Biol 121- 2010.02.12- Genetics- Inheritance in Humans (Ch. 13, 19, 15).docx

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22 Apr 2012

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Biol 121 225
Freeman 286-289, 401-405, 316-318 Feb. 12, 10
-to understand transmission of human traits, must analyze human crosses that already exist b/c
making specific crosses will take too long (and ethical issues)!
-a pedigree (like a family tree) records genetic relationships among the individuals in a family along
with each person’s sex and phenotype with respect to the trait in question
-pedigree may reveal whether trait is due to dominant or recessive allele; and whether it is on a
sex chromosome or autosome
Autosomal recessive traits
-if a phenotype is due to an autosomal recessive allele, then
1) Individuals with the trait must be homozygous (recessive)
2) If parents of affected individual do not have the trait, they are both heterozygous
-a carrier is a heterozygous individual who carries a recessive allele for an inherited disease they
transmit the allele even though they do not exhibit signs of the disease
3) When carriers mate, they produce offspring with the recessive phenotype 25% of the time
Autosomal dominant trait
-individuals with at least one allele for the trait must have the phenotype
-unless new mutation occurs, any child with the trait must have a parent with the trait
-e.g. Huntington’s disease
-a marker is a trait that appears in people that have a specific disease
-e.g. in Huntington’s disease, people have many CAG repeats on chromosome 4 which
is unstable for the protein
-researchers found many CAG repeats in people with the disease and also found that
affected individuals always have this trait this means that this is the gene for the
disease, which they named IT15 and the protein product huntingtin protein.
Autosomal or sex-linked?
-if a trait appears about equally often in males and females, then likely to be
-if males much more likely to be affected, then probably X-linked
-e.g. Haemophilia is due to an X-linked recessive allele
-for X-linked recessive alleles, the trait appears in all males that carry it
-females may be carriers and are therefore heterozygous in such case
-for X-linked dominant alleles, every individual with the defective allele will have the
-a good indicator: affected male has all affected daughters but no affected sons
-this is because affected male must pass his X chromosome to his daughters but
passes only Y to his sons
Metabolic pathways
-a metabolic pathway is a pathway by which a specific compound is synthesized via a
series of steps
-for example, arginine synthesis:
Precursor Enzyme 1-> Ornithine Enz 2-> Citrulline Enz-> Arginine
-enzymes 1, 2, and 3 must be working in order for arginine to be synthesized properly
-if only enzyme 1 and 2 working, then only Citrulline can be produced
-if individual with defect for Enzyme 3 is crossed with individual with a defect for
Enzyme 1, then offspring may have all working enzymes
-if individual does not have working Enzyme 1, but has a food source of Ornithine or
Citrulline, then Arginine can be produced
Chimeras and Mosaics
-chromosomal abnormalities: Caster Semenya, woman or half-woman?
-chimeras are individuals that have two or more separate populations of cells
-e.g. zygote + additional sperm or egg
-two zygotes fuse
-each population contributes genetically to the zygote
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