Polygenic Inheritance, Pleiotropy and Environmental Effects on Gene Expression

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  • Created by: rosieevie
  • Created on: 23-05-17 13:07

Polygenic Inheritance (Multiple Genes)

Inheritance showing continuous variation

Examples - height, weight, skin colout

Phenotypes quantifies by measurement

Two or more genes each with two alleles that are either additive or neutral 

Effect of each individual allele small but together a large significant effect on phenotype

Analysis of polygenic inheritance requires very large progeny

Dihybrid polygenic inheritance example - kernal colour in wheat

Trihybrid polygenic inheritance example - human skin pigmentation

Many gene inheritance - corolla length in tobacco

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Multiple Alleles

Single genes w/ more than two alleles

Different mutations in same gene result in range of allele phenotype

Unlike multiple genes, one allele expressed at a time (diploid - pair)

Example - human blood groups

Example - White eye in drospohila

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Penetrance and Expressivity

Phenotype can be product of complex interaction between an organisms' genetic make up and environment 

Example - retinoplastoma

Caused by dominant allele

Not all individuals develop the tumour (incomplete penetrance in population) and tumours can be different severities (different expressivity between individuals)

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Penetrance

Penetrance - proportion of individuals showing expected phenotype in population

In 100% penetrance, all individuals express phenotype

Example - Tay-Sachs disease shows penetrance as all homozygous develop disease

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Expressivity

Expressivity - degree of expression of a phenotype of a particular gene in individual

Example - human polydactyl controlled by dominant allele - not all individuals express phenotype

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Environmental Effects on Gene Expression - Tempera

Example - Himalayan allele in rabbits

  • Dark fur when reared at low temperature = black tips to ears, feet, tail
  • If reared in hot - no dark fur
  • Cold pack put on rabbit - dark fur induced
  • Change in fur colour because pigment allele expressivity altered

Example

  • Recessive allele codes for temperature sensitive tyrosine enzyme
  • Inactive at cat core body temperature
  • Induced in cold temperatures
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Environmental Effects on Gene Expression - Nutrit

Example - lactose intolerance

  • Young children produce lactase = break down lactose
  • In some adults capacity drops 
  • Intolerance phenotype symptoms - diarrhoea, abdominal cramps
  • Prevalent in many Asian countries
  • Reduced using soya or reduce lactose milk
  • Expressivity different in different individuals

Example - Human phenylketonuria (PKU)

  • Autosomal recessive disorder
  • Defect in enzyme metabolising phenylalanine
  • Amino acid build up causes brain damage in children
  • Low phenylalanine diet - prevents mental retardation
  • Nutrition alters expressivity in individuals
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Pleiotropy

Pleiotropy - single gene controls or influences multiple (possibly unreleated) phenotypic traits

Example - PKU

  • Mental retardedness
  • Blue eyes
  • Light skin colour

Example - sickle cell anaemia

Example - Siamese cats and white tigers tend to be cross eyed

Example - cats w/ white fur and blue eyes often deaf

  • Melanin fur pigment produced by melanocytes
  • Melanocytes regulate correct ion balance inear for transmission of signals to brain
  • Lack of melanocytes = white fur and deafness
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