Inheritance: Mendel’s Inheritance: Mendel’s ExperimentsExperiments

Ideas on Inheritance Before MendelIdeas on Inheritance Before Mendel

PangenesisPangenesis Particles in the body are transmitted to the Particles in the body are transmitted to the

reproductive cellsreproductive cells

BlendingBlending Black horse X White horse =Gray horseBlack horse X White horse =Gray horse

Homunculus (small human)Homunculus (small human) Spermists believed a small fully formed Spermists believed a small fully formed

human was inside the sperm and the human was inside the sperm and the female contributed nothing but her womb!female contributed nothing but her womb!

Gregor MendelGregor Mendel(1822-1884)(1822-1884)

Augustinian monkAugustinian monk Studied the inheritance of traits in over 20,000 Studied the inheritance of traits in over 20,000

pea plantspea plants Kept meticulous notes on his dataKept meticulous notes on his data No knowledge of DNA or chromosomes No knowledge of DNA or chromosomes Father of geneticsFather of genetics

His genius was not recognized until after his deathHis genius was not recognized until after his death

The Lingo of a GeneticistThe Lingo of a Geneticist GeneGene

The unit of heredity (ex. Gene for flower color)The unit of heredity (ex. Gene for flower color) AllelesAlleles

Alternate forms of a gene (ex. Purple or white flower Alternate forms of a gene (ex. Purple or white flower color)color)

Dominant vs. RecessiveDominant vs. Recessive One allelic form “masks” the presence of anotherOne allelic form “masks” the presence of another ex. Purple flowers are dominant over white flowers ex. Purple flowers are dominant over white flowers

(white is recessive)(white is recessive) Dominant =capital lettersDominant =capital letters Recessive =lower case lettersRecessive =lower case letters

PPnn and F and Fnn

Parental and Filial generations Parental and Filial generations

More LingoMore Lingo Genotype vs. PhenotypeGenotype vs. Phenotype

Genotype is what alleles an individual has (ex. One purple flower Genotype is what alleles an individual has (ex. One purple flower allele and one white flower allele)allele and one white flower allele)

Phenotype is what the individual looks like (ex. Purple flowers)Phenotype is what the individual looks like (ex. Purple flowers) Homozygous vs. HeterozygousHomozygous vs. Heterozygous

Homo =sameHomo =same Hetero =differentHetero =different

True-breeding True-breeding Phenotype of parent is always seen in offspringPhenotype of parent is always seen in offspring The phenotype counterpart to the genotype term “homozygous”The phenotype counterpart to the genotype term “homozygous”

If an individual has a homozygous genotype, it will be true breedingIf an individual has a homozygous genotype, it will be true breeding Monohybrid vs. DihybridMonohybrid vs. Dihybrid

A monohybrid cross looks at just one trait in parents who are A monohybrid cross looks at just one trait in parents who are hybridshybrids

A dihybrid cross looks at more than one trait in parents who are A dihybrid cross looks at more than one trait in parents who are hybrids hybrids

Hybrid is another way of saying “heterozygous”Hybrid is another way of saying “heterozygous”

Monofactorial InheritanceMonofactorial Inheritance

A.k.a. Mendelian traitsA.k.a. Mendelian traits

Inheritance of a trait is determined by one Inheritance of a trait is determined by one genegene

Does not really existDoes not really exist Virtually all traits are affected to some degree Virtually all traits are affected to some degree

by other genes and by environmental factorsby other genes and by environmental factors

Pea Plant TraitsPea Plant Traits

Flower color

White (p)

Purple (P)

Mendel’s ExperimentsMendel’s Experiments(Pea Sex)(Pea Sex)

The Gene for Stem LengthThe Gene for Stem Length

The tall allele is dominant (T)The tall allele is dominant (T) A true-breeding tall plant is homozygous A true-breeding tall plant is homozygous

dominant (TT)dominant (TT)

The short allele is recessive (t)The short allele is recessive (t) A true-breeding short plant is homozygous A true-breeding short plant is homozygous

recessive (tt)recessive (tt)

Homozygous (True-breeding) Homozygous (True-breeding) DominantDominant

Tall plant X Tall plantTall plant X Tall plant

XX

All offspring will be tall

Homozygous (True-breeding) Homozygous (True-breeding) RecessiveRecessive

Short stem plant X Short stem plantShort stem plant X Short stem plant

XX

All offspring will be short

Non-True-BreedingNon-True-Breeding

Tall plant X Tall plantTall plant X Tall plant

XX

Some offspring are tallSome offspring are short

??

Monohybrid Cross –Stem LengthParental generation (P1)

Tall X Short

(Both true-breeding)

F1

All Tall

(Non-true-breeding)

a.k.a. hybrids

F2

¼ Short ¾ Tall

True-breeding Tall

True-breeding Short

Non-true-breeding Tall

Monohybrid Cross -Flower ColorMonohybrid Cross -Flower Color

Mendel’s Law of SegregationMendel’s Law of Segregation Elementen separate into gametesElementen separate into gametes

We now know “elementen” as genesWe now know “elementen” as genes

Each individual carries 2 alleles for every geneEach individual carries 2 alleles for every gene One on each chromosome One on each chromosome

One allele on the chromosome from mom and one allele on the One allele on the chromosome from mom and one allele on the chromosome from dadchromosome from dad

Possibilities for any given genePossibilities for any given gene Homozygous Dominant (AA) (True breeding)Homozygous Dominant (AA) (True breeding) Heterozygous (Aa) (Non-true-breeding)Heterozygous (Aa) (Non-true-breeding) Homozygous Recessive (aa) (True-breeding)Homozygous Recessive (aa) (True-breeding)

Gametes carry just ONE allele for each geneGametes carry just ONE allele for each gene Possibilities are A or aPossibilities are A or a

Our Old Friend MeiosisOur Old Friend Meiosis

Germline Cells

Phenotypic ratio

The genotypic ratio is 1:2:1

The Punnett SquareThe Punnett Square

Used to determine phenotypic and genotypic ratiosUsed to determine phenotypic and genotypic ratios

Represents possible alleles in gametes and how Represents possible alleles in gametes and how gametes may combinegametes may combine

Probability of having an offspring with “tt” is the same (1/4) for every pregnancy/fertilization.

#s are ratios, not # of offspring

ExampleExample

True-breeding (homozygous) dominantTrue-breeding (homozygous) dominant

Dominant and Recessive TraitsDominant and Recessive Traits

A dominant trait requires only one allele in A dominant trait requires only one allele in order for the dominant trait to be order for the dominant trait to be expressed expressed

Flower color

White (p)

Purple (P)

There MUST be 2 recessive alleles There MUST be 2 recessive alleles present in order for a recessive trait to be present in order for a recessive trait to be expressed (phenotype)expressed (phenotype)

ExamplesExamples

Ex. 1: True-breeding (homozygous) recessive Ex. 1: True-breeding (homozygous) recessive

Ex. 2: Homozygous dominant X heterozygousEx. 2: Homozygous dominant X heterozygous

Ex. 3: Monohybrid (heterozygous) crossEx. 3: Monohybrid (heterozygous) cross

Genotype vs. PhenotypeGenotype vs. PhenotypeThe Test CrossThe Test Cross

XX

ExampleExample

Unknown X Homozygous RecessiveUnknown X Homozygous Recessive a.k.a “the test cross”a.k.a “the test cross”

Autosomes vs. Sex ChromosomesAutosomes vs. Sex Chromosomes

Autosomal traits are found on non-sex chromosomes so Autosomal traits are found on non-sex chromosomes so (they are not found on the X or Y sex chromosomes)(they are not found on the X or Y sex chromosomes)

Modes of InheritanceModes of Inheritance

Autosomal dominant inheritanceAutosomal dominant inheritance If a child exhibits the trait (phenotype) at least If a child exhibits the trait (phenotype) at least

one of the parents also exhibits the trait one of the parents also exhibits the trait (phenotype)(phenotype)

Autosomal recessive inheritanceAutosomal recessive inheritance Heterozygotes are carriers (genotype) and do Heterozygotes are carriers (genotype) and do

not exhibit the trait (phenotype)not exhibit the trait (phenotype)

Autosomal DominantAutosomal Dominant

Autosomal RecessiveAutosomal RecessiveStraight Hair as an ExampleStraight Hair as an Example

Autosomal RecessiveAutosomal RecessivePhenylketonuria as an ExamplePhenylketonuria as an Example

Mendel’s ConclusionsMendel’s Conclusions

There are discrete units of inheritance (elementen)

We now call these genes

Offspring inherit a unit (gene) for each trait from each parent

Different units (genes) assemble independently in the gametes