Genetics, Genomics and Epigenetics

Genetics

The study of individual genes, their roles and how airborne traits or conditions are inherited through generations

Genomics

The study of an organism’s better genome (all DNA combined) including how different genes interact with one another and the environment.

Epigenetics

The study of how environmentaln factors, lifestyle and physiological stress can modify gene expression without altering the underlying DNA sequence itself.

How does understanding the definitions of genetics, genomics and Epigenetics help midwives?

Understanding these definitions allows midwives to better communicate with families during screening counselling, interpret genetic test results and appreciate how maternal health and environmental exposure influence fetal gene expression.

Where is the human genome stored?

Stored in the nucleus of our cells in the form of deoxyribonucleic acid (DNA)

What are the structural sides of the DNA made of?

Structural sides of the DNA ladder are composed of fluting units of a five carbon sugar called deoxyribose and a phosphate group.

What bond is between the sugar phosphate chain?

The repeating sugar-phosphate chain forms a backbone linked upstart by strong covalent bond. This provides structural stability to the molecule.

Double helix

DNA is synthesised from nucleotide building blocks twisted into a double helix structure.

Base pairing rules

The rungs of the DNA ladder are found by four chemical bases that bond via hydrogen bonds.

Coding genes

Acts as recipes that code for the synthesis of thousand of vital functional proteins required by the body to function.

Non coding genes

Act as a structural regulators for protein synthesis.

Protein synthesis

Code undergoes transcription to create single stranded messenger RNA/mRNA followed by translation. Translation- reads the mRNA strand to synthesise the corresponding functional protein.

Chromosome

Tightly packed bundles of DNA and proteins forming chromatin

Normal karyotype

Humans possess 46 chromosomes grouped into 23 homologous pairs inside somatic (body) cells. This includes 22 pairs of autosomes and 1 pair of sex chromosomes (XX for biological females; XY for biological males).

Diploid

Body cells are diploid (containing 2 copies of each chromosome)

Haploid

gametes (sperm and ovum) are haploid because they contain only 23 individual chromosomes

Stages of mitosis

Prophase

Metaphase

Anaphase

Telophase

Cytokinesis

Prophase

Chromosome condense and become visible the nuclear envelope breaks down.

Metaphase

Chromosome align along the equatorial plane (middle) of the cell

Anaphase

Sister chromatids are pulled apart by spindle fibres to opposite poles of the cell.

Telophase

Nuclear membranes arm around two new identical nuclei at opposite ends of the cell.

Cytokinesis

The cytoplasm splits to complete division into two independent, genetically identical diploid daughter cells.

Interphase

Here the cell grows, performs its routine metabolic functions and painstakingly creates an identical copy of its entire DNA blueprint.

Meiosis

Cell division dedicated to sexual reproduction and generating genetic diversity.

Meiosis I (Reduction Division)

Homologous chromosome pairs align, exchange genetic material via crossing over in Prophase I, and separate into two haploid cells.

Meiosis II (Equational Division)

Sister chromatids separate in a process similar to mitosis, resulting in four non-identical haploid gametes.

Recombination

pieces of DNA are broken and recombined to produce new combinations of alleles

Independent assortment

production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs.

Nondisjunction

where homologous chromosomes or sister chromatids fail to separate properly

Oogenesis

the biological process of forming mature female gametes (ova or egg cells) from precursor germ cells inside the ovaries

Alleles

Different variations or forms of the same gene

Homozygous

Inheriting two identical alleles for a specific gene or trait (inserting 2 copies of a mutated gene or 2 copies of a normal gene)

Heterozygous

Inheriting two different alleles for a specififc gene or trait. The dominant allele will be expressed over a recessive one. Recessive phenotypes are only observed if the individual is homozygous recessive.

Genotype

specific genetic makeup and allele combination present inside the cells

Phenotype

observable physical characteristics, biological trait or clinical condition produced as a result.

What is understanding genotype/phenotype essential for?

Antenatal screening and carrier testing

Silent mutations

Most copying errors occur in non-coding regions of DNA or do not alter the resulting amino acid sequence, having no observable effect on protein function or health.

Environmental triggers

Environmental mutagens such as ionising radiation, ultraviolet (UV) light, alcohol, and tobacco smoke increase the frequency of DNA copying errors and strand breakage.

The paternal link

New gene mutations are predeominantly inherited from the father (80%). Spermatogenesis requires high rate mitotic divisions throughout adult life so spontaneous mutation rate in sperm is much higher than in ova. This risk increases with advanced paternal age (>35 years) or paternal smoking within the 6 months prior to conception.

Autosmal dominant

Expressed when a single copy of the mutated gene is present, typically appearing in every generation. (Huntingtons’s disease)

Autosomal recessive

Requires 2 copies of the mutated gene to be expressed and can skip generations; parents can be unaffected carriers (CF/Sickle CEll)

Sex-linked recessive

Carried on the X chromosome. Primarily affects biological males beacuse they lack a second X chromosome to mask the recessive trait. (Haemophilia)

How are single gene disorders caused?

Caused by a mutation within a single gene sequence. They display highly predictable inheritance patterns easily tracked on pedigree charts.

Trisomy 21 (Downs Syndrome)

Presenece of 3 copies of chromosome 21

Trisomy 18 (Edward’s Syndrome)

Presence of 3 copies of chromosome 18

Trisomy 13 (Patau’s Syndrome)

Caused by the present of 3 copies of chromosome 13

Turner Syndrome eA

genetic configuration where a female baby inherits only 1 normal X chromosome (45,X)

Mosaicism

when a chromosomal division mistake happens after fertilisation during early embryonic development. The fetus develops with a mixed population of normal diploid cells and abnormal aneuploid cells within their organs.