3.4 Genes and the environment Flashcards
Phenotype
The characteristics of an organism
which result from the genes the organism
possesses and the environment in which it lives.
Himalayan rabbits have white fur but black feet,
ears and tail. Cross-breeding pure-bred
Himalayan rabbits produces young rabbits with
these characteristics. The black pigment will
only develop, however, in parts of the body
which are at a low temperature. A Himalayan
rabbit must have the gene for making black fur.
The rabbit must also be in the right environment
for this black pigment to develop.
Genotype
The genotype describes an organism in
terms of the alleles it contains. In pea plants the
allele for tall plants, T, is dominant to that for
short plants, t. A short plant always has the
genotype tt. This means each of its cells contains
two copies of the t allele. The genotype of a tall
plant, however, may be either TT or Tt.
Discontinuous variation
Variation in which
individuals fall into distinct categories. In peas,
for example, plants are either tall or short. There
are no intermediates. Discontinuous variation
results from the genes that an organism inherits.
Environment has little or no effect.
Continuous variation
Variation in which there is
a complete range from one extreme to the other.
Human height is an example of continuous
variation. A person can be any height within the
human range. Continuous variation often occurs
when a character is controlled by many different
genes, in other words, it is polygenic.
Polygenic inheritance
The form of inheritance
where a particular characteristic is controlled by
a number of genes. Characteristics such as
human height and skin colour and clinical
conditions such as schizophrenia and
Alzheimer’s disease are polygenic. Continuous
variation often results when a characteristic is
polygenic.
Multifactorial
A condition, e.g. human height, in
which several genes and one or more
environmental factors are involved. Many
diseases are also multifactorial.
Melanin
A dark pigment found in the skin, hair
and eyes. It is made in special cells called
melanocytes. Production of melanin is increased
by ultraviolet light. This is why sunlight results
in the skin becoming darker in colour. Melanin
protects the DNA of cells from the harmful
effects of ultraviolet light.
Melanocyte
A cell found mainly in the skin,
which is responsible for making the dark
pigment, melanin. These cells are activated by a
hormone called melanocyte-stimulating
hormone (MSH). Ultraviolet light increases the
amount of MSH, which in turn, causes
melanocytes to produce more melanin.
Melanocyte-stimulating hormone, MSH
A
hormone which activates special skin cells
called melanocytes and causes them to make
more of the dark pigment, melanin. Ultraviolet
light increases the amount of MSH produced.
This leads to more melanin being made and the
skin becoming darker in colour.
Melanosome
An organelle found in skin cells.
Melanosomes contain the pigment melanin.
Tyrosinase
An enzyme that catalyses the first
step in the pathway where the amino acid
tyrosine is changed to melanin, a dark pigment
found in the skin. Some people (and other
animals) have a mutant allele for the tyrosinase
gene. As a result they are unable to make
tyrosine so cannot produce melanin. Such people
are described as albinos. They have white hair
and skin, and no pigment in their eyes.
Tumour
A swelling resulting from the
uncontrolled division of cells. If these abnormal
cells rapidly increase in number, invade and
destroy the surrounding tissues, they are
described as malignant (cancerous). In some
tumours, individual cells eventually break away
from the original tumour. They are carried by
the blood or the lymphatic system to other areas
of the body where they form new tumours. A
benign tumour grows slowly, does not invade
surrounding tissue and does not spread. It is not
cancerous but may require treatment if it presses
on surrounding tissues.
Tumour suppressor gene
Tumour suppressor
genes are genes that code for proteins that stop
the cell cycle at appropriate checkpoints.
Mutations may affect these genes and stop them
from working. If this happens, the cell loses
control of the cell cycle. A tumour may result
and the person may develop cancer.
Oncogene
A gene that codes for a protein that
stimulates the cell cycle. Mutations in these
genes can lead to the cell cycle being continually
active. This may result in cancer.
