Human Genome
Vocabulary: karyotype, sex chromosome, autosome, pedigree, sex-linked gene, DNA fingerprinting, gene therapy
Humans are identified by their 46 chromosomes (23 pairs of homologous chromosomes). We receive 23 chromosomes via the egg cell from our mother and 23 chromosomes via the sperm cell from our father. Sperm and egg cells are haploid, meaning that they contain only a single set of chromosomes. When the sperm and egg unite during fertilization, the resulting cell (the zygote) has 46 chromosomes. A zygote is diploid because it contains both sets of homologous chromosomes.
Karyotypes
A karyotype is a photograph of chromosomes in order
from largest to smallest in pairs. By looking at a karyotype, you can decipher
whether the person is male or female, and often you can see if there are any
chromosome abnormalities. For example, if a person has Down's Syndrome, their
karyotype would have an extra 21st chromosome. See the karyotype below.
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The red circle highlights the extra chromosome. The red arrow is pointing to the sex chromosomes (XX). This karyotype is of a female with Down's Syndrome. |
Sex Chromosomes
Two of our 46 chromosomes are known as sex
chromosomes and are identified as the 23rd pair in a karyotype. Females have two
large X chromosomes and males have one X and one small Y chromosome. The other
22 pairs of chromosomes are known as autosomes.
Pedigrees
A pedigree is a chart that scientists use to show the relationships
within a family. A genetic counselor may use a pedigree to see how traits are
passed from one generation to the next, making inferences about the genotypes of
the different individuals. Pedigrees are usually used to trace a single-gene
trait through a family (ex: white forelock).
Many traits are either polygenic (controlled by many genes) or are influenced by the environment. This means that the expression of your genes can be affected by the environment you are exposed to (ex: your nutrition plays a large role in what your height and weight will be).
There are many genetic disorders that humans can inherit. Some of them include PKU, Down's Syndrome, sickle cell anemia, and Huntington's disease. There are many ways to diagnose genetic disorders including blood tests, amniocentesis, urine tests, and karyotype analysis.
Sex-Linked Genes
Genes that are found on the sex chromosomes (the X and Y) are known as
sex-linked genes. Many genetic disorders are found on the X chromosome.
Females would need two recessive copies on their X chromosomes for disorders
like hemophilia and colorblindness to be expressed. Since males only have one X
chromosome, all alleles located on the X chromosome are expressed. Due to the
male's single X chromosome, disorders like colorblindness are found more
commonly in males.
DNA Fingerprinting
To identify individuals, scientists use a method called DNA fingerprinting.
The DNA that is chosen to be analyzed serves little to no function to the
individual, but these sections can vary greatly from person to person. DNA is
cut by a restriction enzyme and undergoes gel electrophoresis, which separates
the DNA by size. The resulting banding pattern is unique to each person, and the
information is extremely useful in convicting criminals or in overturning
wrongful convictions.
Gene Therapy
Since scientists completed work on the Human Genome Project in 2000, we can use
the information gathered to help treat genetic disorders. Gene therapy is
when a faulty gene is replaced with a proper working gene. A similar scenario
would be adding a gene in where one is missing. This can be accomplished with
the help of a virus, since they are able to enter the DNA of a cell. First the
virus is modified so it cannot cause disease. Next, a piece of DNA containing
the normal functioning gene is inserted into the DNA of the virus. Lastly, the
patient is "infected" with the virus (that has the modified DNA), introducing
the proper working gene.
Ethical Issues
As we develop new technology and are able to alter the human genome, many
ethical issues arise. Should we use the knowledge we have to just cure diseases,
or is it alright to start changing whatever it is we don't like about ourselves?
Should we be able to choose the traits we want for our babies before they are
even born? Where do we draw the line?
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