GeneticsGeneticsHow Traits are Passed from How Traits are Passed from

Parents to OffspringParents to Offspring

I. Gregor Mendel and his pea plants

A. Studied how traits were passed from parent to offspring in pea plants

1. Why did he study pea plants?

*Large number of

offspring

*7 easily seen traits(flower color, height, seed color, seed

texture, pod color, pod appearance, flower location)

2. Observations from pea plants:

* Created pure bred pea plants for a particular trait (always produced offspring with that trait)

Example: If two pure bred purple plants were crossed you would end up with….

Purple X Purple = all purple offspring (100%)

X=

Pure bred plants are called the P1 generation

* Performed specific test crossesExample: What would happen if a pure bred

white flower was crossed with a pure bred purple flower?

Purple (P1) X White (P1)= all purple offspring (100%)

X =

What if we crossed 2 of the offspring from this cross?

P1

P1P1

P1 F1

Let’s Try It!F1 Purple X F1 Purple = 75% Purple and 25% White

X =

F1 F1

F2

B. Mendel’s Conclusions (4)

1. Traits are controlled by 2 factors (genes)

Gene for eye color from mom

Gene for eye color from dad

Offspring receives 2 genes for each trait – one from each parent

2. There are dominant and recessive genes

Pure Purple Flower X Pure White Pure Purple Flower X Pure White FlowerFlower

X

PP pp

All offspring are purple because the P is dominant over the p

Pp

3. Law of Segregation: genes separate during meiosis (gamete production)

*So, you have one copy of each gene in the sperm

and one copy of each in the egg

4. Law of Independent Assortment: genes for different traits are

distributed into gametes randomly

C. Vocabulary:C. Vocabulary: Genetics: study of heredityGenetics: study of heredity Heredity: the passing on of traits from parents to offspringHeredity: the passing on of traits from parents to offspring Trait: a specific characteristic (eye color)Trait: a specific characteristic (eye color) Gene: factors that are passed on from one generation to the Gene: factors that are passed on from one generation to the

nextnext Allele: different forms of a gene (blue vs. brown)Allele: different forms of a gene (blue vs. brown) Dominant Allele: gene that will mask the recessive alleleDominant Allele: gene that will mask the recessive allele Recessive Allele: not seen if dominant allele is presentRecessive Allele: not seen if dominant allele is present Genotype: alleles present for a certain geneGenotype: alleles present for a certain gene

* Homozygous Dominant: both alleles are dom. (TT)* Homozygous Dominant: both alleles are dom. (TT)

* Homozygous Recessive: both alleles are rec. (tt)* Homozygous Recessive: both alleles are rec. (tt)

* Heterozygous: one allele is dom. and one is rec. (Tt)* Heterozygous: one allele is dom. and one is rec. (Tt) Phenotype: the physical characteristic shown from the genotypePhenotype: the physical characteristic shown from the genotype

True-Bred or Pure Bred: an organism who will always produce True-Bred or Pure Bred: an organism who will always produce offspring identical to themselvesoffspring identical to themselves

P1: parental generationP1: parental generation F1: offspring from P1F1: offspring from P1 F2: offspring from F1F2: offspring from F1 Law of independent assortment: homologous chromosomes arrange Law of independent assortment: homologous chromosomes arrange

themselves randomly during meiosis 1themselves randomly during meiosis 1 Law of segregation: homologous chromosomes separate during Law of segregation: homologous chromosomes separate during

meiosis 1meiosis 1 Probability: the likelihood of an event occurringProbability: the likelihood of an event occurring Monohybrid cross: cross that involves one traitMonohybrid cross: cross that involves one trait Dihybrid cross: cross that involves 2 traitsDihybrid cross: cross that involves 2 traits Punnett Square: used to predict offspringPunnett Square: used to predict offspring Genotypic Ratio: ratio of homozygous dominant: heterozygous: Genotypic Ratio: ratio of homozygous dominant: heterozygous:

homozygous recessivehomozygous recessive Phenotypic Ratio: ratio of dominant trait: recessive traitPhenotypic Ratio: ratio of dominant trait: recessive trait Complete Dominance: in the heterozygous form, only the dominant Complete Dominance: in the heterozygous form, only the dominant

trait is expressedtrait is expressed Incomplete Dominance: in the heterozygous form, there is an in Incomplete Dominance: in the heterozygous form, there is an in

between traitbetween trait Codominance: in the heterozygous form, both traits are expressedCodominance: in the heterozygous form, both traits are expressed

II. Punnett SquaresII. Punnett SquaresA. Used to predict all possible offspring

1. Dad’s genes on top, mom’s on the left

2. Fill in squares to determine possible offspring

*Example: What are the chances of having a boy?

What can dad give?

What can mom give?

X Y

X

X

XX XY

XX XY50% chance of having a boy

What would you get if you crossed a homozygous purple flower with a homozygous white flower?

*First you must figure out what the genes for the “parents” are.

Homozygous purple=

Homozygous white=

PP

pp

* Second, place the genes on a punnett square and fill P P

p

p

Pp Pp

Pp Pp

Third: write the ratios (genotypic and phenotypic)

Genotypic RatioHomozygous Dominant (PP): Heterozygous (Pp): Homozygous Recessive (pp)

PpPp PpPp

PpPp PpPp

P P

p

p

0 : 4 : 0

Phenotypic Ratio

Dominant Trait: Recessive Trait

PpPp PpPp

PpPp PpPp

P P

p

p

4 : 0

Now you set up the following Now you set up the following test cross and solve for the test cross and solve for the

ratiosratiosCross a homozygous Cross a homozygous

purple flower with purple flower with a heterozygous a heterozygous purple flower.purple flower.

PPPP PPPP

PPpp PPpp

P P

P

p

Geno. Ratio

2:2:0

Pheno. Ratio

4:0

Now you set up the following Now you set up the following test cross and solve for the test cross and solve for the

ratiosratiosCross two Cross two

heterozygous tall heterozygous tall plantsplants

TTTT TtTt

TtTt tttt

T t

T

t

Geno. Ratio

1:2:1

Pheno. Ratio

3:1

Dihybrid Cross: cross involving 2 traits

Homozygous green wrinkled seed

Genotype: yyss

Heterozygous yellow, Heterozygous smooth seed

Genotype: YySs

*Now we must find all possible combinations for each parent plant

Seed color: Y=yellow, y=green

Seed texture: S= smooth, s=wrinkled

Parent #1: yyss

ys ysys ys

ys ys ys ys

Parent #2: YySs

What are all of the possible allelic combinations from this parent?

YS, Ys, yS, ys

ys ys ys ys

YS

Ys

yS

ys

Now fill in the chart!

YySsYySs YySsYySs YySsYySs YySsYySs

YyssYyss ?? YyssYyss YyssYyss

yySsyySs yySsyySs ?? yySsyySs

yyssyyss ?? yyssyyss yyssyyss

ys ys ys ys

YS

Ys

yS

ys

III. Types of DominanceIII. Types of Dominance

A. Complete dominance: one trait is A. Complete dominance: one trait is completely dominant over another completely dominant over another

(Rr= Red)(Rr= Red)

B. Incomplete Dominance: heterozygous B. Incomplete Dominance: heterozygous offspring will have a phenotype in offspring will have a phenotype in between the dominant and recessive between the dominant and recessive trait (Rr= pink)trait (Rr= pink)

C. Codominance: heterozygous offspring C. Codominance: heterozygous offspring show both traits (Rr= red and white hair)show both traits (Rr= red and white hair)

Incomplete Dominance

CodominanceCodominance