Modifications of Mendel - L3+4

What is incomplete dominace

when hetrozygotes expression are a intermediate of dominant homo zygotes

Non - human example

• In 4 o’clock plant Mirabilis japonica → normally red buts some are white. If cross red x white all Pink showing incomplete dominance, if Red was dominant all F1 would be red . Then self ed F1 got ratio of 1R: 2P : 1W showing incomplete dominance

• In Mendles peas also see incomplete dominance, hetrozygotes are an intermediate between smooth and wrinkled. this is because recessives lack enzyme that build up starch and so fall back on themselves

Human example

• Tays sachs disease (loose muscle tone) allele common in Ashkenazi Jewish population

• it is a homozygouse recessive disease

• lack Hexosamiadase enzyme which is important for cell machinery

• homozygouse dominate for this allele have high activity of enzyme

• hetrozygotes have intermediate activity of enzyme

• no cure

• can be tested for it using heel prick

Codominance

genes alleles are both pheotyptically expressed in hetrozygotes

Non- human example

• in cows if cross red and white cow

• get a cow with red and white patches roan cow

• sowing co dominance both phenotypes are expressed

Multiple alleles

when 3 or more alleles can occupy a particular locus

Non - human example

• in clover leaves there are 5 allels that can determine that pathhern on them

  • In dorsophilia wild type +/+ is red eyes w/w is white eyes and know that there are other alleles at other loci which leads to different eye color

Human example

• in humans eye color is determined by multiple alleles at different loci

• The ABO blood setermiens the antigens on blood cells, there are 3 alleles IA, IB, IO (A and B dominant to O)

• multiple alleles for blood type A can be IAIA or IAIO same for blood type B

• A → anti B antibodies

• B → anti A antibodies

• O → anti A + B antibodies

• O is universal blood Donner

• need to give right blood in transfusions if not can lead to coagulation

• was used as paternity testing

Lethal alleles

• homozygotes may be lethal

Non-human example

• yellow mice crossed with normal mice offspring was 1:1

• when crossed yellow with yellow got 2:1 ration Y:N

• when dissected mother found another embryo YY that was dead showing YY togther lethal

• In Manx cat have no tail, having 2 Manx alleles is lethal

Human example

• Brachadactly short hands and stubby fingers - when homozyotes lethal

• Achondrplasia → form of dwarfism, short hand and legs struggle to breath as small lung capacity, homozygotes die

Pleiotropy

one gene effect many phenotypes/ many different effects

Non - human example

• all blue eyes cats are deaf because of reduced melanin so effecting melanin pathway

• In Tabby cats, tabby gene makes them more aggressive, tabby gene increases amount of melanin in brain and so creates more aggressive behavior

Human example

• Sickle cell anemia single base cahneg form A → T cahnges amino acid from Glu to Val which leads ot change in haemaglobin and its molecular properties

• this base change causes polymerization of Haemoglobin causing it to clump together which can lead to it to collapse in low oxygen level

• cells shape is sickle cell

• symptoms include → heart failure, anemia and also malaria resistance in hetrozygotes

Sex-limitation

expression of a phenotype is dependent on sex due to differences in hormones between M and F

Non - human example

• In peacocks both males and females have genes that code for elaborate tails but only expressed in males as they have testosterone

• females pick males based on tails

• tails help men to fight

Human examples

• in males have sex limited male pattern baldness which causes baling in males in presence of testosterone

• female don’t get this but can pass it on to males - can loose hair during menopause because of this when homo zygotes

• males more likely to get mental disorders e.g. autism and Schizophrenia

• Females with BRACA1 mutation more likely to develope breast cancer due to interaction with cell division genes and estrogen

Gene interaction

one genes presence modifies effect of another gene

Non - human examples

• Sex limitation: male and female peacocks

• A-B- → agouti

• A- bb- → cinimmon (new colour due to gene intercation)

• aaB- → black

• aabb → brown

• cc → while fur inhibits color example of epistasis

• ss → pieplad , DWS turns down pigment in Certain areas of body

Human exmples

• Sex limitation : male pattern boldness and BRACA1 mutation

• Hemoglobin HbF gene stops polymerization of HbS which helps with oxygen- so not collapse in low levels of oxygen helps with symptoms of sickle cell anemia

• Se are secraetors causing them to secraete H substances whihc can be identified in bodily fluids and saliver but sese are not secraetors → intercation with ABO blood system

Completmenation

different homozygouse recessive mutations complement each other

Non human examples

• Foxglove → normally purple but if cross white line with another area white line F1 = all white , F2 get 9 W : 7 P showing epistasis ww stops pigment from forming

• Complementaion testing → cross homozygouse green eyes with another line that is homozygouse blue eyed line:

  • If F1 red eyes thsi means 2 loci are involved and complement ecah other, they are hetrozygouse at both loci

  • if F1 blue or green eye they are at the same locus

• this was the way they worked out biochemical pathway in drosphillia 2 patwhyas ; one makes red pigment and another makes brown pigment

Human examples:

• Deafness can be caused by number of different loci : inner ear, cilia, bones , outer ear …

• deafness can be reccesive or domiant or can be syndormic or non-syndromic

• Syndromic deafness → comes with other health issues e.g Wardenburg syndrome, deafness associated with white stripe in hair and mental abndormalities

• Non-syndormic deafness → normal deafness , just deaf

Complementation

• If parents are homozygouse for 2 muations that cause deafness and have a baby - if ababy is hetroygouse at both loci child is not deaf, most children of deaf parents are not deaf

Envirnmental influence

• Iodine deficiency in pregnancy leads to deafness

• Viral infection e.g. Rubella, measles can lead to deafness

• long exposure to load noise can lead to deafness

  Gene-environment interaction:

  • Skin pigmentation may affect susceptibility: darker-skinned individuals may be more resistant to noise-induced hearing loss