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Chapter 12
Mendel and Heredity
Pre-class Question 3/30
How do plants reproduce, sexually or
asexually?
Depends
Homework
Chapter 12 Vocabulary due Thursday 4/2
“In the Glass House” article, due tomorrow
Species Comparison
Character and Trait
A character is a recognizable inherited feature or
characteristic of an organism
Example: eye color
A trait is one or two or more forms of a character.
Example: blue, brown, or hazel eye color
Genetic Trait Activity
Earlobe Attachment
Hitchhiker’s Thumb
Normal
Free
Attached
Mid Digital Hair
More Genetic Traits
Widow’s Peak
Cleft Chin
Pre-class Question 3/31/09
What type of plant did Mendel work with?
Pea Plant (Pisum)
Homework (due tomorrow) – Active Reading, “Mendel’s Theory”
Reminder – Chapter 12 Vocabulary due Thursday
Pea Plant Characteristics and Traits
Characterisitics
Traits
Variation of Traits
Some traits are controlled by a single gene,
which gives rise to few varieties.
Some traits are controlled by more than one
gene (polygenic) which gives rise to many
varieties.
“In the Glasshouse” Homework
1. What were the seven characteristics of
pea plants that Mendel examined in his
experiments?
Mendel chose the common garden pea, of the genus Pisum, to
experiment on. First he chose seven characteristics to look at: the
position of the flowers on the stem, the stem length, the color of
the unripe pod, the shape of the ripe seed, the color of the seed
coat, the shape of the ripe pod, and the color of the ripe seed.
Then he planted his first generation of peas and watched them grow.
For two years he kept planting the seeds to make sure they were pure
breeding, that is, they always produced offspring that had the same
characteristics as their parent plants. Then he began his experiments.
2. What percentage of seeds that Mendel
produced from crossing round seeds and
wrinkled seeds in his first experiment
were round?
The first of the experiments that he conducted crossed
plants with round seeds and plants with wrinkled seeds.
The resulting plants produced all round seeds! The next
year he grew plants from these round seeds and allowed them
to self-fertilize and produce more seeds. This time there were
5474 round seeds and 1850 wrinkled seeds, making a 3:1 ratio
of round to wrinkled seeds.
3. What seed type reappeared in the 2nd
generation plants that had seemed to
disappear in the 1st generation?
Mendel was fascinated! It seemed that the wrinkled trait, which
seemed to disappear in the first generation of offspring, reappeared
in the second generation. He called the round trait "dominating" and the
wrinkled trait "recessive."
4. What combination of letters represents a
pure dominant trait, a hybrid trait and a
recessive trait?
Mendel was the first person to use a single letter to represent
each trait, with a capital letter (A) to represent the dominant
trait and a lowercase letter (a) to represent the recessive trait.
When two pure-breeding parent plants were crossed, their
hybrid offspring were represented as Aa. For every four peas
produced in the second generation of offspring (when the
hybrids, represented by Aa, were allowed to self-fertilize),
one pea could be called AA (pure dominant round), two peas
could be called Aa (round but also carrying the potential to
produce wrinkled peas), and one pea could be called aa (pure
recessive wrinkled).
5. What ratio of seed traits did Mendel
observed for each seed characteristic he
studied?
Mendel repeated the experiment with the other six
characteristics and got the same 3:1 ratios. He also tried
crosses considering two different characteristics at the same
time. For example, he crossed plants producing round yellow
peas with plants producing green wrinkled peas. Finally, he
did backcrosses, in which first generation hybrids were
crossed with either pure-breeding dominants or purebreeding recessives.
Mendel’s First Experiments (3 Steps)
Mendel performed monohybrid crosses to study one pair of
contrasting traits.
Step 1 (Parental Generation)
Mendel allowed plants to self pollinate to create truebreeding plants (ones that produced the same offspring over
and over again.
Step 2 (F1 Generation)
Mendel crossed true-breeding plants that had contrasting
traits.
Step 3 (F2 Generation)
Mendel allowed the F1 generation plants to self-pollinate.
Mendel’s Results
3:1
Pre-class Question
4/1
What did the Punnett Square wear to school?
His Jeans
Using Punnett Squares to Explain
Mendel’s Results
p
p
P
P
Pp
Pp
Pp
Pp
F1 Generation
P
p
P
PP
Pp
p
Pp
pp
F2 Generation
Mendel’s Theory of Simple Inheritance Alleles
Alternate versions of genes
The alleles for flower color in pea plants are P and p
P is the dominant allele and p is the recessive allele
Whenever P is present, the flower color is purple
The only way a flower can be white is if both alleles are p (pp)
Genotype vs. Phenotype
Genotype is the actual combination of genes
PP
Pp
pp
Phenotype is the expression of those genes or trait
Homozygous vs. Heterozygous
Homozygous = Same
Homozygous Dominant = PP
Homozygous Recessive = pp
Heterozygous = Different
Heterozygous = Pp
Law of Segregation of Alleles
When an organism produces gametes,
each pair of alleles is separated and each
gamete has equal chance of receiving one
of the alleles.
Mendel’s Second Experiments
Dihybrid Crosses (involves two traits)
Law of Independent Assortment
During gamete formation, the alleles of
each gene segregate independently. The
inheritance of one trait is not affected by the
inheritance of another trait.
Genes Linked on Chromosomes
Genes that are located close to one
another on a chromosome do not separate
independently. Some genes are linked to
together.
The genes that Mendel studies were far
apart from each other.
Pre-Class Question 4/2
Characteristic = Spots
Traits S=spots
s= no spots
Perform the following cross: Ss x ss
Pre-class Question 4/13
Genes for Hitch hikers thumb
N=normal thumb n=hitch hikers thumb
Genotypes
Homozygous Dominant
NN
Heterozygous
Nn
Phenotypes
Homozygous Recessive
nn
Pre-class Question 4/14/09
What do you think the term “polygenic
character” means?
A character that is controlled by more than one gene
Homework
The Relationship Between Genotype and Phenotype
(Picture and Announcement Due Thursday)
Vocabulary Quiz Friday
Pre-class Question 4/15/09
What is a pedigree?
A diagram that shows the occurrence of a genetic
trait in several generations of a family
Homework
Worksheet, Baby Picture and Announcement Due Tomorrow
Vocabulary Quiz Friday
Pre-class question 4/20
If you were to perform a cross between two individuals
with linked genes using a punnett square, how many
different combinations of genes could you create.
GgHh x HHgg
Homework
Bikini Bottom Genetics 2 worksheet, due Wednesday
Chapter 12 Test Friday
Pre-class Question 4/22
A cross between a red-flowered plant and a whiteflowered plant results in offspring that have
pink flowers. This is an example of
_____________.
A. Polygenic inheritance
B. Incomplete Dominance
C. Multiple Alleles
D. Codominance
Chapter 12 Test Friday