CAM101 Lecture Notes - Lecture 11: Color Blindness, Genetic Disorder, Water–Electrolyte Imbalance

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8 Jun 2018
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Learning Objectives
To understand the basic principles of genetics, including:
Dominant and recessive inheritance
Sex linked inheritance
Complex/polygene inheritances
To relate the basic principles of genetics to the inheritance of human traits
Genetic Variation
Recombination
Exchange of chromosome parts during meiosis I
Independent Assortment of Chromosomes
~8.5 million (223) different gametes can potentially result from the independent assortment of
homologous chromosomes
Random Fertilisation of Egg by Sperm
Born with ~1 million eggs
100 million sperm/mL
Allele
Each chromosome contains many genes
Each gene occupies a defined position on a specific chromosome (locus)
Any two matched genes at the same locus are called alleles
One is maternal and other paternal
Code for the same (homozygous) or alternate (heterozygous) forms of a given trait
Versions of the same gene (one paternal one maternal)
Homozygous - identical alleles
Heterozygous - different alleles
Genotype and Phenotype
Genotype
Sequence of DNA
Phenotype
Physical manifestation of the genotype (measurable trait/observation)
Pedigrees
Genetic representation of a family tree which details inheritance patterns for a particular
disease/phenotype. Inheritance of a trait depicted via family tree.
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Mendelian Inheritance
Mendelian Traits
Single gene
Clear pattern of inheritance
Complete penetrance
Law of Segregation
Allele pairs segregate during gamete formation so each gamete only carries one allele for each gene.
i. A gene can exist in more than one form
ii. Two alleles are inherited for each trait
Law of Independent Assortment
Genes for different traits segregate independently in gamete formation
Law of Dominance
If one dominant allele is present, the phenotype will express dominant allele
Types of Inheritance
Autosomal Dominant
Dominant allele is visible even if paired with a recessive allele
Heterozygous with one copy of the dominant allele are affected
AA and Aa genotypes = same phenotype
If variant/mutant gene copy (A) results in increased function or gain of a deleterious function, then
only one copy is needed to generate disease phenotype e.g. Huntington’s disease
Autosomal Recessive
A recessive allele is only visible when paired with another recessive allele
Homozygotes with two copies of the recessive allele are affected
Different example genotypes
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Aa = Carrier
aa = affected
If variant or mutant gene copy (a) represents a loss of activity, need BOTH mutant alleles to generate
disease phenotype (aa) (e.g. cystic fibrosis, loss of function)
Mendelian Inheritance
Punnet square with 2 heterozygous parents
Potential offspring genotypes: ¼ BB, ½ Bb, ¼ bb
Co-Dominance
Contribution of both alleles is visible in phenotype
Eg. ABO blood grouping is determined by 3 alleles, A, B and O
A and B are co-dominant and produce different surface proteins
O produces no protein
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