Drasophila may be monosomic for chromosome \(4,\) yet remain fertile. Contrast the \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2}\) results of the following crosses involving the recessive chromosome 4 trait, bent bristles: (a) monosomic \(\mathrm{IV}\), bent bristles \(\times\) normal bristles; (b) monosomic \(\mathrm{IV},\) normal bristles \(\times\) bent bristles.

Monosomic organisms are individuals that have only one copy of a particular chromosome instead of the usual two. A normal set of chromosomes is referred to as 'diploid', but when one chromosome is missing, the condition is termed 'monosomy'. In humans, most monosomies are lethal, but there are exceptions in other species. For example, Drasophila flies can be monosomic for chromosome 4 and remain fertile.

In the case of Drasophila, when a monosomic individual is crossed with a diploid individual, the offspring will inherit the single version of the chromosome from the monosomic parent and a normal pair from the other parent. In the exercise provided, the monosomic fly with bent bristles crosses with a fly with normal bristles to produce a first-generation (F1) with a mix of genotypes but a uniform phenotype displaying only normal bristles, due to the dominance of the relevant allele.

The terms 'genotype' and 'phenotype' are fundamental in understanding genetics. A genotype refers to the genetic makeup of an organism—the actual alleles present. On the other hand, a phenotype is the observable traits of an organism, which result from the interaction of the genotype with the environment.

In the exercise, the monosomic genotype causes the bent bristle phenotype to be expressed only when two recessive alleles are present (bb). When crossed with a fly having normal bristles (BB or Bb), the offspring's genotype influences their phenotype. The dominance of the B allele for normal bristles will mask the presence of a b allele, thus the phenotype of the F1 generation will show normal bristles even in Bb heterozygotes.

Inheritance patterns refer to the predictable manner in which genes and traits are passed from parents to offspring. Classic Mendelian inheritance describes patterns such as dominant-recessive and monohybrid crosses, which can be modeled using Punnett squares to predict genotypes and phenotypes of progeny.

In monosomic organisms, these patterns can vary slightly. With Drosophila's chromosome 4 monosomy, the F1 generation from both crosses in our exercise will be heterozygous (Bb) but will exhibit the dominant normal bristle phenotype. Intercrossing these F1 individuals gives rise to the F2 generation, where the standard 3:1 Mendelian phenotypic ratio reappears. This shows that even with abnormal chromosome numbers, the basic principles of genetics still apply.

Recessive traits are those that need two copies of the same recessive allele (one from each parent) to be expressed in an organism's phenotype. Recessive alleles are often symbolized by lowercase letters, as in 'b' for bent bristles in Drasophila. The bent bristles trait is only visible in the phenotype when an individual is homozygous recessive (bb).

Although the F1 generation of the initial cross in the exercise exhibits the dominant trait, the recessive trait can still be seen in the F2 generation. Here, 25% display bent bristles because these flies inherit two copies of the recessive allele (bb), one from each parent. This demonstrates how recessive traits can disappear in one generation but reappear in the next, due to the inheritance of hidden recessive alleles.