Glossary

Gene, chromosome and cells

Cells: The smallest independent parts of organisms

Nucleus: the part of the cell that contains our genetic information

Genome: the total genetic information contained in a the nucleus and/or mitochondria of our cells. The human genome is inherited from the biological parents

DNA: double helix molecule that is our "blueprint" or template. It is the basis of genetic inheritance. The template from which we are made that enables our cells, tissues and organs to grow, develop and function.

Chromosome: organised structure of DNA and protein found in cells

Gene: the basic unit of heredity. Contains the information required to build and maintain cells and to pass genetic traits to offspring

Allele: one member of a pair or series of different forms of a gene. Usually denotes coding sequences, but sometimes in PGD is extends to non-coding sequences. 

Genotype: for that gene, is the set of alleles it happens to possess, for most genes, the two alleles present on the respective chromosomes constitute the genotype.

Phenotype:  is any observable characteristic or trait, such as morphology, development, biochemical or physiological properties, or behaviour.

Trait: a feature of a person- physical or otherwise.

Mutation: Deleterious change in a gene that may lead to disease

Polymorphism:having multiple alleles of a gene within a population, usually expressing differing phenotypes

Protein: compound comprised of amino acids arranged in a linear chain and joined together by bonds between the sidechain of adjacent amino acids, responsible for basic cellular functions.

Amino acid: the building blocks of proteins.

IVF, reproduction and chromosome disorders

IVF: In vitro fertilisation: a process by which egg cells (ova) are fertilised by sperm outside of the womb

Pre-implantation genetic diagnosis (PGD): "IVF for genetic disorders", enables genetic testing of an embryo created using IVF technology

Fertilisation: conception

Gamete: a cell that fuses with another gamete during fertilisation ie sperm and eggs are gametes

Blastocyst: Stage of pre-implantation embryo development of the day 5-6 embryo that has differentiated into two cell types correlating with the future placenta and the future fetus/child. 

Karyotype: cytogenetic chromosome analysis with direct visualisation of the chromosomes to determine chromosome number and structure/content

Array Comparative genomic hybridisation (CGH): Molecular chromosome analysis that provides a detailed assessment of chromosome content

Copy Number Variation: loss or gain of genetic material (as detected on aCGH)

Deletion: Loss of chromosomal/genetic material

Duplication: Gain of chromosomal/genetic material

Aneuploidy syndrome: disorder of chromosome number

Down syndrome: trisomy 21, typically arising from the presence of an extra copy of chromosome 21

Mosaicism: the presence of two populations of cells with different genotypes in one individual, who has developed from a single fertilized egg.

HLA: Human leukocyte antigens - system of self-recognition by immune system. Used for transplantation and tissue typing.

Patterns of genetic inheritance

Autosomal – genes are situated on one of the 22 non-sex chromosomes ("autosomes")
Sex-linked – genes are situated on a sex chromosome (X or Y chromosome)
X-chromosomal – genes are situated on the X chromosome (the more common case

Y-chromosomal – genes are situated on the Y chromosome
Mitochondrial – genes are situated in the mitochondrial DNA
Dominant inheritance: a single faulty gene or chromosome is sufficient to cause disease
Autosomal recessive inheritance: the presence of two faulty alleles (genes) is required to cause disease

X-linked recessive inheritance:   single faulty gene on the X-chromosome in a male is sufficient to cause disease. Females are usually "protected" by the presence of the normal allele (gene) on the second X chromosome (acting as a "back-up copy").

Epigenetics: heritable changes in phenotype or gene expressed caused by mechanisms other than changes in the underlying DNA sequence eg maternal or paternal "imprinting"

Factors influencing genetic inheritance

Fully-penetrant: inheriting a faulty gene leads to development of the disorder in all individuals

Incomplete penetrance: Not all individuals who inherit a faulty gene develop the disorder

Variable expression: the disease manifestations may vary from person-to-person ie age of onset, disease severity, progression, organ systems affected

Sex-limited expression: disorders where males and females are affected differently with different chance of developing a disease and different expression of a disorder

DNA changes

Mutation: Changes to the nucleotide sequence of the genetic material
Missense mutation: alters the amino acid in the mature protein
Nonsense mutations:  change in the code creating a "stop signal" that typically truncates the protein
Germline mutation: gene change transmissable to offspring
Somatic mutation: change in cells outside of the gonadal cells, not usually transmitted to offspring.
de novo mutation: a new mutation in a child or somatic cell, that is not inherited from a parent
Gonadal mosaicism: Presence of two populations of cells with different genotyes in the gonads of an individual. Can cause recurrence of a genetic disorder where testing may indicate a low chance of recurrence. A mode of inheritance that is difficult to exclude in clinical practice.

A heterozygous mutation is a mutation of only one allele.
A homozygous mutation is an identical mutation of both the paternal and maternal alleles.
A compound heterozygous mutation is two different mutations in the paternal and maternal alleles.
A wildtype sequence is one in which the allele is non-mutated (normal)