our school years or perhaps in medical offices. Unfortunately, these diagrams very often lead to misunderstandings.
The diagrams are mathematical calculations of the odds that one gene or the other in a pair of genes will be passed on to a child during any particular conception.
These are the same kinds of calculations one would make if asked to predict the chances of a coin landing as heads or tails. With each coin toss (assuming the coin isn’t weighted and the conditions are otherwise impartial), the chances that the coin will land in one position or the other are 50 percent.
In reality, if you were to toss a coin six times, you might come up with any number of combinations: All your tosses might be heads, or five could be heads with one tails, or four might be tails with two heads.
In fact, every coin toss was a new set of odds: 50 percent heads, 50 percent tails. The second coin toss wasn’t the least bit influenced by the first, nor the third by the first two, nor the sixth by the previous five.
So it is with the conception of children. If the odds of passing on a certain gene (say a gene on the X chromosome that carries a mutation versus a gene on the other X chromosome that doesn’t) are 50 percent for each conception, they remain 50 percent no matter how many children you have.
Don’t be misled by an orderly diagram that shows one out of two children getting each gene so that a family of four children has two children with and two children without the gene in question.
Like the coin toss where six tosses turned up six heads, you could have six children who all inherit the gene, or none who inherit the gene.