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UC Davis Children’s Hospital

UC Davis Children’s Hospital

Cleft and craniofacial glossary

Autosomal dominant

Autosomal dominant inheritance means that the gene involved is not on one of the sex chromosomes and so males and females have the same chance of being affected. It also means that a person only needs to have one changed gene before they have the disorder. The normal gene is not able to make up for the gene with a change in it.

If a child is affected but neither parent is affected, it is likely that the gene change happened for the first time in the child. When a person with an autosomal dominant disorder has children, they may either pass on the normal gene or the changed gene. There is no way to control which gene is passed on.

View a diagram which shows autosomal dominant inheritance when only one parent is affected.

 

Autosomal recessive

Autosomal recessive inheritance means that the gene involved is not on one of the sex chromosomes, and so males and females have the same chance of being affected. It also means that in order for a person to have the disorder the genetic change must be present in both copies of the gene (one from the mother and one from the father).

If a child has an autosomal recessive disorder and neither parent has that disorder, the most likely explanation is that the child got one gene with a change in it from each parent. So both parents each have one normal copy of the gene and one copy that has a change. The normal copy is able to make up for the gene that has a change and so the parents do not have the disorder. When they have children they will either pass on the normal gene or the changed gene. There is no way to control which gene is passed on.

View a diagram which shows autosomal recessive inheritance.

 

Chromosome

Chromosomes are strands of genetic material. We all have 46 chromosomes in almost every cell of our body. Chromosomes can be arranged in 23 pairs, one from the mother and one from the father. The pairs are numbered from 1 to 22. The 23rd pair are the sex chromosomes which are given letters instead of numbers: XX is a female and XY is a male.

A chromosome can be compared to a volume of an encyclopedia. When looking at the spine of an encyclopedia you can only tell if it is much larger or smaller than it should be. You can’t tell if there is only a small change in one of the chapters. When looking at a chromosome you can only see large changes and not changes in individual genes.

 

DNA

DNA is the genetic code that is contained in genes. If a chromosome can be compared to a volume of an encyclopedia and a gene to a chapter in that volume, then DNA are the letters that make the words in the chapter. If there is a spelling mistake in a word, then that word may no longer makes sense. It may change the whole meaning of the sentence, just as a change in the code that makes a gene can alter what that gene does or make the gene not work at all.

 

Gene

There are thousands of genes on each chromosome. If chromosomes can be compared to a volume of an encyclopedia then a gene can be compared to the chapter in that volume. The only way you would know if there was a small change in that chapter is by specifically looking at the chapter in question. The same is true for a gene. You have to look at a particular gene to find a change.

Genes are the blueprints for our body. They determine the color of our hair, the way we look and the way our body is put together. If there is a mistake in a gene that is responsible for the formation of a part of the body, then that part of the body may not form in the way it is supposed to.

 

Syndrome

A syndrome is “a combination of signs and/or symptoms that forms a distinct clinical picture indicative of a particular disorder.” (from Oxford Concise Medical Dictionary).

 

X linked

X linked inheritance means that the gene involved is on the X chromosome. The X chromosome is one of the chromosomes that determine the sex of a child. The other chromosome involved in determining the sex of a child is the Y chromosome. A male is XY and a female is XX.

Males only have one X chromosome, so if there is a change in a gene on this chromosome, males do not have another type to compensate.

Females have two X chromosomes so if they have a change on one X usually the other X can compensate. As a result, in X-linked disorders males are usually affected, while females “carry” the disorder and are usually not affected.

If a boy has an X-linked disorder, a change could have occurred for the first time in him, or the mother could be a carrier. If there is more than one affected boy in the family, it is likely that the mother is a carrier.

View a diagram which shows X linked inheritance with a carrier mother and unaffected father. 

If an affected male has children with a female that is not a carrier, then all their daughters will be carriers, as he only has an X with a gene change on it to give them. All their boys will be unaffected because, to have a son, the father passes on his Y chromosome and the mother passes on her X chromosome.