Chapter 8

DNA Detective
Complex Patterns of Inheritance, and DNA Fingerprinting

8.1 Extensions of Mendelian Genetics

    Incomplete Dominance is when a heterozygote expresses a phenotype intermediate between both alleles.

   For example, RR produces red flowers, Rr produces pink flowers and rr produces white flowers

    Codominance is when two alleles are expressed at the same time.

   ABO blood type is an example of this with both the dominant A and B being expressed in a type AB person.

    Multiple allelism occurs when there are more than two alleles of a gene.

   ABO blood types exhibit this.


8.1 Extensions of Mendelian Genetics

    ABO blood group has three alleles of one gene:

   IA and IB are codominant to each other; i is recessive to both other alleles.

   An individual will have two of these alleles.

    Another blood group is called the Rh factor.

    Rh+ is caused by a dominant allele.

     Rh+Rh+ or Rh+Rh-

    Rh- is caused by 2 recessive alleles.


    Blood typing can be used to exclude potential parents.

   E.g., an AB parent can never have an O child and two Rh- parents can never have a Rh+ child.

   See Table 8.2 for compatibilities of blood types.


8.1 Extensions of Mendelian Genetics

    Pleiotropy is the ability of a single gene to cause multiple effects on the individuals phenotype.

    Hemophilia is an example of pleiotropy.

   The inability to clot blood normally due to the absence of a clotting factor

   Gene for this clotting factor is on the X chromosome

   Prince Alexis suffered from this and inherited the hemophilia allele from his mother


8.2 Sex Determination and Sex Linkage

    Humans have 22 pairs of autosomes and one pair of sex chromosomes

   Women: two X chromosomes

   Men: one X and one Y chromosome

    Sex-linked genes: genes located on the sex chromosomes

   X-linked: located on the X chromosome

   Y-linked: located on the Y chromosome

  SRY gene which leads to the development of the testes

    Males always inherit their X from their mother

    Males are more likely to express recessive X-linked traits than females due to carrying only 1 X.

    Females are less likely to express X-linked traits since they have to have 2 copies of the bad Xs.

    X inactivation guarantees that all females receive only 1 dose of the proteins by the X chromosomes.

    Inactivation is irreversible and inherited during cell division.

    It is caused by RNA wrapping around the X chromosome.


8.3 Pedigrees

    Pedigree: a family tree, showing the inheritance of traits through several generations

    Symbols commonly used in pedigrees are circles and squares

    Pedigrees reveal modes of inheritance

8.4 DNA Fingerprinting

    No two individuals are genetically identical except for identical twins.

   Small differences in nucleotide sequences of their DNA

    This is the basis for DNA fingerprinting

   Unambiguous identification of people

   When sample size is small it is necessary to copy the genetic material to increase the quantity available for testing.


8.4 DNA Fingerprinting

    Small amounts of DNA can be amplified using PCR (polymerase chain reaction)

    DNA is mixed with nucleotides, specific primers, Taq polymerase, and then is heated

    Heating splits the DNA molecules into two complementary strands

    Taq polymerase builds a new complementary strand

    DNA is heated again, splitting the DNA and starting a new cycle.

    Each cycle, the amount of DNA doubles.

    DNA is cut into fragments using restriction enzymes, which cut around DNA sequences called VNTRs (variable number tandem repeats).

    Gel electrophoresis separates DNA fragments on basis of their sizes

   Electric current is applied to an agarose gel

   Smaller fragments run faster through the gel

    Fragments are transferred to a sheet of filter paper

    Labeled probe reveals locations of fragments containing VNTRs


8.4 DNA Fingerprinting

    Each persons set of fragments is unique.

    All of a childs bands must be present in one
or both of the parents.

    To see if parents and their children were Romanovs, DNA fingerprints were prepared for relatives of tsar and tsarina.

    Adult male skeleton (related to the children) was related to George, the tsars brother.

    Adult female skeleton (related to the children) was related to Prince Philip, the tsarinas grand-nephew.

    Conclusion: the grave contained the tsar, tsarina, three of their children, and four servants.