
<ul>
  <li>The pattern of inheritance carried by genes on the sex chromosomes</li>
  <li>X-linked recessive: only expressed in females if homozygous</li>
  <ul>
    <li>Gene transmitted from an affected man to all his daughters</li>
    <li>Never from father to son</li>
    <li>
      Females can be affected if homozygous or if they are &ldquo;manifesting&rdquo;
      heterozygotes/carriers
    </li>
  </ul>
</ul>
<Note>
  <p>
    Where the trait is fatal (eg. Duchenne&rsquo;s), selection drives against inheritance of the
    trait, therefore new mutations comprise a large proportion of cases
  </p>
</Note>
<ul>
  <li>
    In some X-linked recessive conditions, a small number of female heterozygotes are affected due
    to lyonization: inactivation of one member of a pair of X chromosomes in every female somatic
    cell.
  </li>
  <ul>
    <li>
      The inactivated X chromosome is known as the Barr body and inactivation occurs early therefore
      all subsequent cells have the same inactive X chromosome. Summary of Lyon hypothesis:
    </li>
    <ul>
      <li>One X chromosome is active in somatic cells</li>
      <li>The second is the inactive Barr body</li>
      <li>Inactivation occurs from between days 3 and 10 after fertilisation</li>
      <li>Inactivation is random</li>
      <li>All clonal descendents of a cell have the same inactive X</li>
      <li>
        Females are mosaics with respect to X-linked genes (hence there is phenotypic variability in
        X-linked disorders)
      </li>
      <li>
        This explains the fact that a single X produces the same quantity of product in males as two
        X chromosomes do in females
      </li>
      <li>Some segments of the Barr body may still be active</li>
    </ul>
  </ul>
</ul>
<ul>
  <li>
    Females have random expression of the either the paternal or maternal X chromosome so there may
    be variable expression of mutant X-linked genes in carriers&nbsp;
  </li>
  <ul>
    <li>
      Including in X-linked retinitis pigmentosa, choroidaemia, X-linked Nettleship-Falls ocular
      albinism, Fabry disease and colour vision disorders
    </li>
  </ul>
  <li>
    X-linked dominant: used to describe conditions in which heterozygous females are commonly
    affected. Disease is usually very severe in males eg. Aicardi syndrome (male foetus rarely
    survives)
  </li>
  <ul>
    <li>Hypophosphataemic rickets</li>
    <li>Aicardi syndrome. Triad</li>
    <ul>
      <li>Chorioretinal lacunae</li>
      <li>Agenesis of the corpus callosum</li>
      <li>Infantile spasms</li>
      <li>Also: coloboma, microphthalmos</li>
    </ul>
    <li>Incontinentia pigmenti</li>
    <li>Alport&rsquo;s syndrome (5%, the majority being X-linked recessive)</li>
  </ul>
</ul>


Genetics

X-Linked Inheritance

Intro

FRCOphth Part 1 Study Notes

Orbit and ocular adnexae

Orbit

Extraocular Muscles

Orbital Blood Vessels

Paranasal Sinuses

Lacrimal Glands

Tear Film

Eyelids

Blinking

Anterior segment - ocular surface

Conjunctiva

Conjunctivitis

Sclera

Cornea

Corneal Structure And Development

Corneal Physiology

AC to Lens

Angle

Ciliary Body

Aqueous Humour Dynamics

Iris

Pupil

Lens

Posterior segment

Choroid

Vitreous

Retinal Macrostructure

Neurosensory Retina

Retinal Pigment Epithelium

Neuroanatomy

Venous Sinuses

Cerebral Ventricles

Cerebral Blood Supply

Supranuclear Gaze Pathways

Head And Neck Autonomic Nervous Supply

Head And Neck Glands

Osteology

Nerves

Muscles And Pharyngeal Arches

Extraocular Nerves

Sensory Nerves

Physiology

Physics Of Blood Flow

Neuronal Control Of Blood Vessels

Oxygen-Haemoglobin Dissociation Curve

Cytochrome p450

Pituitary Gland

Thyroid Hormone Production

Insulin

Glucagon

Parathyroid Hormone

Corticosteroids

Catecholamines

Cardiac Cycle

Renal Physiology

Fluid Balance

Acid-Base Balance

Muscle Physiology

Sensory Fibres

Cutaneous Receptors

Endocytosis

Glycosaminoglycans

Condensed Ocular Biochemistry

Eye Movements

Accomodation

Vision

Visual Cycle And Phototransuction

Visual Pathway

Visual Acuity

Colour Vision

Binocular Vision

Pathology

Collagen

Tissue Damage

Inflammation

Disease Entities

Wound Healing

Thrombosis / Atherosclerosis

Cell Growth

Neoplasia

Histopathology

Nystagmus

Free Radicals And Antioxidants

Immunology

Ocular Immunology

Microbiology

Viral Infections

Bacterial Infections

Fungal Infections

Protozoa

Nematodes

Antimicrobials

Chromosomes

Mitosis

Meiosis

Genes

Polypeptide Synthesis

Histones

MicroRNA

Hardy-Weinberg Equilibrium

Mutations

Chromosome Defects

Autosomal Inheritance

Mitochondrial Inheritance

Multifactorial Inheritance