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Mendel
BiologyChapter 10.1p. 258-268
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10.1 Mendel’s Laws of Heredity
p. 259-268
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Gregor Mendel
Austrian monk Considered the “father of genetics” The first person to succeed in
predicting how traits would be transferred from one generation to another using the garden pea plant
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Genetics
The branch of biology that studies heredity Heredity
The passing on of characteristics from parents to offspring
Traits Characteristics that are inherited
i.e. eye color, height, etc.
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Why Mendel chose the garden pea? Easily cultivated Short generation time Reproduces sexually and can be cross-
pollinated Gametes (sex cells)
Male = sperm; female = egg Pollination
Transfer of the male pollen grains to the pistil of a flower
Fertilization When the male gamete unites with the female gamete
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Parts of a Flower
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Types of Pollination
Cross-pollination
self-pollination
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How Mendel studied the garden pea plant?
Controlled his experiments Studied only one trait at a time He analyzed his data mathematically He chose true breeding garden pea
plants Offspring are identical to parents
He studied 7 traits of the garden pea
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Pea traits that Mendel studied
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Mendel’s Monohybrid Cross Hybrid
The offspring of parents that have different forms of a trait, such as tall and short
Monohybrid cross (mono = one) The two parent plants differed by a single
trait – height P1 – parent generation F1 –first generation F2 – second generation
P = parent F = “filial” son or daughter
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Mendel’s Monohybrid Cross
The First Generation Crossed 2 true
breeding plants 1 tall and 1 short
All offspring of the 2 parent plants were tall
P1
F1
X
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Mendel’s Monohybrid Cross
The Second Generation Self-pollinated the plants from the first
generation ¾ the offspring were as tall as the tall
plants in the parent and first generation
¼ the offspring were as short as the short plant in the parent generation
3:1 ratio tall to short
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Second Generation
X
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Mendel’s Monohybrid Cross
The rule of unit factors Each organism has 2 factors that
control each of its traits These factors are genes
Genes exist in alternative forms called alleles
Ex. Plant height – one alleles is for tall and another is for short
One comes from the mother and one from the father
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Mendel’s Monohybrid Cross The rule of dominance
Each trait has an allele that will be observed more than the other
Dominant (gene) The observed trait Tall plant
Recessive (gene) The trait that disappeared Short plant Only shows when both alleles are
recessive
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Dominate and Recessive
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Mendel’s Monohybrid Cross
Recording the results for crosses Dominate allele is always written first Uppercase letter is used for dominate
T – tall Lowercase letter is used for recessive
t – short
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Mendel’s Monohybrid Cross Law of segregation
During fertilization, male and female gametes randomly pair to produce 3 combinations of alleles.
Concluded that each plant in the F1 generation carried one dominate allele and one recessive allele and the F2 generation either received 2 dominate; 2 recessive; or one of each
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Phenotypes and Genotypes Two organisms can look alike but have
different underlying gene combinations Phenotype
The way an organism looks or behaves What you see
Genotype The gene combination an organism contains The genetic makeup
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Phenotypes and Genotypes
Homozygous The two alleles for the trait are the same
TT or tt Heterozygous
The two alleles for the trait are different
Tt
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Can you determine the phenotype?
White and purple garden pea flowering plants Purple is dominate (P) White is recessive (p)
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P P
P
P
P P
P P P P
PP
Homozygous Dominate Cross
Cross = Purple X Purple
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p p
p
p
p p
p p p p
pp
Homozygous Recessive Cross
Cross = White X White
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P p
P
p
P P
P p p p
pP
Heterozygous Cross
Cross = Purple X Purple
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Mendel’s Dihybrid Crosses
Cross where the peas differ in 2 traits Ex. Seed color and Seed shape
A cross involving two traits
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Mendel’s Dihybrid Crosses
The First Generation Two true breeding plants (P1)
RRYY = round yellow seed (homozygous dominate)
rryy = wrinkled green seed (homozygous recessive)
When they were crossed all the plants had round yellow seeds (F1)
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Mendel’s Dihybrid Crosses
RY RY RY RY
ry RrYy RrYy RrYy RrYy
ry RrYy RrYy RrYy RyYy
ry RrYy RrYy RrYy RrYy
ry RrYy RrYy RrYy RrYy
Dihybrid Cross = round yellow X wrinkled green
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Mendel’s Dihybrid Crosses
The Second Generation (F2) Self-pollinated plants from the first
generation Resulted in 9 round yellow, 3 round
green, 3 wrinkled yellow, 1 wrinkled green
A ratio of 9:3:3:1
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Mendel’s Dihybrid Crosses
The law of independent assortment Genes from different traits are
inherited independently of each other Ex. A pea plant that is RrYy, the alleles
will separate and the traits will separate
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Mendel’s Dihybrid Crosses
RY Ry rY ry
RY RRYY RRYy RrYY RrYy
Ry RRYy RRyy RrYy Rryy
rY RrYY RrYy rrYY rrYy
ry RrYy Rryy rrYy rryy
Heterozygous Cross = round yellow X round yellow
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Punnett Squares
Devised by an English biologist Reginald Punnett in 1905
Short hand way of finding the expected ratio of genotypes
The phenotype can also be determined by the Punnett Squares
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Punnett Squares
Monohybrid crosses Cross between two plants but only
looking at one trait Alleles of each parent are represented
in the cross One parent is on the top the other is on
the side
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Monohybrid Cross
T t
T TT Tt
t Tt tt
T t
T
t
Heterozygous tall parent = Tt
Cross = Tt X Tt
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Punnett Squares
Dihybrid crosses Cross between two plants, and you
are looking at two traits Both traits will be represented in the
cross RrYy X RrYy (both are heterozygous)
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Dihybrid Cross
RY Ry rY ry
RY
Ry
rY
ry
RY Ry rY ry
RY RRYY RRYy RrYY RrYy
Ry RRYy RRyy RrYy Rryy
rY RrYY RrYy rrYY rrYy
ry RrYy Rryy rrYy rryy
Heterozygous round yellow seed parents = RrYy
Cross = RrYy X RrYy
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Probability
Genetic is like flipping a coin it can go either way
The Punnett Square is only able to show us the chance/probability that the offspring will be a certain way All the offspring could be the same
PRACTICE PUNNETT SQUARES
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Monohybrid Punnett Square
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Dihybrid Punnett Square
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Dihybrid Punnett Square
