BIO 30 - Genetics LE 1

Genetics

The study of heredity and variation.

Heredity

The transmission of traits from generation to generation.

Variation

Deals with genetic differences between organisms.

Cell division

The process involved in heredity and variation.

Cell cycle

The physical and metabolic activities of cells occurring in a regular cycle and in a repetitive manner.

Interphase

The non-dividing phase of the cell cycle, or the process in which a cell may double its entire content in preparation for cell division.

M-phase

The phase of the cell cycle where the cell contents are distributed into daughter cells. May be either mitosis or meiosis.

Nucleoli

One or several small round dense bodies within the nucleus.

Chromatin

A granular network of darkly staining chromatin.

G1 phase

Phase wherein the nucleus and cytoplasm are enlarging toward mature size. The cell increases in volume by imbibing water and nutrients and by building new protoplasm. Cytoplasmic organelles are formed. Secretory and storage granules and cell wall materials are formed. The chromatin, if stretched, would resemble a long chromatin fiber.

S phase

Phase wherein there is active synthesis of DNA (replication) and histones, the components of chromatin. Each chromatin fiber is replicated/doubled.

DNA content at S phase

2n = 4c.

G2 phase

Phase wherein there is active synthesis of RNA and proteins necessary for chromosome synthesis. Mitotic spindles (i.e. spindle fibers) are also formed. The doubled chromatin fiber is folded to form a chromosome, the stained intracellular structured during cell division.

Parts of a chromosome

a. Chromosome arm

b. Chromattid

c. Kinetochore

d. Centromere

e. Telomere

Centromere

Site of constriction in the chromosome; attachment site of kinetochore proteins. Also known as primary constriction.

Kinetochore

Site of attachment of spindle fibers.

Telomere

The tips of chromosome arms; provides stability to the chromosome.

Metacentric

Classification of chromosome when centromere is median.

Submetacentric.

Classification of chromosome when centromere is submedian.

Acrocentric

Classification of chromosome when centromere is subterminal.

Telocentric

Classification of chromosome when centromere is terminal.

Mitosis

Division phase undergone by all somatic and germ cells.

Equational division.

Genome

The complete set of chromosomes from either parent.

Homologue

The corresponding pair of a chromosome in one set.

Fertilization

Transmission of the chromosomes from parent to offspring, accomplished through the union of gametes.

Prophase

Chromosomes shorten, thicken and become visible as thick rods. The chromatids are closely pressed against each other throughout their length. Nucleoli and the nuclear membrane then disappear; then, spindle fibers would radiate from microtubule organizing centers.

Chromatid

Half of a chromosome.

Metaphase

The chromosomes, which are maximally condensed, align at the equatorial plane. The chromatids in each chromosome separate from each other except at the centromere. The spindle fibers attach to the kinetochore proteins on opposite sides of the centromere.

Anaphase

Centromeres of chromosome become functionally double. Each member of the doubled chromosome begins to move toward opposite poles. The poleward movement of the chromosomes is due to the depolymerization of spindle fibers attached to the kinetochores at the centromere.

Telophase

Upon reaching the opposite poles, the chromosomes regroup into two nuclear regions. They begin to uncoil and lengthen, and finally lose their visible identity. Cytokinesis usually follows.

Genetic content at prophase

2n = 4c

Genetic content at metaphase

2n = 4c

Genetic content at anaphase

2n = 4c per pole

Genetic content at telophase

2n = 4c per cell

Meiosis

A type of cell division that reduces the chromosome number by half, resulting in four genetically distinct gametes from one parent cell.

Meiosis I

Reductional division; involves the separation of homologous chromosomes resulting in two cells each with only a haploid chromosome number.

Meiosis II

Equational division; sister chromatids are separated in two haploid cells and four haploid cells are produced via a process that is identical to mitosis.

Prophase I

Has 5 substages:

a. Leptonema

b. Zygonema

c. Pachynema

d. Diplonema

e. Diakinesis

Leptonema

“thin thread”; The chromatin in the nucleus appear as long thin threads with many bead-like structures (chromomeres) along their lengths.

Zygonema

“adjoining thread”; Synapsis or pairing of homologous chromosomes begins, The paired chromosomes form a bivalent (II) consisting of four chromatids. Synapsis proceeds in a zipperlike fashion. Synaptonemal complex is formed once the homologues are attached to each other.

Pachynema

“thick thread”'; The chromosomes are thicker due to further coiling. The two chromosomes in a bivalent are closely appressed to each other. Chromatid breaks and their repair occur along the chromosome. Their repair may entail crossing over.

Crossing over

Exchange of segments between sister and non-sister chromatids in a bivalent.; cytologically observed through the formation of a chiasma/chiasmata at the point of exchange. Generates further genetic variation.

Diplonema

“two thread”; With the synaptonemal complex no longer functional, the longitudinal separation of homologues in a bivalent starts from the centromere and proceeds towards both ends except at the chiasma.

Diakinesis

“across movement”; Bivalents are maximally condensed and are distributed throughout the nucleus. Terminalization of the chiasmata between homologues that underwent crossing over occurs at this stage resulting in complete separation of the homologues. By the end of this stage, both the nucleolus and the nuclear membrane have disappeared, the spindle has formed, and Prophase I is completed.

Metaphase I

The bivalents align at the equatorial plane. The bivalents are oriented such that homologues are on opposite sides of the equatorial plane with spindle fibers attached to their kinetochores.

Anaphase I

The univalents or homologues in each bivalent separate from each other. Movement of the univalents to opposite poles is due to depolymerization of the spindle fibers. The homologues segregate from one another so that each anaphase group is composed of a haploid (n) number of chromosomes. This process accounts for the reductional phase of meiosis I.

Telophase I

The chromosomes regroup and their coiled structures begin to relax. The nuclear membrane and the nucleolus reappear forming two haploid daughter nuclei. Cytokinesis may then follow.

Interkinesis

A short phase between Meiosis I and Meiosis II where no DNA replication occurs.

Meiosis II

Equational division resulting in four genetically distinct haploid gametes from two haploid cells.

3 Interrelated Fields Of Genetics

a. Transmission Genetics

b. Molecular Genetics

c. Population Genetics

Transmission Genetics

Classical Genetics; A field that encompasses the basic principles of genetics; studies relationship between chromosomes and heredity.

Molecular Genetics

A field that studies the biochemical nature of genes; how a gene is encoded, replicated, and expressed.

(Central Dogma of Molecular Bio - Replication, Transcription, Translation).

Dependent on physical chemistry, biophysics, biochemistry.

Population Genetics

A field that studies the genetic composition of a species; evolution. Also a branch of genetics that deals with the fate of genes in a population, and factors affecting gene frequencies (migration, mutation, selection).

Dependent on ecology, statistics, mathematics.

Cytogenetics

A branch of genetics that deals with the behavior of chromosomes.

Dependent on optics, staining technology (e.g. acetocarmine)

Developmental Genetics

A branch of genetics that deals with gene regulation during development; switching on and off of genes.

Dependent on physiology, morphoanatomy, biochemistry.

Evolutionary Genetics

A branch of genetics that deals with genetic changes within or between species.

Dependent on ecology, mathematics, statistics, biochemistry.

Biochemical Genetics

A branch of genetics that deals with the role of enzymes/proteins in the production of genes.

Dependent on biochemistry.

Behavioural Genetics

A branch of genetics that deals with the study of behavioural traits that are inherited.

Dependent on psychology, biochemistry.

Quantitative Genetics

A branch of genetics that deals with the analysis of quantitative genes (genes controlled by environmental factors / inheritance of expression of quantitative traits).

Dependent on mathematics, statistics.

Aristotle

Coined the Theory of Pangenesis (all organs contribute copies of themselves to the sex cells).

Jean Baptiste de Lamarck

Coined the Theory of Inheritance of Acquired Characteristics (acquired body modifications are inherited by the offspring).

August Weismann

Coined the Germplasm Theory (opposite of pangenesis; plans of entire body are found in and are contributed to only by the sex cells).

Kolreuter, Gardner, Naudin, Darwin, Dzierzon

Found that there was uniformity in F1 and variation in F2.

Gregor Mendel

Gave the concept of the gene (elementen); explained the presence of a discrete hereditary unit that passed from parents to offspring; responsible for two principles of genetics (segregation, independent assortment).

Carl Correns (Germany)

Erick Von Tschermak (Austria)

Hugo de Vries (Netherlands)

Individuals who duplicated Mendel’s experiments on other plants (peas, maize, primrose, poppies); rediscoverers of Mendel’s work.

William Bateson

Saunders

Lucien Cuenot

Individuals who observed Mendelian principles in animals.

Walter Sutton (USA)

Theodor Boveri (Germany)

Individuals who came up with the Chromosome Theory of Inheritance.

Thomas Hunt Morgan

Calvin Bridges

Individuals who established association between specific genes and specific chromosomes.

Oswald Avery

Colin Macleod

Maclyn McCarty

Individuals who discovered that DNA is the hereditary material.

James D. Watson

Francis H. Crick

Individuals who elucidated DNA double helix structure.

Applications of Genetics

a. Microbial plants and animal improvement

b. Medicine

c. Genetic Counselling

d. Legal applications

e. Genetic engineering/Recombinant DNA tech

Eugenics

Application of genetic knowledge for the improvement of humanity; coined by Francis Galton in England. (Negative: Restricted reproduction of developmentally delayed people).

Euphenics

Replaced eugenics; medical intervention designed to reduce impacts of defective genotypes on individuals.

Genes

Inherited factor on the chromosome responsible for a trait.

Locus (pl. loci)

Location of a gene on a chromosome.

Genotype

Genetic constitution of a cell or an individual.

Allele

Alternative forms of a gene

Phenotype

Physical/morphological, physiological, biochemical, and behavioral traits of an individual; determined by genotype and interaction with the environment.

Dominant

Gene exerting full effect despite the presence of another allele of the same gene.

Recessive

Gene not expressed due to the presence of a dominant gene.

Homozygous

Two identical alleles; can be dominant (AA) or recessive (aa).

Heterozygous

Two differing alleles (Aa).

F1

First filial generation; 1st generation produced after mating of homozygous parents.

F2

Second filial generation; Generation produced by selfing or sib-mating F1 individuals.

Emasculation

Removal of anther from female plant to prevent self-fertilization.

Principles of Genetics

a. Law of Independent Segregation

b. Law of Independent Assortment

Backcross / Testcross

Crossing a heterozygous individual with one of its parents.

Law of Independent Segregation

Alleles in a gene pair separate cleanly and completely from each other in meiosis.

Law of Independent Assortment

Alleles of different gene pairs separate independently from each other and randomly combine during meiosis.

TEGI / TEPI

Take Each Gene Pair/Phenotype Independently

Monohybrid Cross

A cross between homozygous individuals that are different from each other at one gene locus. Fertilization results in an F1 generation with heterozygous genotype.

Dihybrid Cross

A cross between two homozygous individuals that are different from each other at two gene loci.

Sum Rule of Probabilities

In the case of a group of mutually exclusive events, the combined probability is the sum of the individual probabilities of each of the events.

e.g. P(heads or tails) = P(heads) + P(tails)

Gene Interaction

Non-allelic interaction of 2 or more genes; results in a modified phenotypic ratio.

Epistasis

Interaction of 2 or more genes; determined by observing certain phenotypic ratios in the progeny of heterozygous-heterozygous parents.

Types of non-allelic interactions

1. Novel phenotype

2. Recessive epistasis

3. Dominant epistasis

4. Complementary gene action

5. Duplicate gene action

Types of Allelic Interactions

1. Complete Dominance

2. Codominance

3. Lethal Genes

4. Overdominance

5. Incomplete Dominance

Novel phenotypes

Two gene pairs responsible for one trait. New phenotypes result from interaction between dominant + homozygous recessive.

Phenotypic ratio of novel phenotypes

9:3:3:1

Recessive Epistasis

The homozygous recessive gene hides expression of other gene; 2 gene pairs responsible for 1 phenotype