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Summary of IB Biology Topic 3 (Genetics)
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Summary of IB Biology Topic 3 (Genetics)
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Topic 3 - Genetics3.1 - Genes
3.1.1 - Genes and Loci
Gene: sequence of DNA that codes for a speci c traits (traits may also be in uenced by multiple
genes)
Locus: position of gene on a particular chromosome
3.1.2 - Alleles
Alleles: alternative forms of a speci c gene that code for variations of a speci c trait
(e.g. gene: hair color, allele: brown)
Alleles only di er from each other by one or a few bases.
3.1.3 - Mutations
Gene mutation: change in nucleotide sequence
New alleles are formed by mutations.
Types of mutations:
• Missense mutations: bene cial, create new variations
• Nonsense mutations: detrimental, abrogate with normal function of trait
• Silent mutations: neutral, no e ect on functioning of feature
Sickle cell anaemia: a disorder caused by gene mutations
Arose from base substitution mutations (GAG to GTG on non-transcribed strand —> Glu
to Val on polypeptide chain).
Altered structure of haemoglobin —> formation of brous strands (sickle shape) —>
cannot carry oxygen as e ectively
May form harmful blood clots and are destroyed more rapidly (anaemia: low cell count)
3.1.4 - Genome
Genome: totality of genetic information (genes + introns + promoters + etc)
Human genome:
• 46 chromosomes
• 3 million base pairs
• 21,000 genes
Human Genome Project: project to sequence the human genome
• Mapping (number, location, size and sequence)
• Screening (detect disease)
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,• Medicine (new treatments discovered)
• Ancestry
3.1.5 - Gene Comparisons
Number of genes ≠ level of complexity
Rice has more genes than humans.
3.2 - Chromsomes
3.2.1 - Prokaryote Genetics
Prokaryotes:
• No nucleus
• Nucleoid: region where genetic material exists
• Genophore: genetic material in prokaryotes (circular)
• Naked DNA (not associated with proteins)
• Plasmids: additional DNA (small, circular)
• Capable of self-replications
• Bacterial conjugation: exchange of plasmids between bacteria (via pili)
• Allows bacteria to develop new traits within one generation (one organism)
3.2.2 - Eukaryote Genetics
3.2.3 - Homologous Pairs
Sexually reproduced organisms possess two copies of each chromosome (one maternal, one
paternal) known as homologous chromosomes.
Homologous chromosomes share:
• Same structural features (e.g. size)
• Same genes at same loci positions (same genes, maybe di erent alleles)
Homologous chromosomes separate in gametes prior to reproduction (meiosis II:
chromosomes —> chromatids)
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, 3.2.4 - Diploid Versus Haploid
Diploid: nuclei possessing pairs/two sets of chromosomes (2n)
• Somatic cells are diploid
Haploid: nuclei possessing only one set of chromosomes (n)
• Sex cells are haploid
3.2.5 - Autosome Versus Heterosome
Sex is determined by sex chromosomes:
• Female: XX
• Male: XY
Y chromosome: contains genes for developing male sex characteristics (SRY gene)
In absence of Y (XX), female characteristics develop.
Father is always responsible for sex of o spring (mother gives only X chromosomes, father gives
either X or Y).
Heterosomes: sex chromosomes
Autosomes: non-sex chromosomes
3.2.6 - Karyograms
Karyotype: number/type of chromosomes in eukaryotic cell
Process:
• Harvest cells
• Induce cell division, pause mitosis process
• Determine which chromosomes appear with sister chromatids
Karyogram: visual pro le of chromosomes in cell
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