Gregor MendelWhen two heterozygous

plants are crossedthe expected andobserved phenotypicratio will always be3:1

This was the same for all 7 of his tests

Gregor MendelMendel concluded that the two alleles for

each characteristic separate during gamete production.

This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis

Known as the law of segregation

Gregor MendelGenotype of alleles:

R = red flowerr = orange flower

All genes occur in pairs, so 2 alleles affect a characteristic

Possible combinations are:

Genotypes RR Rr rr

Phenotypes RED RED ORANGE

PracticeFor each genotype: heterozygous (He) or

homozygous (Ho)

Which of the genotypes would be purebred?

__________________________________________________

Which of the genotypes would be hybrid?

__________________________________________________

TT_____ Bb______ DD______ Ff ______ tt _____

Dd_____ ff ______ Tt ______ BB______ dd_____

HoHo

HoHo

HoHoHe

HeHe

He

TT, ff, DD, BB, tt, dd

Dd, Bb, Tt, Ff

PracticeDetermine the phenotype for each genotype

Yellow body color is dominant to blue

YY_______________ Yy _______________yy _______________

Square shape is dominant to round

SS________________Ss_______________ ss________________

Yellow Yellow Blue

Square Square Round

PracticeFor each phenotype, give the possible genotypes

A tall head (T) is dominant to short (t)

Tall_______________ Short_______________

Pink body color (P) is dominant to yellow (p)

Pink_______________ Yellow______________

TT, Tt tt

PP, Pp pp

Genetic CrossesMonohybrid cross: cross involving a single

traitFlower color, plant height

Dihybrid cross: cross involving two traitsFlower color & plant height

Punnett squares help determine the possible combinations of genotypes that can occur in the offspring.

It also shows the probability of each genotype occurring

Genetic Crosses

Genetic CrossesSolving Punnett squares only takes a few steps:1. Determine the genotypes of the parent

organisms2. Write down your ‘cross’3. Draw your Punnett square4. ‘Split’ the letters of the genotype for each

parent & put them ‘outside’ the Punnett square

5. Determine the possible genotypes of the offspring by filling in the Punnett square

6. Summarize the results (both genotype and phenotype of the offspring)

Genetic Crosses1. Sometimes the question already tells you,

other times you need to understand the vocabulary. Read carefully!

2. Write down the genotypes of the parentsEx: Tt x tt

3. Draw the Punnett square

Genetic Crosses4. Split the letters of our cross Tt x tt

This ‘split’ represents the process of meiosisThe vertical represents the male gameteThe horizontal represents the female gamete

Genetic Crosses5. Each box represents a possible genotype of

the offspring

Genetic Crosses6. Summarize the results of the offspring; both

genotype and phenotype

Genotype:50% TT 50% tt

Phenotype:50% tall 50% short

SpongeBob is heterozygous for his square shape, but SpongeSusie is round. Create a Punnett square to show the possibilities if they had children. (look at old questions!)

Cross: ___________ x ___________Punnett Square

Genotypes: Genotypic ratio:Phenotypes: Phenotypic ratio:

Practice

____ ____

____

____

ssSs

S

s

s s

Ss

Ss

Ss

ss

Square, Round

Ss, ss 3:1

3:1

Patrick met Patti at a dance. They are both heterozygous for their pink body color, create a Punnett square to show the possibilities if they had children. (look at old questions!)

Cross: ___________ x ___________Punnett Square

Genotypes: Genotypic ratio:Phenotypes: Phenotypic ratio:

Practice

____ ____

____

____

PpPp

P

p

P p

PP

Pp

Pp

pp

Pink, yellow

PP, Pp, pp 1:2:1

3:1

Gregor MendelIn one dihybrid cross, Mendel studied the

inheritance of seed color and seed shape

The allele for yellow seeds (Y) is dominant to the allele for green seeds (y).

The allele for round seeds (R) is dominant to the allele for wrinkled seeds (r).

Gregor MendelHe crossed true-breeding plants that has

yellow, round seeds (YYRR) with true-breeding plant that had green, wrinkled seeds (yyrr)

One possibility is that the two characteristics are transmitted from parents to offspring as a packageThe Y and R alleles and the y and r alleles

stay together

Gregor MendelThe Y and R alleles and

the y and r alleles staytogether

This was not consistentwith Mendel’s results

Gregor MendelAn alternative hypothesis is that the two

alleles segregate independently of each otherKnown as the law of independent

assortment

The presence of one specific allele for one trait has no impact in the presence of a specific allele for the second trait.

Gregor MendelIn our example, the F1 offspring would still

produce yellow, round seeds. (YyRr)

However, when the F1’s produced gametes, genes would be packaged into gametes with all possible allelic combinations.

Four classes of gametes (YR, Yr, yR, and yr) would be produced in equal amounts.

Gregor Mendel