DNA – double helix

• DNA coils to form a chromosome

• You receive 1 set of chromosome from mom & one from dad

• Chromosomes have areas that code for a gene

• 1 gene = 1 protein

The Central Dogma!

Number of Genes on Chromosomes

Introduction to GeneticsChapter 9

Section 1 – Mendel’s LegacySection 2 – Genetic Crosses

Mendel’s Legacy

Gregor Mendel was a priest in the mid 19th Century. Studied different pea plants Father of GENETICS – the scientific

study of heredityWhat is heredity?-passing down of

physical characteristics from parent to offspring

Mendel’s ExperimentsSelf pollinated

Developed a method of cross pollination

P generation, F1 generation, F2 generation

TRAITS: plant height, flower position, pod color, pod appearance, seed texture, seed color, flower color

The Outcome of Mendel’s WorkIn all of Mendel’s experiments

First generation ONLY produced one form of the trait (i.e. tall)

Second generation the “lost” form the trait reappeared in about ¼ of the plants

Mendel’s Findings TODAYFactors that control the traits are called

GENES.Alleles – contrasting forms of a gene

i.e. Tall vs. Short; round vs. wrinkled Inherited from parents Dominant Allele – trait always shows up

when the allele is present Recessive Allele – “masked” or hidden by

the dominant trait

TERMS TO KNOW Phenotype – the physical characteristic

displayed by the organism (PHYSICAL = PHENOTYPE)

Genotype – the actual combination of alleles held by the organism (GENOTYPE = GENES)

Genotype = Rr

Phenotype = Red

Mendel concluded that the paired “factors” or alleles separate when making reproductive cells This means that each gamete only gets one

copy of each allele When fertilized, the offspring would have 2

copies

Law of Segregation Alleles are segregated or separated during

the formation of gametes.

Mendel also observed that dominant traits do not always appear together.

Mendel concluded that these factors are not connected

Law of Independent Assortment“Factors” or alleles for different

characteristics are distributed to gametes independently

Explain this in YOUR words

Representing AllelesUse letters to represent alleles

Use the same letter for one trait (T for height)

Capital letter for dominant (T = Tall) Lowercase letter for recessive letter (t =

short)T T

t Tt Tt

t Tt Tt

Purebred Tall Parent

Purebred Short Parent

Hybrid Offspring

Genetic Crosses

What is the chance that a coin will land on heads?

Probability – a number that describes how likely it is that an event will occur.

Interdependence of Events

If you toss a coin once and then toss again, the first toss does not affect the second

Each TIME you toss the coin, you have a 1 in 2 chance of getting heads.

Probability & GeneticsHow is probability related to

Genetics? (Think about Mendel’s results)

Each time Mendel repeated his experiment, he got similar results. WHY?

Mendel was the first to discover that you could PREDICT the outcome of a cross.

Purebred Tall Plant X Purebred Short Plant

ALL TALL PLANTS

Punnett SquaresTool used by scientists to determine

the outcome of a genetic crossShows all the possible combinations

of offspring

T T

t Tt Tt

t Tt Tt

Purebred Tall Parent

Purebred Short Parent

Hybrid OffspringOUTCOME: TALL OFFSPRING

Monohybrid Crosses Involves ONE PAIR of contrasting traits Homozygous – purebred (has the same

type of alleles) Heterozygous – hybrid (has two types of

alleles)

T T

t

t

Genetic cross between a tall plant and a short plant

Genetic cross between 2 heterozygous tall plants

T t

T

t

TERMS TO KNOW Phenotype – the physical characteristic

displayed by the organism (PHYSICAL = PHENOTYPE)

Genotype – the actual combination of alleles held by the organism (GENOTYPE = GENES)

Genotype = Rr

Phenotype = Red

Ratio’s Phenotypic Ratio – the ratio of

phenotypes that appear in the offspring Genotypic Ratio – the ratio of genotypes

that appear in the offspring

Genotypic Ratio = 4:0

Phenotypic Ratio = 4:0

Monohybrid Punnett Square Review

Heterozygous Tall x Heterozygous TallGive phenotypic and genotypic ratios

TT x TtGive phenotypic and genotypic ratios

Dihybrid Crosses Involves TWO PAIRS of contrasting traits Homozygous – purebred (has the same

type of alleles) Heterozygous – hybrid (has two types of

alleles)Genetic cross between a Purebred tall

plant, purple flower and a Purebred short plant, white flower

TTPP x ttpp

What are the possible gametes?

(ALL THE POSSIBLE COMBINATION OF GAMETES)

Determining Gametes – all possible combinations

PARENT 1 TtPp

TP Tp tP tp

PARENT 2 TtPp

TP Tp tP tp

What are the possible gametes for these parents?

Heterozygous Tall plant w/ purple flower (TtPp) x Heterozygous Tall plant w/ purple flower

(TtPp)TP Tp tP tp

TP

Tp

tP

tp

Other Types of DominanceIncomplete Dominance

Two or more alleles influence the phenotype

An intermediate phenotype results; a blending of traits Red Flower x White Flower

(RR) (rr)

Pink Flower

(Rr)

Red Flower x White Flower

Outcome =ALL PINK FLOWERS

R R

r Rr Rr

r Rr Rr

Incomplete dominance Punnett Squares

Pink Flower x Pink Flower

Outcome =Red, White & Pink

Flowers

R r

R RR Rr

r Rr rr

Other Types of DominanceCodominance

Two or more alleles are expressed in the phenotype

A combination phenotype results

Red Cow x White Cow

(RR) (WW)

Roan Cow

(RW)

Red Cow x White Cow

Outcome =ALL ROAN COWS

R R

W RW RW

W RW RW

Codominance Punnett Squares

Roan Cow x Roan Cow

Outcome =Red, White & Roan Cows

R W

R RR RW

W RW WW