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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.
•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.
•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
•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.
•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
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
•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.
•In modern terms, the law of independent assortment holds that during gamete formation,
the alleles of each gene segregate independently.