Download Mendel and his Peas Sept.1, 2010 Lecture Learning Objectives: You

Biosc 0150
Foundations of Biology I
Mendel and his Peas
2010
Sept.1, 2010
Lecture Learning Objectives: You should be able to…
LO1– To describe Mendel’s work and the hypotheses it generated
LO2--To explain how Mendel’s results lead to the principles of particulate inheritance and
independent inheritance.
Observations Driving Mendel’s Work
1. Plant and Animal Breeders noticed that offspring (hybrids) of 2 different individuals
often displayed an intermediate like a mix of parental traits.
2. Hypothesis: Blending inheritance—
3. Blending Hypothesis Predicts:
Mendel’s Model Organism: Pisum sativum—garden peas
Advantages:
1. Short growing time
2. Produces large number of seeds
3. Self-pollinating but possible to cross pollinate
4. Seven observable discrete traits:
1
Biosc 0150
Foundations of Biology I
2010
Mendel’s Monohybrid Crosses
Blending Inheritance Hypothesis:
Mendel’s Results
1.Parental Generation= pure breeding lines
2. Offspring – F1 generation–
3.Self pollination of F1 generation = F2
What did Mendel conclude about the inheritance of discrete traits?
Mendel’s unfortunate name choices
Definitions from theAmerican Heritage Dictionary of the English Language
English Definitions.
Dominant—(1) exercising the most influence or control, (2) pre-eminent in position or
prevalence
Recessive—Tending to go backward or to receede
Genetic Definitions
Dominant—producing the same phenotype whether paired with an identical or dissimilar allele
Recessive—an allele that does not show a phenotype when paired with a dominant allele
2
Biosc 0150
Foundations of Biology I
2010
Does Gender Influence Trait Inheritance: The Reciprocal Cross?
Critically important question to ask
– does the source of the trait matter
for its expression?
What was the result of this cross?
Did the experiment succeed or fail?
Results of Mendel’s 8 Years of Experiments
1. What do you notice about the ratio of dominant to recessive
phenotype in each cross?
2. Is the distribution of offspring dependent on the trait studied?
3
Biosc 0150
Foundations of Biology I
2010
Mendel’s Monohybrid Cross: Particulate Inheritance
Parental Generation= pure
breeding lines
Offspring – F1 generation–
phenotype of male parent
Phenotype of female parent not
present
Prediction of Blending Hypothesis
The phenotypes are not blending
The wrinkle seed trait is not present
What is going on?
Where did the Wrinkled Trait Go ?
Think About It……
Thanks to digital photography and programs like Photoshop, photography studios
can claim to predict what a couple’s child would look like based upon images of
the putative parents.
Explain in genetic terms why this is nonsense.
4
Biosc 0150
Foundations of Biology I
2010
Your Turn: With a partner work through the following monohybrid crosses.
1. You are given 2 blue beetles and 2 black beetles:
a. Cross 1: You mate blue beetle #1 to black beetle # 1 and obtain 220 black beetles in the
F1 generation
 What is the dominant phenotype?

What is the recessive phenotype?

What are the genotypes of the parent beetles?
b. Cross 2: You mate blue beetle #2 to black beetle #2 and obtain55 blue beetles and 65
black beetles. What are the genotypes of the parents and the offspring?
c. Cross 3: You mate 2 black offspring from Cross 2.
What are the possible genotypes and phenotypes of the offspring?
What are the expected proportions of these genotypes and phenotypes?
2.Consider some flowers in which color is controlled by one gene and green color is dominant to
blue; G is the symbol for the dominant phenotype, g is the symbol for the recessive phenotype, blue
flowers.
Give the expected ratios of offspring from the following crosses:
GG x GG—
ggxgg—
Gg x gg—
Ggx Gg—
Green x Green –
Blue xBlue – bbxbb =
5
Biosc 0150
Foundations of Biology I
2010
Mendel Describes Results
(with unfortunate results for students)
Dominant =Phenotype that appears in F1 generation
Recessive =Phenotype that reappears in F2 generation
Mendel did not mean that the dominant phenotype was superior or more common.
Dominant & recessive does not imply presence or absence of a trait, it only refers
to a phenotype
Does Gender Influence Trait Inheritance?
The Reciprocal Cross
Does the source of the phenotype affect its
expression
Results of Mendel’s 8 Years of Experiments
What do you notice about the ratio of dominant to
recessive phenotype in each cross?
Is the distribution of offspring dependent on the trait
studied?
6
Biosc 0150
Foundations of Biology I
2010
The Dihybrid Cross and Independent Assortment
Do the parental alleles remain togther when gametes are formed?
Predict the Gametes for Dependent Assortment
Female Parent: rryy ( green seeds, wrinkled)
Male parent: RRYY (yellow seeds, smooth
Gametes(eggs) produced by Female Parent:
Gametes produced by Male Parent:
Phenotypes of F1 Generation:
Phenotypes of F2 Generation:
7
Biosc 0150
Foundations of Biology I
2010
Dihybrid Cross Problems
1. You are following two single gene traits in peas: seed color and seed shape. Green peas
are dominant to yellow peas, smooth peas are dominant to wrinkled peas
a. What are the genotypes of the parents using the convention that the dominant allele is
given by an upper case letter and the recessive allele uses the lower case of the same
letter?
b. complete the Punnett square for this cross.
c. In a dihybrid cross of two F1 plants, what is the phenotypic rations you expect in the
offspring?
2. Consider a hypothetical flowering plant that can have red or green flowers and tall or
short stems. For the flower color, green is dominant to red, and tall stems are dominant to
short stems. Predict the phenotypic ratios for the following crosses
a. Green tall (GGTT) x red short (ggtt)
b. Green tall (GgTt) x red short (ggtt)
c. Green short (Ggtt) x red tall (rrTt)
d. Green short(GGtt) x green tall (GgTt)
8
Biosc 0150
Foundations of Biology I
2010
But What If You Know The Phenotype But Not The Genotype: The Test Cross
A test cross reveals an unknown genotype
Why do we care?
Contribution of homozygous
recessive parent known.
Allows determination of 2nd
parental phenotype.
Lecture Summary
What hypothesis did Mendel’s work reject and why?
What evidence supports the principle of particulate inheritance?
What evidence supports the principle of independent segregation?
9