1. THEORETICAL GENETICS
Topic 4.3
IB Biology
Miss Werba

2. TOPIC 4 - GENETICS 4.1 CHROMOSOMES, ALLELES, GENES & MUTATIONS 4.4 4.2
MEIOSIS
4.3
THEORETICAL GENETICS
4.4
GENETIC ENGINEERING & BIOTECHNOLOGY
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3. THINGS TO COVER Definitions of terms Construct a Punnett grid
Multiple alleles for single genes
ABO blood groups & co-dominance
Gender & sex chromosomes
Sex linkage
Calculate & predict the genotypic and phenotypic ratios of offspring of monohybrid crosses
Deduce the genotypes or phenotypes of individuals in pedigree charts
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4. MORE GENETIC DEFINITIONS Command term = DEFINE
4.3.1
MORE GENETIC DEFINITIONS Command term = DEFINE
Genotype: the alleles of an organism.
Can be homozygous or heterozygous.
Phenotype: the characteristics of an organism.
Phenotype = Physical manifestation of genotype
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5. MORE GENETIC DEFINITIONS Command term = DEFINE
4.3.1
MORE GENETIC DEFINITIONS Command term = DEFINE
Dominant allele: an allele that has the same effect on the phenotype whether there are one or two copies present.
Represented with an uppercase letter (A)
Recessive allele: an allele that only has an effect on the phenotype when two copies are present.
Represented with a lowercase letter (a)
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6. MORE GENETIC DEFINITIONS Command term = DEFINE
4.3.1
MORE GENETIC DEFINITIONS Command term = DEFINE
Homozygous: Two identical alleles of a gene. Can be homozygous dominant (BB) or homozygous recessive (cc).
Heterozygous: Two different alleles of a gene. Only the dominant allele in the heterozygous genotype (Aa) will be expressed.
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7. MORE GENETIC DEFINITIONS Command term = DEFINE
4.3.1
MORE GENETIC DEFINITIONS Command term = DEFINE
Inheritance can be:
Completely dominant: One allele can mask another. eg. Brown allele (B) always expressed when present
Codominant: Pairs of alleles that both affect the phenotype when present in a heterozygote. eg. Type AB blood from A and B alleles eg. red allele + white allele = pink phenotype
Please note: the term incomplete dominance is no longer used
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8. MORE GENETIC DEFINITIONS Command term = DEFINE
4.3.1
MORE GENETIC DEFINITIONS Command term = DEFINE
Locus: the position of a gene on homologous chromosomes.
Carrier: an individual that has one copy of a recessive allele that causes a genetic disease in individuals that are homozygous for this allele. Will show no symptoms of the disorder, but can pass it on to the next generation.
Test cross: testing a suspected heterozygote by crossing it with a known homozygous recessive.
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9. 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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10. 4.3.2
4.3.11
PUNNETT SQUARES
Example:  App: Gene Screen If we start with a pure bred tall plant and a heterozygote, what will be the phenotype & genotype ratios of the offspring?
Let T = Tall; t = not tall
Parents: TT and Tt
Gametes: T and T , t
F1 genotype ratio: 1 TT : 1 Tt (50% TT : 50% Tt)
F1 phenotype ratio: 100% Tall T TT t Tt
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11. MULTIPLE ALLELES Command term = STATE
4.3.2
MULTIPLE ALLELES Command term = STATE
Some genes have more than two alleles (ie. multiple alleles).
eg. ABO blood grouping
Controlled by 3 alleles:
A (ΙA)
B (ΙB) and
O (i)
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12. MULTIPLE ALLELES and CODOMINANCE
4.3.4
MULTIPLE ALLELES and CODOMINANCE
2 of the alleles show codominance:
means that both alleles affect the phenotype and neither is dominant or recessive
ΙA and ΙB
specific notation (symbols) given because of codominant type of inheritance
base letter with allele superscript
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13. MULTIPLE ALLELES and CODOMINANCE
4.3.4
MULTIPLE ALLELES and 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 i (O)
specific notation given because of complete type of inheritance
uppercase letters for base and A and B alleles
lowercase for O
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14. MULTIPLE ALLELES and CODOMINANCE
4.3.4
MULTIPLE ALLELES and CODOMINANCE
Allows for six genotypes and four 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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15. 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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16. SEX- LINKAGE Command term = DEFINE
4.3.7
SEX- LINKAGE Command term = DEFINE
Sex linkage: when a gene controlling a characteristic is found on a sex chromosome
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17. SEX- LINKED GENES Command term = STATE
4.3.6
SEX- LINKED GENES Command term = STATE
Some genes are present on the X chromosome that are absent from the shorter Y chromosome in humans.
This means that most sex-linked conditions are usually X-linked.
Special notation is used for X-linked genes:
eg. XA Xa not Aa
Please note: as the Y chromosome have no corresponding alleles, they are not given a superscript allele
eg. XAY or XaY
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18. GENDER & SEX-LINKED TRAITS Command term = STATE
4.3.9
4.3.10
GENDER & SEX-LINKED TRAITS Command term = STATE
Females can be homozygous or heterozygous with respect to x-linked traits:
Homozygous XNXN or XnXn
Heterozygous XNXn ( = carriers )
Males only have one X chromosome. They cannot be carriers. They are either affected or unaffected.
Unaffected XNY
Affected XnY
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19. SEX- LINKED TRAITS Two x-linked traits are: Colour blindness
4.3.8
SEX- LINKED TRAITS
Two x-linked traits are:
Colour blindness
Haemophilia
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20. COLOURBLINDESS  App: ColorTest
4.3.8
COLOURBLINDESS
 App: ColorTest
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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21. HAEMOPHILIA Relates to ability to make a blood clotting factor
4.3.8
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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22. 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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23. Sample questions
Q1. The pedigree chart shows the blood types of three members of a family.
Which could be the blood types of individuals 1 and 2?
Individual 1 Individual 2
A. A AB
B. AB B
C. O B
D. B A
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24. Sample questions
Q2. The ACHOO syndrome is an inherited condition that leads to sneezing in response to bright light and is hypothesized to be inherited in an autosomal (not sex-linked) dominant fashion. The following is a pedigree chart that shows three generations from one family.
[Source: “Autosomal dominant pedigree chart. In Autosomal Dominance the chance of receiving and expressing a particular gene is 50% regardless of the sex of parent or child.” Jerome Walker. 22 July 2006.]
What evidence from the pedigree chart confirms that the ACHOO syndrome is not X-linked dominant?
A. Four females in the pedigree chart are affected and X-linked conditions do not affect females.
B. There is an affected male in generation II.
C. There is an affected female in generation II.
D. If the condition is X-linked dominant, the affected mother in generation II could not produce an unaffected son.
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25. Sample questions
A1. D
A2. B
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26. Sample questions
Q3. A boy inherited red-green colour-blindness from one of his grandfathers. Deduce, giving your reasons, which of his two grandfathers was also colour-blind. (3)
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27. Sample questions
A3.
red-green colour-blindness is a sex-linked character / gene located on X- chromosome;
boy inherits X chromosome from mother, only inherits Y chromosome from father;
boy’s mother must have been carrying the gene;
he inherited it from mother’s father / maternal grandfather; 3 max
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