Download Mendel`s Principles

Complex InheritanceExploring More About
Genetics
The inheritance of biological
characteristics is determined by individual
units known as genes and genes are
passed from parents to their offspring
 In cases of alleles, some may be
dominant, others recessive
 In most sexually reproducing organisms,
each adult has two copies of each geneone from each parent
 The alleles for different genes usually
segregate independently

Mendel’s Principles
The segregation of one pair of alleles does
not affect the segregation of another pair
 Mendel performed experiments with two
different genes (i.e. round/wrinkled and
yellow/green) and crossed the hybrids
(RrYy)
 He found that different traits segregate
independently during the formation of
gametes
 This forms genetic variability
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Independent Assortment
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First introduced by Mendel
Used to determine if an individual expresses
a homozygous dominant or heterozygous
trait
It involves breeding the individual with an
individual who expresses the recessive
version of the trait
If all offspring display the dominant
phenotype, the individual is homozygous
dominant
If the offspring display both phenotypes, the
individual is heterozygous
Test Cross
There are exceptions to most of Mendel's
principles
 Not all genes show simple patterns of
dominant and recessive alleles-most of
the time its much more complicated
because most genes have more than two
alleles
 Some traits are controlled by more than
one gene

Beyond Dominant and
Recessive
Incomplete dominance- cases in which
one allele is not completely dominant over
another
 Codominance- both alleles contribute to
the phenotype
 Multiple alleles- genes that have more
than two alleles
 Polygenic traits- traits controlled by two
or more genes

Exceptions to the Rule:
Vocabulary
Incomplete Dominance
Example

A red 4 o’clock
(RR) breeding with
a white 4 o’clock
(WW), neither red
nor white is
dominant thus the
offspring will be a
mixture of the two
colors or pink
R
R
W
RW
pink
RW
pink
W
RW
pink
RW
pink

In chickens the allele for black feathers is
codominant with an allele for white
feathers, the heterozygous chickens have
a color termed erminette (both color
feathers); the two colors appear
separately and are not blended
Codominance Example
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It means that there are more than two
possible alleles exist in a population
Rabbit color is the best known example, it is
determined by a single gene that has at least
4 alleles, colors include:
◦ Full color (brownish)
◦ Chinchilla (greyish)
◦ Himalayan (white body, dark colored ears, nose and
paws)
◦ Albino (no color, white)

Blood type is also considered multiple alleles
Multiple Alleles Examples
Phenotype
Genotype
A
IAIA or IAi
B
IBIB or IBi
AB
IAIB only
O
ii only
Examples of Blood Type
Crosses
Blood Typing Overview
Poly means many so the term mean
having many genes
 Fruit flies have at least three genes
involved in making their reddish-brown
eye pigment and different combinations of
the three genes produces variations in the
eye color
 Eye color, hair color and skin color are
also controlled by more than one trait
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Polygenic Traits Examples
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True-breeding-plants that self-pollinate
creating clones of themselves
Hybrid-offspring of crosses between
parents with different traits
Segregation-separation of alleles
Gametes-sex cells
Allele-one form of a gene
Antigens-substance that prompts the
generation of antibodies
Glossary