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A gene is a length of DNA sufficient to code for a protein.
are believed to have approximately 100,000 genes. The
genetics studies the transmission of these genes from
Suppose we have a plant with RED flowers which we breed or cross
with a plant with WHITE flowers. What will the offspring or next
generation look like?
One possible result will be that all the offspring have PINK
flowers. This can happen and is a situation known as Incomplete
Dominance or Blending Inheritance. Roughly half the genes of an
organism are transmitted in this manner. The offspring are
simply the combination of the two parental types (Red + White =
Pink). This, however, is only one possible result and this is
all we need to know about Blending Inheritance.
The other possible result is that all the offspring will
one or the other of the parents. Since dark colors and
structures tend to dominate over light colors and small
structures, most probably, the offspring would all have Red
flowers. This would be an example of what is termed a
Dominance/Recessive situation. The Dominant trait is expressed
and the Recessive trait is not.
We can express the two possibilities above using symbols to
represent the genes involved. For instance, we can say there is
a gene R which causes red flower formation and a gene r which
causes white flower formation. Alternate forms of a gene are
Therefore, we can represent the situations above in the following
RR X rr = Rr
Since most higher organisms are diploid (two of every kind of
chromosome carrying the genes), we need to use two symbols for
each of the parents. If the inheritance is Incomplete Dominance,
the Rr would produce Pink flowers. In the case of
Dominance/Recessiveness, the Rr would produce Red flower, but
notice that the offspring still carry the recessive gene r which
simply is not expressed.
"The way an organism looks" is called its phenotype.
it carries" is called its genotype.
Now let's look in detail at what would happen in the next
generation (Rr X Rr = ?). From here on, we will only consider
You need to remember that diploids have to undergo meiosis first
before they can mate. As in the key concept list before, in
meiosis I, the homologous chromosomes separate and each gamete
will only have one copy of a given gene. In this example, each
parent will produce two kinds of gametes, 50% carrying R and 50%
If R joins with R at fertilization, the offspring's genotype will
be RR and red flowers.
If R joins with r at fertilization, the
offspring's genotype will
be Rr and red flowers.
If r joins with R
at fertilization, the offspring's genotype will
be Rr and red flowers.
If r joins with r at fertilization, the offspring's genotype will
be rr and white flowers.
These are the only possibilities. Notice that genotypically, a
1-RR:2-Rr:1-rr ratio is produced and phenotypically a
3-red:1-white ratio is produced.
A diploid organism which has the same allele on each of its
homologous chromosomes is called Homozygous. They can be
homozygous dominant (RR) or homozygous recessive (rr). A diploid
organism which has different alleles on each of its homologous
chromosomes is called Heterozygous (Rr).
Therefore, using the example given above, we can say in words
that whenever two heterozygous organisms mate (Rr X Rr), their
offspring or progeny will appear phenotypically in a 3:1
phenotypic ratio and genotypically in a 1:2:1 ratio. This is an
invariant rule of what is called Mendelian genetics named after
Gregor Mendel, the Austrian monk who first worked out these rules
while studying sweet pea plants.
If you can understand the above material, you have the basis for
understanding Mendelian genetics. Most problems simply involve:
(1) determining the parental genotypes and phenotypes, (2)
determining the gametic products of each parent and (3)
recombining the gametes to form the offspring.
There are a number of special circumstances where Mendelian
rules do not apply.
For Mendelian genetics to apply, genes that are being examined
to be on different chromosomes. If we had for instance, two
genes, A & B on the same chromosome, then at meiosis I, when the
homologous chromosomes move apart, the two genes would move
together. This is referred to as Linkage.
Recombination through Crossing-Over:
Linkage, however, can be broken by an event which can occur in
meiosis known as crossing over. Realize, that before meiotic
anaphase, we have the homologous chromosomes paired up
side-by-side by synapsis. Each of the chromosomes consists of
two chromatids each so that we have a total of four chromatids
lined up side-by-side at this time. This is called the Tetrad
Occasionally, non-sister chromatids (those of different
homologues) can physically cross over one another at which time
pieces of the DNA of those chromatids can be exchanged. For
instance, if we had two homologues, one with linked A & B genes
and the other with linked a & b genes, we would normally not
expected to find A & b or a & B combinations. But
crossing over, these non-parental combinations do occur
only a small percentage of the time (which makes it easy to
recognize like on the Final Exam). The results of crossing over
can be seen in the meiotic cells as the presence of strange
chromosome forms known as chiasma or chiasmata.
We have said that humans cells have 46 chromosome which consist
two sets of 23. Of those 23 chromosomes in a haploid set, 22
have nothing to do with the determination of sex during fetal
development. They are called autosomes. However, the
chromosome is called a sex chromosome which can be found in
different forms: X and Y. Normal females have XX
Normal males have XY combinations. We never find a
combination since at least one X is needed for survival.
The Y chromosome does not have many known genes and is different
from the X. However, the X and Y chromosomes act as if they are
homologues during meiosis and pair up during synapsis. The XY
combination is called hemizygous. The Y chromosome does have
special genes which when activated at the right time of fetal
development cause the fetus to develop as a male. Females
develop as females since Y-specific genes are not present.
Occasionally, in genetic males (XY), the genes do not turn on at
the correct time in development or are faulty, and therefore,
also will develop as females.
There are some individuals which have other than the correct
numbers of X's and Y's. In female appearing individuals,
Turner's Syndrome includes individuals with only a single X or
multiple X chromosomes other than 2. In male appearing
individuals, Klinefelter's Syndrome and Jacobs Syndrome, involve
extra X chromosomes or extra Y chromosomes, respectively. These
disease conditions are caused by lack of separation, called
Non-Disjunction, of the sex chromosomes during meiosis. Only one
autosome can be tolerated in one extra copy, that being
21, giving rise to Downs Syndrome.
The importance to genetics is that males with only one X
chromosome, express whatever genes are present on their X which
they inherit from their mothers. Since females have two X
chromosomes, they can carry a faulty gene on one which will be
compensated for by a normal gene on the other.
A good example of this is hemophilia, a blood disease where
clotting factors are not produced and it is difficult to control
bleeding. The hemophilia gene is carried on the X chromosome.
We will use H to designate the normal gene, h to designate the
abnormal gene and 0 to represent no gene present..
If a female has one X with H (X-H) and one X with h (X-h), she
appear normal but be a carrier for the disease. If she
with a normal male X-H and Y-0, the following offspring can
Females: X-H/X-H and X-h/X-H -- one totally normal and one
carrier as herself
and X-h/Y-0 -- one totally normal and one
The disease is to be expected in one half of her male offspring
one half of her daughters will be carriers. Disregarding
one fourth of her children will have the disease.