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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