Inheritance
"She has her mother's eyes". Some characteristics
obviously "run in families" with similarities between the
children and parents being quite noticeable.
It was initially thought these inherited features were passed through
the blood (hence the term "blood brothers"). Later the sperm
was credited with carrying all the necessary information, with the ovum
simply being a food supply! Now we know that both the egg and sperm
carry information about inherited features.
The first true geneticist
In the 1860s, however, the Austrian monk Gregor
Mendel carried out an extensive cross-breeding program with garden peas
and he concluded that the inheritance of each characteristic of an organism
is determined by hereditary units (genes), which can be passed from
one generation to another.
What is a gene?
A gene is actually a set of instructions in DNA (deoxyribonucleic acid)
for assembling a specific protein which affects one aspect of the organism's
structure, function or behaviour. Each characteristic of an organism
is coded by at least one gene.
Genes are made of DNA with a specific code. Mutations in gametes slightly
alter this code which changes the individual's ability to make that
specific protein.
Genes are located on the chromosomes found in the nucleus of each cell.
The position of a gene along a chromosome is called the gene locus (pl.
loci).
Alleles
When a mutation occurs in the DNA of a gene it may generate a new alternative
form of the gene called an "allele". Many gene loci have at
least two alternative forms or alleles.
Phenotype and genotype
The genotype of an individual is made up of the alleles at each locus
that the individual has inherited from its parents. One copy of the
gene comes from the mother's (maternal) gamete, and the other copy comes
from the father's (paternal) gamete.
The phenotype is the outward expression of the gene: the appearance,
behaviour, biochemical and physiological properties of an individual
which is influenced by the genotype, as well as the environment. For
example, sunlight (an environmental factor), can change light brown
hair (genetically determined) to several shades lighter during summer.
Dominant and recessive phenotypes
Suppose a gene for coat colour in dogs has two alleles B for black and
b for brown. When two different alleles at a particular gene locus occur
in the one organism, in this case Bb, the genotype of this organism
is described as "heterozygous" at this locus. When you look
at the phenotype (appearance) of this individual it will tell you which
phenotype is dominant. In this case, the heterozygous dog will always
be black, so black is the dominant phenotype. The brown phenotype is
called the "recessive" phenotype, as it is not seen in heterozygous
individuals. An individual with two b alleles ( called "homozygous")
will be brown as there is no B allele to "dominate". We talk
of dominant and recessive phenotypes rather than alleles because it
is the expression of the allele in the phenotype which we see.
Genes in gametes
During the formation of parents' gametes (sperm and eggs), the pairs
of genes are separated so that each gamete contains only one of each
gene pair. At fertilisation, when the sperm and egg nuclei meet, the
first cell (zygote) has a full complement of two genes for each characteristic.
These genes are found in the cell nucleus with all the others needed
to determine all the characteristics of an individual. They are arranged
on the chromosomes which can be seen as paired threads during cell division.
Widow's peak or no widow's peak?
(An example of an inherited characteristic)
If you observe the hairline of a group of individuals, some will have
a straight hairline and others will have a widow's peak or 'V' shape
in the middle of their hairline. This is an inherited trait. The alleles
are designated W for widow's peak and w for no widow's peak. The capital
letter represents the allele for the dominant phenotype.
There are three possible genotypes for this gene locus WW, Ww and ww.
WW and ww are described as homozygous. The genotypes WW and Ww will
produce a widow's peak (the dominant phenotype) and the ww genotype
will produce no widow's peak (the recessive phenotype).
Inheritance
If we know the genotype of the parents, we can predict the possible
genotypes and phenotypes in their offspring (first generation).
Genotypes of parents |
|
WW
|
|
|
|
ww
|
|
Phenotypes of parents |
|
widow's
peak
|
|
|
|
no
widow's
peak
|
|
Gametes produced |
W
|
and
|
W |
|
w
|
and
|
w
|
Fertilisation involves
one |
|
gamete from each parent |
Ww
|
|
Ww |
|
Ww
|
|
Ww
|
All the offspring will have a widow's peak.
If two individuals both heterozygous for this locus (Ww) had children,
we can predict the possible genotypes and phenotypes in the offspring.
Parental genotype |
Ww
|
|
Ww
|
Parental phenotype |
widow's peak
|
|
widow's peak
|
Gametes formed |
W
|
and
|
w
|
|
W
|
and
|
w
|
|
|
At fertilisation |
WW
|
|
Ww
|
|
Ww
|
|
ww
|
Offspring phenotype |
widow's
peak
|
|
widow's
peak
|
|
widow's
peak
|
|
no
widow's
peak
|
Of the four possible combinations of gametes, three will result in the
dominant widow's peak phenotype and only one for the straight hairline.
The probability is 75% for widow's peak, and 25% for no widow's peak.
At each pregnancy, this couple has a 1/4 chance of having a child without
a widow's peak. The chance is exactly the same for each pregnancy.
If parents are known to come from families with a history of recessive
genetic disorders (e.g. Cystic Fibrosis), they may attend genetic counselling
sessions. If parents are known to both be heterozygous for a recessive
disorder, they will be told that at each pregnancy there is a 1/4 chance
that their child will have the disorder, and a 3/4 chance it will not
(but it may carry the allele and perhaps pass it on to its children).
|