In many species, individuals have two alleles of each
gene. The principle of segregation states that prior to
the formation of eggs and sperm, the allele of each gene
separate so that each egg or sperm cell receives only one
The principle of independent assortment states that
alleles of different genes are transmitted to egg cells and
sperm cells independently of each other.
Genes are located on chromosomes:
o Principle of segregation separation of
homologous chromosomes in anaphase of meiosis
o Principle of independent assortment
genes found on different chromosomes and is
explained by chromosomes lining up randomly in
metaphase of meiosis I
There are important exceptions and extensions to the
basic patterns of inheritance
1865 Gregor Mendel, an Austrian monk, worked out the
rules of inheritance through a series of brilliant
experiments on garden peas
Walter Sutton and Theodor Boveri formulated the
chromosome theory of inheritance which proposes that
meiosis causes the patterns of inheritance that Mendel
Genetics is the branch of biology that focuses on
Mendel was interested in heredity. Heredity is the
transmission of traits from parents to offpsprings
Addressing the basic question of why offspring
resemble their parents and how transmission of traits
occurs Two hypotheses:
o Blending inheritance: parental traits blend such
rhat heir offspring have intermediate traits
o Inheritance of acquired characteristics: parental
traits are modified and then passed on to their
Mendel chose the common garden pea:
- easy to grow
- short reproductive cycle
- produces large number of seeds
- matings are easy to control
- traits are easily recognizable
Peas normally pollinate themselves aka self-fertilization
Mendel could prevent this by removing the male
reproductive organs (stamen) containing pollen from
each flower. He then used this pollen to fertilize the
female reproductive organs (pistil) of flowers on
different plants, thus performing cross-pollination
Mendel worked with pea varieties that differed in seven
easily recognizable traits: seed shape, seed color, pod
shape, pod color, flower color, flower and pod position,
and stem length.
An individuals observable features comprise its
phenotype. Mendels pea population had two distinct
phenotypes for each of the seven traits.
Mendel worked with pure lines that produced
identical offspring when self-pollinated. He used these
plants to create hybrids by mating two different pure
lines that differed in one or more traits.
Mendel's first experiments involved crossing pure lines
that differed in just one trait.
The adults in the cross were the Mendels first
experimented with crossing plants that differed in only one trait.
When Mendel crossed pure line plants with round
seeds and pure line plants with wrinkled seeds, all of
the F offspring had round seeds.
This contradicted the hypothesis of blending
inheritance. The genetic determinant for wrinkled
seeds seemed to have disappeared.
Mendel allowed the F p1ogeny to self-pollinate.
The wrinkled seed trait reappeared in the next F2g
Mendel called the genetic determinant for wrinkled
seeds recessive and the determinant for round seeds
In modern genetics, the terms dominant and recessive identify only which phenotype is observed in
individuals carrying two different genetic determinants.
Mendel repeated these experiments with each of the
other traits. In each case, the dominant trait was
present in a 3:1 ratio over the recessive trait in th2 F
Mendel wanted to determine if gender influenced
He performed a reciprocal cross, in which the
mother's phenotype in the first cross is the father's
phenotype in the second cross, and the father's
phenotype in the first cross is the mother's phenotype
in the second cross.
The results of the two crosses were identical. This
established that it does not matter whether the genetic
determinants for seed shape are located in the male or
Hereditary determinants for a trait are now called
Mendel also proposed that each individual has two
versions of each gene. Today these different versions of
a gene are called alleles. Different alleles are
responsible for the variation in the traits that Mendel
The alleles found in an individual are called its
genotype. An individuals genotype has a profound
effect on its phenotype.
Mendel developed the principle of segregation: the
two members of each gene pair must segregatethat is,
separateinto different gamete cells during the
formation of eggs and sperm in the parents.
Mendel used a letter to indicate the gene for a particular
trait. For example, R represented the gene for seed
shape. He used uppercase (R) to show a dominant allele