Everything you need to know for the grade 12 genetics section. Topics covered include: Transmission of hereditary information, Genes in gametes, Monohybrid crosses (complete dominance, incomplete dominance and co-dominance), Mendel's laws, Dihybrid crosses (with example), Sex-linked disorders, Ped...
Genetics
Transmission of hereditary information
Genes are responsible for the transmission of hereditary information.
Somatic cells in humans have 23 pairs of chromosomes. In each homologous pair
one chromosome is maternal and the other paternal.
Human reproduction
Characteristics are controlled by 2 genes.
When both genes carry the same information they are homozygous.
When two genes carry contrasting information, each gene is called an allele. The
contrasting pair is known as heterozygous.
Genes in Gametes
Gametes are haploid and formed during meiosis.
Meiosis
Genes are not paired in gametes.
After fertilization the zygote will have 23 pairs of chromosomes. in each
homologous pair one chromosome is maternal and the other paternal.
When the zygote undergoes mitosis every somatic cell has one gene from each
parent.
Monohybrid crosses
A hybrid is an individual that results from a cross between parents that are
genetically different.
A Monohybrid cross involves a single pair of contrasting characteristics.
Mendel's Laws
Mendel's first law (Segregation)
The pair of alleles on homologous chromosomes separate during meiosis
(anaphase) so that only one allele of each pair is presenting the gamete.
Hence the offspring can acquire one allele from each parent.
Mendel's Second law (Independent arrangement)
, The various genes are separate entities and do not influence each other
in any way. They sort themselves out independently during gamete
formation, since homologous chromosomes arrange themselves
randomly along the equator of the cell during metaphase I
[Meiosis]
Complete dominance
With Complete dominance generally two characteristics are present and one
takes over (Dominates) and the individual displays one of the two characteristics
only.
Only one of the two characteristics show in the organism.
Complete dominance B b
B BB Bb
b Bb bb
Incomplete dominance
Neither are dominant or recessive
Blending takes place:
RY = O (Red + yellow = orange)
Incomplete dominance R R
Y RY RY
Y RY RY
Co-dominance
Both are dominant
No blending takes place.
Both genes appear (E.g. brown rabbit with white spots, BW)
E.g. Red + white = roan (both show)
Co-dominance W W
R RW RW
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