Mendel’s TheoryMendel’s Theory

Section 8-2Section 8-2

Mendel’s Work Became a Mendel’s Work Became a Theory of HeredityTheory of Heredity

Mendel’s experiments changed people’s belief Mendel’s experiments changed people’s belief that an offspring got his/her characteristics that an offspring got his/her characteristics from blending of the two parents’ from blending of the two parents’ characteristics.characteristics.

Mendel stated that each parent has two Mendel stated that each parent has two separate “heritable factors” for each trait – separate “heritable factors” for each trait – one from each parent.one from each parent.

When gametes form, only one of the two When gametes form, only one of the two factors for each trait is given to a gamete.factors for each trait is given to a gamete.

When gametes fuse during fertilization, the When gametes fuse during fertilization, the resulting offspring has two factors for each resulting offspring has two factors for each trait. trait.

Mendel’s HypothesesMendel’s Hypotheses

Mendel developed four hypotheses Mendel developed four hypotheses that were based directly on the that were based directly on the results of his experiments.results of his experiments.

These four hypotheses now make up These four hypotheses now make up the Mendelian theory of heredity.the Mendelian theory of heredity.

Hypothesis OneHypothesis One

For each inherited trait , an For each inherited trait , an individual has two copies of the gene individual has two copies of the gene – one from each parent.– one from each parent.

Hypothesis TwoHypothesis Two

There are alternative versions of There are alternative versions of genes. Today these are called genes. Today these are called alleles.alleles.

In the case with flower color, the In the case with flower color, the alleles would be purple or yellow.alleles would be purple or yellow.

One allele comes from each parent.One allele comes from each parent.

Hypothesis ThreeHypothesis Three

When two different alleles occur When two different alleles occur together, one of them may be completely together, one of them may be completely expressed, while the other may have no expressed, while the other may have no observable effect on the organism’s observable effect on the organism’s appearance.appearance.

DominantDominant – the expressed form of the – the expressed form of the traittrait

RecessiveRecessive – the form of the trait that – the form of the trait that was not expressed when the dominant was not expressed when the dominant form is present.form is present.

Hypothesis FourHypothesis Four

When gametes are formed, the alleles for When gametes are formed, the alleles for each gene in an individual separate each gene in an individual separate independently of one another. Thus, independently of one another. Thus, gametes carry only one allele for each gametes carry only one allele for each inherited trait. inherited trait.

When gametes unite during fertilization, When gametes unite during fertilization, each gamete contributes one allele.each gamete contributes one allele.

Parents can only contribute one allele Parents can only contribute one allele because of the way gametes are formed because of the way gametes are formed in meiosis.in meiosis.

Mendel’s Findings in Mendel’s Findings in Modern TermsModern Terms

Dominant alleles are represented by the Dominant alleles are represented by the first letter of the trait as a capital letter.first letter of the trait as a capital letter.

Recessive alleles are represented by the Recessive alleles are represented by the same letter as the dominant trait, but in same letter as the dominant trait, but in lower case.lower case.

HomozygousHomozygous – when two alleles are the – when two alleles are the same for a particular genesame for a particular gene

HeterozygousHeterozygous – when the two alleles – when the two alleles for a particular gene are differentfor a particular gene are different

In heterozygous individuals, only the In heterozygous individuals, only the dominant allele is expressed.dominant allele is expressed.

The recessive allele is present, but not The recessive allele is present, but not expressed.expressed.

The set of alleles an individual has is The set of alleles an individual has is called its called its genotypegenotype..

The physical appearance of a trait is The physical appearance of a trait is called a called a phenotypephenotype..

Phenotypes (physical appearance) are Phenotypes (physical appearance) are determined by the genotype (set of determined by the genotype (set of alleles).alleles).

Dominant form of trait is written first, Dominant form of trait is written first, then the recessive form.then the recessive form.

If purple is the dominant trait (P) If purple is the dominant trait (P) and white is the recessive trait (p) and white is the recessive trait (p) what would the following be?what would the following be?

Homozygous Recessive Homozygous Recessive

pp and whitepp and white Homozygous DominantHomozygous Dominant

PP and purplePP and purple HeterozygousHeterozygous

Pp and purplePp and purple

Mendel’s Ideas Gave Rise to Mendel’s Ideas Gave Rise to the Laws of Hereditythe Laws of Heredity

Mendel’s ideas or hypotheses are often Mendel’s ideas or hypotheses are often referred to as the Laws of Heredityreferred to as the Laws of Heredity

The Law of SegregationThe Law of Segregation Describes the behavior of chromosomes Describes the behavior of chromosomes

during meiosis, when homologous during meiosis, when homologous chromosomes and then chromatids are chromosomes and then chromatids are separatedseparated

States that the two alleles for a trait States that the two alleles for a trait segregate (separate) when gametes are segregate (separate) when gametes are formedformed

The Law of Independent AssortmentThe Law of Independent Assortment Mendel studied whether the inheritance of one Mendel studied whether the inheritance of one

trait influenced the inheritance of a different trait influenced the inheritance of a different trait.trait.

He conducted dihybrid crosses to determine this.He conducted dihybrid crosses to determine this. Dihybrid crossDihybrid cross – a cross that considers two – a cross that considers two

pairs of contrasting traitspairs of contrasting traits Found that the inheritance of one trait does not Found that the inheritance of one trait does not

influence the inheritance of another traitinfluence the inheritance of another trait States that the alleles of different genes separate States that the alleles of different genes separate

independently of one another during gamete independently of one another during gamete formationformation

We now know this only applies to genes located We now know this only applies to genes located on different chromosomes or that are far apart on different chromosomes or that are far apart on the same chromosome.on the same chromosome.

SummarySummary Different versions of a gene are called alleles. Different versions of a gene are called alleles.

An individual usually has two alleles for a An individual usually has two alleles for a gene, each inherited from a different parent.gene, each inherited from a different parent.

Individuals with the same two alleles for a Individuals with the same two alleles for a gene are homozygous; those with two gene are homozygous; those with two different alleles for a gene are heterozygous.different alleles for a gene are heterozygous.

The law of segregation states that two alleles The law of segregation states that two alleles for a trait separate when gametes are for a trait separate when gametes are formed. The law of independent assortment formed. The law of independent assortment states that two or more pairs of alleles states that two or more pairs of alleles separate independently of one another during separate independently of one another during gamete formation.gamete formation.

HomeworkHomework

Section 8-2 Section 8-2

Review Questions Review Questions

p. 167 p. 167

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