Genetic System

Gene

• A gene is the basic physical and functional unit of heredity. Genes are small sections of DNA that carry genetic information (1^✔ ✔Trusted Source
  Basic Genetics Information
  
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• The human genome contains approximately between 20,000 and 25,000 genes (2^✔ ✔Trusted Source
  What is a gene?
  
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• Every person has two copies of each gene, one inherited from the mother and one from the father (3^✔ ✔Trusted Source
  Why is genetics important to my family and me?
  
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• All humans are 99.9% genetically identical; only 0.1% of our genetic make-up differs (4^✔ ✔Trusted Source
  Genetics vs. Genomics Fact Sheet
  
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Chromosome

• Chromosomes exist in pairs. Human cells normally have 23 pairs of chromosomes (total 46 chromosomes).
• Chromosomes are divided into 22 numbered pairs (autosomes) and one sex chromosome pair (5^✔ ✔Trusted Source
  Chromosomal DNA and Its Packaging in the Chromatin Fiber
  
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Sex Chromosomes

• Sex chromosomes (X and Y) are specialized chromosomes that carry genes that determine an individual's sex (6^✔ ✔Trusted Source
  Sex Chromosome
  
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• Females have two X chromosomes, and males have one X and one Y chromosome (7^✔ ✔Trusted Source
  Chromosome
  
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• The child inherits an X chromosome from his mother. The father may provide an X or a Y. The father's chromosome determines whether the child is born male or female.
• The X chromosome is around three times larger than the Y chromosome, with approximately 900 genes, whereas the Y chromosome contains only 55 genes (8^✔ ✔Trusted Source
  Y Chromosome Infographic
  
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Chromosome 9

• The gene that determines the blood group of an individual is found on chromosome 9 (9^✔ ✔Trusted Source
  A Brief History of Human Blood Groups
  
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Siblings

• Siblings share an average of 50% of their genes (10^✔ ✔Trusted Source
  Why are siblings so different? The significance of differences in sibling experiences within the family
  
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Identical Twins

• Identical twins have identical DNA but not identical fingerprints (11^✔ ✔Trusted Source
  Are fingerprints determined by genetics?
  
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• On average, dizygotic or non-identical twins share 50% of their genes, whereas identical or monozygotic twins share 100% of their genes (12^✔ ✔Trusted Source
  Twin Studies: A Unique Epidemiological Tool
  
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Genetic Disorders

• Genetic conditions are caused by mutations in one or more genes that are passed down from parents to offspring (13^✔ ✔Trusted Source
  Genetic Disorders
  
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• Consanguineous marriage (cousin marriage) increases the risk of genetic diseases in offspring (14^✔ ✔Trusted Source
  Genes and genetics
  
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