Definition 2

Dr. Foster: ‘We’ve established that Jake is a carrier for thalassemia, meaning he has one recessive gene but does not show any symptoms. If Eleonore has no copy of this recessive gene, then her genotype is most likely AA, while Jake’s is Aa.’

Eleonore (Medical Student): ‘Exactly, so how can you be so sure our child won’t have the disease?’

Dr. Foster: ‘Let’s illustrate this using a Punnett square. Picture the alleles: A represents the normal allele and a the thalassemia allele. One parent is Aa, the other is AA. Separating the alleles, you place them into the Punnett square columns and rows. Each child inherits one allele from each parent.”

[She draws the square on a tablet.]

Dr. Foster: ‘All resulting genotypes are either AA or Aa. No child will be aa, which means no child will actually have thalassemia. However, there is a fifty percent chance each child could be a carrier, Aa, just like Jake.’

Eleonore (Medical Student): ‘So none of our children will be affected, but half may carry the gene?”

Dr. Foster: ‘That’s correct. Remember, carriers often show no symptoms. Still, it’s important for carriers to be aware of their status in case they have children with another carrier or an affected individual. This same principle applies to a variety of other recessive disorders we’ll talk about in subsequent laboratory sessions.’

Which Punnet square do you think is correct?

Punnett square 5:

Punnett square 6: