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Chapter 4: Problem 1

In this chapter, we focused on many extensions and modifications of Mendelian 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 sex 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

Expert verified
Short Answer: Early geneticists determined inheritance patterns not fitting Mendelian ratios through observation of traits in different organisms and controlled crosses, leading to discoveries of complex inheritance phenomena. They analyzed offspring phenotypes in genetic crosses to identify multiple gene-pair interactions and used correlations with sex to locate genes on sex chromosomes. By studying inheritance patterns, they distinguished X-linked genes from sex-limited or sex-influenced inheritance. Extracellular inheritance was discovered upon observing traits that did not follow Mendelian inheritance patterns and were not linked to sex chromosomes, leading to the identification of organelle genomes, like mitochondrial and chloroplast DNA.

Step by step solution

01

(a) Inheritance patterns not fitting Mendelian ratios

Early geneticists were able to ascertain inheritance patterns that did not fit typical Mendelian ratios through the observation of traits in different organisms and the use of controlled crosses within these organisms. These studies, along with statistical analysis, allowed them to see that not all traits followed simple Mendelian inheritance patterns and that some traits exhibited more complex phenomena like incomplete dominance, codominance, polygenic inheritance, and gene-environment interactions.
02

(b) Determination of the involvement of multiple gene pairs

Geneticists were able to determine that the inheritance of some phenotypic characteristics involved the interactions of two or more gene pairs through careful analysis of offspring phenotypes in various genetic crosses. When they noticed that traits didn't segregate according to typical Mendelian ratios, they experimented with different cross combinations to determine if multiple genes were interacting. They monitored the phenotypic ratios in the offspring from these crosses and used statistical analysis to determine the number of gene pairs involved in producing the various phenotypes.
03

(c) Location of specific genes on sex chromosomes

To determine if specific genes are located on sex-determining chromosomes rather than autosomes, geneticists performed crosses with organisms that had visibly different sex chromosomes (such as Drosophila), and observed the inheritance patterns of traits in the offspring. When they noticed that the inheritance of certain traits consistently correlated with the sex of the organism, they inferred that these genes were located on sex chromosomes.
04

(d) Distinguishing X-linked genes from sex-limited or sex-influenced inheritance

Geneticists could identify if a gene was X-linked, as opposed to exhibiting sex-limited or sex-influenced inheritance, through comparing the phenotypic ratios between male and female offspring in various genetic crosses. If the inheritance pattern of a trait was consistently tied to the sex of an individual and followed more predictable ratios (such as 1:1), the gene was likely X-linked. In contrast, for a gene to be considered sex-limited or sex-influenced, it would need to demonstrate different expression patterns between males and females, but the inheritance of the trait would still follow Mendelian ratios.
05

(e) Discovery of extranuclear inheritance

Extracellular inheritance was discovered when geneticists observed traits that didn't follow Mendelian inheritance patterns and were not linked to sex chromosomes. They saw that certain traits, such as those involving mitochondria and chloroplasts, did not exhibit the expected segregation patterns in offspring. Further cytological and experimentation studies led them to the conclusion that these traits were inherited through elements outside of the cell nucleus, specifically through organelle genomes like mitochondrial and chloroplast DNA.

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Most popular questions from this chapter

While vermilion is X-linked in Drosophila and causes eye color to be bright red, brown is an autosomal recessive mutation that causes the eye to be brown. Flies carrying both mutations lose all pigmentation and are white-eyed. Predict the \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2}\) results of the following crosses: (a) vermilion females \(\times \quad\) brown males (b) brown females \(\times\) vermilion males (c) white females \(\times\) wild males

What genetic criteria distinguish a case of extranuclear inheritance from (a) a case of Mendelian autosomal inheritance; (b) a case of \(\mathrm{X}\) -linked inheritance?

A geneticist from an alien planet that prohibits genetic research brought with him two true-breeding lines of frogs. One frog line croaks by uttering "rib-it rib-it" and has purple eyes. The other frog line croaks by muttering "knee- deep knee-deep" and has green eyes. He mated the two frog lines, producing \(\mathrm{P}_{1}\) frogs that were all utterers with blue eyes. A large \(\mathrm{F}_{2}\) generation then yielded the following ratios: \(27 / 64\) blue, utterer \(12 / 64\) green, utterer \(9 / 64\) blue, mutterer \(9 / 64\) purple, utterer \(4 / 64\) green, mutterer \(3 / 64\) purple, mutterer (a) How many total gene pairs are involved in the inheritance of both eye color and croaking? (b) Of these, how many control eye color, and how many control croaking? (c) Assign gene symbols for all phenotypes, and indicate the genotypes of the \(P_{1}, F_{1},\) and \(F_{2}\) frogs. (d) After many years, the frog geneticist isolated true-breeding lines of all six \(\mathrm{F}_{2}\) phenotypes. Indicate the \(\mathrm{F}_{1}\) and \(\mathrm{P}_{2}\) phenotypic ratios of a cross between a blue, mutterer and a purple, utterer.

Two different genes, located on two different chromosomes, are responsible for color production in the aleurone layer of com kernels. For color production (either purple or red), the dominant alleles of these two genes \((C \text { and } R\) ) must come together. Furthermore, a third gene, located on a third chromosome, interacts with the \(C\) and \(R\) alleles to determine whether the aleurone will be red or purple. While the dominant allele ( \(P\) ) ensures purple color, the homozygous recessive condition (pp) makes the aleurone red. Determine the \(\mathrm{P}_{1}\) phenotypic ratio of the following crosses: (a) \(C C r r P P \times \operatorname{ccRRp} p\) (b) \(C c R R p p \times C C R r p p\) (c) \(\operatorname{CcRrPp} \times\) CcRrpp.

In cattle, coats may be solid white, solid black, or black-andwhite spotted. When true-breeding solid whites are mated with true-breeding solid blacks, the \(\mathrm{F}_{1}\), generation consists of all solid white individuals. After many \(\mathrm{F}_{1} \times \mathrm{F}_{1}\) matings, the following ratio was observed in the \(\mathrm{F}_{2}\) generation: \(12 / 16\) solid white \(3 / 16\) black-and-white spotted \(1 / 16\) solid black Rxplain the mode of inheritance governing coat color by determining how many gene pairs are involved and which genotypes yield which phenotypes. Is it possible to isolate a true-breeding strain of black-and-white spotted cattle? If so, what genotype would they have? If not, explain why not.
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