What Are the Chances of People With Down's Syndrome Passing Their Condition to Their Children?

Question

Down's syndrome is caused by an extra copy of the 21st chromosome, which somehow interferes with development.

What are the chances that a person with Down's syndrome will pass this extra chromosome to their offspring?

Answer 1

Genes are present in every cell of the body. They are inherited from our parents and are responsible for our development from fertilized egg to fully grown adult. They provide the chemical information that is needed to maintain our body and keep it working well.

People with Down's syndrome have the same genes as anyone else; they just have 1% extra. This is enough to change the finely-tuned balance of the body and produce the physical and intellectual characteristics found in people with Down's syndrome.

Cells

Genes

Chromosomes

Cell division

Eggs and sperm

Overview of genetics and Down's Syndrome

Trisomy 21 - also known as standard, regular or free trisomy

Translocation and partial trisomy

Mosaicism

Some questions we have been asked

Summary

Cells

The human body is made up of cells. Each cell is like a tiny factory which makes the materials needed for growth and maintenance of the body. Different parts of the body have specialised cells with special tasks, for example muscle cells are different from nerve cells, heart cells are different from brain cells.

The production and output of each cell is controlled by genes. The genes are the same in every cell of the body. Not all the genes are active in any one cell at any one time, however. The active genes in a cell are only those appropriate to the cell type and its functions.

The body grows by making new cells. It does this by making an exact copy of the genes in a cell. The cell then divides in two with one copy of the genes in each new cell.

Genes

Genes contain the body's instructions for making its parts and for its day to day running. Genes therefore control or influence such things as:

Our physical appearance

The way babies and children grow and develop, even before they are born

The timing of the milestones of growth and development

The body's supply and use of building and maintenance materials

The way the body ages

A person's resistance to illness

Genes are passed from parents to children. Most people have two copies of every gene - in general one copy comes from the mother and the other from the father.

The genes are made of DNA (deoxyribonucleic acid). The arrangement of chemicals in the DNA is different for every gene. These chemical instructions tell the body what components to make, and how, when and in what quantity to make them, in order to ensure normal working of the body. There are also long stretches of DNA, the precise role of which is still a mystery.

Chromosomes

The genes are grouped together in long, thin, thread-like structures called chromosomes. Thus chromosomes carry our genetic information.

There are 46 chromosomes (in 23 pairs) in all body cells (except the sperm and egg cells). Each pair of chromosomes has different genes. One chromosome of each pair comes from the mother and one from the father. A person's characteristics are determined by the interaction of the copies of the genes from each parent.

The chromosome pairs are numbered 1-22 according to size. Chromosome 1 is the largest. Each chromosome has a long arm and a short arm.

The 23rd pair are the s*x chromosomes which are called the X and Y chromosomes. Girls have two X chromosomes and boys have an X and a Y. The s*x chromosomes are different from the others.

In chromosome tests, the chromosomes (usually from blood cells) are stained with dye. Each pair has a different staining pattern. When seen down a microscope the chromosomes lie in a haphazard manner, but when they are photographed, cut out and arranged in pairs, a picture of an individual's chromosomes known as a karyotype is obtained.

The genes, thousands on each chromosome, are spaced along the length of the DNA molecule, in a definite order. Apart from the X and Y chromosomes, the order of the genes on one chromosome exactly matches the order on the other chromosome of the same pair (and on chromosomes of the same number in everyone else). In general, there are only two copies of each gene and in all people they are always on the same chromosome pair.

The two copies of each gene may not be identical. Any changes may be small "misprints" or bigger alterations, with a corresponding effect of the "readability" of the instructions. When this happens, variation occurs between people - this variation is sometimes advantageous, sometimes neutral and sometimes unhelpful. Often a mistake in the one copy of a gene can be "cancelled out" when there is no mistake is the other copy of the same gene. Geneticists believe that everyone carries a number of defective genes which are partnered by a normal copy which "cancels" the defect. They also believe we all carry a number of genes with small variations which are not cancelled out, and which account for some of the differences in appearance or health among people in the general population.

Ordinary cell division

The body grows by making new cells. New cells are made by one exist

Answer 2

Down's Syndrome largely causes infertility. Only 3 men with Down's are known to have fathered children.

Answer 3

more likely than if two normal people had children.

Answer 4

Me8md,

To answer this question it is better to know a few general medical facts. There are three types of DS. The first form is trisomy 21 due to "nondisjunction" and accounts for about 95% of all cases of DS. In this case the extra chromosome is present in the sperm or in the egg, or in the first cellular division and, as a consequence, each cell that forms will have three chromosomes in the couple 21. This form is strictly linked to maternal age not to hereditary genetic factors. The second form of DS, which is also the least common, is the mosaicism. This represents about 2% of cases. In this form of trisomy 21 the cellular division occurs in a defective way after fertilisation, during the second or third division, or those following. As a consequence, not all the cells of the embryo that is forming contain an extra chromosome. This form also does not depend on hereditary factors. The third form, which is very rare, is partial trisomy 21 due to Robertsonian Translocation, accounts for 2-3% of cases. In this case a part of 21st chromosome breaks up during the cellular division, "moves" and attaches itself to another chromosome, usually the 14th chromosome or the other 21st chromosome. Some extra genetic material is present in both cases. About 2/3rds of translocation occur spontaneously during fertilisation. The translocation is hereditary from one of the two parents in 1/3rd of cases. In this case we have the only form of DS which is linked to a genetic factor from either the father or the mother. The parent who is the carrier of this is normal, but two of his/her chromosomes are united in a way that the total number of his/her chromosomes is only 45 instead of 46. It is important to identify, through the child's karyotype the presence or not of partial trisomy 21 due to translocation because if one of the parents is a carrier the chances that other children with DS can be born into that family are higher. It is advisable that the parents, the other children without DS and the blood relatives of the carrier parent have their chromosomes mapped. So, in conclusion, the chances of a brother or sister of someone with DS having children with DS are only minutely higher than those with no case of DS in their family, and concern only the form of translocation. Therefore, it is important to determine whether the brother/sister has got this form and, in this case, whether they are a healthy carrier. The chromosome map or "karyotype" can be carried out on a simple blood test. Together with the blood test you can ask for a "genetic consultancy", that is a more precise and personalised explanation.

ALL ANSWERS SHOULD BE THOROUGHLY RESEARCHED, IN ANY FORUM AND ESPECIALLY IN THIS ONE. MANY ANSWERS ARE FLAWED.

The information provided here should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed physician should be consulted for diagnosis and treatment of any and all medical conditions.

I add a link with details of this subject

http://kidshealth.org/

parent/medical/genetic/

down_syndrome.html

Hope this helps

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