1.5 Changing Man
Problem 1
Genetic transmission
Chromosomes and genes are located inside the nucleus. Chromosomes carry the genes that code the
aspects of ourselves that we inherit. All of our body cells have 46 chromosomes. An exception are the gg
and sperm cells, which contains only half of this material; 23 chromosomes). Germ cells are the ones
that produce gametes (sperm in males and ova in females) which contain the 23 chromosomes. This is
due to the special form of cell division that happens in the reproductive organs, called meiosis. The paring
is possible because each chromosome contributed by the father´s sperm is homologous (similar in shape
and function) to the mother´s egg chromosomes. Copies of these original 23 homologous pairs of
chromosomes are passed on to every cell in a person´s body apart from the reproduction cells (which
only has 23 single chromosomes).
During meiosis (when a male´s or females set of chromosomes is halved to produce a reproductive cell;
sperm or egg) has random DNA from the mother and father because of the mixing process by a process
called crossing over. The germ cell first duplicates its 46 chromosomes. Then adjacent duplicated
chromosomes cross and break at one or more points along their length, exchanging segments of genetic
material (cross over). This transfer of the genes during crossing over creates a new and unique hereditary
combinations. Next, pairs of duplicated chromosomes segregate into new cells, each of which contains
46 chromosomes. Finally, the new cell divides so that each of the resulting gametes contain 23 single
chromosomes. At conception then, a sperm with 23 chromosomes unites with an ovum with 23
chromosomes, producing a zygote with a full set of 46 chromosomes.
The process by which a human being is formed after conception is called mitosis. Here a cell duplicates
its chromosomes and the divides into daughter cell s that have the exact same number of chromosomes
as their parent cell until the zygote becomes a multicellular organism.
The binding element of a chromosomes is a long, thin molecule of deoxyribonucleic acid, or DNA. This
molecule, which stores genetic information and transmits it during reproduction is made up of building
blocks called nucleotides that are held together by two long, twisted parallel stands that resemble the
two side rails of a spiral staircase. Only bases that are compatible with each other ill bond together;
adenine and thymine forma a bond, s do cytosine and guanine. Chromosome carry the units of hereditary
information via genes which are located at particular sites on the chromosome where they code for the
production of certain kinds of proteins. Genes trigger the production of proteins only when a particular
change in the environment signals them to activate. Now the gene, splits down the middle so that it’s
not attached to it pairs, and in turn attaches to another pair to form a new connection. The resulting
copy of the gene called mRNA or messenger ribonucleic acid then travels from the cell nucleus to the
body cell where the protein synthesis takes place.
Gene expression
A genotype is the specific makeup of the individual, while phenotype is the individual´s observable
characteristics. Genotype is present from conception, abut phenotype can be affected both by other
genes and by the environment. The study of how genes come to be expressed is known as epigenetics.
, Researchers have discovered that only 2% of our DNA produces proteins while the rest of it regulates
how protein-coding DNA is expressed. For example, it inhibits the transcription into mRNA until a
regulation factor (substance in the environment) switches the inhibition off.
Alleles are the single strands of chromosomes (from the 23 pairs). If the alleles from both parents are
the same, the person is said to be homozygous for that particular gene or for the trait associated with it.
If the two alleles are different, the person is heterozygous for that particular characteristic. The possible
outcomes when the alleles are heterozygous are that they will produce an intermediate outcome, they
will combine but not blend (codominance), or the characteristic associated with only one of the alleles
will be expressed (dominant and recessive allele). An example of codominance is type AB blood.
An exception of the rule that genes have two alleles are the sex chromosomes. In every human being, 22
of the pairs of chromosomes are autosomes, while 1 of the 23 pairs, are called sex chromosomes. This
chromosomes determine the individual´s sex, and they differ in males (XY) and females (XX). Fathers are
the ones that determine the sex of their children since half of the father´s sperm contains the X
chromosome and half the Y chromosomes, while the mother has both X chromosomes.
It is important to mention that the Y chromosome isn’t homologous or doesn’t have the same genetic
material as the X chromosome. As a result, any recessive X-linked genes will automatically be expressed.
Because of these differences between the sex chromosomes, genetic disorders that occur in the sex
chromosomes are more common in men than in women. One sex chromosome abnormality is called
fragile X syndrome. People with this syndrome carry an X chromosome that appears to be pinched in
some areas, causing it to be fragile. Because females are more likely to have at least one proper
functioning X chromosome (they have two X chromosomes), the syndrome is more frequent in males
than in females. Thus, the higher rates of mortality in males compared with females are partly attributed
to this disorder.
Some genes call for the production of amino acids (which forms enzymes and other proteins that are
necessary for the formation and functioning of new cells), other guide cell differentiation (which cells
form part of the brain and which ones for the digestive system, for example), and others regulate the
timing of development. There are four main patterns of genetic expression
Simple dominant-recessive inheritance: the strong gene is called dominant allele and the weaker
gene is called recessive allele
Codominance: two genes are equally dominant so they coexist
Sex-linked inheritance: traits that are determined by genes located on the sex chromosomes.
Polygenic inheritance: human characteristics influenced by many pairs of alleles. Some examples
include weight, height, intelligence, skin color, temperament and susceptibility to cancer.
Another thing is that genes usually have multiple effects on coding so they are pleiotropic.
Some negative effects genes can have on development are:
A major reason potentially harmful alleles survive is that they are not harmful in the heterozygous state
since the dominant (healthy) allele will always win. The problem is when the alleles are recessive
homozygous (the same) gene. However, some potentially harmful alleles may survive because they are
beneficial in combination with a normal allele. For example, the sickle cell anemia is a disease that, the
people that have it in Africa, can combat better against malaria than the ones that don’t have it.
