Genes, Alleles, and Cell Cycle Questions
What is the relationship between a gene and an allele? Explain how two different alleles can result in different versions of the same trait.
When an organism inherits two copies of these alleles (one from each parent), the combination (genotype) determines the functional enzyme levels and the resulting trait.
A gene is a specific DNA sequence that codes for a trait, while an allele is a specific variant of that sequence. Two different alleles yield distinct versions of a trait because variations in their DNA sequences produce proteins with altered structures and functional activity levels.
What is a phenotype?
This term refers to the observable physical or biological characteristics of an organism, which are determined by the expression of its alleles.
When crossing two heterozygous green-flowered plants, the resulting F2 generation exhibits a 1:2:1 ratio of blue, green, and yellow flowers, demonstrating this type of dominance.
Incomplete dominance
Define what complete dominance is and what incomplete dominane is. Provide examples.
Complete Dominance: A condition where the dominant allele completely masks the presence of the recessive allele in a heterozygote (Aa). The heterozygous phenotype is identical to the homozygous dominant phenotype.
Example: Seed shape in peas (R=round, r=wrinkled). An Rr plant produces smooth, round seeds, completely indistinguishable from an RR plant.
Incomplete Dominance: A condition where neither allele is fully dominant over the other, resulting in a heterozygous phenotype (A1A2) that is a quantitative blend or intermediate between the two homozygous parental phenotypes.
Example: Flower color in snapdragons (CR=red, CW=white). Crossing a red plant (CR) with a white plant (CW) yields all pink offspring (CRCW) in the F1 generation.
Why are most mutations neutral?
If it doesn’t happen in a protein-coding region (which is only 2% of our DNA) it won’t affect the phenotype that affects the fitness of an individual.
Also, some phenotypes don’t affect the fitness of an individual, like cilantro tasting like soap
Explain how the information in a DNA sequence of a gene can ultimately result in the production of a protein. In your answer, include the roles of transcription and translation.
A gene’s DNA sequence is first transcribed in the nucleus into messenger RNA (mRNA) by RNA polymerase, using complementary base pairing to copy the genetic instructions. Next, the mRNA moves to a ribosome in the cytoplasm, where it undergoes translation: transfer RNA (tRNA) molecules read the mRNA three bases at a time (codons) to deliver specific amino acids. The ribosome links these amino acids into a polypeptide chain, which then folds into a functional protein.
What is Ploidy? (bonus points +50 if you can give a method of determining what the ploidy of a cell that is 3n=9)
Ploidy is the number of complete sets of chromosomes present in a single cell. It is designated by the letter n:
3n=9 -> triploid cell
In an X-linked recessive pedigree, an affected female will pass the trait along to this percentage of her biological sons:
100%
Define autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, and Y-linked modes of inheritance.
Autosomal Recessive: Requires two recessive alleles; can skip generations, affects both sexes equally, and is more likely with inbreeding.
Autosomal Dominant: Requires one dominant allele; typically does not skip generations, affects both sexes equally, and can pass from father to son.
X-Linked Recessive: Recessive allele on the X chromosome; more common in males, can skip generations, and has no father-to-son transmission.
X-Linked Dominant: Dominant allele on the X chromosome; affected fathers pass it to all daughters but none of their sons.
Mitochondrial: Mutation in mitochondrial DNA; affected mothers can pass it to all offspring, but affected fathers do not transmit it.
Y-Linked: Allele on the Y chromosome; only males are affected, and affected fathers pass it to all their sons.
Define and contrast fitness, natural selection, and gene flow, and genetic drift
Fitness is an organism’s ability to survive and reproduce, passing its alleles to the next generation. Natural selection occurs when individuals with advantageous heritable traits reproduce more successfully, changing allele frequencies. Gene flow is the movement of alleles between populations through migration and reproduction. Genetic drift is a random change in allele frequencies due to chance; bottleneck effect.
If almost every cell in an organism contains the same DNA, how can different types of cells have different structures and functions? Use gene expression in your explanation.
Even though almost every cell in a multicellular organism contains the same DNA, different cells in the body develop distinct structures and functions through selective gene expression (turning genes on and off).
If an organism's diploid cell is 2n=122n=12, a cell from this organism in Metaphase of Mitosis will contain this total number of sister chromatids.
24
A plant with a dominant purple flower phenotype (P_P_) of unknown genotype is crossed with a white-flowered plant (pp), yielding 24 purple and 22 white offspring. Which genotype belongs to the purple parent?
Pp
Define and provide the names of generations in a pedigree
From a word problem with a given F1 cross, and F2 generation, these are the steps to finding the mode of inheritance
Choose a trait (if given more than one)
Hypothesize what your mode of inheritance is (for your chosen 1 trait)
Define your genes /alleles
Use a punnet square to make predictions (via genetic crosses) to make a phenotypic ratio
Compare predicted phenotypic frequency to observed results (your phenotypic ratio)
Write a conclusion to support your hypothesis- Is the mode of inheritance supported?
How can changes in chromatin structure affect whether a gene is expressed? Explain the difference between open and tightly packed chromatin. What can cause this?
Chromatin structure dictates gene expression by controlling access to DNA: open chromatin is loosely wound, allowing RNA polymerase and transcription factors to reach genes and express them, whereas tightly packed chromatin physically blocks access, keeping genes silenced.
Epigenetics- environmental factors
Define what a Gene and DNA, as well as what a chromosome is
DNA is the double-stranded molecule that stores an organism's genetic code.
A gene is a specific segment of that DNA that provides instructions for building a protein.
A chromosome is a single long strand of DNA tightly coiled around proteins to keep the genetic material organized inside a cell.
Crossing two carnations with shaded pink petals yields an F1 generation with 50 red, 104 pink, and 48 white flowers. This fraction represents the proportion of pink offspring exhibiting the heterozygous genotype.
50%
What is a wild type?
The most common or typical phenotype observed in a natural population, used as the reference for comparison with mutant phenotypes.
Explain what deleterious and beneficial mutations are and provide an example of each.
Deleterious mutations: reduce an organism's fitness by reducing the survival/ fertility of an individual; cystic fibrosis
Beneficial mutations: increase survival; camouflage
You track alleles A and a for a single gene on a Chromosome in an Aa individual. If nondisjunction yields a gamete containing both A and a alleles without any crossing over having occurred, it proves nondisjunction happened in _______. However, if a gamete contains two A alleles, which division (Meiosis I or II) points to nondisjunction? Explain your answer.
If nondisjunction yields a gamete containing both A and a alleles without any crossing over having occurred, it proves nondisjunction happened in Meiosis I:
In a normal cell, homologous chromosomes (one carrying A, one carrying a) separate during Anaphase I: If nondisjunction occurs in Meiosis I, the entire homologous pair fails to separate and gets pulled into the same daughter cell. When this cell divides normally in Meiosis II, each resulting gamete inherits both non-identical alleles.
If a gamete contains two A alleles (or two a alleles), it points to nondisjunction in Meiosis II.
After a normal Meiosis I, the A allele and aa allele successfully separate into two distinct cells. The cell receiving A contains two identical sister chromatids. In Anaphase II, these sister chromatids are supposed to separate. If nondisjunction occurs in Meiosis II, both A sister chromatids fail to pull apart and end up in the same gamete, yielding an AA gamete (while another gamete ends up with no copy of that chromosome).
Compare sister chromatids and homologous chromosomes by defining each term and contrasting their roles during mitosis and meiosis.
Sister chromatids are two identical copies of a single chromosome created during DNA replication (S phase of interphase), joined at the centromere.
In Mitosis, sister chromatids separate during Anaphase to ensure each daughter cell receives an identical copy of the genome.
In Meiosis, sister chromatids remain attached through Meiosis I and separate during Anaphase II, resulting in four genetically distinct haploid gametes.
Homologous chromosomes are a pair of similar but non-identical chromosomes inherited from two different parents (one maternal, one paternal).
In Meiosis, homologous chromosomes pair up during Prophase I to form tetrads and undergo crossing over (genetic recombination), creating new allele combinations. During Anaphase I, homologous pairs separate to reduce the chromosome number from diploid (2n2n) to haploid (nn).
Researchers are investigating the inheritance of eye colour in fruit flies (Drosophila melanogaster). Fruit flies can have either red eyes or white eyes. Researchers hypothesize that eye colour is an X-linked trait, with red eyes dominant to white eyes. To investigate this hypothesis, researchers cross a true-breeding red-eyed female with a white-eyed male. The resulting F1 generation consists of 48 red-eyed females and 52 red-eyed males. Next, researchers cross a heterozygous red-eyed F1 female with a red-eyed F1 male. In the resulting F2 generation, researchers observed 46 females with red eyes, 0 females with white eyes, 23 males with red eyes, and 27 males with white eyes.
Does the data support the hypothesis that eye colour in fruit flies is an X-linked trait, with red eyes dominant to white eyes? Show your work
Yes, the data does support that this is an x-linked trait (x linked recessive, where white eyes are recessive)
In a family pedigree tracing a rare neurological disorder, two unaffected parents have three children: an unaffected son, an unaffected daughter, and an affected daughter. This specific mode of inheritance is....
autosomal recessive
Explain and describe the sequency of the cell cycle:
The cell cycle proceeds through interphase (G1: cell growth, S: DNA replication, G2: preparation for division), followed by mitosis (prophase, metaphase, anaphase, telophase) and cytokinesis, which divides the cytoplasm to produce two daughter cells.