IB Biology: Theoretical Genetics

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4.3 Theoretical Genetics

Genotype > the alleles possessed by an organism
Phenotype > the characteristics of an organism
Co-dominant Alleles > pairs of alleles that both affect the phenotype when presented in a heterozygote
Carrier > an individual that has a recessive allele of a gene that does not have an effect on phenotypes


Blood Group: Possible Genotype:
A [I^A] [I^A] or [I^A] [i]
B [I^B] [I^B] or [I^B] [i]
AB [I^A] [I^B]
O [i] [i]

Blood Groups

  • Controlled by three alleles; A, B and O
  • Both A and B are co-dominant; they are both dominant over group O
  • O can act as a universal donor whose blood cells can be given to any other genotype without cause agglutination


> individuals of group O can only receive transfusions from a group O donor
> blood cells of group AB cannot be given to any other genotype, although AB individuals may receive blood from any other blood group
> blood cells of group A cannot be given to the recipients of group B and vice versa


Males and Females

  • Two chromosomes determine the gender of a child
  • The sex chromosomes are in pair 23; XX = female and XY = male
  • The Y chromosome is smaller than the X chromosome and so carries fewer genes
  • Some genes are present on the X chromosome and absent from the shorter Y chromosome
  • When females reproduce they pass on one X chromosome in the egg
  • When males reproduce they pass on either one X or Y chromosome in the sperm

 Sex Linkage

  • The association of a characteristic with gender, because the gene is controlling the characteristic is located on a sex chromosome; examples include red-green color vision and the production of Factor VIII (helps blood clotting)

Pedigree Charts

  • Can be used to deduce whether a character is caused by dominant or recessive alleles and whether it is sex linked or not
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