Cell Division, Genetics, and Patterns of Inheritance

Comprehensive practice flashcards covering eukaryotic and prokaryotic cell division, binary fission, mitosis, cell cycle regulation and cancer, meiosis, nondisjunction, sources of genetic variation, Mendelian genetics, monohybrid and dihybrid crosses, linked genes, and sex-linked traits.

What are the two primary biological purposes of cell division?

Reproduction (occurring in all organisms) and growth and repair (occurring in multicellular eukaryotes).

How do asexual reproduction and sexual reproduction differ in the number of parents and genetic identity of offspring?

Asexual reproduction involves $1$ parent producing genetically identical offspring (occurring in prokaryotes and eukaryotes), whereas sexual reproduction involves the fusion of egg and sperm ($2$ gametes) producing genetically unique offspring (occurring in multicellular eukaryotes only).

What three key events must occur in all cells undergoing cellular division?

$1$) The cell increases in size and synthesizes more cellular molecules, $2$) all DNA in the cell is replicated, and $3$) the replicated DNA copies are distributed equally to each new cell.

How does prokaryotic genome organization differ from eukaryotic genome organization?

Prokaryotic cells have a single circular chromosome that is not wound around proteins and lacks a nucleus, whereas eukaryotic cells have multiple linear chromosomes wrapped around proteins (histones) housed within a membrane-bound nucleus.

What is the process and outcome of prokaryotic cell division (binary fission)?

DNA replication starts, the replicated chromosome copies are pulled to opposite sides of the dividing cell, and the cell physically splits through cytokinesis to produce $2$ genetically identical offspring cells.

What distinguishes a haploid cell from a diploid cell in eukaryotic organisms?

A haploid cell contains only $1$ set of chromosomes (such as gametes, moss cells, and fungi), while a diploid cell contains $2$ sets of chromosomes ($1$ set inherited from an egg and $1$ set from a sperm).

What are homologous chromosomes?

A pair of chromosomes (one inherited from the egg and one from the sperm) that carry the same set of genes controlling the same characteristics, although they can possess different alleles for those genes.

What are sister chromatids, and how do their DNA sequences compare to those of non-sister chromatids?

Sister chromatids are identical duplicate copies of a single chromosome generated during DNA replication that remain attached and have identical DNA sequences (alleles). Non-sister chromatids belong to different chromosomes of a homologous pair and can carry different alleles.

A diploid cell has $4$ pairs of homologous chromosomes. How many individual chromosomes does it have before and after DNA replication, and how many pairs of sister chromatids are formed?

It has $8$ individual unreplicated chromosomes before replication, $8$ replicated chromosomes after replication, and $8$ pairs of sister chromatids ($16$ total chromatid strands).

Why do eukaryotic chromosomes condense after DNA replication during cell division?

Condensation prevents the extremely long, thread-like DNA molecules from tangling and breaking during distribution and keeps sister chromatids securely joined so they can be accurately partitioned into separate offspring cells.

What is the role of the mitotic spindle and the nuclear envelope breakdown during mitosis?

The nuclear envelope breaks down to allow mitotic spindle fibers access to the chromosomes; spindle fibers attach to sister chromatids from opposite sides of the cell, align them at the cell center, and pull them apart.

If a cell with $3$ pairs of homologous chromosomes ($6$ total) experiences an error in mitosis where spindle fibers from the same side attach to both sister chromatids of one chromosome, how many chromosomes will each offspring cell receive?

One offspring cell will receive $7$ chromosomes (containing both sister chromatids of that chromosome), and the other will receive $5$ chromosomes.

What are the primary biological roles and outcomes of mitosis?

Mitosis functions in tissue growth and cell repair in multicellular organisms, as well as asexual reproduction; it starts with a haploid or diploid cell and produces $2$ genetically identical offspring cells.

What is cancer at the cellular level, and what types of genes are mutated to cause it?

Cancer is uncontrolled cell division resulting from accumulated mutations in multiple genes that encode proteins regulating the cell cycle, specifically proto-oncogenes (which promote cell division) and tumor suppressor genes (which halt cell division).

Why can cancers caused by mutations in body (somatic) cells not be passed directly from parent to offspring?

Mutations in somatic cells (such as skin, breast, or colon cells) are not packaged into gametes; only mutations present in germline cells (eggs or sperm) can be inherited by offspring.

What is the main biological purpose and outcome of meiosis?

Meiosis produces haploid gametes (eggs and sperm) for sexual reproduction, converting $1$ diploid parent cell into $4$ genetically unique haploid offspring cells.

Why can haploid eukaryotic cells divide by mitosis but NOT by meiosis?

Meiosis requires homologous chromosomes to pair up and separate into different cells to halve the chromosome number; haploid cells have only $1$ set of chromosomes and lack homologous pairs.

Why is it biologically necessary for gametes to be haploid rather than diploid?

Gametes must be haploid ($1$ set of chromosomes) so that when an egg and sperm fuse during sexual reproduction, the resulting zygote restores the correct diploid number ($2$ sets of chromosomes).

How do chromosome behaviors differ between Meiosis I and Meiosis II?

In Meiosis I, replicated homologous chromosomes pair up and separate into $2$ haploid cells with replicated chromosomes. In Meiosis II, sister chromatids separate without prior DNA replication, producing $4$ haploid cells with unreplicated chromosomes.

What meiotic event is depicted between the non-sister chromatids of this homologous chromosome pair?

Crossing over, in which non-sister chromatids of paired homologous chromosomes swap corresponding segments of DNA containing the same genes during meiosis I.

What chromosomal abnormality is indicated by the arrow at chromosome $21$ in this human karyotype?

Trisomy $21$ (Down syndrome), characterized by the presence of three copies of chromosome $21$ instead of the normal two copies.

How do the gametic outcomes differ when nondisjunction occurs in Meiosis I versus Meiosis II?

Nondisjunction in Meiosis I results in $100\%$ abnormal gametes ($2$ gametes with an extra chromosome and $2$ missing that chromosome). Nondisjunction in Meiosis II results in $50\%$ normal gametes, $1$ gamete with an extra chromosome, and $1$ gamete missing that chromosome.

What five mechanisms generate genetic variation in sexually reproducing eukaryotic organisms?

$1$) Mutations, $2$) allele differences between homologous chromosomes, $3$) independent orientation of homologous chromosomes in meiosis I, $4$) crossing over between homologous chromosomes in meiosis I, and $5$) random fertilization of gametes.

How does independent orientation generate genetic diversity during meiosis?

Each homologous chromosome pair lines up at the metaphase plate in Meiosis I independently of every other pair, shuffling maternal and paternal chromosomes into all possible combinations in the resulting gametes ($2^n$ combinations, where $n=23$ in humans yields over $8\text{ million}$ possibilities).

In genetics, what is the distinction between a character and a trait?

A character is an inheritable physical or functional feature (analogous to a gene, e.g., flower color), while a trait is a specific variant of that character (analogous to an allele, e.g., purple or white flower color).

What is the difference between an organism's genotype and its phenotype?

Genotype is the specific combination of alleles an organism possesses in its DNA, whereas phenotype is the physical and functional combination of observable traits exhibited by the organism.

How do environmental conditions influence the phenotype of hydrangeas despite their genetic makeup?

Hydrangeas possess genes allowing them to produce flower pigments, but the exact color displayed (pinkish versus bluish) is directly determined by the acidity and chemical composition of the soil in which they grow.

What were the historical discoveries made by Walter Sutton, Theodor Boveri, Nettie Stevens, and Thomas Hunt Morgan?

Sutton and Boveri (~$1900$) proposed the chromosome theory of inheritance (genes are on chromosomes); Nettie Stevens (late $1800\text{s}$) discovered sex chromosomes differ between sexes; and T.H. Morgan ($1930\text{s}$) identified sex-linked inheritance of eye color in fruit flies.

What characteristics made garden peas (Pisum sativum) an exceptional model organism for Gregor Mendel?

They are inexpensive, easy to grow, have a short generation time, produce large numbers of offspring (allowing statistically reliable probabilities), possess readily distinguishable traits, and their matings can be strictly controlled.

Why is an organism that is true-breeding for a particular trait guaranteed to be homozygous?

True-breeding organisms produce $100\%$ identical offspring for that trait across generations when self-fertilized, meaning they carry only one type of allele for that specific gene.

In Mendelian genetics, what defines a dominant allele versus a recessive allele?

A dominant allele determines the observable phenotype in a heterozygote; a recessive allele only manifests phenotypically when an individual possesses $2$ copies of that recessive allele.

In Mendel's monohybrid cross between true-breeding round ($RR$) and wrinkled ($rr$) pea plants, what are the genotypic and phenotypic ratios of the $F_2$ generation?

Genotypic ratio: $\frac{1}{4}\,RR$, $\frac{1}{2}\,Rr$, and $\frac{1}{4}\,rr$ ($1:2:1$). Phenotypic ratio: $\frac{3}{4}$ round and $\frac{1}{4}$ wrinkled ($3:1$).

How many different gamete genotypes can an individual with the genotype $rrYyEe$ produce if the three genes are on different chromosomes?

$4$ different gamete genotypes ($rYE$, $rYe$, $ryE$, and $rye$), determined by multiplying the allele choices: $1\text{ (for r)} \times 2\text{ (for Y/y)} \times 2\text{ (for E/e)} = 4$.

In a dihybrid cross ($RrQq \times RrQq$) where both genes assort independently, what is the probability of obtaining an $RRQq$ offspring?

$\frac{2}{16}$ (or $\frac{1}{8}$ / $12.5\%$), as there are $2$ squares out of the $16$ total in the Punnett square that yield the $RRQq$ genotype.

A butterfly inherits alleles $A$ and $B$ from one parent and $a$ and $b$ from the other. What gametes are produced if the genes are on DIFFERENT chromosomes versus the SAME chromosome (without crossing over)?

If on different chromosomes, independent orientation yields $4$ gametes: $AB$, $Ab$, $aB$, and $ab$ ($25\%$ each). If on the same chromosome with no crossing over, only $2$ gametes are formed: $AB$ and $ab$ ($50\%$ each).

Why are alleles located on the same chromosome NOT always inherited together into gametes?

Crossing over during meiosis I allows non-sister chromatids of homologous chromosomes to exchange corresponding segments of DNA, recombining linked alleles into new combinations.

What is the difference between autosomal genes and sex-linked genes?

Autosomal genes reside on autosomes (chromosomes $1$ through $22$ in humans) that are common to both sexes, whereas sex-linked genes reside on the sex chromosomes ($X$ or $Y$) that determine biological sex.

Why do gametes with an $X$ chromosome produced by a male always have identical alleles for $X$-linked genes, while female gametes can have different alleles?

Males have only one $X$ chromosome ($XY$), so sister chromatids separated into sperm are identical copies generated by replication. Females have two $X$ chromosomes ($XX$) and can be heterozygous, producing eggs with different alleles.

Why is it impossible to determine whether an $X$-linked trait is dominant or recessive by examining male offspring alone?

Dominance is defined by the phenotype expressed when two different alleles are present in a heterozygote. Males are hemizygous (they only possess a single $X$ chromosome) and express whatever allele they carry regardless of dominance.

In fruit flies, red eyes ($X^R$) are dominant to white eyes ($X^r$). If an $F_1$ female ($X^R X^r$) is crossed with an $F_1$ male ($X^R Y$), what is the probability that an offspring will be male AND have white eyes?

$25\%$ (or $\frac{1}{4}$), because only $1$ of the $4$ equally likely offspring genotypes is $X^r Y$.

Mr. Purple is red-green color-blind ($X^b Y$). His partner has normal vision ($X^B X^b$) because her father was color-blind. Can they have a color-blind child, and of what sex?

Yes, either a son or a daughter can be color-blind. Punnett square analysis yields a $50\%$ chance of color blindness for daughters ($X^b X^b$) and a $50\%$ chance for sons ($X^b Y$).