Topic 4.3: Theoretical genetics

1. Topic 4.3: Theoretical genetics

2. Assessment Statement • 4.3.1: Define genotype, phenotype, dominant allele, recessive allele, codominant alleles, locus, homozygous, heterozygous, carrier and test cross • 4.3.2: Determine the genotype and phenotype 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 human • 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 color blindness and haemophilia as examples of sex linkages • 4.3.9: State that a human female can be homozygous or heterozygous with respect to sex-linked genes • 4.3.10: Explain that female carriers are heterozygous for x-linked reciessive alleles • 4.3.11: Predict the genotypic and phenotypic ratios of offspring of monohybrid crosses involving any of the above patterns of inheritance • 4.3.12: Deduce the genotypic and phenotypic ratios of offspring of monohybrid crosses involving any of the above patterns of inheritance • 4.3.13: Deduce the genotypes and phenotypes of individuals in pedigree charts

3. Who is Gregor Mendel • Published results of his experiments on how garden pea plants passed on their characteristics • Among the questions Mendel asked: • How can I be sure that I will get only smooth peas and no wrinkled ones? • How can I be sure that the resulting plants will be short or tall? • How can I be sure to obtain only flowers of a certain color?

4. Who is Gregor Mendel • Mendel used artificial pollination in a series of experiments in which he carefully chose the pollen of various plants to fertilize other plants

5. Key Terminology • Genotype: The symbolic representation of pair of alleles possessed by an organism, typically represented by two letters • Phenotype: The characteristics or traits of an organisms • Dominant allele: An allele that has the same effect on the phenotype whether it is paired with the same allele or different one. • Recessive allele: An allele that has an effect on the phenotype when present in a heterozygote

6. Key Terminology • Codominant alleles: Pairs of alleles that both affect the phenotype when present in a heterozygote • Locus: The particular position on homologous chromosomes of a gene. Each gene is found at a specific place on a specific pair of chromosomes • Homozygous: Having two identical alleles of a gene • Heterozygous: Having two different alleles of a gene

7. Key Terminology • Carrier: An individual who has a recessive allele of a gene that does not have an effect on their phenotype • 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 Squares • Practice!!!!

9. Multiple alleles • With two alleles, three different genotypes are possible which produce two different phenotypes • Genetics are not always this simple; 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

10. Multiple alleles • Blood Type • ABO blood type system in humans has four possible phenotypes • A, B, AB, and O • To create these four blood types there are six alleles of the gene. • These three alleles can produce six different genotypes • The gene for the ABO blood type is represented by the letter I. To represent more than just two alleles (i and I) • Two capital letters represent codominance

11. Multiple alleles • Blood Type • IAIA or Iai gives phenotype of type A blood • IBIB or Ibi gives phenotype of type B blood • IAIB gives type AB (codominance) • Ii give type O blood

12. Sex Chromosomes: X and Y • The 23rd pair of chromosome are called the sex chromosome because they determine if a person is a male or a female. • The X chromosome is longer than the Y chromosome and contains many more genes • Unlike the other 22 pairs of chromosome, this is the only pair in which it is possible to find chromosomes that are very different in size and shape • XX= female • XY= male

13. Sex Chromosomes: X and Y • Genes carried on the sex chromosome • Because the Y chromosome is significantly smaller than the X chromosome, it has fewer loci and therefore fewer genes than the X chromosome • This means that sometimes alleles present on the X chromosome have nothing to pair up with. • For example, a gene whose locus is at an extremity of the X chromosome would have no counterpart on the Y chromosome because the Y chromosome does not extend that far from its centromere

14. Sex Chromosomes: X and Y • Sex Linkage • Any genetic trait whose allele has its locus on the X or the Y chromosome is said to be sex linked • Often genetic traits which show sex linkage affect one gender more than the other. • Two examples of genetic traits which have this particularity are color blindness and hemophilia

15. Sex Chromosomes: X and Y • Alleles and genotypes of sex-linked traits • Since the alleles for both color blindness and hemophilia are found only on the X chromosome • Xb=recessive allele for color blindness • XB=allele for the ability to distinguish • XBXB=give the phenotype of a non-affected female • XBXb=gives the phenotype of a non-affected female who is a carrier • XbXb=gives the phenotype of an affected female • XBY=gives the phentype of a non-affected males • XbY=gives the phenotype of an affected male

16. Sex Chromosomes: X and Y • Carriers of sex-linked traits • Sex-linked recessive alleles such as Xbare rare in most populations of humans worldwide

17. Pedigree Charts • The term pedigree refers to the record of an organisms ancestry • Pedigree charts are diagrams which are constructed to show biological relationships • In genetics, they are used to show how a trait can pass from one generation to the next • Used in this way for humans, a pedigree chart is similar to a family tree complete with parents, grandparents, aunts, uncles and cousins

18. Pedigree Charts • To build such a chart, symbols are used to represent people. • Male (affected) • Female (affected) • Male • Female • Vertical line-represents offspring • Horizontal line-represents marriage