Genetics Genetics IntroductionIntroduction

GeneticsGenetics

The study of heredity -The study of heredity -the transmission of the transmission of characteristics from characteristics from parents to offspringparents to offspring

* Gregor Mendel* Gregor Mendel

(1822-1884)(1822-1884)– First person to First person to

scientifically attempt to scientifically attempt to discover the laws of discover the laws of heredityheredity

Mendel’s work went unrecognized until 16 yrs after his death…

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

Experimental DesignExperimental Design

Step 1 - Allowed plants Step 1 - Allowed plants to self-pollinate to to self-pollinate to ensure plants were true-ensure plants were true-breeding for a particular breeding for a particular trait.trait.– All the offspring the All the offspring the

same (a purebred)same (a purebred)– Called his parental Called his parental

purebreds the P purebreds the P generationgeneration

Did you know?? Peas have “complete” flowers – both male and female parts.

Some plant’s flowers are called “imperfect” – have either male or female parts but not both

Experimental DesignExperimental Design

Step 2 – Cross-Step 2 – Cross-pollinated two different pollinated two different purebred P generation purebred P generation flowersflowers

11stst generation offspring generation offspring are called Fare called F11

Step 3 – Allowed FStep 3 – Allowed F11 to to self pollinateself pollinate

22ndnd generation are F generation are F22

F stands for filial means offspring

Repeated experiment Repeated experiment with 7 different with 7 different characteristicscharacteristics

Discovered some traits Discovered some traits were dominant and some were dominant and some recessiverecessive

DominantDominant – a trait that is expressed when it is – a trait that is expressed when it is homozygous or heterozygous. (PP or Pp)homozygous or heterozygous. (PP or Pp)

RecessiveRecessive – does not turn up in the F1 generation – does not turn up in the F1 generation (pp)(pp)

Homozygous verses Homozygous verses HeterozygousHeterozygous Homozygous-Homozygous-

organism has two organism has two genes for the same genes for the same trait that are alike trait that are alike (PP or pp)(PP or pp)

HeterozygousHeterozygous--the two genes the two genes for a trait are for a trait are different (Pp)different (Pp)

PP pp

Pp

Genotype verses Genotype verses PhenotypePhenotype GenGenotypeotype-What the -What the

gengenes actually say-es actually say-gene combination in gene combination in an organism (alleles) an organism (alleles) ex: GG, Gg or ggex: GG, Gg or gg

PhPhenotypeenotype - - phphysical appearanceysical appearance ex: purple or whiteex: purple or white

Law of Dominance

When parents with contrasting genes (alleles) for traits are crossed, the effect of one gene is often seen in the offspring.

• Dominant-Capital Letter-When present, this gene is expressed. It masks the recessive trait.

• Recessive-Lower case letter-Masked by dominant trait.

Punnett SquaresA format that simplifies our understanding of how genes are transmitted to offspring.

Examples of Punnett Squares

TT is Tall, Tt is Tall, and tt is short

Short pea plants crossed with short pea plants.

tt tt

tt tt

t t

t

t

GENOTYPE: 100 % tt

PHENOTYPE:100% short

Another Example of Punnett Squares

TT is Tall, Tt is Tall, and tt is short

Pure Breeding Tall with Pure Breeding Tall

T T

T

T

TT TTTT TT

GENOTYPE: 100 % TT

PHENOTYPE:100 % Tall

Another Example of Punnett SquaresTT is Tall, Tt is Tall, and tt is short

Heterozygous Tall with Heterozygous Tall

TT TtTt tt

T

T

t

t

GENOTYPE: 25% TT50% Tt25% tt

PHENOTYPE: 75% Tall25% Short

Another Example of Punnett SquaresTT is Tall, Tt is Tall, and tt is short

Heterozygous Tall with homozygous short

Tt ttTt tt

T

t

t

t

GENOTYPE:50% Tt50% tt

PHENOTYPE:50% Tall50% Short

Dihybrid Cross Dihybrid Cross ChallengeChallenge A dihybrid A dihybrid

cross involves cross involves two alleles per two alleles per trait for two trait for two traits, for a traits, for a total of four total of four alleles alleles

Dihybrid CrossDihybrid Cross

A dihybrid cross involves two A dihybrid cross involves two alleles per trait for two traits, for alleles per trait for two traits, for a total of four alleles a total of four alleles

What are the phenotypes for this dihybrid cross?

Note: this 9:3:3:1 ratio is typical for parents that are both heterozygous individuals for the traits.