Genetics and Genetics and MendelMendel

Gregor MendelGregor Mendel

• Austrian Monk • Studied peas 1860

(Darwin’s voyage ended 1836)

• Developed laws of inheritance of traits

• Did not know about genes or DNA

Mendel’s ExperimentsMendel’s Experiments

Mendel’s PeasMendel’s PeasAfter many test crosses of plants Mendel decided some traits were dominant (expressed) and some

traits were recessive (not expressed) but still resided in the organism

Law of DominanceLaw of Dominance

• Only the dominant trait will appear in the offspring

• The other trait does not appear and is recessive

When crossing two purebred individuals:

Mendel’s Laws of Mendel’s Laws of InheritanceInheritance

• Each individual has two factors for each traitEach individual has two factors for each trait• The factors segregate (separate) during the The factors segregate (separate) during the

formation of gametesformation of gametes• Each gamete contains only one factor from each Each gamete contains only one factor from each

pair of factorspair of factors• Fertilization gives each new individual two factors Fertilization gives each new individual two factors

for each traitfor each trait• Factors sort independently of each otherFactors sort independently of each other

• All possible combinations of factors can occur in All possible combinations of factors can occur in the gametesthe gametes

Modern GeneticsModern Genetics• Each person has two alleles for each trait, one

on each homologous chromosome• Meiosis separates the alleles when gametes are

formed• Each gamete receives one of the alleles

• Fertilization gives each new individual two alleles for each gene of a trait

• During metaphase I the tetrads arrange randomly at the equatorial plane

• All possible combinations are in the gametes

AllelesAlleles• Alternate forms of the same gene• Dominant alleles –

– expressed – capital letter used

• Recessive alleles – – suppressed by the dominant allele – not expressed– Lower case letter used

• Gene locus - – specific location on homologous

chromosomes

Homozygous vs. Homozygous vs. HeterozygousHeterozygous

• Homozygous –– Two identical alleles for the trait

• Heterozygous –– Two different alleles for the trait

Homozygous dominant for shape

Heterozygous for pod color

Genotype vs. PhenotypeGenotype vs. Phenotype

• Genotype – Genotype – – Alleles received Alleles received – Genetic code of the Genetic code of the

individualindividual

• PhenotypePhenotype– Physical Physical

appearanceappearance– Gene expressionGene expression

Genotype is homozygous dominant for

shapePhenotype is

Round

Genotype is heterozygous

for color Phenotype is Green pod

color

Genotype vs. PhenotypeGenotype vs. Phenotype

Genotypes? Phenotypes?

Homozygous

recessive

Heterozygous

Wrinkled

Green pod

color

• Humans Humans Alleles for finger length:Alleles for finger length:– Short is dominantShort is dominant– Long is recessiveLong is recessive

• What letter will indicate the dominant allele?What letter will indicate the dominant allele?

S (capital)S (capital)

• What letter will indicate the recessive allele?What letter will indicate the recessive allele?

s (lower case)s (lower case)

Genotype vs. PhenotypeGenotype vs. Phenotype

What are the possible genotypes and phenotypes?What are the possible genotypes and phenotypes?

GenotypesGenotypes Phenotypes Phenotypes

Genotype vs. PhenotypeGenotype vs. Phenotype

SS

Ss

ss

Short Fingers

Short Fingers

Long Fingers

• Mendel discovered that tall pea plants Mendel discovered that tall pea plants (T) were dominant over short pea (T) were dominant over short pea

plants (t). plants (t).

• If the phenotype of a pea plant is short, If the phenotype of a pea plant is short, what is its genotype?what is its genotype?

• Explain:Explain:The short gene is recessive. To be The short gene is recessive. To be

expressed, both recessive genes must be in expressed, both recessive genes must be in the genotypethe genotype

Genotype vs. PhenotypeGenotype vs. Phenotype

tt

Predicting ResultsPredicting ResultsPunnett Square: predicts the probability of the

offspring having a specific genotype and phenotype

STEPS:

1) Assign a symbol for each allele

2) Determine genotype of each parent

3) Determine all possible gametes each parent can produce (set up Punnett square)

  4) Determine all allele combinations of offspring

  5) Determine phenotype of each offspring from genotype

Given:

Purple flowers are dominant over white flowers Two heterozygous purple flowers are crossed

Predicting ResultsPredicting Results

1)Assign a symbol for each allelePurple: PWhite: p

2)Determine genotype of each parentHeterozygous: Pp

Given: Purple flowers are dominant over white flowers Two heterozygous purple flowers are crossed

Predicting ResultsPredicting Results

3) Determine all possible gametes each parent can produce 4) Determine all allele combinations of offspring

Father’s gametes

Mother’s gametes

P

P

p

p p

p

ppP

PP PFirst write the letters at the top of each column in each square below

Then write the letters at the beginning of each row in each square across

Separate and write one parent’s alleles across the top of each columnPlace the other parent’s alleles down the side in front of each row

Predicting ResultsPredicting ResultsGiven: Purple flowers (P) are

dominant over white flowers (p) Two heterozygous purple flowers are crossed P

P

p

p p

p

ppP

PP P5) Determine phenotype of

each offspring from genotype

PP = 1 Purple

Pp = 2 Purple

pp = 1 WhiteWhat percentage of offspring are purple? What percentage of offspring are white?

¾ = 75%¼ = 25%

Genotypes vs. Genotypes vs. PhenotypesPhenotypes

Which color is dominant?

Which color is recessive?

Which plants are homozygous?

Which plants are heterozygous?

GenerationsGenerations

• The original parents: – Called the P generation

– Produce P gametes

• Offspring of P generation: – Called the F1 generation

– Produce F1 gametes

Two Trait InheritanceTwo Trait Inheritance

Yellow (YY)

Round (RR)

(YYRR)

Green (yy)

Wrinkled (rr)

(yyrr)

P gametes YR yr

YyRrF1 Generation

Only combinations possible since they are homozygous for both traits

Two Trait InheritanceTwo Trait InheritanceYellow, Round (YyRr)

F1 gametes: YR Yr yR yr

YR

YR

Yr

Yr

yR

yR

yr

yr

Y

Y

Y

Y

Y

Y

Y

Y

RR

RR

RR

RR

r

r

r

r

r

r

r

r

y

y

y

y

y

y

y

yY R Y R Y R Y R

Y r Y r Y r Y r

y R y R y R y R

y r y r y r y r

FF22 Generation Generation

YR

YR

Yr

Yr

yR

yR

yr

yr

YYRR

YYRr

YyRR

YYrr YyRr Yyrr

YyRrYyRRYYRr

YyRr

yyRR

Yyrr

YyRr

yyrryyRr

yyRr

How many: Yellow Smooth? Yellow wrinkled? Green Smooth? Green wrinkled?

9 33 1

What to Know:What to Know:

• Know and apply terminology for geneticsKnow and apply terminology for genetics

• Describe genotypes and phenotypesDescribe genotypes and phenotypes

• Identify and explain dominant and Identify and explain dominant and recessive traits recessive traits

• Identify gametes of parents for one and Identify gametes of parents for one and two traitstwo traits

• Set up and complete Punnett squares for Set up and complete Punnett squares for both one and two traitsboth one and two traits