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Summary Genetics & Inheritence
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Summary and notes needed for grade 12, Relative to both IEB and NSC
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&- Heredity → transmission of characteristics from parents to offspring. - Definition → alternative forms of a gene found at the same locus &
- Genetics → study of heredity & variations that occur in transmission of providing different expressions of a characteristic.
hereditary characteristics. - Dominant allele → always expressed in the phenotype.
- Genes → particular length of DNA at a specific location (locus) on the - Recessive allele → not expressed in phenotype; only shows up in
chromosome that influences a particular characteristic. homozygous combo as it is masked by dominant allele when in a
- Genes control structural characteristics (eye colour, blood type etc.) or heterozygous combo.
physiological characteristics (rate of metabolism, susceptibility to diseases - Represented using letters of the alphabet (dominant = capital; recessive =
/ disorders etc.) lowercase)
- Monogenic inheritance → characteristic is controlled by a single gene at a - Eg. flower colour is controlled by a single gene with 2 alleles: one allele
single locus on the chromosome. for red colour & one for white colour.
- Polygenic inheritance → characteristic is controlled by more than one - When homologous chromosomes separate during meiosis, this results in
gene at different loci on one or more chromosomes. only one allele ending up in each new cell ∴ gametes have one allele each
- Phenotype → external appearance of an organism; determined by ∴ when they fuse, offspring will have a combination of alleles from both
genotype. parents.
- Genotype → combination of alleles that an individual possesses for a - Homozygous (pure-breed) → 2 copies of the same allele for a specific
particular characteristic. characteristic. (eg. RR = homozygous dominant or rr = homozygous
- Gregor Mendel = “Father of Genetics” recessive)
- Heterozygous (hybrid) → different alleles for a specific characteristic (eg.
Rr)
, MONOHYBRID CROSSES: TYPES OF DOMINANCE:
- Definition → cross between parents concerning the inheritance of one
characteristic / trait. (eg. height) - Alleles are EITHER dominant or recessive
- Dominant allele is FULLY expressed in phenotype.
- Uppercase letter = dominant allele; lowercase letter = recessive allele
P1 generation / parents: Phenotype: Tall pea plant x Short pea plant
Genotype: TT x tt
Meiosis - 2 different alleles combine to create an intermediate phenotype
Female - Offspring look like a blend of the characteristics of the 2 parents (neither
Gametes: T ; T x t ; t
of the parents’ characteristics actually appear in the phenotype!)
- Eg. Red flower (CRCR)
Can’t be just one capital & one small letter → would
Male White flower (CWCW)
Gametes suggest that one is dominant and one is recessive.
T T Pink flower (CRCW)
Fertilisation t Tt Tt
- Both alleles of a specific characteristic are equally as dominant & are both
t Tt Tt fully expressed in the phenotype of a heterozygous offspring.
- Eg. Blood groups:
> A , B , AB , O
> Controlled by a single gene (I → immunoglobulin) with many alleles.
F1 generation / offspring: Phenotype: 4 x Tall
> This gene produces antigens on the surface of red blood cells (RBC).
Genotype: 4 x Tt
Punnet square IAIA or IA i = Group A A & B are dominant to O ∴ if O is with one
IBIB or I Bi = Group B of them, complete dominance is shown.
IAIB = Group AB
ii = Group O A & B are both present and are equally
dominant ∴ co-dominance is shown and
both produce their antigens.
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