Genetics Part.1 Dr. Mohammed Hussein M.B.Ch.B, MSC, PhD, DCH (UK), MRCPCH Genetics Part 1: Single-Gene Disorders Part 2: Population Genetics
Part 3: Cytogenetics Single-Gene Disorders Basic Definitions 46 Chromosomes 22 pairs of autosomes
(autosomal chromosomes) 1 pair of Sex chromosome The two chromosomes in each diploid pair are said to be homologs, or homologous chromosomes.
X and Y chromosomes, have some homologous regions but the majority of genes are different. No. of chromosomes Gametes (ova and sperms) are haploid, have one copy of each type of chromosome (122, X or Y). Somatic cells (cells other than gametes) are diploid, having two copies of each type of autosome (122) and
either XX or XY Gene. Physically a gene consists of a sequence of DNA that encodes a specific protein. Locus. The physical location of a gene on a chromosome is termed a locus. Alleles. Variation (mutation) in the DNA sequence of a gene produces a new allele at that locus. Many genes have multiple alleles.
Polymorphism. When a specific site on a chromosome has multiple alleles in the population, it is said to be polymorphic (many forms). Allele for brown eye color Locus for eye color gene
Homologus pair of chromosomes Allele for blue eye color Homozygous vs. Heterozygous alleles Homozygous if the individual has the same allele on both homologous chromosomes at that locus. Heterozygous if the individual has different alleles on
the two homologous chromosomes at that locus. Hemizygous if the individual has one allele on one of the two homologs. Genotype vs. Phenotype The genotype is the specific DNA sequence at a locus The phenotype is generally understood as the expression of the genotype in terms of observable
characteristics Mutations The Recurrence Risk The recurrence risk is the probability that the offspring of a couple will express a genetic disease.
Pedigrees (Family Tree) A patients family history is diagrammed in a pedigree. I 1 2
II 1 3 2 4
5 III 1 2 3
4 IV 1 2 3
5 6 Child 2 in Generation IV IV-2 4
Child 11 in Generation III III-11 Mendelian Inheritance 1. 2. 3.
4. Autosomal Dominant Inheritance Autosomal Recessive Inheritance Sex - linked Dominant Inheritance Sex - linked Recessive Inheritance Autosomal Dominant Inheritance This is the most common mode of Mendelian inheritance.
Caused by alterations in only one copy of a gene pair. This mean the condition occurs in the heterozygous state despite the presence of an intact copy of the relevant gene. AD genes are located on the autosomal chromosomes, so males and females are equally affected. Suppose that A is a dominant allele that cause a disease
And a is a recessive allele that is normal Homozygous AA , aa Heterozygous Aa
AA and Aa has disease aa is normal aa Aa Normal
Diseased Aa A Diseased (Hetrozygous) a
a A a a a a A a
a a 50 % Disease 50 % Normal aa Normal
Punnett Square Aa A Diseased (Hetrozygous) a
A A A A a 75 % Disease
a A a a a 25 % Normal Aa
Diseased (Hetrozygous) AA A Diseased (Homozygous) A
a A a A a a
A a A a aa Normal 100 % Disease
Achondroplasia (Dwarfism) Autosomal Dominant Pedigree AA Aa aa aa
AA Aa aa Familial hypercholesterolemia Huntington disease Neurofibromatosis type I
Marfan syndrome Achondroplasia Spherocytosis Neurofibromatosis type I Autosomal Recessive Inheritance AR alleles are clinically expressed only in the homozygous state.
The offspring must inherit one copy of the disease causing allele from each parent. Suppose that a is the disease allele and it is recessive.
So the normal allele is the dominant (A) Homozygous AA , aa Heterozygous Aa AA no disease aa has the disease Aa has no disease (but he/she is a carrier for the
abnormal gene), Heterozygous carrier Aa A Heterozygous carrier a
50 % Heterozygous Carrier A A A A a
75 % No Disease a A a a a 25 % Disease
Aa Heterozygous carrier Aa A Heterozygous carrier
a A A A A a
A A A A a AA Homozygous normal
50 % Normal 50 % Carrier Aa Aa
aa Carrier Risk This person wants to know his carrier risk I II III
IV IV-1 Carrier risk for the individual IV-1 is: a) 25% b) 33% c) 50% d) 67% e) 75%
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