1. Theoretical Genetics
Topic 4.3

2. Assessment Statements
4.3.1 Define genotype, phenotype, dominant allele, recessive allele, codominant alleles, locus, homozygous, heterozygous, carrier and test cross.
4.3.2 Determine the genotypes and phenotypes of the offspring of a monohybrid cross using a Punnett grid.
4.3.3 State that some genes have more than two alleles (multiple alleles).
4.3.4 Describe ABO blood groups as an example of codominance and multiple alleles.
4.3.5 Explain how the sex chromosomes control gender by referring to the inheritance of X and Y chromosomes in humans.
4.3.6 State that some genes are present on the X chromosome and absent from the shorter Y chromosome in humans.
4.3.7 Define sex linkage.
4.3.8 Describe the inheritance of colour blindness and hemophilia as examples of sex linkage.
4.3.9 State that a human female can be homozygous or heterozygous with respect to sex-linked genes.
Explain that female carriers are heterozygous for X-linked recessive alleles.
Predict the genotypic and phenotypic ratios of offspring of monohybrid crosses involving any of the above patterns of inheritance.
Deduce the genotypes and phenotypes of individuals in pedigree charts.

3. Gregor Mendel
Austrian monk who published results of garden pea plants inheritance in 1865
Used artificial pollination in a series of experiments by using a small brush to place the pollen on the reproductive parts of the flowers

4. Key terminology
Genotype – symbolic representation of pair of alleles possessed by an organism, typically represented by two letters
Ex: Bb, GG, tt
Phenotype – characteristics or traits of an organism
Ex: five fingers on each hand, color blindness, type O blood
Dominant allele – an allele that has the same effect on the phenotype whether it is paired with the same allele or a different one; always expressed in phenotype
Ex: Aa give dominant trait A b/c the a allele is masked; the a allele is not transcribed and translated during protein synthesis

5. Locus – particular position on homologous chromosomes of a gene
Recessive allele – an allele that has an effect on the phenotype only when present in the homozygous state
Ex: aa gives rise to the recessive trait b/c no dominant allele is there to mask it
Codominant allele – pairs of alleles that both affect the phenotype when present in a heterozygote
Ex: parent with curly hair and parent with straight hair can have children with different degrees of curliness as both alleles influence hair condition when both are present in the genotype
Locus – particular position on homologous chromosomes of a gene

6. Homozygous – having two identical alleles of a gene
Ex: AA is a genotype which is homozygous dominant whereas aa is the genotype which is homozygous recessive
Heterozygous – having two different alleles of a gene
Ex: Aa is a heterozygous genotype
Carrier – an individual who has a recessive allele of a gene that does not have an effect on their phenotype

7. Test cross – testing a suspected heterozygote plant or animal by crossing it with a known homozygous recessive (aa). Since a recessive allele can be masked, it is often impossible to tell if an organism is AA or Aa until they produce offspring which have the recessive trait.

8. Punnett grid
Shows all possible combinations of genetic information for a particular trait
Monohybrid cross shows the results for one trait only
Don’t use Cc, Ff, Oo, Pp, Ss, Uu, Vv, Ww, Yy, Zz

9. Constructing a Punnett grid
Choose and indicate a letter to show the alleles
Determine the parents’ genotypes
Determine the gametes which the parents could produce
Draw a Punnett grid
Deduce the chances for each genotype and phenotype

10. BREED!!!

11. Multiple alleles
Sometimes there are three or more alleles for the same gene
This is the case for the alleles which determine the ABO blood type in humans
4 possible phenotypes: A, B, AB, O
Three alleles of the gene which can produce six different genotypes
Gene represented by letter I

13. The sex chromosomes: X and Y
Determines gender
X much longer and contains many more genes
Female: XX
Male: XY
Always 50% chance of each gender
Chances remain the same no matter how many boys or girls already exist

14. Genes carried on the sex chromosomes
Sometimes alleles present on the X chromosome have nothing to pair up with
X and Y are not alleles, so terms such as dominant and recessive do not apply

15. Sex linkage
Any genetic trait whose allele has its locus on the X or Y chromosome is said to be sex linked
Colorblindness
Inability to distinguish certain colors (often green and red)
Two colors look the same
Hemophilia
Disorder in which blood does not clot properly

16. Alleles and genotypes of sex-linked traits
Since the alleles for both color blindness and hemophilia are found only on the X chromosome, the letter X is used in representing them:
Xb = allele for colorblindness
XB = allele for ability to distinguish colors
Xh = allele for hemophilia
XH = allele for the ability to clot blood
In both cases, there is no allele on the Y chromosome
What are the possible genotypes for color blindness?
Can men be carriers for X-linked alleles?

17. Dragon Genetics

18. TOK moment
Mendel was not recognized or commended for his work during his lifetime
Why is it that the scientific community is so reluctant to take on new ideas and paradigm shifts?
Does this have a positive or negative impact on the advancement of science?
Is it acceptable that a scientist’s new ideas are not adopted just b/c he or she is not a well-known person with an established reputation?

19. Pedigree Charts
Pedigree refers to the record of an organism’s ancestry
Constructed to show biological relationships
Used to show how a trait can pass from one generation to the next

20. The Royal Disease Pedigree

21. Homework Create a pedigree chart of your family
Include yourself, siblings, parents, maternal and paternal aunts and uncles, cousins, maternal and paternal grandparents

22. Huntington’s disease Caused by a dominant allele (H)
Genetic condition causes severely debilitating nerve damage
Symptoms do not show until the person is about 40
Why is this relevant to natural selection?
Within a few years the person loses complete control of his or her muscles and dies and early death

23. Pedigree chart showing members of a family affected by Huntington’s disease?

24. Codominance in flower color
Can create more than two colors
Pedigree can help keep track of how the offspring got their phenotypes
C refers to the gene that codes for flower color in snapdragons
Superscript refers to the specific color R or W
CRCR makes red flowers
CWCW makes white flowers
CRCW makes pink flowers

25. Pedigree chart for flower color in snapdragonsB C E G D F I K H J

26. Codominance in the shape of rbcs
Hb refers to the gene that codes for hemoglobin
Superscript letter for the typical shape of hemoglobin is A for normal and S for the shape that causes sickle cells
HbAHbA = normal
HbSHbS = sickle cell anemia
HbAHbS = some of each type

27. TOK
If someone is a carrier, should they have children knowing that they might transmit the gene?
How would it feel knowing – or not knowing – if you had the gene?
How might parents feel when the find out they have passed the gene on to their offspring?
What about marriage? Career? Life insurance?
Should your individual genetic makeup be disclosed?