GeneticsGenetics HeredityHeredity – the – the

passing of traits passing of traits from parent to from parent to offspringoffspring

GeneticsGenetics- the study - the study

of heredityof heredity

Part I: Mendel and Part I: Mendel and the Gene Ideathe Gene Idea

I. Gregor MendelI. Gregor Mendel He was a monk, a gardener, and a He was a monk, a gardener, and a

trained mathematiciantrained mathematician 11stst to apply statistical analysis: to apply statistical analysis:

Selective breeding was an old art…Selective breeding was an old art… Published his work on pea plant Published his work on pea plant

inheritance patterns in the inheritance patterns in the 1860’s.1860’s.(nothing was known about the (nothing was known about the

cellular mechanisms for cellular mechanisms for

inheritance…)inheritance…)

Why was the pea a good Why was the pea a good choice?choice?

1) Cheap and available 1) Cheap and available

2) Produce offspring quickly/short 2) Produce offspring quickly/short generation timegeneration time

3) Characteristics are determined on one 3) Characteristics are determined on one gene with only two alleles.gene with only two alleles.

Ex. Flower color: purple or whiteEx. Flower color: purple or white

4) No blending of traits 4) No blending of traits

5) Can control breeding because of 5) Can control breeding because of access to male and female sex partsaccess to male and female sex parts

Mendel’s QuestionsMendel’s Questions

Mendel crossed (bred) two different Mendel crossed (bred) two different plants to discover what traits the plants to discover what traits the offspring would show. offspring would show.

For instanceFor instance: Will a purple flower : Will a purple flower plant crossed with a purple flower plant crossed with a purple flower plant produce plant produce allall purple offspring? purple offspring?

II. Basic II. Basic genetic genetic concepts concepts

A. Mendel studied 7 A. Mendel studied 7 different different characterscharacters in peas:in peas:

Ex)Ex)

HeightHeight: tall vs. dwarf: tall vs. dwarf

Seed shapeSeed shape: smooth : smooth vs. wrinkledvs. wrinkled

Review of chromosomesReview of chromosomes

A. Each = one DNA moleculeA. Each = one DNA molecule

B. B. GeneGene – DNA sequence found on a – DNA sequence found on a particular chromosome, that codes for a particular chromosome, that codes for a particular trait.particular trait.

C. Each may have thousands of genes for C. Each may have thousands of genes for a particular set of genesa particular set of genes• Ex. Human chromosome 11 has the genes Ex. Human chromosome 11 has the genes

for making the enzymes hemoglobin, for making the enzymes hemoglobin, catalase, and insulin (and thousands of catalase, and insulin (and thousands of other genes)other genes)

Genes express Genes express allelesalleles

AllelesAlleles are different forms of the are different forms of the same gene.same gene.

gene that codes for brown pigmentgene that codes for brown pigment

gene that codes for black pigmentgene that codes for black pigment

gene that codes for red pigmentgene that codes for red pigment

More about alleles:More about alleles:Dominant alleleDominant allele – the “stronger” allele. In a – the “stronger” allele. In a

heterozygous combination the dominant heterozygous combination the dominant allele will be expressed.allele will be expressed.

Symbol is a capital letterSymbol is a capital letterEx) Ex) traittrait: pea plant height: pea plant height

tall is dominant, tall is dominant, tall = Ttall = T

Recessive alleleRecessive allele – “weaker” allele. In the – “weaker” allele. In the heterozygous individual these are hidden, heterozygous individual these are hidden, and the dominant form of the trait will be and the dominant form of the trait will be expressed.expressed.

Symbol is a lowercase letterSymbol is a lowercase letterEx) Ex) traittrait: pea plant height: pea plant height

dwarf is recessive, dwarf is recessive, dwarf = tdwarf = t

Allele combinations for Allele combinations for genesgenes

HOMOZYGOUSHOMOZYGOUS – both gene are the – both gene are the same.same.

Ex) Seed coat trait:Ex) Seed coat trait:

wrinkledwrinkled seed allele seed allele

and and wrinkledwrinkled seed allele seed allele HETEROZYGOUSHETEROZYGOUS – genes are different – genes are different Ex) seed coat traitEx) seed coat trait

wrinkledwrinkled seed allele and seed allele and roundround seed seed allele fallele f

GenotypeGenotype: the alleles an individual has: the alleles an individual has PhenotypePhenotype: the way those alleles are : the way those alleles are

expressed expressed Ex) pea plant heightEx) pea plant height

Possible Possible genotypesgenotypes: T T, T t, or t t: T T, T t, or t t Possible Possible phenotypesphenotypes: tall or dwarf: tall or dwarf

Possible combinations: Possible combinations: Ex) pea plant heightEx) pea plant height

Homozygous dominantHomozygous dominant = TT, tall = TT, tall Homozygous recessiveHomozygous recessive = tt, dwarf = tt, dwarf HeterozygousHeterozygous = Tt, tall = Tt, tall

Example of an individual’s Example of an individual’s allele combinations on three allele combinations on three

gene locigene loci

Mendel’s ExperimentsMendel’s Experiments True-breederTrue-breeder – an individual that always – an individual that always

produces offspring with the same produces offspring with the same expression for a given trait.expression for a given trait. Ex) purple flower plant always produces purple Ex) purple flower plant always produces purple

flower offspringflower offspring

P1P1 – the parent generation. Two true – the parent generation. Two true breeders with different traits for the same breeders with different traits for the same character are crossedcharacter are crossed Ex) true breeding purple flower pea X true Ex) true breeding purple flower pea X true

breeding white flower peabreeding white flower pea F1F1 – first filial generation. The offspring – first filial generation. The offspring

(progeny) of the P1(progeny) of the P1 F2F2 – second filial generation. The offspring – second filial generation. The offspring

of two individuals from the F1 generation.of two individuals from the F1 generation.

ExperimenExperimentt

ResultsResults

C. Mendel’s ConclusionsC. Mendel’s Conclusions

Mendel’s Rules of Inheritance – Mendel’s Rules of Inheritance – generalizations made by Mendelgeneralizations made by Mendel

Law of SegregationLaw of Segregation Law of Independent AssortmentLaw of Independent Assortment

Mendel’s Rules of InheritanceMendel’s Rules of Inheritance

1)1) Different versions of genes account Different versions of genes account for variety in organisms.for variety in organisms.

2) For each character, an organism 2) For each character, an organism inherits two alleles, one from each inherits two alleles, one from each parent.parent.

3) If two alleles differ, then one 3) If two alleles differ, then one (dominant) is fully expressed, and (dominant) is fully expressed, and the other (recessive) has no effect. the other (recessive) has no effect.

Mendel’s Laws of Mendel’s Laws of InheritanceInheritance

Law of SegregationLaw of Segregation - The two alleles a parent - The two alleles a parent has for each character segregate during has for each character segregate during gamete production. gamete production.

Ex. Heterozygote pea for flower colorEx. Heterozygote pea for flower color Pp : half of gametes get P, other half get pPp : half of gametes get P, other half get p

Law of Independent AssortmentLaw of Independent Assortment – for each – for each gene the alleles separate independently of gene the alleles separate independently of alleles for other genesalleles for other genes

Ex) heterozygote for flower color (Pp) and Ex) heterozygote for flower color (Pp) and seed color (Gg)seed color (Gg)

some gametes will get PG, some will get Pg, some gametes will get PG, some will get Pg, pG, or pg, in a ¼ ratiopG, or pg, in a ¼ ratio Refers to Refers to two or moretwo or more genes/characters genes/characters

A Test CrossA Test Cross WHY?WHY? To determine whether an organism To determine whether an organism

that has the dominant trait is homozygous that has the dominant trait is homozygous dominant or heterozygous.dominant or heterozygous.

HOW?HOW? Cross with a homozygous recessive Cross with a homozygous recessive organism. Does the recessive phenotype organism. Does the recessive phenotype show up?show up?

How to read the results?How to read the results? 100% dominant offspring = Parent with 100% dominant offspring = Parent with

unknown heritage is homozygous dominant.unknown heritage is homozygous dominant. ANY recessive offspring = Parent with unknown ANY recessive offspring = Parent with unknown

heritage is heterozygous.heritage is heterozygous.

Patterns of Inheritance NOT Patterns of Inheritance NOT revealed by Mendel’s studiesrevealed by Mendel’s studies

Incomplete dominanceIncomplete dominance CodominanceCodominance Multiple allelesMultiple alleles Polygenic traitsPolygenic traits

Incomplete dominanceIncomplete dominance In heterozygotes both alleles are In heterozygotes both alleles are

expressed so the trait blends.expressed so the trait blends. ExampleExample: flower color in : flower color in

snapdragonssnapdragons allele F allele F R R = red= red allele F allele F WW = white = white In heterozygotes (F In heterozygotes (F R R F F WW) = pink) = pink

CodominanceCodominance In heterozygotes both alleles are In heterozygotes both alleles are

expressed in separate distinguishable expressed in separate distinguishable waysways

ExampleExample: Roan horse: Roan horse RR – there are red hairsRR – there are red hairs rr – there are white hairsrr – there are white hairs Rr – some hairs are white and some Rr – some hairs are white and some

are redare red

Multiple allelesMultiple alleles There are more than two possible There are more than two possible

alleles for a character.alleles for a character. ExampleExample: human ABO blood group 3 alleles IA, IB, and i (allele = O)

O is recessive, A and B are codominant 4 possible phenotypes

Type A blood (genotypes: IAIA or IAi) Type B blood (IBIB or IBi) Type O blood (ii) Type AB blood (IAIB)

Polygenic inheritancePolygenic inheritance

-many genes affect one phenotype.-many genes affect one phenotype.

Ex) human heightEx) human height

Sex-linked genesSex-linked genes – – genes that are on the X chromosome genes that are on the X chromosome

show unique inheritance patterns show unique inheritance patterns The Y chromosome (ONLY male The Y chromosome (ONLY male

gender). The X chromosome has other gender). The X chromosome has other genes on it.genes on it.

Sex-linked traits Sex-linked traits can pass from mom to sons OR daughterscan pass from mom to sons OR daughters can pass from the dad to ONLY daughterscan pass from the dad to ONLY daughters

Mendelian Inheritance in Mendelian Inheritance in humanshumans::- from dominanat/recessive - from dominanat/recessive alleles on alleles on one gene.one gene.ExamplesExamples::• Widow’s peakWidow’s peak• Attached or free earlobesAttached or free earlobes• Recessive disordersRecessive disorders*: Ex.s) cystic *: Ex.s) cystic

fibrosis, Tay-Sachs disease, sickle-cell fibrosis, Tay-Sachs disease, sickle-cell diseasedisease

• Dominant disordersDominant disorders*: Ex.) Huntington’s*: Ex.) Huntington’s• *Determined with pedigree analysis, genetic *Determined with pedigree analysis, genetic

testing and counselingtesting and counseling