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IB Biology Objectives
Topic 3: Genetics


Topic 1: Cells
Topic 2: The Chemistry of Life
Topic 3: Genetics
Topic 4: Ecology and Evolution
Topic 5: Human Health and Physiology
Topic 7: Cell Respiration and Photosynthesis
Topic 8: Genetics
Topic 9: Human Reproduction
Topic 10: Defense Against Infectious Disease
Topic 11: Nerves, Muscles, and Movement
Topic 12: Excretion
Topic 13: Plant Science
Option E: Neurobiology and Behaviour
Option H: Further Human Physiology

3.1.1. State that eukaryote chromosomes are made up of DNA and protein.


Eukaryote chromosomes are made up of DNA and protein.


3.1.2. State that in karyotyping, chromosomes are arranged in pairs according to their structure.


In karyotyping, chromosomes are arranged in pairs according to their structure.


3.1.3. Describe one application of karyotyping.


Karyotyping can be used not only to diagnose aneuploidy, which is responsible for Down, Turner's, and Klinefelter's syndromes, but also to identify the chromosomal aberrations associated with solid tumours such as Wilms' tumour, meningioma, neuroblastoma, retinoblastoma, renal-cell carcinoma, small-cell lung cancer, and certain leukemias and lymphomas.


3.1.4. Define gene, allele, and genome.


Gene-A hereditary unit consisting of a sequence of DNA that occupies a specific location on a chromosome and determines a particular characteristic in an organism. Genes undergo mutation when their DNA sequence changes.


            Allele-One of two alternative forms of a gene, differing from other alleles by one or a few bases only, that can have the same place on homologous chromosomes and are responsible for alternative traits.


            Genome-One haploid set of chromosomes with the genes they contain; the full DNA sequence of an organism.


3.1.5. Define gene mutation.


Gene Mutation-A mutation, in the base sequence of the DNA, due to an intramolecular reorganization of a gene, which ultimatly causes genetic diversity.


3.1.6. Explain the consequence of a base substitution mutation in relation to the process of transcription and translation, using the example of sickle cell anemia.


Base substitutions (aka point mutations) occur when 1 base on a DNA strand is replaced with a different one. This can affect the resulting protein in various ways. Great distortions can arise because during transcription and translation, the DNA that is being read is wrong, and thus do not correctly produce a needed protein. An incorrect sequence of amino acids may alter the shape of the protein, the most important characteristic that defines a proteins function. In the case of sickle cell anaemia, the haemoglobin molecule is crystalline and makes the red blood cell crescent-shaped. Even a slight change in the sequence may be enough to completely alter the information-


3.2.1. State that meiosis is a reduction division in terms of diploid and haploid numbers of chromosomes.


Meiosis is a reduction division in terms of diploid and haploid numbers of chromosomes.


3.2.2. Define homologous chromosomes.


            Homologous Chromosomes- A pair of chromosomes, that correspond in proportion, value, and structure, in the same individual that carry the same type of information (eye color) but not necessarily the same alleles (blue or brown). One of these "homologs" comes from the individual's mother and one from the father.



3.2.3. Outline the process of meiosis, including pairing of chromosomes followed by two divisions, which results in four haploid cells.


The process of meiosis essentially involves two cycles of division, involving a gamete mother cell (diploid cell) dividing and then dividing again to form 4 haploid cells.  These can be subdivided into four distinct phases which are a continuous process.


1st Division

            Prophase-homologous chromosomes in the nucleus begin to pair up with one another and then split into chromatids (one half of a chromosome) where crossing over can occur.  Crossing over can increase genetic variation

            Metaphase-Chromosomes line up at the equator of the cell, where the sequence of the chromosomes lined up is at random, though chance, increasing genetic variation via independent assortment.

            Anaphase- The homologous chromosomes move to opposing poles from the equator.

            Telophase-A new nuclei forms near each pole alongside its new chromosome compliment


At this state two haploid cells have been created from the original diploid cell of the parent.


2nd Division

            Prophase II The nuclear membrane disappears and the second meiotic division is intitiated.

            Metaphase II Pairs of chromatids line up at the equator

            Anaphase II -  Each of these chromatid pairs move away from the equator to the poles via spindle fibres

            Telophase II- Four new haploid gametes are created that will fuse with the gametes of the opposite sex to create a zygote.


Overall, this process of meiosis creates gametes to pass genetic information from parents to offspring, continuing the family tree and the species as a whole.  Each of these gametes possess unique genetic information due to situations in meiosis where genetic diversity is increased, all of which is elaborated upon on the next page.


3.2.4. Explain how the movement of chromosomes during meiosis can give rise to genetic variety in the resulting haploid cells.


Chromosones are inherited as a group; that is, during cell division they act and move as a unit rather than independently. The existence of linkage groups is the reason some traits do not comply with Mendel's law of independent assortment (recombination of genes and the traits they control); i.e., the principle applies only if genes are located on different chromosomes. Variation in the gene composition of a chromosome can occur when a chromosome breaks, and the sections join with the partner chromosome if it has broken in the same places. This exchange of genes between chromosomes, called crossing over, usually occurs during meiosis, when the total number of chromosomes is halved.



3.2.5. Explain that non-disjunction can lead to changes in chromosome number, illustrated by reference to Down's syndrome (trisomy 21).


Congenital disorder caused by an extra chromosome on the chromosome 21 pair, thus giving the person a total of 47 chromosomes rather than the normal 46. Persons born with Down syndrome are characterized by several of the following: broad, flat face; short neck; up-slanted eyes, sometimes with an inner epicanthal fold; low-set ears; small nose and enlarged tongue and lips; sloping underchin; poor muscle tone; mental retardation; heart or kidney malformations or both; and abnormal dermal ridge patterns on fingers, palms, and soles. The mental retardation seen in persons with Down syndrome is usually moderate, though in some it may be mild or severe. Congenital heart disease is found in about 40 percent of people with Down syndrome.


Most persons with Down syndrome have an extra (third) chromosome--a condition known as trisomy--associated with the chromosome 21 pair. Almost all individuals with Down syndrome have this trisomy, but a small number (perhaps 4 percent) have an abnormality called translocation, in which the extra chromosome in the 21 pair breaks off and attaches itself to another chromosome. The cause of the chromosomal abnormalities in Down syndrome remains unknown.


3.2.6. State Mendel's law of segregation.


During gamete formation each member of the allelic pair separates from the other member to form the genetic constitution of the gamete.



3.2.7. Explain the relationship between Mendel's law of segregation and meiosis.


Meiosis is the process that separates allele pairs to create the gametes (sex cells; sperm, egg) that later fuse together during fertilisation.

In meiosis I, the chromosome pairs are separated. However, the two alleles for a character are still together and not separated. They are only separated in meiosis II when the sister chromatids separate and are packed into separate gametes.


3.3.1. Define: genotype, phenotype, dominant allele, recessive allele, codominant alleles, locus, homozygous, heterozygous, carrier and test cross.


            Genotype- The actual genes for a trait present in an individual.


Phenotype- An individual's observable traits (how the organism looks, behaves, etc.).


Dominant Allele- An allele which, if present, masks the effect of any recessive allele paired with it. Indicated by a capital letter.


Recessive Allele- An allele which must be homozygous for it's effect to be observed. Indicated by a lowercase letter.


Codominant Alleles- Allelees which have a partial effect on the phenotype when present in heterozygotes but a greater effect in homozygotes


Locus- The physical location of the alleles of a gene on it's chromosome (See the definition for chromosome for an image).


Homozygous- Both alleles of a gene in a homologous pair are identical.


Heterozygous- The two alleles of a pair are not identical (for example: one dominant and one recessive allele for the color trait in roses).


Carrier- An individual that has a recessive allele of a gene that does not have an effect on the phenotype


Test Cross- Testing a suspected heterozygote by crossing with a known homozygous recessive.