Topic 4.3: Theoretical genetics

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

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?

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

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

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

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.

Punnett Squares Practice!!!!

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

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

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

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

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

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

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

Sex Chromosomes: X and Y Carriers of sex-linked traits Sex-linked recessive alleles such as Xb are rare in most populations of humans worldwide

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

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