Codominance & Sex-Linked Characteristics | Biology for GCSE/IGCSE - Year 11 PDF Download

Codominance

• Codominance is observed when both alleles in heterozygous organisms jointly influence the physical traits displayed.
• An illustrative instance of codominance is the inheritance of blood groups, showcasing this genetic phenomenon.
• In the case of blood group inheritance, a departure from the usual genetic pattern occurs, involving three alleles governing the process instead of the typical two.
• Notably, alleles I_A and I_B exhibit codominance, exerting dominance over allele IO.
• The gene is represented by 'I', with 'A', 'B', and 'O' denoting the respective alleles.
• Allele I_A prompts the production of antigen A within the blood system, while I_B triggers the generation of antigen B. Conversely, IO results in the absence of any antigens.
• These distinct alleles offer various genotypic and phenotypic outcomes, showcasing the complexity and diversity of genetic inheritance.

• We can utilize genetic diagrams to forecast the results of crosses involving codominant alleles.

When a parent with blood group A and another with blood group B come together, they can have offspring with blood group O.

• The parent with blood group A possesses the genotype I^AI^O.
• The parent with blood group B has the genotype I^BI^O.
• These genotypes are confirmed, as they can produce a child with blood group O, indicating the child inherits an allele for group O from each parent.
• Parents with these blood types hold a 25% likelihood of having a child with blood type O.

Sex-Linked Characteristics

• Alleles on the same chromosome are said to be linked
• When alleles controlling a specific trait are located on the sex chromosomes, it leads to what we call 'sex-linked' inheritance. In most cases, these alleles are found on the X chromosome, as the Y chromosome is significantly smaller.
• Males, possessing only one X chromosome, are at a higher risk of displaying sex-linked recessive conditions like red-green color blindness and hemophilia.
• Females, with two X chromosomes, typically inherit at least one dominant allele that conceals the effects of the recessive allele.
• A female who carries one recessive allele in a masked state is termed a carrier. Although she does not exhibit the disease, there's a 50% chance of passing it on to her offspring.
• If the offspring is male, he will express the disease.
• When a normal male mates with a female carrier of color blindness, specific outcomes are observed:

• When using a Punnett square for colorblindness, there is a 25% probability of producing a colorblind male.
• There is also a 25% chance of producing a female carrier, a 25% chance of a normal female, and a 25% chance of a normal male offspring.