Chapter 14:Chapter 14:Mendel & The Gene Mendel & The Gene IdeaIdea

Quantitative Quantitative approach to approach to sciencescience

Pea plantsPea plants

Austrian MonkAustrian Monk

Mendel vs. Conventional Mendel vs. Conventional Inheritance Inheritance

““Blending”hypothBlending”hypothesisesis

Traits of offspring Traits of offspring are a blending of are a blending of parents genetic parents genetic materialmaterial

““Particulate” Particulate” hypothesishypothesis

Parents pass on Parents pass on discrete heritable discrete heritable unitsunits

GeneGene

Mendel’s Mendel’s hypothesishypothesis

Why Peas?Why Peas?

They are available in many varietiesThey are available in many varieties

They have They have heritableheritable features or features or character character (flower color).(flower color).

Traits,Traits, or variations of heritable or variations of heritable features are obvious(flower color- features are obvious(flower color- purple vs white)purple vs white)

Mendel could easily control which traits Mendel could easily control which traits he wanted to crosshe wanted to cross

Mendelian geneticsMendelian geneticsTrue-bred True-bred

Results of true breeding???Results of true breeding???

Mendelian geneticsMendelian genetics

Hybridization Hybridization The crossing of two true breeding The crossing of two true breeding varietiesvarieties

P generation (parents)P generation (parents)Purple flowers X White flowersPurple flowers X White flowers

FF11 generation (first filial generation) generation (first filial generation)All purpleAll purple

Mendelian Genetics:Mendelian Genetics:

Self pollinate the F1 generation Self pollinate the F1 generation produces the F2 generation.produces the F2 generation.

Second filial generationSecond filial generation3: 1 ratio3: 1 ratio3 purple: 1 white 3 purple: 1 white

Leading to the Leading to the Law of Law of SegregationSegregation

1. Alternative versions of genes 1. Alternative versions of genes (alleles) account for variations in (alleles) account for variations in inherited characteristicsinherited characteristics

2. For each character, an organism 2. For each character, an organism inherits 2 alleles, one from each inherits 2 alleles, one from each parentparent

3. If the two alleles differ, then one, 3. If the two alleles differ, then one, the dominant allele, is fully the dominant allele, is fully expressed in the organism’s expressed in the organism’s appearance; the other, the recessive appearance; the other, the recessive allele, has no noticeable effect on allele, has no noticeable effect on the organism’s appearancethe organism’s appearance

4. The alleles for each character 4. The alleles for each character segregate (separate) during gamete segregate (separate) during gamete production (meiosis). production (meiosis).

Mendel’s Mendel’s Law of SegregationLaw of Segregation

Genetic vocabulary…….Genetic vocabulary…….Punnett square: predicts the Punnett square: predicts the results of a genetic cross results of a genetic cross between individuals of known between individuals of known genotypegenotype

Homozygous: pair of identical Homozygous: pair of identical alleles for a characteralleles for a character

Heterozygous: two different Heterozygous: two different alleles for a genealleles for a gene

Phenotype: an organism’s Phenotype: an organism’s traitstraits

Genotype: an organism’s Genotype: an organism’s genetic makeupgenetic makeup

Problem:Problem:

How could you determine the How could you determine the genotype of a dominant trait?genotype of a dominant trait?

Example: purple flower are Example: purple flower are dominant to white flowersdominant to white flowers

We know the genotype of the white We know the genotype of the white flower to be homozygous recessive (pp). flower to be homozygous recessive (pp). What is the genotype of the purple What is the genotype of the purple flower? Pp or PP?flower? Pp or PP?

Test Cross:Test Cross:

breeding of a recessive breeding of a recessive homozygote X dominate homozygote X dominate phenotype (but unknown phenotype (but unknown genotype)genotype)

The The Law of Independent Law of Independent AssortmentAssortment

Law of Segregation Law of Segregation involves 1 character. involves 1 character. What about 2 (or more) What about 2 (or more) characters?characters?

Monohybrid cross vs. Monohybrid cross vs. dihybrid crossdihybrid cross

The two pairs of alleles The two pairs of alleles segregate segregate independently of each independently of each other.other.

Mendel’s Mendel’s Law of Law of Independent Independent AssortmentAssortment

Dihybrids:Dihybrids:

Are two characters, i.e. Are two characters, i.e. flower and seed color, flower and seed color, transmitted from parents to transmitted from parents to offspring as a package?offspring as a package?

Trihybrid:Trihybrid:What would the F1 generation look like if you were What would the F1 generation look like if you were to cross three true breeding traits? such as: to cross three true breeding traits? such as:

Flower color (PP, pp)Flower color (PP, pp)

Seed color (YY, yy) Seed color (YY, yy)

Seed shape (RR, rr)Seed shape (RR, rr)

What would the F1 gametes look like?What would the F1 gametes look like?

Mendel’s inheritance is Mendel’s inheritance is governed by the laws of governed by the laws of

probability:probability:

Multiplication ruleMultiplication rule::Multiplying the probability of one Multiplying the probability of one event by the probability of the event by the probability of the other eventother event

Mendel’s inheritance is Mendel’s inheritance is governed by the laws of governed by the laws of

probability:probability:Addition ruleAddition rule: :

the probability that any one of the probability that any one of two or more mutually exclusive two or more mutually exclusive events will occur is calculated by events will occur is calculated by adding together their individual adding together their individual probabilitiesprobabilities

Complex inheritance Complex inheritance patterns: patterns:

Complete dominance:Complete dominance:One allele dominant over the One allele dominant over the otherother

Homozygous dominantHomozygous dominant

Heterozygous dominantHeterozygous dominant

Complex inheritance Complex inheritance patterns: patterns:

Codominance:Codominance:The two alleles affect the phenotype The two alleles affect the phenotype in separate and distinguishable way.in separate and distinguishable way.

Human blood: ABHuman blood: AB

Complex inheritance Complex inheritance patterns: patterns:

Incomplete dominance:Incomplete dominance:The phenotypic expression falls The phenotypic expression falls between the two allelesbetween the two alleles

Flower color of snapdragonsFlower color of snapdragonsRed x white = pinkRed x white = pink

Complex inheritance Complex inheritance patterns: patterns:

Multiple alleles: Multiple alleles: more than 2 more than 2 possible alleles for a gene. possible alleles for a gene.

Ex: human blood typesEx: human blood types

Complex inheritance Complex inheritance patterns: patterns:

Complex inheritance Complex inheritance patterns: patterns:

Pleiotropy: Pleiotropy: genes genes with multiple with multiple phenotypic effect. phenotypic effect.

sickle-cell anemiasickle-cell anemia

Complex inheritance Complex inheritance patterns: patterns:

Epistasis: Epistasis: a gene at one locus (chromosomal a gene at one locus (chromosomal location) affects the phenotypic expression location) affects the phenotypic expression of a gene at a second locus. of a gene at a second locus.

Epistatic vs.Epistatic vs. hypostatic hypostatic

mice coat colormice coat color

Combs in ChickensCombs in Chickens

Complex inheritance Complex inheritance patterns: patterns:

Polygenic Inheritance: Polygenic Inheritance: an additive an additive effect of two or more genes on a effect of two or more genes on a single phenotypic character single phenotypic character

human skin pigmentation and heighthuman skin pigmentation and height

The relationship between The relationship between dominance and phenotype:dominance and phenotype:

No interactions between dominant No interactions between dominant and recessive alleles:and recessive alleles:

Pathway to expressionPathway to expressionExample: round peas vs. wrinkledExample: round peas vs. wrinkledExample: Example: Tay SachsTay Sachs(recessive)- (recessive)- depends on the level at which the depends on the level at which the phenotype is examinedphenotype is examined

Frequency:Frequency:Dominance does not Dominance does not necessarily mean more necessarily mean more frequent.frequent.

Example: polydactly- dominant Example: polydactly- dominant trait affects 1 out of 400 born trait affects 1 out of 400 born in USin US

Polydactly is also an example Polydactly is also an example of of variable expressivityvariable expressivity- the - the degree to which a gene is degree to which a gene is expressedexpressed

Recessively inherited Recessively inherited disorders: disorders:

Always homozygousAlways homozygous

One recessive allele inherited One recessive allele inherited from each parentfrom each parent

Parents are usually carriersParents are usually carriers

Can be life threatening or Can be life threatening or harmlessharmless

Recessively inherited Recessively inherited disorders: disorders:

Albinism:Albinism:Lack of pigment- relatively Lack of pigment- relatively mild- may lead to blindnessmild- may lead to blindness

Recessively inherited Recessively inherited disorders: disorders:

Cystic Fibrosis: Cystic Fibrosis: LethalLethal

Sickle Cell AnemiaSickle Cell AnemiaCan also be lethalCan also be lethal

Dominantly Inherited Dominantly Inherited Disorders: Disorders:

Homozygous or Heterozygous Homozygous or Heterozygous individuals express the disorderindividuals express the disorder

Much less common than Much less common than recessive disordersrecessive disordersLess chance of being passed Less chance of being passed from one generation to the next from one generation to the next if lethalif lethal

No heterozygous carrierNo heterozygous carrier

Dominantly Inherited Dominantly Inherited Disorders: Disorders:

Achondroplasia: dwarfismAchondroplasia: dwarfismOne out of 25,000One out of 25,00099.99% of population is 99.99% of population is homozygous recessivehomozygous recessive

Dominantly Inherited Dominantly Inherited Disorders: Disorders:

• Huntington’s disease: Huntington’s disease: – Degenerative disease of the Degenerative disease of the

nervous systemnervous system• No obvious phenotype until 35 – 40 No obvious phenotype until 35 – 40

yrs. yrs. • 1 out of 10,0001 out of 10,000• Children with a parent who has this Children with a parent who has this

disease will have a 50% chance of disease will have a 50% chance of acquiring the diseaseacquiring the disease

Testing: Testing:

•Amniocentesis: Amniocentesis:

•Chorionic Villus Sampling: Chorionic Villus Sampling: CVSCVS

•Genetic CounselingGenetic Counseling

Pedigree:Pedigree:

• A family tree describing the A family tree describing the interrelationships of parents and interrelationships of parents and children across generations for a children across generations for a particular trait. particular trait.

Human disordersHuman disorders