HMB265H1 Lecture Notes - Lecture 4: Melanin, Albinism, Consanguinity

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Lecture 4: Mendelian Traits in Humans & Human Pedigree Analysis
Mendelian inheritance and humans: same principles apply
Albinism: autosomal recessive condition
o Two copies of A produce Tyrosine enzyme melanin
Aa is also sufficient to produce melanin
aa no tyrosine albino
Autosomal Inheritance
Human autosomal traits are located on the non-sex chromosomes (1-22)
They may be inherited as:
o Autosomal dominant
o Autosomal recessive
Homozygous dominant and heterozygotes exhibit the
affected phenotype
Males and females are equally affected and may
transmit the trait
Affected phenotype does not skip a generation
Autosomal Recessive
Only homozygous recessive individuals exhibit the affected phenotype
Males and females are equally affected and may transmit the trait
May skip generations
2 recessive alleles are needed for the phenotype to be expressed
Pedigrees
Cannot do controlled breeding experiments on humans, use model organisms and human
pedigrees to dissect Mendelian traits of interest
o Humans have a very long generation time
Pedigrees are an orderly diagram of a family’s relevant genetic features extending through multiple generations
Pedigrees help us infer it a trait is from a single gene and if the trait is dominant or recessive, autosomal or sex-
linked
Dominant Traits
Affected children always have at least one affected parent
As a result, dominant traits show a vertical pattern of inheritance: the trait shows up in every generation
Two affected parents can produce unaffected children, if both parents are heterozygotes
Vertical pattern of inheritance in pedigrees
o Affected individual in every generation
Recessive Traits
Affected individuals can be the children of two unaffected carriers, particularly as the result of consanguineous
matings
All the children of two affected parents should be affected
Rare recessive traits show a horizontal pattern of inheritance: the trait first appears among several members of one
generation and is not seen in earlier generations
Recessive traits may show a vertical pattern of inheritance if the trait is extremely common in the population
Most traits are recessive
Dominant deleterious traits do not continue to manifest
Gene loadlethal recessive alleles
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HMB265H1 Full Course Notes
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