CH 4- extensions from mendelian genetics

What happens if there are more than 2 alleles

• Wild type(most common version)→ letters with a + superscript or just +

• Other alleles→ letters with a letter superscript (d^a etc)

Multiple alleles math rules for # alleles, kinds of genotypes, kinds of homozygs, kinds of heteros

• n= # of alleles

• Number of possible genotypes for a single gene with n alleles, includes both homozygotes and heterozygotes.

  • So not TTYyRr because these are 3 different genes (colour, height, etc)

• n(n+1)/2= kinds of genotypes

• n= kinds of homozygotes

• n(n-1)/2= kinds of heterozygotes

• Think of blood type IAIA, etc

Complete dominance

• 1 allele is dominant to the other

• Heterozygote phenotype same as homozygote

• ex) AA, Aa both same phenotype

Complete recessiveness

• recessive allele is phenotypically only expressed when its homozygous

Codominance

• 2 alleles at a locus produce different and detectable gene products in heterozygote

  • Both expressed

• ex) AB blood type

Blood antigen mechanism

• ABO blood group gene encodes glycosyltransferases

  • this adds sugars to pre-existing glycolipids

• Glycolipids are associated with rbc membranes

• SO H antigen→ A antigen when a-N acetyl galactoasmine added

• H antigen→ B antigen when galactose added

What is the H antigen

• a pre-existing glycolipid

• Encoded by a dominant H allele

  • Bombay blood type lack H allele (Fut 1 gene)

  • (have 2 inactive h alleles (hh))

Incomplete dominance

• When 1 allele not completely dominant to another allele of the same gene

  • get a blend

• Phenotype is of the heterozygote

  • (lies bw either heterozygote)

• ex) snapdragons- red RR x white rr = pink Rr

• ex) Tan Palomino horse→ hetero so it’s tan

Codominance at the molecular level

• products result from both alleles

Incomplete dominance at the molecular level

• with loss of function alleles

• only 1 allele product gets expressed

  • 1 allele produces nothing/little so you dont get enough to produce the full phenotype→ resulting in an intermediate phenotype

Gene sufficiency

• =How many copies of a gene are required to give a normal phenotype

• Haplosufficient- only 1 gene copy is enough to give a normal phenotype

• Haploinsufficient- 1 gene copy not enough to give full colour for instance→ results in incomplete dominance

• Threshold effect

  • The gene’s activity must stay above a certain level to maintain a normal phenotype. Falling below that level (as in haploinsufficiency) causes the phenotype to appear

Essential genes

• genes that when mutated can result in a lethal phenotype

• required for surviva;

Lethal allele

• allele that results in the death of an organism

• Can be dominant/recessive

• can be lethal at

  • diff life stages

  • only under certain envt conditions

Recessive lethal alleles

• Death occurs when lethal allele homozygous

• will produce a phenotypic ratio out of 3 instead of 4 bc one died (if it was dominant then ratio would be small so 1/1)

Example of human recessive lethal allele

• Tay sachs disease

• caused by lack of the enzyme that breaks down lipids in lysosomes

  • hexosaminidase A

• Results in excess lipids in CNS (neurons die)

• death bw 3-4 age

Human example of dominant lethal alleles

• (only takes 1 copy so Aa or AA you’re done)

• Huntington disease

• huntington protein has too many glutamine AAs leading to progressive degeneration of brain cells

• 30-40 yrs age