Question

In this chapter, we focused on many extensions and modifications of Mendellan principles and ratios. In the process, we encountered many opportunities to consider how this information was acquired. Answer the following fundamental questions: (a) How were early geneticists able to ascertain inheritance patterns that did not fit typical Mendelian ratios? (b) How did geneticists determine that inheritance of some phenotypic characteristics involves the interactions of two or more gene pairs? How were they able to determine how many gene pairs were involved? (c) How do we know that specific genes are located on the sexdetermining chromosomes rather than on autosomes? (d) For genes whose expression seems to be tied to the gender of individuals, how do we know whether a gene is X-linked in contrast to exhibiting sex- limited or sex-influenced inheritance? (e) How was extranuclear inheritance discovered?

Short answer

Answer: Early geneticists were able to identify non-Mendelian inheritance patterns by observing the phenotypes of offspring resulting from controlled crosses between organisms with different traits and analyzing deviations from expected Mendelian ratios. To differentiate between various forms of inheritance like X-linked traits or extranuclear inheritance, geneticists studied the patterns of inheritance in families or controlled crosses, considering the traits' correlation with sex, and further investigating the genetic mechanisms underlying specific patterns, such as the involvement of mitochondrial DNA in extranuclear inheritance.

Step-by-step solution

(a) Identifying non-Mendelian inheritance patterns

Early geneticists were able to ascertain inheritance patterns that did not fit typical Mendelian ratios by observing the phenotypes of offspring resulting from controlled crosses between organisms with different traits. When the observed ratios of phenotypes deviated from the expected Mendelian ratios, they realized that different forms of inheritance must be at work, such as incomplete dominance, codominance, or multiple-alleles, among others.

(b) Discovering gene interactions

Geneticists determined that inheritance of some phenotypic characteristics involves the interactions of two or more gene pairs by analyzing the results of controlled crosses. They observed that some traits didn't follow Mendelian inheritance patterns, leading them to investigate the possibility of multiple genes affecting the same characteristic. By analyzing the phenotype ratios of subsequent generations and making careful deductions, geneticists were able to figure out how many gene pairs were involved in the inheritance of a particular characteristic.

(c) Genes on sex-determining chromosomes

Geneticists discovered that specific genes are located on sex-determining chromosomes rather than autosomes by studying the patterns of inheritance of sex-linked traits in humans and other organisms. They observed that some traits correlated with the sex of the individual and were inherited in a very specific manner, which could not be explained by their presence on autosomes. Further studies, like those on fruit flies, helped establish that these sex-linked traits were indeed located on sex chromosomes.

(d) X-linked vs. sex-limited/influenced inheritance

To know whether a gene is X-linked or exhibits sex-limited or sex-influenced inheritance, geneticists study the patterns of inheritance in families or controlled crosses. X-linked traits will show a specific inheritance pattern, with males being more affected than females, and females often acting as carriers. In contrast, sex-limited or sex-influenced traits have different expression patterns between males and females, but they are autosomal traits and won't show a skewed inheritance pattern based on sex.

(e) Discovery of extranuclear inheritance

Extracellular inheritance was discovered when geneticists found that some traits did not seem to fit any known patterns of inheritance, even when considering sex chromosomes. An example of this was the discovery of the "petite" mutation in yeast, which affected the size and growth of yeast colonies. Further studies revealed that the inheritance pattern for this trait was not based on nuclear DNA but rather on the DNA contained within the mitochondria, leading to the understanding of extranuclear inheritance.