These notes are a 5-page summary of GTS161 study unit 4. They provide a detailed but concise overview of the lecture notes with colour images to aid understanding. They were written by a student who obtained a distinction in the module.
Study unit 4 - Chapter 5
Mendelian Interac-ons
• Assump-ons - 2 alleles per gene / complete dominance / genes located on different
chromosomes
• 1 gene, 2 alleles, complete dominance - GR: 1:2:1 / PR: 3:1 (monohybrid)
• 2 genes, 2 alleles, complete dominance - GR: 1:2:1:2:4:2:1:2:1 / PR: 9:3:3:1 (dihybrid)
Impacts on Mendel’s principles
• Allelic interac-ons - at a single gene varia-ons on dominance and number of alleles
• Gene interac-ons - 2 or more genes code for one phenotypic character
• Sex-based influences - expression of genes determined by the presence of make of
female hormones
What influences mendelian ra-os for a single gene
1. Types of dominance
§ Intralocus interac-on
§ Phenotype is determined by the interac-on between the proteins produced by
alleles A and a of the same gene
§ Complete dominance - 1 copy of the dominant allele is sufficient to produce the
dominant phenotype
§ Incomplete dominance - heterozygote phenotype is a mix between the two
homozygous
§ co-dominance - clear expression of both alleles
§ MN blood group - LM allele codes LM an-gen / LN allele codes LN an-gen
2. Level of phenotype observed may affect dominance type
§ molecular (cells) / physiological (what is happening to body)/ organismal (overall
phenotype)
3. Penetrance and expressivity
§ Penetrance - percentage of individuals having a par-cular genotype that express
the expected phenotype
§ Expressivity - the degree to which the trait is expressed
§ influenced by other genes and environmental factors
4. lethal alleles
§ causes death at early stage of development (embryonic development)
§ 2:1 phenotypic ra-o
§ some genotypes don’t appear
§ a true dominant lethal allele can only be transferred if expressed late
5. Mul-ple alleles
§ for some loci - more than two possible alleles in a popula-on
, § inheritance paYern same as 1 gene, 2 alleles
§ number of possible genotypes = n(n+1)/2 (n= alleles)
ABO blood group
• An-gen - glycolipid / protein expressed on the surface of a cell (gives a biochemical
iden-ty)
• An-body - produced by body and recognizes and binds to specific an-gen (destroys
foreign cells)
• IA - enzyme that adds 6th sugar (encodes for A an-gen) (pink)
• IB - another enzyme adding the 6th sugar (purple)
• ii - 5 sugars (no sixth sugar added)
• Donor an-bodies are diluted so much that agglu-na-on of recipients red blood cells
is not an issue
• O - universal donor
• AB - universal recipient
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