Chromosomes & Alleles




Humans have 23 pairs of chromosomes:

  • 22 pairs of autosomes
  • Final pair determines sex 
  • XX or XY
  • Ova have X chromosome, sperm determines X or Y


Homozygous dominant (AA) – dominant allele expressed

Homozygous recessive (aa) – recessive allele expressed

Heterozygous (Aa) – dominant allele expressed

Codominant alleles: IA & IB are A & B proteins, I0 = no proteins 

I0I0 = blood group O

IAIA or IAI0 = blood group A

IBIB or IBI0 = blood group B

IAIB = blood group AB (codominance)

Parents AABB and aabb > F1 will all be AaBb > F2 will give 9:3:3:1 ratio of phenotypes

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