What Are Genes?
- GENES are segments of DNA that contain
the information for an hereditary trait. They are
located in the chromosomes
- Genes are transmitted from parents to
offspring through the gametes.
- Each gene has alternatives called
ALLELES. In the population there may be
many, but in an organism there are two, one
in each homologous chromosome.
- The position a gene has in a chromosome
is called LOCUS (plural: loci)
Individuals that have two identical alleles for a gene are called
HOMOZYGOUS (AA or aa)
If the two alleles are different, they are HETEROZYGOUS or
HYBRID for that trait (Aa)
GENOTYPE is the group of genes of an
individual. For a trait it may be AA, Aa or aa.
PHENOTYPE is the external expression of
the genotype, that is, the observable trait.
Phenotype does not only depend on the
genotype, but also on the environment in
which the individual develops.
Multiple alleles make possible multiple phenotypes in a population
For each gene, there are three possible genotypes. For the
gene that regulates the production of melanine, for example:
MM-------------------------------------- normal (pigmented skin)
Mm-------------------------------------- normal (pigmented skin)
mm--------------------------------- ---- albino
This happens because the allele M is DOMINANT over the
allele m, that is RECESSIVE.
Dominant alleles have the same effects whether they are in
homozygosis (MM) or heterozygosis (Mm).
Recessive alleles are expressed only in homozygosis (mm)
Sometimes there is no complete
dominance of an allele over the other,
but an INCOMPLETE DOMINANCE, or
INTERMEDIATE INHERITANCE. Then
the heterozygous phenotype is
intermediate between the dominant and
Sometimes both the alleles express simultaneously (CODOMINANCE).
An example of this happens with the bloodtypes.
GAMETES are HAPLOID (n), so they only have one chromosome
for each homologous pair. This means they only have one alelle
for each gene.
Genotype of the individual
Gametes it produces
A heterozygous will produce 50% of the gametes with each
In fertilization, gametes
GAMETES PRODUCED M
Mendel's Good Move:
Mendel's success in discovering the laws of
inheritance depended on his wise decision to choose
the pea (Pisum sativum) for his experiments. This was
- it has a short life cycle
- it produces many offspring in one cross
- it allows both self and cross-fertilization and it is easy
- it has easily observable traits. He focused on 7, and
studied its transmission with the aid of statistics:
The traits Mendel studied in peas (Pisum sativum):
MENDEL'S FIRST LAW:
UNIFORMITY OF THE FIRST GENERATION
When crossing two homozygous parents
(or pure lines), the resulting offspring (F1
generation) is uniform.
100% phenotype R
MENDEL'S SECOND LAW:
During gamete formation each allele of a pair is
separated (=segregated) from the other
member, and will join again with another one
75% phenotype R
25% phenotype r
First Law: Uniformity of F1
First Law: Uniformity of F1
What happens with more than one trait?
To study this, Mendel crossed two pure lines (homozygous) for two traits:
Seed colour: yellow (A) > green (a)
Seed shape: round (B) > wrinkled (b)
MENDEL'S THIRD LAW:
Genes for each trait are transmitted
and inherited independently.
- They are diagrams used to predict the outcome of a breeding or
- In these diagrams, we represent in a table the possible alleles
produced by the two individuals that are being crossed. We
assume that the probability of inheriting copies of each parental
allele is independent (according to Mendel's Laws).
Sex-linked (X-linked) recessive traits:
Some traits (diseases like hemophilia or color-blindness are
determined by alleles whose loci are in the X chromosome.
Males will inherit only one allele from their mother (and always
express it), while females will receive two alleles, one from their
mother and other from his mother.
Tracing Hemophilia through Queen Victoria's Pedigree
Down Syndrome: trisomy 21
Abnormalities in sex chromosomes:
Turner syndrome: X_
Klinefelter syndrome: XXY