Biology
November 16, 2012
11.3 Exploring Mendelian Genetics
I. Independent Assortment
A. Two-Factor Cross: F1
Mendel crossed true-breeding plants
that produced only round yellow peas
(genotype RRYY) with plants produced
wrinkled green peas (genotype rryy)
Result: F1 Offspring – all round and
yellow peas
Dominent alleles: round/yellow
B. Two-Factor Cross: F2
Produced 556 seeds
1) 315 seeds - round/yellow
2) 32 seeds – wrinkled/green
3) 2 parental phenotypes
4) 209 seeds – combination
phenotypes (alleles) – not found in
either parent
Independent Assortment
Genes that segregate independently
(seed shape/color) will not influence
each other’s inheritance
Ratio 9: 3: 3: 1
***Principle of independent
assortment states that genes for
different traits can segregate
independently during the formation of
gamates. Independent assortment
helps account for the many genetic
variations observed in plants, animals,
and other organisms.
A Summary of Mendel’s Principles
Form of basis of the modern science
of genetics.
1) The inheritance of biological
characteristics is determined by
individual units known as genes.
Genes are passes from parents to
their offspring.
2) In cases in which two or more
forms (alleles) of the gene for a
single trait exist, some forms of the
gene may be dominant and others
may be recessive.
3) In most sexually reproducing
organisms, each adult has two
copies of each gene—one from
each parent. These genes are
segregated from each other when
gametes are formed.
4) The alleles for different genes
usually segregate independently of
one another.
II. Beyond Dominant and Recessive Alleles
Exception to Mendel’s Experiment
Some alleles are neither dominant nor
recessive, and many traits are
controlled by multiple alleles or
multiple genes.
Incomplete Dominance
1) One allele is not completely
dominant over another
2) Heterozygous phenotype is
somewhere in between the two
homozygous phenotypes
Codominance
Both alleles contribute to the
phenotype
Ex. Allele can be both black and white
feathers on a chicken – chicken will
have a combination of both black and
white feathers
Multiple Alleles
Genes that have more than two alleles
**Can exist in a population not
necessarily in an individual.
Polygenic Traits
“having many genes”
Many traits are produced by the
interaction of several genes
III. Applying Mendel’s Principles
Thomas Hunt Morgan used Mendel’s
Principles to flies. He found that it
worked not only with plants but all
organisms.