Final answer:
Recessive alleles become expressed phenotypically when they exist in a homozygous pair without a dominant allele to mask them. This commonly occurs with autosomal recessive disorders and X-linked recessive traits, affecting males more often due to their single X chromosome. Even if the trait isn't expressed, carriers can still pass the recessive allele to their offspring.
Step-by-step explanation:
Expression of Recessive Alleles
Recessive alleles often have an effect or show themselves in the phenotype of an organism when they are present in a homozygous pair, meaning the individual carries two copies of the recessive allele (e.g., aa genotype). This is because the recessive allele is not expressed when paired with a dominant allele in a heterozygous condition (e.g., Aa genotype). However, if an individual is homozygous recessive, the trait linked to the recessive allele will be visible because there are no dominant alleles to mask its effects.
In the case of X-linked recessive traits, such as color blindness or hemophilia, males are more commonly affected because they carry only one X chromosome. Thus, if a male inherits a recessive allele for an X-linked trait, he will express the condition since there's no corresponding allele on the Y chromosome to counteract it. In autosomal recessive disorders like cystic fibrosis or Tay-Sachs disease, the disorder will manifest in individuals who are homozygous for the recessive allele, requiring that both parents contribute a copy of the defective gene.
Moreover, even if the phenotype is not exhibited, carriers of recessive alleles can pass these genes to their offspring, which is significant for predicting the inheritance of certain genetic conditions. The likelihood of inheriting recessive traits can be calculated using Punnett squares and understanding Mendelian inheritance patterns, which include the random segregation of gametes.