Transcript Mendel`s Theory

Mendel’s Theory
Section 8-2
Mendel’s Work Became a Theory of
Heredity
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Mendel’s experiments changed people’s belief that an
offspring got his/her characteristics from blending of
the two parents’ characteristics.
Mendel stated that each parent has two separate
“heritable factors” for each trait – one from each
parent.
When gametes form, only one of the two factors for
each trait is given to a gamete.
When gametes fuse during fertilization, the resulting
offspring has two factors for each trait.
Mendel’s Hypotheses
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Mendel developed four hypotheses that were
based directly on the results of his experiments.
These four hypotheses now make up the
Mendelian theory of heredity.
Hypothesis One
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For each inherited trait , an individual has two
copies of the gene – one from each parent.
Hypothesis Two
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There are alternative versions of genes. Today
these are called alleles.
In the case with flower color, the alleles would
be purple or yellow.
One allele comes from each parent.
Hypothesis Three
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When two different alleles occur together, one
of them may be completely expressed, while the
other may have no observable effect on the
organism’s appearance.
Dominant – the expressed form of the trait
Recessive – the form of the trait that was not
expressed when the dominant form is present.
Hypothesis Four
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When gametes are formed, the alleles for each
gene in an individual separate independently of
one another. Thus, gametes carry only one allele
for each inherited trait.
When gametes unite during fertilization, each
gamete contributes one allele.
Parents can only contribute one allele because
of the way gametes are formed in meiosis.
Mendel’s Findings in Modern Terms
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Dominant alleles are represented by the first
letter of the trait as a capital letter.
Recessive alleles are represented by the same
letter as the dominant trait, but in lower case.
Homozygous – when two alleles are the same
for a particular gene
Heterozygous – when the two alleles for a
particular gene are different
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In heterozygous individuals, only the dominant
allele is expressed.
The recessive allele is present, but not expressed.
The set of alleles an individual has is called its
genotype.
The physical appearance of a trait is called a
phenotype.
Phenotypes (physical appearance) are
determined by the genotype (set of alleles).
Dominant form of trait is written first, then the
recessive form.
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If purple is the dominant trait (P) and white is
the recessive trait (p) what would the following
be?
Homozygous Recessive
pp and white
Homozygous Dominant
PP and purple
Heterozygous
Pp and purple
Mendel’s Ideas Gave Rise to the
Laws of Heredity
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Mendel’s ideas or hypotheses are often referred
to as the Laws of Heredity
The Law of Segregation
Describes the behavior of chromosomes during
meiosis, when homologous chromosomes and then
chromatids are separated
 States that the two alleles for a trait segregate
(separate) when gametes are formed
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The Law of Independent Assortment
Mendel studied whether the inheritance of one trait
influenced the inheritance of a different trait.
 He conducted dihybrid crosses to determine this.
 Dihybrid cross – a cross that considers two pairs of
contrasting traits
 Found that the inheritance of one trait does not influence
the inheritance of another trait
 States that the alleles of different genes separate
independently of one another during gamete formation
 We now know this only applies to genes located on
different chromosomes or that are far apart on the same
chromosome.
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Summary
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Different versions of a gene are called alleles. An
individual usually has two alleles for a gene, each
inherited from a different parent.
Individuals with the same two alleles for a gene are
homozygous; those with two different alleles for a
gene are heterozygous.
The law of segregation states that two alleles for a
trait separate when gametes are formed. The law of
independent assortment states that two or more pairs
of alleles separate independently of one another
during gamete formation.
Homework
 Section
8-2
Review Questions
p. 167
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