Biological basis of child health 1: understanding the cell and genetics

Abstract

This article is the first of a series that outlines the fundamental aspects of the biological basis of child health. Cells and genes are the basic units of life. Therefore, it is essential that nurses have knowledge of how cells function to understand normal physiology and pathophysiology, and how specific conditions are inherited. This article describes the components of the human cell, detailing their structure and function. It also discusses genetics, providing examples of inherited diseases including those caused by mutations that affect specific components of the cell. The aim is to provide children's nurses with an accessible introduction to cell biology and genetics linked to their clinical practice.

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Aim and intended learning outcomes
This article aims to review the fundamental
aspects of cell biology and how cellular
dysfunction underlies various human diseases.
It describes the common features of human
cells, including the structure and function of
different organelles (specialised cell structures
that perform specific tasks). The article
examines what happens when different cell
components are functionally impaired, linking
this to developmental disease and other
conditions of childhood. After reading this
article and completing the time out activities
you should be able to:
»Describe the different components and
functions of a human cell.
»Discuss the main consequences of
dysfunction of various cellular systems and
link these to conditions of childhood that
you may encounter in your clinical practice.
»Outline the stages of the cell life cycle, and
the role of deoxyribonucleic acid (DNA) and
ribonucleic acid (RNA).
»Describe the differences between dominant
and recessive inheritance.
»Explain various chromosomal disorders and
their causes.
Introduction
Cells are the basic building blocks that
make up the human body, and there
are about 37 trillion cells in the body
(Bianconi et al 2013). Although all cells share
common components, for example the cell
membrane, cytoplasm and organelles, and
have a similar general structure, they have
adaptations that specialise them to specific
functions. As a result, humans have about
200 different cell types. These range from
motor neurons, which can extend axons
more than 1 metre long to conduct nerve
impulses (for example the axon that stretches
from the base of the spine to a muscle in
the big toe), to red blood cells that are
biconcave in shape to aid the absorption and
transport of oxygen.
Citation
Davies K , Meimaridou E (2020)
Biological basis of child
health 1: understanding the
cell and genet ics. Nursing
Children and Young People.
doi: 10.7748/ncyp.2020.e1047
Peer review
This art icle has been subject to
open peer rev iew and has been
checked for plagiarism using
automated s oftware
Correspondence
kate.davies@lsbu.ac.uk
\@sausmash
Conflict of interest
None declared
Accepted
28 October 2019
Published online
March 2020
Why you should read this article:
● To improve your knowledge of the fundamentals of cell division and genetics
● To understand how errors during deoxyribonucleic acid (DNA) replication can cause mutations in genes
● To learn the laws of inheritance and how genes are passed from parents to offspring
Biological basis of child health 1:
understanding the cell and genetics
Kate Davies and Eirini Meimaridou
Abstract
This article is the first of a series that outlines the fundamental aspects of the biological basis of child
health. Cells and genes are the basic units of life. Therefore, it is essential that nurses have knowledge
of how cells function to understand normal physiology and pathophysiology, and how specific
conditions are inherited. This ar ticle describes the components of the human cell, detailing their
structure and function. It also discusses genetics, providing examples of inherited diseases including
those caused by mutations that affect specific components of the cell. The aim is to provide children’s
nurses with an accessible introduction to cell biology and genetics linked to their clinical practice.
Author details
Kate Davies, senior lecturer in non-medical prescribing, London South Bank University and honorary
research fellow in paediatric endocrinology, Barts and The London School of Medicine and Dentistry,
Queen Mary University of London, London, England; Eirini Meimaridou, reader, School of Human
Sciences, London Metropolitan University, London, England
Keywords
child health, ethical issues, genetic disorders, genetic testing, genetics

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Key points
● The nucleus is the
control centre for
cells in the body,
and contains the
genetic material
deoxyribonucleic
acid (DNA), which is
packaged as 23 pairs of
chromosomes
● Mitosis is the process
of cell division,
where the two nuclei
produced are identical
to the ‘mother’
nucleus. Meiosis is the
process of cell division
specific to oocytes and
sperm cells
● Biological inheritance
relies on DNA
replication. Sometimes
during replication,
there is change in the
base sequence, called
a mutation
● The information
stored in DNA that
determines hereditary
characteristics is
separated into genes.
The different copies
of the gene that occur
on each of a pair of
chromosomes are
known as alleles
● In dominant
inheritance, a mutant
gene from one parent
causes disease, even
when the copy of
the gene the child
inherits from the other
parent is normal. In
recessive inheritance,
both copies of a gene
must be mutated to
cause disease
© RCN Publishing Company Limited 2020
TIME OUT 1
Consider the ways you have attempted to describe cells
to patients or their parents. What did you wish to convey
in your explanation? Why did an understanding of the
cell seem important in relation to the patient ’s illness or
treatment that you were hoping to explain? Note down
your answer; we will return to this later with reference to
pathophysiology
Cell structure and function
Human cells are between 5 and 50
micrometres in diameter; for reference,
10 micrometres is about 200 times smaller
than the diameter of the head of a pin.
Figure 1 shows the structure of a mammalian
cell. Using a basic light microscope, it is
possible to see the nucleus and surrounding
cytoplasm, which consists of membrane-bound
organelles and intracellular fluid (Tortora and
Derrickson 2010).
Cell membrane: the external boundary
The cell membrane, also known as the plasma
membrane, maintains the integrity of the
cell by separating the cytoplasm from the
external environment. It is semi-permeable,
allowing some substances, such as oxygen and
carbon dioxide, to pass freely by diffusion
(the movement of a substance from a region
of higher concentration to a region of lower
concentration) (Tortora and Derrickson 2010),
while controlling the entry of other substances.
Nucleus: the control centre
The largest organelle is the nucleus. It contains
the genetic material DNA, which encodes all
of the information necessary to make a cell
and guide cellular processes (Peate and Nair
2015). The DNA is packaged as 23 pairs of
chromosomes (46 chromosomes in total),
which will be discussed in detail later in this
article. A cell with 23 pairs of chromosomes is
known as a diploid cell.
Mitochondria: the power station
Cellular processes require a source of energy.
In animals, this energy comes in the form of
calories from the food they consume, but the
molecular systems inside cells cannot use these
calories directly. Instead, the energy must be
kept in a readily available form that can fuel
cells. This is achieved by cells using an energy
carrier molecule called adenosine triphosphate
(ATP) (Ward and Linden 2013), which can be
broken down to form another molecule called
adenosine diphosphate (ADP), providing an
accompanying release of energy.
Mitochondria are the power stations of
cells, functioning to produce ATP through
the process of cellular respiration. They
comprise two membranes.
Mitochondrial disease
Mitochondrial disease encompasses a group
of hereditary disorders that can develop in
adulthood, but frequently manifest at birth or
early infancy. These are multi-system disorders
that frequently affect organs with high energy
demands such as the brain, heart and muscles.
Clinical features include suboptimal growth,
muscle weakness and neurological conditions,
for example the neurodegenerative disease
Leigh syndrome and mitochondrial DNA
depletion syndrome (Tuppen et al 2010).
The clinical features and age of onset of
mitochondrial diseases vary significantly.
Since there is no cure for mitochondrial
disease, researchers have developed the ‘three-
person baby’ fertility technique as a risk-
reduction treatment for mitochondrial disease
(Amato et al 2014). In this technique, if the
mother has mitochondrial disease, the nucleus
from a fertilised egg is transplanted into the
egg of a healthy donor where the nucleus
has been removed, before it is implanted into
the uterus. As a result, the baby is born with
normal mitochondrial DNA from the female
donor and nuclear DNA from the parents.
Mitochondrial disease is the most common
inherited neurometabolic disorder of
childhood, with an estimated incidence of 1 in
5,000 live newborns (Pérez-Albert et al 2018).
Diagnosis relies on the characteristic clinical
features, whereby three or more body systems
are affected without an underlying diagnosis,
for example liver failure, encephalopathy,
uncontrolled seizures and an increase in muscle
weakness (Senger et al 2018).
Endoplasmic reticulum: the protein factory
The endoplasmic reticulum (ER) consists of
an interconnected network of membrane-
enclosed tubules and sacs, known as cisternae,
which are continuous with the outer layer
of the nuclear envelope. The ER functions
in the production of proteins and lipids and
can be divided into two types – smooth ER
and rough ER – which differ in structure and
function (Peate and Nair 2015). Rough ER
works as a ‘factory’ for synthesising proteins
and molecules. Smooth ER is important for the
production of lipids and can also have a role in
metabolising these to make other biomolecules,
including steroid hormones, such as oestrogens
and testosterone, and fatty acids.
The ER protein factory has an inbuilt quality
control system that can detect proteins that
have not been manufactured correctly. One
of the most studied examples of this is cystic

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fibrosis, a genetic disease caused by mutations
in the membrane protein cystic fibrosis
transmembrane conductance regulator (Lukacs
and Verkman 2012).
Golgi apparatus: the processing plant
and distribution centre
The Golgi apparatus consists of a stack of
cisternae. It is usually located near the ER and
there may be more than one Golgi apparatus
per cell. The Golgi apparatus functions as
a processing plant and distribution centre for
proteins and lipids that are synthesised in the
ER, and its main function is to modify and
package proteins (Tortora and Derrickson
2010). For example, to enable some proteins
to function effectively, they need to be
chemically modified by having specific sugar
molecules attached.
Lysosomes: membrane-enclosed vesicles
that contain digestive enzymes
Human cells can have several hundred
lysosomes. The inside of lysosomes contains
enzymes that break down biomolecules
including nucleic acids, proteins, lipids and
carbohydrates. Lysosome disorders are linked
to a group of approximately 50 rare inherited
disorders. These are known as lysosomal
storage diseases and occur because genetic
mutations cause loss of function of specific
lysosomal enzymes. This means the cell is
unable to break down a specific biomolecule,
for example a lipid or glycoprotein, leading to
its accumulation.
Examples of lysosomal storage diseases
include Gaucher’s disease, where the function
of the enzyme beta-glucocerebrosidase is lost,
and Tay-Sachs disease, which can result in
the destruction of cells in the spinal cord and
brain. For some lysosomal storage diseases,
there are potential therapies that aim to replace
the missing enzyme, and gene therapy is also
being explored (Sun 2018).
Cytoskeleton: the scaffolding
and train tracks
The cytoskeleton is a network of filaments
that is found throughout the cytoplasm of
animal cells (Ramaekers and Bosman 2004).
It controls the shape of cells (Peate and Nair
2015) and how organelles are organised
within the cells. This includes facilitating the
movement of organelles and other cellular
components along microtubules, which act
as cellular train tracks. It also provides the
mechanical support that enables cells to
divide and move.
Primary cilia: the signalling antenna
The majority of vertebrate cells have
a single slender sensory organelle, known
as the primary cilia, that emanates from
their surface (Marieb and Hoehn 2007a).
Cilia function to regulate several important
signalling pathways in cells, including
pathways important in human development
and homeostasis. Mutations in the genes that
are required for normal cilia function lead to
a group of disorders with overlapping clinical
phenotypes that have been collectively termed
ciliopathies (Reiter and Leroux 2017). One of
the most studied of these diseases is Bardet-
Biedl syndrome, which can be caused by
mutations in more than 14 genes (Zaghloul
and Katsanis 2009), and is characterised by
retinal degeneration, obesity, polydactyly
(additional fingers or toes), hypogonadism,
renal anomalies (kidney malformations and/or
malfunctions) and learning disabilities.
Cell life cycle
To understand how genes carried in
chromosomes are passed on, it is important
to understand the cell life cycle and cell
division. The cell life cycle can be described as
a series of changes a cell must undergo until
it reproduces (Marieb and Hoehn 2007b). It
includes interphase, where the cell grows and
continues with its usual functions, and cell
division, which is also known as the mitotic
phase. Oocyte and sperm cells undergo
a separate type of cell division, known as
meiosis. It is useful to understand the stages of
Nucleus
Smooth endoplasmic
reticulum
Cytosol
Lysosome
Mitochondrion
Centrioles
Microvilli
Microfilaments
Microtubules Intermediate filaments
Chromatin
Peroxisome
Golgi apparatus
Ribosomes
Rough
endoplasmic
reticulum
Plasma membrane
Nucleolus
Nuclear envelope
Figure 1. Structure of a mammalian cell
©Francesca Corra

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Figure 2. Stages of mitosis
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mitosis and meiosis because errors are likely to
occur at these stages.
Mitosis
Mitosis is the process by which two ‘daughter’
nuclei are produced, with exactly the same
genes as the ‘mother’ nucleus (Marieb 2014).
DNA replication occurs shortly before mitosis
begins, so, for a short while, the mother
nucleus has a ‘double dose’ of genes. The
stages of mitosis are shown in Figure 2.
Meiosis
In meiosis, the diploid cells containing two
complete sets of chromosomes are separated,
and each oocyte or sperm cell ends up with
one chromosome (haploid), rather than two
chromosomes (Cedar 2012).
Meiosis occurs in relation to reproduction,
and it forms gametes in preparation for
fertilisation, leading to the formation of
a zygote, which eventually develops into
a fetus (Boore 2016). Meiosis is split into
meiosis 1 and meiosis 2. Instead of producing
two daughter cells, four daughter cells are
reproduced. This results in spermatogenesis
and oogenesis. In males, spermatogonia
develop into spermatozoa through meiosis,
which takes approximately 64 days. In
contrast, in females, oogenesis begins in the
fetus at week 12 of gestation, but stops at
week 20, when the primary oocytes remain in
the prophase stage until ovulation at puberty.
Genetics
Many diseases and conditions that children’s
nurses encounter in their clinical practice are
caused by, or are a result of, an issue at cellular
level. Therefore, it is important that you
understand genetics and the genetic diseases
that can manifest in childhood.
The main function of genes is heredity, and
they store information in the form of DNA
in the cells (Knight and Andrade 2018a).
The study of genetics involves exploring
specific traits that are passed down from one
generation to the next.
Genes comprise DNA. Every single person’s
DNA – apart from identical twins – is unique.
Each gene has codes for a specific protein, and
it details which amino acids must be joined
in a certain order. Inside a cell’s nucleus are
chromosomes, which are made up of long
strands of DNA.
Knowledge of the role of DNA and
chromosomes enables children’s nurses to
understand disease processes and how traits
are passed down.
Deoxyribonucleic acid
DNA is a long, double-stranded polymer:
that is, a double chain of nucleotides (Marieb
and Hoehn 2007a). Nucleotides are the basis
of nucleic acids, and are made up of three
subunit molecules:
»A nitrogen-containing base.
»A five-carbon sugar.
»At least one phosphate group.
The double helix structure of DNA is
composed of two strands of DNA, with two
linked, coiled chains of nucleotides. There are
four nucleotide bases in DNA:
»Adenine (A).
»Guanine (G).
»Cytosine (C).
»Thymine (T).
These nucleotide bases pair up with each
other – A with T and G with C – to form
base pairs. These base pairs, along with sugar
and phosphate, form the nucleotides. Two
strands of long nucleotides wind around
each other to form a double helix, and there
are ten pairs of nucleotides per complete
turn of the helix.
DNA is found mainly in the nucleus
of the cell (nuclear DNA), but a small
amount can be found in the mitochondria
of the cell (mitochondrial DNA). DNA has
two main roles:
»It replicates itself before cell division,
ensuring that any information in the next
cells is identical.
»It provides instructions on how to build
proteins in every cell of the body.
Interphase Prophase
Metaphase Anaphase Telophase
PaternalMaternal

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Once DNA synthesis begins, it continues
until all the DNA has been replicated.
The basis for biological inheritance relies
on DNA replication; a process by which
the DNA double helix uncoils and slowly
separates into its two nucleotide chains,
then acts as a template for the new joining
strand (Marieb 2014).
Once reformed, the new double helix should
be identical to the one before. Sometimes
during this replication process, there may be
a change in the base sequence (McLafferty et al
2012), known as a mutation. Mutations can be
minor or major, with major mutations having
a significant effect on protein synthesis, and
therefore on the cell’s function. Because there
are so many nucleotide bases – about three
billion base pairs – it is possible that errors will
occur (Knight and Andrade 2018b).
Ribonucleic acid and protein synthesis
DNA in genes provide instructions to make
proteins, but first the information in a specific
part of the DNA must be transcribed to
make a molecule known as messenger
RNA (mRNA). This process takes place
in the nucleus.
Transcription
DNA itself is not involved in protein synthesis;
an intermediate molecule known as mRNA
is required, which is made through a process
known as transcription. RNA is a single-
stranded molecule. Its structure is similar to
that of DNA, except the deoxyribose – the
modified sugar – is replaced with ribose
– a simplified, five-carbon ‘normal’ sugar
(Knight and Andrade 2018b). In addition,
thymine (T) is replaced with uracil (U).
Translation
Once mRNA exits the nucleus and enters the
cytoplasm, it binds to ribosomes in the cell
cytoplasm or ER to initiate translation. In
essence, the ‘language’ of nucleic acids (base
sequence) is ‘translated’ into the language of
proteins (an amino acid sequence) and follows
on from transcription.
Protein synthesis is one of the most
fundamental biological processes that occurs
in all organisms, and knowledge of this can
assist nurses to understand medical and
pharmacological processes. For example,
some genetic disorders involve errors at the
level of RNA splicing, where a nucleotide
sequence is removed and exons (the DNA
sequence and the corresponding sequence
in RNA transcripts) are joined just before
translation. The antibiotic rifampicin, which
is used to treat severe bacterial infections such
as tuberculosis or meningitis, blocks bacterial
transcription. Other antibiotics also target the
translation phase, such as: chloramphenicol,
which is commonly used for eye infections;
tetracycline, which is used to treat severe
acne; and erythromycin, which is commonly
used to treat respiratory infections (Pritchard
and Korf 2013).
TIME OUT 2
Have you used antibiotics such as rifampicin,
chloramphenicol, tetracycline or erythromycin in your
clinical practice? Does knowledge of where these
antibiotics work enable you to understand the disease
process, and explain the purpose of the medicine to
patients and their families? Practise explaining this
with a colleague
Chromosomes
A chromosome is a DNA molecule packaged
into thread-like structures tightly coiled around
proteins called histones. Chromosomes carry
the genetic information in the genes.
All cells in the human body (except gametes,
for example ova and sperm) contain 46
chromosomes, and are arranged in pairs.
In men and women, there are 23 pairs of
chromosomes: there are 22 identical, or
homologous pairs, which are called autosomes.
An autosome is any chromosome that is
not a sex chromosome. In females, the
23rd pair is identical: XX, which is two X
chromosomes. However, in males, there are
two different chromosomes: XY, which is
one X chromosome and one Y chromosome
(Pritchard and Korf 2013).
Every chromosome makes a substance
called chromatin. The main functions
of chromatin are:
»To ensure DNA is packaged densely.
»To enable mitosis.
»To prevent damage to DNA.
»To assist in controlling DNA replication.
During interphase (Figure 2), the chromosome
is in a relaxed state and is partially
unwound. This assists in DNA replication
and protein synthesis. However, once the
cell prepares to divide, the chromosome
begins to tighten and package, to facilitate
the separation of the chromosomes. During
early mitosis, the chromosome can be visible
under a microscope. The shorter arm of the
chromosome is called the p arm and the longer
arm is the q arm. At the tips are telomeres,
which form a kind of ‘cap’ to protect the
chromosome from degradation. Scientists
can ‘read’ chromosomes, examining three
main features:

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»Size.
»Banding pattern.
»Position of the centromere.
Banding patterns are patterns of light and dark
transverse bands that become apparent when
stained under a microscope, and are similar to
a barcode. These bands describe the location of
specific genes on a chromosome. For example,
a reading of 7q31 would indicate that it is
chromosome 7, on the long q arm, and band
position number 31 – the cystic fibrosis gene.
A picture of chromosomes is called
a karyotype, which cytogeneticists use to
examine the chromosomal features under
a microscope. Obtaining a karyotype involves
a simple blood test.
Laws of inheritance
The basic principles that determine how genes
are inherited are determined by the laws of
inheritance (Boore 2016). The understanding
of how characteristics are passed on was
derived from Gregor Mendel’s experiments
with plants (Pritchard and Korf 2013), which
demonstrated that characteristics combine
and segregate in types of mathematical
proportions. The information stored in DNA
that determines hereditary characteristics is
separated into genes. Because humans have
two copies of each chromosome, and therefore
two copies of each gene in the corresponding
position (or genetic locus) on each
chromosome, they are described as diploid.
The different copies of the gene that occur on
each of a pair of chromosomes are known as
alleles. There can be a difference in the DNA
code between the alleles, and when this occurs
the individual is described as heterozygous for
that gene. If both alleles of the gene are the
same, they are known as homozygous.
The laws of inheritance are as follows
(Pritchard and Korf 2013):
»Law of dominance – alleles occur in pairs,
but one allele can be dominant and the other
recessive.
»Law of segregation – during meiosis, allele
pairs separate, so each gamete has only one
allele from each pair. They are restored again
at fertilisation.
»Law of independent assortment – genes of
different traits can segregate independently
during the formation of gametes. However,
this can vary because some genes can be
on the same chromosome, and therefore
can be inherited together. Figure 3 shows
the Punnett Square (Boore 2016), which
demonstrates the probability of the offspring
of two heterozygous parents having
a particular genetic make up.
Dominant inheritance
In dominant inheritance, a mutant gene from
one parent causes disease, even when the
copy of the gene the child inherits from the
other parent is normal. The abnormal gene
is therefore described as dominant (Knight
and Andrade 2018c). Its expression depends
on that gene ‘winning’, for example being
replicated, when the other parent’s gene is
added. Some genetic diseases are only carried
on dominant genes. Conditions that are
known to be autosomal dominant include
achondroplasia, Marfan syndrome and
Huntington’s disease (Cedar 2012).
Recessive inheritance
In recessive inheritance, both copies of a gene
must be mutated to cause disease. People
with only one defective allele in the gene are
known as carriers. When both parents are
carriers, there is a 25% chance that a child
will inherit two mutant copies of the gene (one
from each parent) and develop disease. The
chance of the child inheriting one normal and
one abnormal copy of the gene, such that they
are also a carrier of the disease, is 50%, and
the chance of them inheriting no mutant copy
of the gene is 25%. In both these instances,
the child will not inherit the recessive disease.
Each pregnancy will have a 1 in 4 chance of
the offspring having the condition. Examples
of autosomal recessive conditions include
cystic fibrosis, sickle cell anaemia and
phenylketonuria.
Heterozygous parent Aa
Heterozygous
parent Aa
AA
aa
Aa
Aa
A
A
a
a
A = dominant gene; a = recessive gene
»AA (homozygous dominant) — offspring will show the dominant
characteristic
»Aa (heterozygous) — offspring will show the dominant
characteristic
»aa (homozygous recessive) — offspring will not show the dominant
characteristic
Figure 3. Punnett Square demonstrating the
probability of the offspring of two heterozygous
parents having a particular genetic make up

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TIME OUT 3
Access the Cystic Fibrosis Trust charity website at:
tinyurl.com/CFT-cascade-screening, and read the area
that explains how cystic fibrosis is diagnosed and passed
on. Do you think the website relays the information in a
way that means families can understand the process?
Does it relate to the information in this article? Practise
explaining the inheritance process with a colleague
Chromosomal disorders
There are various chromosomal disorders that
can lead to a range of clinical conditions. It
is estimated that about 50% of miscarriages
are because of a chromosomal disorder
(Robinson 2010). Chromosomal disorders can
occur as a result of errors at the chromosome
pairing stage of meiosis (Pritchard and Korf
2013), when the chromosomes cross over, or
at disjunction (where the chromatids – one
half of a replicated chromosome – separate
in anaphase). Errors at disjunction can
cause aneuploidy (an abnormal number of
chromosomes in a cell). Additional or missing
chromosomes in a cell is a common cause of
genetic disorders.
Monosomy
Monosomy arises if there is a missing
chromosome. It is rare but can be seen in
children with monosomy of the X chromosome,
which is known as Turner syndrome. This is
the only survivable condition where an entire
chromosome is missing (Knight and Andrade
2018c). Defects and complications associated
with Turner syndrome include short stature,
webbed neck, cardiac anomalies, swollen
hands and feet and an increased risk of hip
dysplasia. An individual’s intellectual abilities
are not usually affected, although their lifespan
can be shortened.
Many monosomy conditions occur as
a result of partial losses of chromosomes,
where a part is missing but may be attached
to another part of a chromosome, which is
known as translocation. Other monosomy
conditions can occur as a result of errors
during mitosis, which are usually caused by
chemical exposure or some types of cancer
(Robinson 2010).
Trisomy
Trisomy occurs when there is an additional
chromosome. The most common of these
conditions is trisomy 21, or Down’s syndrome,
where there are three copies of chromosome
21. Down’s syndrome affects about one
in every 800 children, and is caused by
errors in meiosis.
Other trisomy conditions include trisomy
18 (Edwards syndrome) and trisomy 13
(Patau syndrome). Trisomy 18 is a life-
limiting disorder characterised by severe
psychomotor and growth retardation,
microcephaly, microphthalmia, malformed
ears and micrognathia or retrognathia.
Trisomy 13 is rare and usually incompatible
with life, with the average survival being
about three days. The infant can be affected
by holoprosencephaly, polydactyly, flexion of
the fingers, rocker-bottom feet, facial clefts,
neural tube defects and cardiac anomalies.
Chromosomal rearrangements
Chromosomal rearrangements involve the
occurrence of large-scale changes, such as:
»Duplication – occurs when parts of
the chromosome are copied more than
once, therefore increasing the length
of the chromosome. These can also be
classified as partial trisomies. One form
of autism is considered to result from
duplication of part of chromosome 8
(Papanikolaou et al 2006).
»Inversion – occurs when part of the
chromosome has been inverted, which
results in a reversal of the gene sequence.
Haemophilia type A may be caused
by an inversion in the X chromosome
(Tantawy 2010).
»Deletion – where large parts of the
chromosome are missing. This happens
either during the interphase of the cell life
cycle, or because of unequal crossings over
during meiosis. For example, Cri-du-Chat
syndrome occurs as a result of a deletion
on chromosome 5, estimated to affect
one in every 15,000 to 50,000 live births
(Liverani et al 2019). Children with Cri-du-
Chat syndrome often have a characteristic
cry like a kitten, hence the name. Deletion
of part of the long arm of chromosome
15 results in Prader-Willi syndrome
(Angulo et al 2015).
»Translocation – involves large parts of
the chromosome swapping over. It can be
reciprocal, where the exchange is balanced
(Figure 4a), and is the most common
type of chromosomal rearrangement.
Conversely, the translocation can be non-
reciprocal, where the exchange is unbalanced
(Figure 4b), and thereby resulting in some
deletions. Chromosomes 11 and 22 are
often involved in reciprocal translocations,
resulting in birth defects such as cardiac
abnormalities and cleft palate, as well
as a hereditary form of breast cancer
(Robinson 2010).

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Genetics of sex
The sex phenotype of a human is determined
by two chromosomes: X and Y. Usually,
females have two X chromosomes (XX),
while males have one X and one Y
chromosome (XY). The X chromosome
contains approximately 900-1,200 genes and
is important for development; if it is missing,
the developing zygote cannot progress. X
chromosome genes are responsible for red
blood cell formation, blood clotting and
muscle development and function.
Embryological development for sexual
differentiation begins at about week 4 of
gestation. How these genes develop next
depends on whether there is ovarian or
testicular tissue present. If there is no Y
chromosome, two genes work together to give
the embryo the female phenotype.
In comparison to the X chromosome, the
Y chromosome is small. Individuals can
survive with one X and no Y chromosome,
which has led geneticists to believe that the Y
chromosome is not required for survival. Most
of the genes on the Y chromosome are involved
in male sex determination and sexual function.
Sex-linked inheritance
Traits that are determined by genes on the sex
chromosomes are said to be ‘sex linked’.
X-linked inheritance
Females can have dominant or recessive
properties of their X-linked genes
(Pritchard and Korf 2013). Most sex-linked
conditions are caused by X-linked recessive
alleles in males.
A man (XY) receives his X chromosome
from his mother, and would pass it to every
daughter he has, resulting in mother and
daughter being ‘obligate carriers’ of any
X-linked recessive gene that has been expressed
by the male. The rules of X-linked recessive
inheritance are (Pritchard and Korf 2013):
»The ‘mutant allele’ is passed from an affected
man to every daughter.
»A carrier mother passes the mutant allele to
half her sons who express it, and half her
daughters who do not.
»The mutant allele is never passed from
father to son.
The rules of X-linked dominant conditions are
(Pritchard and Korf 2013):
»The condition is expressed and transmitted
by males and females.
»The condition occurs twice as frequently in
females as it does in males.
»An affected man always passes the condition
to his daughter but never to his son.
»An affected woman passes the condition to
half her sons and half her daughters.
X-linked recessive conditions are usually
expressed in males; since females have two
copies (XX) and males only one (XY),
males are usually more likely to be affected.
Therefore, there is a chance of females having
a healthy gene to ‘compensate’ for a defective
gene. However, females may be carriers of
these conditions.
Well known X-linked recessive conditions
include Duchenne muscular dystrophy,
Fragile X syndrome, haemophilia and
red-green colour blindness (Pritchard and
Korf 2013). X-linked dominant conditions
are much less common, and include Rett
syndrome, a rare neurological disorder that
is fatal in males, and Alport syndrome, which
affects the kidneys and hearing (Knight and
Andrade 2018c).
Y linked inheritance
Y linked inheritance is where genes are
inherited from the Y chromosome and occurs
only from fathers to sons. The sex-determining
region Y (SRY) gene is responsible for the
Figure 4. Balanced and unbalanced chromosomal
translocation
Chromosome A
Chromosome B
Normal chromosomes of
parent 1
Chromosomes of parent 2
with balanced translocation
4a) Balanced chromosomal translocation
4b) Unbalanced chromosomal translocation

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evidence & practice / CPD / cell biology
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sex determination. Another trait passed down
on the Y chromosome is one for hairy ears
(Marieb and Hoehn 2007c, Robinson 2010).
Future of genetics
In the future, genetics and how it is applied
in medicine will be at the forefront of medical
science. Future developments will include
gene therapy (Cedar 2012), where genetic
material is inserted into cells to compensate
for abnormally structured or functioned genes.
In addition, gene editing will also be possible
in the future, whereby scientists can modify an
organism’s DNA.
Ethical issues will need to be considered
carefully, because decisions will need to be
made about whether genes are ‘faulty’ and
what is ‘normal’ in society. For example,
advances in prenatal testing have led to
suggestions that the incidence of Down’s
syndrome would fall as a result. However,
further research is beginning to identify
ways to improve lives of people with
Down’s syndrome, such as improvements in
learning ability and memory, through stem
cell research (Diamandopoulos and Green
2018). Alternatively, types of screening are
performed as part of in vitro fertilisation (IVF)
to identify if fertilised eggs contain a defective
gene, and the parents can decide whether to
have these embryos implanted.
Conclusion
It is important that children’s nurses have
knowledge of cell biology and genetics to
support their understanding of certain diseases
and treatments, and to enable them to explain
these to parents. This article has provided
an overview of genetics, emphasising the
importance of understanding the fundamental
genetic processes that are particularly relevant
to nurses’ clinical practice. Since it is likely
that children’s nurses will be involved in caring
for children with inherited conditions, it is
essential that they have an awareness of how
inheritance works. As technology advances,
the study of diseases where multiple genes have
a contribution, as well as the ethics of gene
editing, will provide future challenges.
TIME OUT 4
Consider how understanding cell biology and
genetics relates to The Code: Professional Standards
of Practice and Behaviour for Nurses, Midwives and
Nursing Associates (Nursing and Midwifery Council
2018) or, for non-UK readers, the requirements of your
regulatory body
TIME OUT 5
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Biological basis of child health 1:
understanding the cell and genetics
TEST YOUR KNOWLEDGE BY COMPLETING THIS MULTIPLE-CHOICE QUIZ
1. Cells:
a) Are the basic building blocks that make up the
human body c
b) Share common components, for example the cell
membrane, cytoplasm and organelles c
c) Have a similar general structure, but have adaptations
that specialise them to specific functions c
d) All of the above c
2. Which statement is false?
a) Mitochondria produce adenosine triphosphate (ATP) c
b) Mitochondria are single membrane organelles c
c) Mitochondria are the power stations of cells c
d) There is no cure for mitochondrial disease c
3. What is the main function of the Golgi apparatus?
a) To translate ribonucleic acid (RNA) during protein
synthesis c
b) To degrade damaged organelles and proteins c
c) To modify and package proteins c
d) To produce the cellular energy source ATP c
4. What happens during interphase?
a) The cell grows and continues with its usual functions c
b) The cell produces two daughter nuclei c
c) The cell splits c
d) The chromosomes of the cell re-form c
5. Which of these is not a nucleotide in
deoxyribonucleic acid (DNA)?
a) Adenine c
b) Guanine c
c) Thymine c
d) Uracil c
6. What is an autosome?
a) A sex chromosome c
b) Any chromosome that is not a sex chromosome c
c) The centre of a chromosome c
d) A chromosome with only one copy c
7. The Law of Independent Assortment states that:
a) Alleles occur in pairs, but one allele can be more
dominant than the other, and the other recessive c
b) Genes of dierent traits can segregate independently
during the formation of gametes c
c) During meiosis, allele pairs separate so each gamete
has only one allele from each pair. They are restored
again at fertilisation c
d) The sex phenotype of a human is determined
by two chromosomes: X and Y c
8. What is recessive inheritance?
a) Where both copies of a gene must be mutated to
cause disease c
b) Where a gene received from the father is defective c
c) Where a mutant gene from one parent causes disease c
d) Where the mother is a carrier of a disease c
9. Translocation is a type of chromosomal
rearrangement in which:
a) Parts of the chromosome are copied more than once c
b) Large parts of the chromosome are missing c
c) Part of the chromosome has been inverted c
d) Large parts of the chromosome are swapped over c
10. Y linked inheritance:
a) Occurs only from mothers to daughters c
b) Is expressed and transmitted by males and females c
c) Occurs only from fathers to sons c
d) Is a recessive form of inheritancec
1. d 2. b 3. c 4. a 5. d 6. b
7. b 8. a 9. d 10. c