Situs inversus is a congenital condition in which the major visceral organs are reversed from their normal positions. Investigations into the genetics of this abnormality revealed that individuals with at least one dominant allele (SI) of an autosomal gene are normal but, surprisingly, of individuals that are homozygous for a recessive allele (si), 1/2 are situs inversus and 1/2 are normal.

What genotypes and phenotypes are expected in progeny from a cross of two si si individuals?

Key Concepts

Genotype and Phenotype

Genotype refers to the genetic makeup of an organism, specifically the alleles it possesses for a particular gene. Phenotype, on the other hand, is the observable physical or biochemical characteristics of an organism, which result from the interaction of its genotype with the environment. In the context of the question, understanding the difference between these two concepts is crucial for predicting the traits of the progeny from the genetic cross.

Autosomal Dominance and Recessiveness

In genetics, alleles can be classified as dominant or recessive. A dominant allele, such as SI, will express its trait even if only one copy is present, while a recessive allele, like si, requires two copies (homozygous) to express its trait. This concept is essential for understanding how traits are inherited and how the presence of dominant and recessive alleles affects the phenotypes of offspring in genetic crosses.

Punnett Square

A Punnett square is a diagram used to predict the genotypes and phenotypes of offspring from a genetic cross. By organizing the possible gametes from each parent, it allows for a visual representation of the inheritance patterns. In this case, using a Punnett square to analyze the cross between two si si individuals will help determine the expected genotypes and phenotypes of their progeny.