Gregor Mendel
Gregor Mendel
When two heterozygousplants are crossedthe expected andobserved
phenotypicratio will always be3:1
This was the same for all 7 of his tests
Gregor Mendel
Mendel 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 Mendel
Genotype of alleles:R = red flowerr = orange flower
All genes occur in pairs, so 2 alleles affect a
characteristic
Possible combinations are:
Genotypes RR Rrrr
Phenotypes RED RED ORANGE
Practice
For 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_____
Ho
Ho
Ho
Ho
Ho
Ho
He
He
He
He
TT, ff, DD, BB, tt, dd
Dd, Bb, Tt, Ff
Practice
Determine 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
Practice
For 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 Crosses
Monohybrid cross: cross involving a single trait
Flower color, plant height
Dihybrid cross: cross involving two traits
Flower 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
Solving Punnett squares only takes a few steps:
Determine the genotypes of the parent organisms
Write down your cross
Draw your Punnett square
Split the letters of the genotype for each parent & put them
outside the Punnett square
Determine the possible genotypes of the offspring by filling in
the Punnett square
Summarize the results (both genotype and phenotype of the
offspring)
Genetic Crosses
Sometimes the question already tells you, other times you need
to understand the vocabulary. Read carefully!
Write down the genotypes of the parents
Ex: Tt x tt
Draw the Punnett square
Genetic Crosses
Split the letters of our cross Tt x tt
This split represents the process of meiosis
The vertical represents the male gamete
The horizontal represents the female gamete
Genetic Crosses
Each box represents a possible genotype of the offspring
Genetic Crosses
Summarize the results of the offspring; both genotype and
phenotype
Genotype:
50% TT50% tt
Phenotype:
50% tall50% 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
________________
ss
Ss
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
________________
Pp
Pp
P
p
P
p
PP
Pp
Pp
pp
Pink, yellow
PP, Pp, pp
1:2:1
3:1
Gregor Mendel
In 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 Mendel
He 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 package
The Y and R alleles and the y and r alleles stay together
Gregor Mendel
The Y and R alleles andthe y and r alleles staytogether
This was not consistentwith Mendels results
Gregor Mendel
An alternative hypothesis is that the two alleles segregate
independently of each other
Known 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 Mendel
In our example, the F1 offspring would still produce yellow,
round seeds. (YyRr)
However, when the F1s 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.
