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Understanding patterns of inheritance This presentation builds on session 1 exploring patterns of inheritance.

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Presentation on theme: "Understanding patterns of inheritance This presentation builds on session 1 exploring 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 inherited Understand the differences between the inheritance patterns associated with Autosomal dominant, Autosomal recessive, X- linked recessive and chromosomal abnormalities Understand that the environment can impact on some common complex conditions

3 So how are genes passed on from parent to child? Genes in the cell nucleus are physically located on 23 pairs of chromosomes One set of 23 chromosomes is inherited from each parent Therefore, of each pair of genes, one is inherited from a person’s mother, and one from their father Chromosome Gene Diagram 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 Male Alterations in single genes Variants in genes Chromosomal imbalance Single Gene Disorders Multifactorial diseases Chromosome disorders + environment

5 Some 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 dominant Autosomal recessive X-linked Single gene disorders

6 Recessive Homozygotes must have two copies of the altered gene to be affected Dominant These individuals are called Heterozygotes with one copy of the altered gene they are affected X-linked recessive Males with an altered gene on the X- chromosome are always affected Male

7 Examples of Autosomal Dominant Conditions Huntington disease Neurofibromatosis type 1 Marfan syndrome Familial hypercholesterolemia Familial Adenomatous Polyposis (FAP) Prader-willi Autosomal dominant inheritance

8 Fig. 3.2 ©Scion Publishing Ltd Marfan syndrome (a) Arachnodactyly (long fingers). (b ) Dislocated lens. Autosomal dominant inheritance

9 Parents Gametes Autosomal dominant inheritance

10 Parents Gametes Autosomal dominant inheritance Affected Unaffected At conception

11 Examples of Autosomal recessive conditions Sickle Cell disease Cystic fibrosis Batten Disease Congenital deafness Phenylketonuria (PKU) Spinal muscular atrophy Recessive blindness Maple syrup urine disease Autosomal recessive inheritance

12 Fig. 1.2 ©Scion Publishing Ltd Photos (a) and (b) courtesy of Dr Tim David Cystic fibrosis (a) The outlook for cystic fibrosis patients has improved over the years but they still need frequent hospital admissions, physiotherapy and constant medications. (b) Chest X-ray of lungs of cystic fibrosis patient. © Erect abdominal film of newborn with meconium ileus showing multiple fluid levels. Photos (a) and (b) courtesy of Dr Tim David, Royal Manchester Children’s Hospital.

13 Fig. 4.1 ©Scion Publishing Ltd Sickle cell disease. (a) Blood film showing a sickled cell, marked poikilocytosis (abnormally shaped red cells) and a nucleated red cell. (b and c) Bony infarcations in the phalanges and metacarpals can result in unequal finger length.

14 Parents AUTOSOMAL RECESSIVE INHERITANCE Parent 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 Parents Sperm/Eggs AUTOSOMAL RECESSIVE INHERITANCE A parent who is a carrier passes on either the usual gene or the altered gene into the eggs or sperm The 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 sperm

16 Parents AUTOSOMAL RECESSIVE INHERITANCE Affected Unaffected (carrier) Unaffected Sperm/Eggs Unaffected (carrier)

17 Examples of X-Linked Recessive Conditions Fragile X syndrome Haemophilia Duchenne muscular dystrophy (DMD) (Becker BMD) Fabry disease Retinitis pigmentosa Alport syndrome Hunter syndrome Ocular albinism Ocular albinism Adrenoleucodystrophy. Adrenoleucodystrophy. X-Linked recessive inheritance

18 Fig. 4.2 ©Scion Publishing Ltd Photos courtesy of Medical Illustration, Manchester Royal Infirmary (a and c), and Andrew Will (b) Effects of haemophilia (a) Bleeding around elbow. (b) A retinal bleed. (c) Repeated bleeds into joints produce severe arthritis.

19 X-linked recessive inheritance One copy of an altered gene on the X chromosome causes the disease in a male. Male An altered copy on one of the X chromosome pair causes carrier status in a female. XX XY Female

20 Daughter Daughter (Carrier) Son Son (Affected) Parents Gametes At conception X-linked inheritance where the mother is a carrier Father Mother X Y X X (Carrier) (Unaffected)

21 Polygeneic Inheritance Single gene disorders are quite rare Single gene disorders either give risk to a condition or they don’t Most traits are Polygenic’ i.e. 1 trait coded by a number of altered and unaltered genes working together

22 Common Polygenic Disorders Alzheimer's Diabetes Cancer Eczema

23 Multifactorial inheritance Inheritance controlled by many genes plus the effects of the environment Congenital malformations Cleft lip/palate Congenital hip dislocation Congenital heart defects Neural tube defects Pyloric stenosis Talipes Adult onset disorders Diabetes mellitus Epilepsy Glaucoma Hypertension Ischaemic heart disease Manic depression Schizophrenia

24 GENETIC ENVIRONMENTAL Duchenne muscular dystrophy Haemophilia Osteogenesis imperfecta Club foot Pyloric stenosis Dislocation of hip Peptic ulcer Diabetes Tuberculosis Phenylketonuria Galactosaemia Spina bifida Ischaemic heart disease Ankylosing spondylitis Scurvy The contributions of genetic and environmental factors to human diseases Rare Genetics simple Unifactorial High recurrence rate Common Genetics complex Multifactorial Low recurrence rate 100% genetic100% Environmental

25 Multifactorial Examples include some cases of cleft lip and palate; neural tube defects; diabetes and hypertension Caused by a combination of genetic predisposition and environmental influences Pattern – more affected people in family than expected from incidence in population but doesn’t fit dominant, recessive or X-linked inheritance patterns

26 Some medical conditions are caused abnormalities in chromosome number or structure. Chromosomal abnormalities

27 Chromosome anomalies Cause 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.

28 Most frequent numerical anomalies in liveborn Autosomes Down syndrome (trisomy 21: 47,XX,+21) Edwards syndrome (trisomy 18: 47,XX,+18) Patau syndrome (trisomy 13: 47,XX+13) Sex chromosomes Turner syndrome 45,X Klinefelter syndrome 47,XXY All chromosomes Triploidy (69 chromosomes)


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