Presentation on theme: "Understanding patterns of inheritance"— Presentation transcript:
1 Understanding patterns of inheritance This presentation builds on session 1 exploring patterns of inheritance
2 Patterns of inheritance The objectives of this presentation are to:Understand how genes are inheritedUnderstand the differences between the inheritance patterns associated with Autosomal dominant, Autosomal recessive, X-linked recessive and chromosomal abnormalitiesUnderstand that the environment can impact on some common complex conditions
3 So how are genes passed on from parent to child? ChromosomeGeneGenes in the cell nucleus are physically located on 23 pairs of chromosomesOne set of 23 chromosomes is inherited from each parentTherefore, of each pair of genes, one is inherited from a person’s mother, and one from their fatherDiagram showing just one pair of the 23 pairs of chromosomes in the cell nucleus. The location of one of the genes on this chromosome is shown.
4 Classification of genetic disorders Single Gene DisordersMaleMutations in single genesMultifactorial diseases+ environmentVariants in genesChromosome disordersChromosomal imbalance
5 Single gene disordersSome medical conditions are caused by a change in just one or both copies of a particular pair of genes. These are called “single gene disorders”.The three common types of single gene disorders are called:Autosomal dominantAutosomal recessiveX-linked
6 Heterozygotes with one copy of the altered gene are affected DominantHeterozygotes with one copy of the altered gene are affectedRecessiveHomozygotes with two copies of the altered gene are affectedMaleX-linked recessiveMales with one copy of the altered gene on the X-chromosome are affected
7 Examples of Autosomal Dominant Conditions Autosomal dominant inheritanceExamples of Autosomal Dominant ConditionsHuntington diseaseNeurofibromatosis type 1Marfan syndromeFamilial hypercholesterolemiaFamilial Adenomatous Polyposis (FAP)
14 AUTOSOMAL RECESSIVE INHERITANCE ParentsParent who are carriers for the same autosomal recessive condition have one copy of the usual form of the gene and one copy of an altered gene of the particular pair
15 AUTOSOMAL RECESSIVE INHERITANCE ParentsSperm/EggsA parent who is a carrier passes on either the usual geneThe other parent who is also a carrier for the same condition passes on either the usual gene or the altered gene into his/her eggs or spermor the altered gene into the eggs or sperm
19 X-linked Recessive Inheritance MaleFemaleXYXXOne copy of an altered gene on the X chromosome causes the disease in a male.An altered copy on one of the X chromosome pair causes carrier status in a female.
20 Y X Parents Gametes X X (Unaffected) (Carrier) Father Mother X-linked inheritance where the mother is a carrierFatherMotherYXParents(Unaffected)(Carrier)GametesXXAtconceptionDaughterDaughter (Carrier)Son (Affected)Son
21 Multifactorial inheritance Inheritance controlled by many genes plus the effects of the environmentClinical clue: One organ system affectedAdult onset disorders Diabetes mellitus Epilepsy Glaucoma Hypertension Ischaemic heart disease Manic depression SchizophreniaCongenital malformations Cleft lip/palate Congenital hip dislocation Congenital heart defects Neural tube defects Pyloric stenosis Talipes
22 Rare Genetics simple Unifactorial High recurrence rate The contributions of genetic and environmental factors to human diseasesGENETIC ENVIRONMENTALDuchennemuscular dystrophyHaemophiliaOsteogenesis imperfectaClub foot Pyloric stenosis Dislocation of hipPeptic ulcer DiabetesTuberculosisPhenylketonuria GalactosaemiaSpina bifida Ischaemic heart disease Ankylosing spondylitisScurvyRare Genetics simple Unifactorial High recurrence rateCommon Genetics complex Multifactorial Low recurrence rate
23 MultifactorialExamples include some cases of cleft lip and palate; neural tube defects; diabetes and hypertensionCaused by a combination of genetic predisposition and environmental influencesPattern – more affected people in family than expected from incidence in population but doesn’t fit dominant, recessive or X-linked inheritance patternsOther patterns of InheritanceCleft lip and palate is an example of a condition which may have a genetic factor but can also have environmental causes. For example, certain drugs used to treat epilepsy are associated with an increased incidence of cleft lipGenetic factors underlie many common diseases such as cancer, heart disease and diabetesWhen there is significant family history, monitoring and preventative regimens can be offered. Genetic testing is currently very limited but much research is underway
24 Chromosomal abnormalities Some medical conditions are caused abnormalities in chromosome number or structure.Chromosome abnormalities was covered in “DNA, genes and chromosomes” presentation in lesson 1. This section provides a refresher of the previous lesson.
25 Chromosome anomaliesCause their effects by altering the amounts of products of the genes involved.Three copies of genes (trisomies)= 1.5 times normal amount.One copy of genes (deletions)= 0.5 times normal amount.Altered amounts may cause anomalies directly or may alter the balance of genes acting in a pathway.
27 Summary of Chromosome Anomalies Change in numbere.g. trisomy 21 Down syndrome; Edwards’ syndrome; Turner syndrome.Usually an isolated occurrence.Change in structuree.g. deletionsMay be inherited.Down’s SyndromeAn example of a chromosomal abnormality which causes learning difficultiesThe karyotype shows a case of Down’s syndrome (trisomy 21)For more information about Down’s syndrome, visit:[NOTES FOR TRAINING FACILITATORS IN CASE OF QUESTIONS ONLYIn total, there are normally 23 pairs of chromosomes, but in a trisomy there is an extra chromosome (tri somy = three bodies)A normal karyotype is written 46,XY or 46,XXThe karyotype, or complement of chromosomes, seen in a regular trisomy 21 is written as 47,XX +21 if female or 47, XY +21 if maleNote that Down’s syndrome can be inherited, although this is rare. These cases are due to a change in chromosome structure (a translocation)]Trisomy 21