Transcript 12.2: Mendel`s Theory

12.2: Mendel’s Theory
Objectives
• What patterns of heredity were explained
by Mendel’s hypotheses?
• What is the law of segregation?
• How does genotype relate to phenotype?
• What is the law of independent
assortment?
Explaining Mendel’s Results
• Mendel developed several hypotheses to explain the
results of his experiments.
• These hypotheses are collectively called the Mendelian
theory of heredity and form the foundation of modern
genetics.
• Mendelian theory explains simple patterns of
inheritance. In these patterns, two of several versions of
a gene combine and result in one of several possible
traits.
• Different traits result from different versions of genes.
Each version of a gene is called an allele.
Explaining Mendel’s Results
• Each allele can lead to a unique trait.
• Traits can come from either parent
because each pair of alleles is separated
when gametes form during meiosis.
• Only one of the pair is passed on to
offspring.
Alleles
Explaining Mendel’s Results
• An allele that is fully expressed whenever it is
present is called dominant.
• An allele that is not expressed when a dominant
allele is present is called recessive.
• A recessive allele is expressed only when there
is no dominant allele present.
• Traits may also be called dominant or recessive.
Random Segregation of Alleles
• Because chromosome pairs split randomly
during meiosis, either one of a pair of
homologous chromosomes might end up in any
one gamete. Chance decides which alleles will
be passed on.
• In modern terms, the law of segregation holds
that when an organism produces gametes, each
pair of alleles is separated and each gamete
has an equal chance of receiving either one of
the alleles.
Mendel’s Findings in Modern Terms
• Scientists use a code of letters to represent the function
of alleles.
• A dominant allele is shown as a capital letter. This letter
usually corresponds to the first letter of the word for the
trait.
• A recessive allele is shown as a lowercase letter.
• Offspring do not show a trait for every allele that they
receive. Instead, combinations of alleles determine
traits
Mendel’s Findings in Modern Terms
• The set of specific combinations of alleles
that an individual has for a character is
called the genotype.
• The detectable trait that results from the
genotype’s set of alleles is called the
phenotype.
• Thus, genotype determines phenotype.
Mendel’s Findings in Modern Terms
• If an individual has two identical alleles of
a certain gene, the individual is
homozygous for the related character.
• If an individual has two different alleles of
a certain gene, the individual is
heterozygous for the related character.
Checkpoint Questions
• What is Mendelian theory of heredity?
• What is the difference between a dominant and
recessive trait?
– When is a dominant trait expressed?
• What type of letter is used to represent a dominant trait?
– When is a recessive trait expressed?
• What type of letter is used to represent a dominant trait?
• In your own words, what is the law of segregation?
• What is the difference between:
– Genotype and phenotype
– Homozygous and heterozygous
STOP HERE FOR TODAY!!!!
Intro Questions
• T= Tall t= short
• Cross a heterozygous Tall pea plant with a short
pea plant.
– What is the genotypic ratio?
– What is the phenotypic ratio?
• What are all the possible combination between g
and r in the following situations
• GGRR
• GgRR
• GgRr
• Ggrr
Mendel’s Second Experiments
• A dihybrid cross involves two characters, such
as seed color and seed shape.
• Mendel used dihybrid crosses in his second
experiments and found that the inheritance of
one character did not affect the inheritance of
another character.
• In modern terms, the law of independent
assortment holds that during gamete formation,
the alleles of each gene segregate
independently.
Dihybrid Cross
Mendel’s Second Experiments
• Genes are said to be linked when they are close
together on chromosomes.
• Scientists now know that many genes are linked to each
other as parts of chromosomes.
• Genes that are located close together on the same
chromosome will rarely separate independently.
• The only genes that follow Mendel’s law of independent
assortment are those that are far apart.
Summary
• Mendelian theory explains simple patterns of
inheritance. In these patterns, two of several
versions of a gene combine and result in one of
several possible traits.
• In modern terms, the law of segregation holds
that when an organism produces gametes, each
pair of alleles is separated and each gamete
has an equal chance of receiving either one of
the alleles.
• Genotype determines phenotype.
Summary
• In modern terms, the law of independent
assortment holds that during gamete
formation, the alleles of each gene
segregate independently.