Genetic and Chromosomal Abnormalities

By King Meera -
Geneotype: genetic makeup containing both expressed and unexpressed characteristics
Phenotype: observable characteristics
  • Mutations: permanent alteration in genes or chromosomes that may produce harmful characteristics (at least 1.6 harmful mutation per person in each generation)
  • Many defects are hereditary and people pass them on 
  • Mutation leads to abnormalities in genes/chromosomes
  • (can occur spontaneously or be induced by environmental hazards like radiation)
  • Many disorders are a result of inherited disposition and environmental factors before and/or after birth - Ex: ADHD is a behavioral disorder transmitted multifactorially -> combination of genetic and environmental factors to produce certain complex traits
  • Not all abnormalities are visible at birth:
  1. Sachs - not visible until 6 months
  2. Sickle cell anaemia -"-
  3. Cystic fibrosis - not seen until age 4
  4. Glaucoma - not seen until middle age
  5. Huntington's disease -"-
  • 1 in every 156 children in Western countries have chromosomal abnormalities
  • some disorders are caused by an extra sex chromosome - XXY or a missing one - XO
DOWN Syndrome is a chromosomal disorder characterized by moderate-to-severe mental retardation and by some physical signs such as a downward sloping skin fold at the inner corner of the eyes
  • Caused by extra 21st chromosome (in 95% cases, from mother's ovum) or translocation of part of the 21st chromosome onto another
  • 1 in every 700 babies born alive has it
  • more likely to be hereditary
  • more than 70% live into their 60s but are at a special risk of developing Alzheimer's
  • many adults with Down Syndrome live in small group homes and tend to do well in structured jobs
DEFECTS TRANSMITTED BY DOMINANT INHERITANCE
  • Mendel discovered that characteristics can be passed on from parent to child by dominant or recessive inheritance
  • Normal genes are usually dominant but at times abnormal genetic traits may dominate
  • Ex: Huntington's Disease and Achondroplasia
  • Recessive defects expressed only if child receives same recessive gene from each biological parent
  • Inbreeding increases chances of genetic defects
  • Transmitted defects through recessive inheritance are more likely to be lethal at an early age
  • Dominant inheritance is less likely to be lethal at an early age 
  • A dominant inheritance-transmitted defect, if eradicated before age of reproduction, saves the next generation
  • Recessive transmitted defects are in the carriers (who may not have it) and are passed on
INCOMPLETE DOMINANCE
  • Partial expression of trait
  • person having partly dominant or partly recessive traits may not suffer from the disease but may show some manifestation of the condition

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