PCB 3063 – Exam 1 (Ch. 1–5) Vocabulary Review

A set of vocabulary flashcards covering key genetic terms and concepts likely to appear on PCB 3063 Exam 1 (Chapters 1–5).

What is an allele?

An allele is one of two or more alternative forms of a gene. These different forms arise from mutations and are found at the same corresponding locus (position) on homologous chromosomes, leading to variations in inherited characteristics.

What is a gene?

A gene is a fundamental unit of heredity in DNA (or RNA in some viruses). It is a specific sequence of nucleotides that typically encodes the instructions for making a functional product, such as a protein or an RNA molecule, which then carries out specific functions in the cell.

What is a locus in genetics?

In genetics, a locus (plural: loci) refers to the specific, fixed physical position or location of a gene or a specific DNA sequence on a chromosome. Knowing the locus is important for mapping genes and understanding their inheritance patterns.

What does 'wild type' refer to in genetics?

The term 'wild type' refers to the most common, standard, or naturally occurring form of a gene, trait, or phenotype within a natural population. It is often used as a reference point when comparing and studying genetic variants or mutants.

When is an individual considered heterozygous?

An individual is considered heterozygous when it possesses two different alleles for a particular gene or locus (e.g., Aa). This means it has inherited a different version of the gene from each parent, and often the dominant allele's trait will be expressed.

Define 'homozygous' in terms of an individual's genotype?

An individual is homozygous when it possesses two identical alleles for a particular gene or locus (e.g., AA or aa). This means both inherited copies of the gene are the same, and the trait associated with that specific allele will be expressed.

What does it mean for an individual to be hemizygous?

For an individual to be hemizygous means that they possess only one copy of a particular gene or chromosomal segment in an otherwise diploid cell. This is typically observed in males for genes located on sex chromosomes, such as X-linked genes in XY males, where they only have one X chromosome.

In genetics, what is the 'heterogametic sex'?

The 'heterogametic sex' is the sex that produces two different types of gametes (sex cells) with respect to the sex chromosomes. For example, in humans, males are heterogametic (XY) because they produce sperm containing either an X or a Y chromosome, determining the sex of the offspring. In birds, females are heterogametic (ZW).

How is the 'homogametic sex' characterized?

The 'homogametic sex' is the sex that produces only one type of gamete with respect to the sex chromosomes. For example, in humans, females are homogametic (XX) because they produce eggs that all contain an X chromosome. In birds, males are homogametic (ZZ).

What is the function of the SRY gene?

The SRY gene (Sex-determining Region of the Y chromosome) is a crucial Y-linked gene in humans. Its primary function is to act as a master switch that initiates the cascade of events leading to male gonadal development, causing the undifferentiated gonads to develop into testes.

Explain a 'recessive lethal allele' and its impact on Mendelian ratios?

A recessive lethal allele is a specific type of allele that, when present in two copies (homozygous recessive state), results in the death of the organism. However, it does not cause death when present in a single copy (heterozygous state). Such alleles often alter expected Mendelian ratios, typically leading to a phenotypic ratio of 2:1 instead of the expected 3:1 in a monohybrid cross.

What is a 'dominant lethal allele' and why is it rarely observed in populations?

A dominant lethal allele is an allele that causes the death of an organism when present in just one copy (heterozygous state). These alleles are rarely observed in a population because individuals carrying them usually die before they are able to reproduce and pass the allele on to their offspring, unless its expression is delayed until after reproductive age.

State Mendel’s Law of Segregation and its significance?

Mendel’s First Law, the Law of Segregation, states that during the formation of gametes, the two alleles for a heritable character segregate (separate from each other) so that each gamete receives only one allele. This ensures that offspring inherit one allele from each parent during fertilization.

Describe Mendel’s Law of Independent Assortment and its conditions?

Mendel’s Second Law, the Law of Independent Assortment, states that alleles for different genes (loci) segregate independently of each other when gametes are formed. This means the inheritance of one gene does not influence the inheritance of another, provided that the genes are located on different chromosomes or are far apart on the same chromosome.

What is genetic recombination and how does it occur?

Genetic recombination refers to the process by which new combinations of alleles are generated on a chromatid. This primarily occurs through crossing-over, an exchange of genetic material between homologous chromosomes, during prophase I of meiosis. Recombination is a crucial source of genetic variation in sexually reproducing organisms.

Which part of meiosis is known as 'reductional division' and why?

Meiosis I is known as 'reductional division' because it is the stage of meiosis where homologous chromosomes separate from each other, resulting in a reduction of the chromosome number from diploid (2n) to haploid (n) in the daughter cells. Each daughter cell receives only one chromosome from each homologous pair.

What is the role of the kinetochore during cell division?

The kinetochore is a complex protein structure that assembles on the centromere of each sister chromatid during cell division (mitosis and meiosis). Its primary role is to serve as the attachment site for spindle microtubules, which are responsible for moving and separating chromosomes accurately to opposite poles of the cell.

Where is the centromere located on a chromosome and what is its function?

The centromere is the constricted region of a chromosome, typically appearing as a narrow waist. It is the critical site where sister chromatids are most closely joined after DNA replication and where the kinetochore protein complex assembles. The centromere is essential for the proper segregation of chromosomes during cell division.

What are telomeres and why are they important for chromosome stability?

Telomeres are specialized regions found at the ends of eukaryotic chromosomes. They consist of repetitive non-coding DNA sequences (TTAGGG in humans) and associated proteins. Telomeres are important because they protect the ends of chromosomes from degradation, fusion with other chromosomes, and erroneous repair mechanisms. They are notably absent on circular prokaryotic chromosomes.

What is the function of cohesin in cell division?

Cohesin is a fundamental ring-like protein complex that plays a crucial role in cell division by holding sister chromatids together from the time of DNA replication until their separation in anaphase. This cohesion is vital for ensuring that sister chromatids align properly during metaphase and segregate accurately into daughter cells.

What is separase and what is its role in chromosome separation?

Separase is a key protease enzyme that plays a critical role in chromosome segregation during both mitosis and meiosis. Its function is to cleave the cohesin complex, which holds sister chromatids together. This cleavage allows sister chromatids (in mitosis and meiosis II) or homologous chromosomes (in meiosis I) to separate and move to opposite poles of the cell.

Explain nondisjunction and its genetic consequences?

Nondisjunction is an error that occurs during cell division (either meiosis or mitosis) where homologous chromosomes (in meiosis I) or sister chromatids (in meiosis II or mitosis) fail to separate properly and move to opposite poles. This leads to the production of gametes (or somatic cells) with an abnormal number of chromosomes (aneuploidy), such as monosomy (2n-1) or trisomy (2n+1), often resulting in genetic disorders.

What is epistasis in genetics?

Epistasis is a type of gene interaction where an allele at one genetic locus masks, alters, or modifies the phenotypic expression of a gene at a different locus. This means that even if a gene for a particular trait is present, its expression can be hidden or changed by the action of another gene.

Describe 'recessive epistasis' with an example?

Recessive epistasis occurs when the presence of two recessive alleles at one gene locus obstructs or masks the phenotypic expression of alleles at a second, different locus. A classic example is coat color in Labrador retrievers, where the homozygous recessive 'ee' genotype at the epistatic locus prevents the expression of black 'B' or brown 'bb' alleles at the pigment locus.

How does 'dominant epistasis' differ from recessive epistasis?

Dominant epistasis is a form of epistasis where a single dominant allele at one gene locus can mask or suppress the expression of alleles at a second, distinct locus. Unlike recessive epistasis, only one dominant allele is needed for the masking effect to occur, leading to different characteristic phenotypic ratios, such as 12:3:1 or 13:3, in dihybrid crosses.

Define codominance and provide an example?

Codominance is an inheritance pattern where both alleles of a gene in a heterozygote are fully and distinctly expressed in the phenotype. Neither allele is dominant or recessive; instead, the heterozygote exhibits the specific traits associated with both homozygous genotypes simultaneously. A common example is the AB blood type in humans, where both A and B antigens are present on red blood cells.

What is incomplete dominance? How does it differ from codominance?

Incomplete dominance is an inheritance pattern in which the phenotype of the heterozygote is intermediate between the phenotypes of the two homozygous parents. Neither allele is completely dominant over the other, resulting in a blended or mixed phenotype. This contrasts with codominance, where both parental phenotypes are distinctly expressed. An example is pink flowers resulting from a cross between red and white snapdragons.

What is a testcross and why is it performed?

A testcross is a specific genetic cross performed to determine the genotype of an individual that expresses a dominant phenotype but whose genotype (either homozygous dominant or heterozygous) is unknown. This is achieved by crossing the unknown individual with a homozygous recessive individual. The phenotypes of the offspring then reveal the genotype of the unknown parent.

What is a gamete and what is its ploidy level?

A gamete is a haploid (n) sex cell (such as a sperm or an egg) that is produced by meiosis. Each gamete carries only one set of chromosomes, so when two gametes fuse during fertilization, they restore the diploid chromosome number in the offspring.

Define fertilization and explain its outcome?

Fertilization is the biological process involving the fusion of two haploid gametes (one sperm and one egg) to form a single diploid cell called a zygote. This crucial event restores the full diploid chromosome number for the species and combines the genetic material from both parents, initiating the development of a new individual.

Describe the process of spermatogenesis?

Spermatogenesis is the complex process of male gamete (sperm) formation that occurs in the testes. It involves a series of mitotic and meiotic divisions, ultimately producing four functionally mature, highly motile, and roughly equally sized haploid sperm cells from a single spermatocyte.

Explain the process of oogenesis and its unique products?

Oogenesis is the process of female gamete (ovum or egg) formation that occurs in the ovaries. It is a more complex process than spermatogenesis, characterized by unequal cytokinesis during meiosis, which results in the production of one large, haploid ovum (egg cell) rich in cytoplasm and nutrients, along with smaller, non-functional polar bodies that eventually degenerate.

What is the genetic maternal effect?

Genetic maternal effect is a phenomenon in which the phenotype of an organism is determined not by its own genotype, but solely by the genotype of its mother. This occurs because the mother's genotype influences the composition of the egg before fertilization, such as providing gene products (proteins or RNA) that direct early embryonic development.

Define cytoplasmic inheritance and identify common examples?

Cytoplasmic inheritance refers to the transmission of genetic material that is located outside the nucleus. This typically involves genes found in organelles such as mitochondria and chloroplasts (in plants). This type of inheritance is predominantly non-Mendelian and is usually transmitted solely through the egg (maternally inherited) because the egg contributes the vast majority of cytoplasm to the zygote.

What is a karyotype and what information can it provide?

A karyotype is an organized and ordered display of the complete set of chromosomes of an organism, typically arranged as homologous pairs, and ordered by decreasing size and centromere position. Karyotypes are used in genetics to identify and analyze chromosomal abnormalities, such as changes in chromosome number (e.g., Down syndrome) or large structural rearrangements.

What does 'diploid' mean in terms of chromosome sets?

The term 'diploid' refers to a cell or organism that contains two complete sets of chromosomes (2n) in its somatic cells. One set of chromosomes is inherited from each parental gamete, thus providing two copies of each gene (except for sex chromosomes in heterogametic individuals).

Characterize a 'haploid' cell and provide an example?

A 'haploid' cell or organism is one that contains only one complete set of chromosomes (n). In sexually reproducing organisms, gametes (sperm and egg cells) are haploid, ensuring that when they fuse during fertilization, the resulting zygote will restore the diploid chromosome number.

What is a chromosome and what is its primary function?

A chromosome is a highly organized DNA-protein structure found within cells that carries the genetic information (genes) of an organism in a linear sequence. Its primary function is to compactly package DNA to fit within the nucleus and to ensure its precise segregation into daughter cells during cell division.

What is a genome?

A genome refers to the complete set of genetic information (DNA or, in some viruses, RNA) present in an organism or a cell. It includes all of an organism's genes, as well as non-coding sequences, regulatory elements, and mitochondrial or chloroplast DNA.

Distinguish between 'genotype' and 'phenotype'?

A genotype refers to the specific allelic composition of an individual, either for a single gene or for its entire genetic makeup. It represents the genetic blueprint or the set of genes that an individual carries, which then contributes to their observable traits (phenotype).

What is a 'phenotype'?

A phenotype refers to the observable traits, characteristics, or physical and biochemical attributes of an organism. It is the outward manifestation of an organism's genotype interacting with its environment. Examples include hair color, height, disease susceptibility, or even behavior.

Describe the key events occurring during prometaphase of mitosis?

Prometaphase is an intermediate stage of mitosis that immediately follows prophase and precedes metaphase. During prometaphase, the nuclear envelope completely breaks down into fragments, and the spindle microtubules (part of the cytoskeleton) extend into the nuclear region and begin to attach to the kinetochores located at the centromeres of sister chromatids.

Explain the concept of 'independent assortment' during meiosis?

Independent assortment is a crucial principle of heredity that occurs during metaphase I of meiosis. It describes the random orientation and alignment of homologous chromosome pairs at the metaphase plate, independent of other pairs. This random arrangement leads to the production of gametes with new and varied combinations of maternal and paternal chromosomes, significantly contributing to genetic diversity among offspring.