A complete set of notes on the Genetics + Genetic Engineering Section of the IEB Matric / Grade 12 Syllabus for Life Sciences (Biology). Includes colourful diagrams as well as detailed descriptions of various processes.
Mendel’s https://www.youtube.com/watch?v=BkwYyLdx-lk
Experiments ● Investigated the transfer of characteristics from one generation to the next
● He realised that ‘something’ was passed on from parent to offspring and that
sexual reproduction combines these ‘somethings’ from each parent to produce
offspring which are unique, yet the same
‘Somethings’ - Chromosomes
Chromosomes 1. Long, thread-like structures that form part of the chromatin network in the
nuclei of cells
2. Made up of a strand of DNA wound around proteins called histones
Gene 1. Unit of inheritance
2. Each chromatid is made up of one, helical DNA molecule
3. A small section of DNA that controls a hereditary trait (DNA - series of genes)
Alleles 1. Genes responsible for controlling different versions of a trait
2. Found in the same locus (position) on homologous chromosomes
Represented: Capital = dominant allele, Lowercase (same letter) = recessive allele
Locus The physical location in a chromosome of a particular gene or allele
Gene pool Set of all genes / genetic info in a population of sexually reproducing organisms
Large → High genetic diversity and increased chances of survival
Small → Low genetic diversity and increased possibility of extinction
Genome All the genes of an organism
Filial Any generation resulting from a genetically controlled mating following parental
Generations generation
Gametes Haploid (n) reproductive cells in sexually reproducing organisms that fuse with one
another during fertilization
Genotype Made up of all the genes an organism carries on its chromosomes which it has
inherited from its parents
Phenotype 1. Physical appearance of an organism - tallness / hair colour
2. Partly programmed by genes (genotype), but also shaped by external factors - diet
/ exercise
Homozygous Pair of alleles at a locus are the same - homozygous for that specific trait
1
, Heterozygous Pair of alleles at a locus are different - heterozygous (hybrid) for that specific trait
Dominant The allele which has an effect in both homozygous and heterozygous conditions
Recessive The allele which only has an effect when homozygous - suppressed by dominant
Hybrid The offspring of genetically dissimilar parents (parents are not related)
Monohybrid A cross between parents with different alleles for a single gene
Crosses
Complete Characteristic that is fully expressed in the phenotype of a heterozygous organism is the
Dominance dominant allele
● When a dominant and a recessive allele are present together, only the dominant
allele has an effect on the phenotype
● The heterozygous phenotype is the same as the homozygous phenotype with
dominant alleles
● The characteristic that is masked when the organism is heterozygous is the
recessive allele and will only be expressed when it is homozygous
Mandel’s Law of Segregation → Alleles segregate during meiosis
During meiosis, allele pairs separate so that the gametes have a single allele for each
characteristic
Mandel’s Law of Dominance → Dominant allele = phenotype
In a cross of parents that are pure for contrasting traits, only the dominant trait will
appear in the phenotype. Recessive alleles will always be masked by dominant
alleles
Mandel’s Law of Independent Assortment → Random segregation of alleles
The alleles of different genes segregate randomly and independently of one another
during gamete formation
Test Cross If an organism shows the dominant phenotype, but we are unaware of whether it is
homozygous / heterozygous. We can perform a test cross:
● Cross the unknown dominant organisms several times with an organism showing
the recessive phenotype ( __ x tt )
● If the offspring all show the dominant characteristic (Tt) = org. was homozygous
● If the cross yields any recessive offspring (tt) = org. was heterozygous
Sex-linked The alleles that are carried on the non-homologous part of an X-chromosome
alleles (remember that Y chromosomes are much shorter)
Men have 1 - genetic diseases can occur more often, Women have 2
2
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