Chapter 7 : DNA Detective

7.1 Extensions of Mendelian Genetics

Incomplete dominance
Some genes show incomplete dominance, where both alleles affect the phenotype. Table 7.1. So homozygous dominant (red flower) looks different from heterozygous (pink flower).
If two heterozygous individuals are crossed the offspring are in the ratio :

        1        :        2        :        1
   homozygous        heterozygous       homozygous
    dominant                             recessive
      (red)             (pink)            (white)

Human Blood groups
There are 3 different alleles : I^A , I^B , and i.
I^A and I^B are codominant ( both affect the phenotype). Essay 7.1.

  Phenotype
(Blood group )       Genotype
    O                i  i
    A                I^A  I^A  or  I^A i	
    B                I^B  I^B   or  I^B i
    AB               I^A  I^B

Blood has antibodies that react against factors that are not present in the blood.
eg group A has antibodies against B.

Blood group O can give blood to anyone ( universal donor ).
Blood group AB can receive blood from anyone ( universal recipient ).

Blood typing is used in paternity cases. It can be used to prove that someone is not the father of a child (for example if the father is blood group AB the child cannot be group O).

7.2 Sex determination

Genes are located on chromosomes. A typical human chromosome has roughly 1000 genes on it.
Gender
Human cells have 22 pairs of non-sex ( autosomal ) chromosomes, that are the same in men and women, and 1 pair of sex chromosomes. Fig 7.1

The sex chromosomes are XX in the female and XY in the male. The egg always contains an X chromosome. Half the sperm contain an X chromosome, half contain a Y chromosome. If a sperm containing the X chromsome fertilizes the egg the baby will be a girl (XX), if the sperm contains a Y chromsome the baby will be a boy (XY).

Sex linked diseases
These diseases mainly affect men. They are caused by recessive genes on the X chromosome. Females have 2 copies of X, males have only 1 copy. Sex-linked diseases are usually passed on from a carrier female to her son. Fig 7.3
examples : colorblindness ( 8 % men colorblind, 0.5% women ). There are several types, the most common is red/green colorblindness.

Hemophilia ( blood does not clot ). Queen Victoria was a carrier. Fig 7.6 Hemophilia

Essay 7.2: Chromosomal abnormalities
Sometimes due to a mistake during meiosis a cell inherits the wrong number of chromosomes. If the number inherited is very different from normal, the embryo does not survive.

Examples ( Table E7.3 ).
Down syndrome : extra copy of chromosome 21 (trisomy 21) .Total of 47 chromosomes.
The risk of having a baby with Down syndrome increases as the mother gets older.

Down syndrome children are usually short, mild to moderately mentally retarded, and live about 55 years. Men with Down syndrome are sterile, women can have children but there is a 50% chance of the child having Down syndrome.

Turner syndrome : missing an X chromosome ( genotype X ).
Total of 45 chromosomes.
They are always female, are typically short and almost always sterile.

Metafemale : extra X chromosome ( XXX ).
Total of 47 chromosomes.
Usually tall females, with normal fertility.

Klinefelters syndrome : genotype XXY
Total of 47 chromosomes.
They have male sex organs, but high levels of female hormone (because of the two X chromsomes), so often grow breasts. Typically they are sterile.

Jacobs syndrome : genotype XYY
Total of 47 chromosomes.
Tall males with very slight mental retardation.

7.3 Pedigrees

Pedigree charts are family trees that are used to show how traits are inherited. Fig 7.4.

Dominant alleles : if a child has that trait, at least one parent must have that trait. eg Polydactyly (having 6 fingers). Fig 7.5
Recessive alleles : children can have that trait even if neither parent shows it (both parents are carriers) eg attached earlobes. Fig 7.5
Sex-linked diseases affect mainly men eg hemophilia Fig 7.5 and 7.6

7.4 DNA fingerprinting

This method cuts DNA into fragments with restriction enzymes, then separates the fragments with electrophoresis. Fig 7.9. This makes a genetic fingerprint. Fig 7.11.

DNA can be extracted from skin, blood, hair, sperm, saliva etc. It is used in paternity cases to prove that someone is the father.

Genetic fingerprints are used in murder and rape cases. Florida was one of the first states to use genetic fingerprinting in court cases. DNA Fingerprinting

Polymerase chain reaction : PCR
This uses DNA polymerase to make many copies of DNA. Fig 7.7.
It is often used where there are small samples of DNA, for example a single drop of blood at a crime scene. PCR can copy that DNA so that a genetic fingerprint can be obtained. PCR

The Russian royal family were executed during the Russian Revolution in 1918. Their bodies were identified using PCR and DNA fingerprinting in the 1990s. Fig 7.12 These tests also showed that a woman who claimed to be Anastasia (one of the daughters of the Tsar) was not related to the royal family.

Last edited March 2009, by David Byres, dbyres@fccj.edu