multiple and missing alleles

• Alleles A and B are not dominant over each other but they are both dominant over O.
• Codominance is when two alleles are both expressed in the same phenotype.
• Sex linked inheritance is when a trait is controlled by a gene on the X chromosome.
• Because males only have one X chromosome, they are much more likely to show sex-linked recessive conditions.
• Females, having two copies of the X chromosome, are likely to inherit one dominant allele that masks the effect of the recessive allele.
• A female with one recessive allele masked in this way is known as a carrier; she doesn’t have the disease, but she has a 50% chance of passing it on to her offspring.
• Punnett square showing the inheritance of colourblindness, an X-linked condition:

  
• If alleles for a disorder appear on the sex chromosomes they are called sex-linked.