1. REVISION: THEORETICAL GENETICS
Topic 4.3
IB Biology
Miss Werba

2. DEFINITIONS Phenotype: Physical manifestation of genotype
4.3.1
DEFINITIONS
Phenotype: Physical manifestation of genotype
Genotype: Combination of alleles present for a particular gene. Can be homozygous or heterozygous.
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3. 4.3.1
DEFINITIONS
Heterozygous: Two different versions of a gene. Only the dominant allele (A) will be expressed.
Homozygous: Two of the same versions of a gene. Can be homozygous dominant (BB) or homozygous recessive (cc).
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4. DEFINITIONS Inheritance can be:
4.3.1
DEFINITIONS
Inheritance can be:
Completely dominant: One allele can mask another. eg. Brown allele (B) always expressed when present
Codominant: Both alleles in heterozygous state are fully expressed. eg. Type AB blood from A and B alleles
Incompletely dominant: Both alleles in heterozygous state are expressed as a blend. eg. red allele + white allele = pink phenotype
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5. 4.3.2
PUNNETT SQUARES
Punnett square: a way of finding the expected ratio of the offspring, given certain parental phenotypes
Steps:
Identify parental genotypes
Identify possible gametes  add to column/row headers
Do the cross – determine possible genotypes of offspring
Work out the F1 genotype ratio (eg. 1:2:1, 25%:50%:25%)
Work out the F1 phenotype ratio (eg. 1:2:1, 25%:50%:25%)
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6. 4.3.2
4.3.11
PUNNETT SQUARES
Example: If we start with a pure bred tall plant and a heterozygote, what will be the phenotype & genotype ratios of the offspring?
Parents: TT and Tt
Gametes: T and T , t
F1 genotype ratio: 1 TT : 1 Tt
F1 phenotype ratio: 100% Tall T TT t Tt
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7. MULTIPLE ALLELES & CODOMINANCE
4.3.3
4.3.4
MULTIPLE ALLELES & CODOMINANCE
eg. ABO blood grouping
3 alleles: A (ΙA), B (ΙB) and O (i)
2 of the alleles show codominance:
means that both alleles have an effect on the phenotype and neither is dominant or recessive
ΙA and ΙB
specific notation given b/c of codominant type of inheritance (base letter with allele subscript)
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8. MULTIPLE ALLELES & CODOMINANCE
4.3.3
4.3.4
MULTIPLE ALLELES & CODOMINANCE
2 of the alleles show complete dominance also:
means that only the dominant allele will be expressed in the phenotype
ΙA and ΙB are both dominant over ί (O)
specific notation given b/c of complete type of inheritance (uppercase letters for base and A and B alleles, lowercase for O)
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9. MULTIPLE ALLELES & CODOMINANCE
4.3.3
4.3.4
MULTIPLE ALLELES & CODOMINANCE
Allows for six possible genotypes.
Allows for four possible phenotypes.
Possible Genotypes
Phenotype or blood group
ΙAΙA or ΙAi
Type A
ΙBΙB or ΙBi
Type B
ΙAΙB
Type AB ii
Type O
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10. SEX DETERMINATION All ova contain an X chromosome
4.3.5
SEX DETERMINATION
All ova contain an X chromosome
Sperm contain either an X or Y chromosome
XX  girl
XY  boy
There is a 50:50 chance in every fertilisation event of a child being a boy or a girl.
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11.
SEX- LINKED GENES
Sex-linked genes are on the sex chromosomes (mostly X)
Two x-linked traits are:
Colour blindness
Haemophilia
Special notation is used for X-linked genes:
eg. XA Xa not Aa
NB. Y chromosome have no corresponding alleles
eg. XAY or XaY
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12. COLOURBLINDESS Relates to ability to make photo-pigments in cone cells
4.3.8
4.3.9
4.3.10
COLOURBLINDESS
Relates to ability to make photo-pigments in cone cells
Inheritance is X-linked recessive
Most common form is red-green colour blindness
Males cannot be carriers of X-linked disorders, but females can
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13. HAEMOPHILIA Relates to ability to make a blood clotting factor
4.3.8
4.3.9
4.3.10
HAEMOPHILIA
Relates to ability to make a blood clotting factor
Inheritance is X-linked recessive
It normally results in excessive bleeding both internally and externally.
Regular injections of clotting factor are needed for survival.
Males cannot be carriers of X-linked disorders, but females can
Females cannot be homozygous recessive – it is a lethal combination
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14. PEDIGREES Genetic family tree
4.3.12
PEDIGREES
Genetic family tree
Gives clues about the type of inheritance:
dominant or recessive
sex-linked or autosomal
Things to look for:
Gender bias of males : females ( sex-linked)
Generation skipping ( recessive)
Affected sons from affected fathers ( autosomal)
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