Transcript Document
Mendelian
Genetics
Unit 1
Genetics
Science that deals with the structure &
function of genes and their transmission
from one generation to the next
(heredity)
Genes
factors
that control traits
Genotype
Genetic
makeup of an organism
Phenotype
Observable
characteristic of an organism
produced by the interaction between its
genotype and the environment
Genotype & Phenotype
Genes provide only the POTENTIAL for
developing a particular phenotype.
The extent to which it is realized depends
upon
Interactions with other genes & their products
Environmental influences
Random developmental events
Genes are only a starting point for
determining structure and function of an
organism.
Gregor Mendel
Father of modern genetics
Experimented using garden pea plants (Pisum
sativum)
Good choice:
Easy to grow
Bears flowers and fruit in the same year a seed is
planted
Produces a large number of seeds
Each trait had two easily distinguishable, alternative
appearances
Mendel’s Work
Pea plants are normally self-fertilizing
He allowed each strain to self-fertilize for
many generations to ensure that the traits
he wanted to study were inherited (truebreeding strains).
Then he prevented self-fertilization and
cross fertilized true-breeding strains of peas
that differed in a single trait
This is a MONOHYBRID CROSS
Used reciprocal crosses to show that the trait
does not depend on the sex of the organism
Example:
Cross smooth female with wrinkled
male
Cross wrinkled female with smooth
male
Cross of the P (parental)
generation:
All the F1 progeny exactly resemble only one of
the parents (not a blend of both)
Mendel planted and allowed the
F1 plants to self-fertilize:
Both smooth and
wrinkled seeds
appeared in the F2
generation (3:1
ratio).
But how can a trait
present in the P
generation disappear
in the F1 generation
and then reappear in
the F2?
Mendel concluded…
The alternative traits in the cross were
determined by what we now call genes.
Factors transmitted from parents to progeny
that carry hereditary information
Each existed in alternative forms (which we
now call alleles)
A true-breeding strain of peas must contain a pair
of identical factors (and each F1 must have
contained both factors).
Because only one of the traits was seen in F1, the
expression of the missing trait must somehow have
been masked by the visible trait: DOMINANCE
The allele for purple (P) is dominant to the allele for
white flowers (p).
The allele for white flowers is recessive because it is
masked.
Individuals that contain two copies of the
same specific allele of a particular gene
are said to be HOMOZYGOUS for that
gene.
Individuals that have two different alleles
of a particular gene are said to be
HETEROZYGOUS.
Punnett Square
Matrix that describes all the possible genotypes of
progeny resulting from a genetic cross
Mendel’s Conclusions:
Results of all reciprocal crosses were the same.
All F1 progeny resembled one of the parental strains
(indicating dominance).
In the F2 generation, the parental trait that had
disappeared in the F1 generation reappeared.
The two members of a gene pair (alleles)
segregate (separate) from each other during the
formation of gametes.
• We now know genes are on chromosomes and the specific
location of a gene on a chromosome is called its locus.
• Gene segregation parallels the separation of homologous pairs
of chromosomes at anaphase I in meiosis.
Confirming the Principle of Segregation
Test Cross
Cross of an individual of unknown genotype (usually
expressing the dominant phenotype) with a
homozygous recessive individual to determine the
unknown phenotype