This aesthetic, in-depth, neatly typed document summarises 3.3 of strand 3 of the IEB grade 12 mind action series textbook: Genetics and genetic engineering. It includes the full 3.3 Genetic engineering syllabus as well as additional class notes, own research notes and all relevant diagrams.
• Each DNA molecule is made up of a series of genes
• Gene: Section of DNA (series of nucleotides/bases) that controls hereditary characteristics (Basic
unit of heredity)
• Each chromosome has between several hundred and several thousand genes (total in humans is
between 20 000 and 25 000)
• Almost all somatic cells have exact copy of all the genes in an organism
• 2 of each chromosome (maternal & paternal) therefore each cell has 2 of each kind of gene
(variations = alleles)
• Gene pool = set of all genes (genetic information) in a population of sexually reproducing
organisms
• Large gene pool = high genetic diversity (increased chances of survival)
• High proportion of genes are non-coding genes (only 2% of DNA codes for protein: Responsible
for genetic traits)
• Hox genes = Master control genes determine how the body develops from single zygote
• Control of genes = epigenetics
,• Human Epigenome Project documents what switches genes on and off
• Eugenics: Gene tampering/manipulation
Alleles:
• Responsible for controlling different versions of a trait/characteristic found in the same locus
(position) on homologous chromosomes eg. eye colour
• Represented by letters:
- Capital letter = dominant allele (Eg. B)
- Lower case letter = recessive allele (EG. b)
• Alleles passed from parents to offspring by chromosomes in gametes made by meiosis in sex
organs
• Normal somatic cells are diploid (2n): Have a pair of homologous chromosomes & 2 alleles which
may be the same or different
• Sex cells (gametes) are haploid (n): Have 1 pair of homologous chromosomes & 1 of each pair of
alleles
, • During fertilisation, male & female gametes with alleles fuse to form diploid zygote
• Zygote divides by mitosis to form new organism with the same set of chromosomes and alleles
Genotypes & phenotypes:
• Genotype: Made up of all genes an organism carries on its chromosomes
• Phenotype: Physical appearance of an organism: Partly programmed by genes, and shaped by
external factors (exercise, diet, environment)
Homozygous & heterozygous:
• Homozygous: Pair of alleles at same locus are the same eg BB
• Heterozygous: Pair of alleles at same locus are different eg Bb (hybrid)
Dominant & Recessive alleles:
In heterozygous pairing, one allele of a pair may be:
• Dominant: Trait is expressed in offspring
• Recessive: Trait is suppressed in presence of dominant allele & not in offspring
• Homozygous dominant (BB)
• Homozygous recessive (Bb)
• Heterozygous
Genotype: Phenotype:
- BB (Homozygous dominant). - Black coat
- Bb (Heterozygous). - Black coat
- bb (Homozygous recessive) - White coat
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