Genes and Alleles
Define a chromatin
Why DNA is packaged into chromosomes
DNA + proteins (histone) = chromatin
A chromatin condenses to form chromosomes
Why must cells divide, and what would happen if a cell continued growing without dividing?
Cells divide for growth, tissue repair, cell replacement, and reproduction. If a cell continued growing without dividing, its volume would increase faster than its surface area, making it less efficient at exchanging nutrients, oxygen, and waste with its environment. Dividing produces smaller daughter cells that can function more efficiently.
A red flower crossed with a white flower produces pink offspring. What inheritance pattern does this demonstrate?
incomplete dominance
What is evolution? Explain at the micro and macro scale.
Microevolution: Small-scale evolutionary changes within a population, such as changes in allele frequencies over generations.
Macroevolution: Large-scale evolutionary changes above the species level, including the formation of new species and the diversification of groups over long periods
A cell containing more than two complete sets of chromosomes is described as this.
Polyploid
What is ploidy? What is a haploid number?
Ploidy is the number of each type of chromosome present in a cell (or the number of “copies” of each chromosome type)
Haploid number (n) refers to the number of different non-homologous chromosomes present in a gamete.
You can also determine the haploid number using the equation that describes chromosomes in a nucleus:
2n = 46 -> 23
A diploid cell has 2n = 8. After DNA replication, how many chromosomes and chromatids does it have?
Diploid vs. replicated (in terms of chromosomes)
Ploidy/chromosome number is not affected by DNA replication; only affects the # of chromatids (# of centromeres remains the same)
An affected father passes an X-linked dominant condition to all his daughters but none of his sons. Why?
Males typically have only one X chromosome. Therefore, a single recessive allele on that chromosome can be expressed because there is no corresponding allele on a second X chromosome to mask it.
Does the individual with the greatest strength necessarily have the highest evolutionary fitness? Explain.
No. Evolutionary fitness depends on reproductive success, not simply strength or survival.
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
An organism has 2n = 12. How many homologous chromosome pairs does a typical diploid cell contain?
6
What is crossing over? What is recombintation? What is non-disjunction?
Crossing over = chromosomes exchange DNA.
Recombination = new combinations of alleles are produced.
Nondisjunction = chromosomes fail to separate correctly.
Crossing over and recombination change the combinations of alleles. HOWEVER, nondisjunction changes the number of chromosomes in the resulting cells.
A heterozygous brown squirrel (Bb) mates with a white squirrel (bb). What percentage of their offspring are expected to have brown fur?
50%
In a snowy environment, squirrels with lighter fur are less visible to predators. Predict how fur-colour frequencies might change over generations.
Lighter fur may become more common because lighter squirrels are less visible to predators, potentially allowing them to survive and reproduce more successfully.
A heterozygous individual displays an intermediate phenotype between the two homozygous phenotypes. Is this complete dominance, incomplete dominance, or codominance? Explain.
Incomplete dominance. The heterozygote has an intermediate phenotype rather than displaying only one allele's phenotype or both phenotypes distinctly.
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
Describe the stages of the cell cycle, including the major checkpoints that regulate progression through the cycle. Explain what happens at each checkpoint and why these checkpoints are important.
The cell cycle consists of interphase and the mitotic (M) phase
G1 phase: The cell grows, carries out normal functions, and produces proteins and organelles. G1 checkpoint: Checks whether the cell is large enough, has sufficient resources, and has sufficiently intact DNA to proceed to DNA replication.
S phase: The cell replicates its DNA, producing two sister chromatids per chromosome.
G2 phase: The cell continues growing and prepares for division. G2 checkpoint: Checks whether DNA replication is complete and whether DNA damage needs to be repaired before mitosis.
M phase: The cell undergoes mitosis, followed by cytokinesis. M checkpoint (spindle checkpoint): Ensures chromosomes are properly attached to spindle fibres before sister chromatids separate.
A squirrel with brown fur has an unknown genotype (BB or Bb). It is crossed with a white squirrel (bb), and some offspring have white fur. What must the brown squirrel's genotype be? Explain.
Bb. The white offspring must have inherited a recessive b allele from each parent. Since the white parent is bb, the brown parent must carry a b allele while expressing brown fur.
Birds and insects both have wings used for flight, but their wings evolved from different ancestral structures. What type of structures are these, and what evolutionary process explains their similarity in function?
Analogous structures. Their similar function evolved independently through convergent evolution.
A cell has 12 chromosomes before S phase. After S phase, how many chromosomes and chromatids does it have?
12 chromosomes, 24 chromatids
As winter approaches, squirrels begin growing thicker fur to help them survive colder temperatures. Scientists studying two squirrels discover that both have the same gene responsible for producing a protein involved in fur growth. However, Squirrel A produces large amounts of this protein and develops thicker fur, while Squirrel B produces very little of the protein and has thinner fur.
Explain why Squirrel A produces more of the protein than Squirrel B, even though both squirrels have the same gene.
Squirrel A produces more of the protein because its chromatin is loosely packed, allowing transcription machinery to access the gene more easily. In Squirrel B, tightly packed chromatin makes the gene less accessible, reducing transcription and protein production.
Epigenetics refers to changes in gene expression that occur without changing the underlying DNA sequence: Exposure to cold temperatures could trigger cellular signalling pathways that alter epigenetic marks.
Mitosis vs. Meosis
Mitosis is a cell division that produces two genetically identical daughter cells with the same chromosome number as the parent cell. It supports growth, repair, and cell replacement.
Meiosis is a two-cell division that produces four genetically different haploid cells, each with half the chromosome number of the diploid parent cell. It is used to produce reproductive cells and increases genetic variation through crossing over and independent assortment.
A heterozygous mother squirrel has an X-linked recessive allele, and the father squirrel is unaffected. What proportion of their squirrel sons is expected to express the trait, assuming equal probabilities of inheriting either maternal X chromosome?
50% of sons are expected to express the trait.
The mother is heterozygous (XᴬXᵃ), and the father is unaffected (XᴬY). Sons inherit the Y chromosome from their father and one X chromosome from their mother. Half are expected to inherit Xᵃ and express the trait.
A population of squirrels develops thicker fur over several generations in a cold environment. Explain the roles (3 necessities) of natural selection in this change.
Variation: Individuals in a population differ in a trait, such as fur thickness.
Heritability: Some of these differences are genetically inherited by offspring.
Differential reproductive success: Individuals with advantageous traits tend to survive and reproduce more, passing those traits to more offspring.
A population of squirrels has variation in fur colour and tail length. A squirrel inherits one homologous chromosome from its mother and the other from its father. During meiosis, the homologous chromosomes pair up and exchange segments of DNA. On one homologous chromosome, the alleles are AB, while on the other, the alleles are ab. Crossing over occurs between the two genes.
A) During which stage of meiosis does crossing over occur?
B) Which chromatids exchange DNA during crossing over?
C) What new combinations of alleles can result from crossing over between the two genes?
D) Explain how crossing over increases genetic variation in the squirrel population.
A) Prophase I of meiosis.
B) Non-sister chromatids of homologous chromosomes.
C) The original allele combinations are AB and ab. Crossing over can produce the recombinant combinations Ab and aB.
D) Crossing over creates new combinations of alleles that were not present together on the original parental chromosomes. This increases genetic variation among gametes and, consequently, potential offspring.