REVISION THEORETICAL GENETICS Topic 4 3 IB Biology

REVISION: THEORETICAL GENETICS Topic 4. 3 IB Biology Miss Werba

4. 3. 1 DEFINITIONS Phenotype: Physical manifestation of genotype Genotype: Combination of alleles present for a particular gene. Can be homozygous or heterozygous. J WERBA – IB BIOLOGY 2

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). J WERBA – IB BIOLOGY 3

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 J WERBA – IB BIOLOGY 4

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 F 1 genotype ratio (eg. 1: 2: 1, 25%: 50%: 25%) Work out the F 1 phenotype ratio (eg. 1: 2: 1, 25%: 50%: 25%) J WERBA – IB BIOLOGY 5

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 T TT TT t Tt Tt F 1 genotype ratio: 1 TT : 1 Tt F 1 phenotype ratio: 100% Tall J WERBA – IB BIOLOGY 6

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) J WERBA – IB BIOLOGY 7

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) J WERBA – IB BIOLOGY 8

4. 3. 3 4. 3. 4 MULTIPLE ALLELES & CODOMINANCE Allows for six possible genotypes. Allows for four possible phenotypes. Possible Genotypes ΙAΙA or ΙAi ΙBΙB or ΙBi ΙA ΙB ii J WERBA – IB BIOLOGY Phenotype or blood group Type A Type B Type AB Type O 9

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. J WERBA – IB BIOLOGY 10

4. 3. 6 4. 3. 7 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 Xa. Y J WERBA – IB BIOLOGY 11

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 J WERBA – IB BIOLOGY 12

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 J WERBA – IB BIOLOGY 13

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) J WERBA – IB BIOLOGY 14