Sickle cell anemia is a hereditary blood disorder characterized by the production of abnormal hemoglobin S (HbS) globins in red blood cells. This results in the cells assuming a sickle shape, leading to a variety of clinical features, including anemia, acute pain crisis, leg ulcers, and increased risk of stroke. Sickle cell anemia is caused by a heterozygous recessive gene combination on both chromosomes, specifically the inheritance of one HBB gene with the sickle cell variant (HBB*), known as HBB^s.
To understand the inheritance patterns of sickle cell anemia, one can use Punnett squares, which are tables that show the possible genotypes and phenotypes that can arise from the mixing of two sets of alleles. A Punnett square for sickle cell anemia would be constructed with two rows and two columns, representing the_male and_female父母的 genotypes, respectively.
Here is an example of a Punnett square for sickle cell anemia:
Male's Genes
HBB^a HBB^s
┌───────┐ ┌───────┐
│ HBB │ │ │
│ * │ │ * │
└───────┘ └───────┘
│ │
Female's Genes
HBB^a HBB^s
┌───────┐ ┌───────┐
│ HBB │ │ │
│ * │ │ * │
└───────┘ └───────┘
Each box in the Punnett square represents a possible combination of alleles that can be passed on to an offspring. The letters above the boxes indicate the alleles present, withcapital letters indicating dominant (HBB^A) and lowercase letters indicating recessive (HBB^s or HBB^0) alleles. The * symbol represents the presence of a wild-type HBB allele, which is necessary to avoid sickle cell anemia in heterozygous individuals.
Given the presence of two HBB^a alleles (which encode for HBB^s), the probability of passing on the HBB^s allele to an offspring is 50%. This is because each parent has one HBB^a and one HBB^s allele, and when they combine the genes, there is a 50% chance that the offspring will end up with an HBB^s allele.
The resulting genotypes and phenotypes calculated from this Punnett square are presented in the second column of the table. As shown, there are four possible offspring genotypes: HBB^aa (healthy), HBB^as (sickle cell trait), HBB^ss (sickle cell anemia), and HBB^AS (sickle cell trait carrier). The percentages next to each genotype indicate the probability that each will occur. In this example, the chance of passing on sickle cell anemia (HBB^ss) is 25%, while the chance of passing on a sickle cell trait (HBB^as or HBB^ss) is 75%.
This example illustrates that sickle cell anemia is a recessive trait, meaning that both parents must carry the HBB^s allele (or one of the two copies of the HBB^a allele) to have a child with the condition. The Punnett square demonstrates that the probability of passing on the recessive HBB^s allele is 50% per pregnancy, regardless of the genetic makeup of previous children.
It's important to note that the Punnett square provides only the probabilities of passing on the HBB^s allele and does not take into account other factors, such as the specific HBB genotypes present in the parents, the possibility of gene interaction, or the effects of environmental influences on gene expression. Additionally, the table does not show the percentage of carriers among the offspring, which is another consideration when discussing the inheritance patterns of recessive traits.
