CAM101 Lecture Notes - Lecture 11: Color Blindness, Genetic Disorder, Water–Electrolyte Imbalance
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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find more resources at oneclass.com