Almost every medical innovation these days seems to involve genes and genetics. I suspect many patients find the vocabulary bewildering.

Cells, the nucleus and chromosomes

Under the microscope the most conspicuous part of a cell is the nucleus — and it is also the most important. It sits at the centre of the cell, wrapped in a membrane that holds all the chromosomes inside.

Each cell holds 23 pairs of chromosomes — one set inherited from the mother and one from the father. This information sits inside every cell of the body.

Diagram of the cell, nucleus and chromosomes

Testing embryos for chromosome number (PGT-A)

PGT-A rests on the principle that only chromosomally normal embryos implant and go on to produce a healthy baby. It is performed by biopsying 5–10 trophectoderm cells from a blastocyst-stage embryo.

Some chromosome abnormalities, however, are still compatible with implantation, with carrying to term, and with the birth of an affected child. These involve:

Chromosome 13 — Patau syndrome
Chromosome 18 — Edwards syndrome
Chromosome 21 — Down syndrome
X/Y — Turner syndrome, Klinefelter syndrome

What genes are

A gene is a package of information specifying a particular characteristic of the body — hair colour, eye colour, how an enzyme works. Some functions depend on several genes working together.

Faults in genes cause inherited disease. These fall into four patterns.

Inheritance pattern	Example diseases
Autosomal dominant	Huntington’s disease, Marfan syndrome
Autosomal recessive	Thalassaemia, cystic fibrosis, sickle cell anaemia
X-linked dominant	Fragile X syndrome, incontinentia pigmenti
X-linked recessive	Haemophilia A, Duchenne muscular dystrophy (DMD)

The human genome contains roughly 20,000 genes, so testing an embryo's chromosomes does not cover this level at all. Specific genes can be tested, though, when a couple is known to be at risk of passing on an inherited disease.

What embryo chromosome testing does tell you

It tells you

Whether the chromosomes of the 5–10 cells that were biopsied are normal or abnormal

It does not tell you

That the whole embryo has normal chromosomes
That the baby born from this embryo will have normal chromosomes
Anything about other inherited diseases
That the child will be free of genetic disease

What embryo chromosome testing does for treatment

It helps

Shortens the time to pregnancy
Lowers the miscarriage rate

It does not help

It does not raise the overall chance of pregnancy
It does not reduce pregnancy complications such as gestational diabetes or pre-eclampsia

Genetics 101 — the infertility edition