Mendelian Genetics
What is Heredity?
The passing on of traits from one generation to the next
X
Female normal wing
Male double wing
P- generation
F1 – normal wing
Early Ideas about Heredity
• Blending Theory (19th century):– Each parent contributed “factors” that were blended in
the offspring
• What was the problem with this theory?– All individuals of a population would eventually look
the same.– Once blended, traits would never separate and show
up in later generations.
Chapter 9, Section 1: Pre-Mendel
Gregor Mendel (1822 – 1884)• Augustinian monk who
studied pea plants• Established the
particulate theory of heredity
• Significance:– Developed pure lines– Counted results and kept
statistical notes (data)
• His work remained undiscovered until 1903.
Chapter 9, Section 1: Mendel
Character Dominant Trait Recessive Trait
Flower Color
Flower Position
Seed Color
Seed Shape
Pod Shape
Pod Color
Stem Length
Purple White
Side Top
Yellow Green
Round Wrinkled
Inflated Constricted
Green Yellow
Tall Dwarf
Mendel’s Research
Why did he research pea plants?
1. They are normally self-pollinated, but can be cross-pollinated.
2. They have several qualitative traits that are easy to distinguish
i.e.,Tall vs. Short
Self Pollination
Cross Pollination
Mendel’s Research
1. Removed stamens from purple flower.
2. Transferred pollen from stamens of white flower to pistil of purple flower.
3. Pollinated flower matured into a pod.
4. Planted seeds from pod.5. Examined offspring:
All purple flowers…
Watch this video clip, and seeif you can explain why…
Mendel’s Conclusions
1. Law of Segregationa. Factors (genes) for a particular trait occur in
pairs
b. For each trait, an organism inherits two genes, one from each parent.
c. Dominant alleles mask recessive onesi. Exception 1: Incomplete Dominance
ii. Exception 2: Co-dominance
d. Two alleles for each trait segregate (separate) during gamete production
Homologous pair of Chromosomes
Locusfor
FlowerColorGene
Allele for White Flowers (p)
Allele for Purple Flowers (P)
Law of Segregation:
Factors for a particular trait occur in pairs
Back to Mendel’s Conclusions
Genes: The “factors” that control traits.
Alleles: Different forms of a gene.
• Each individual is diploid– Diploid: Containing a double-set of chromosomes (2n)
• Each gamete is haploid– Gamete: Reproductive cell (egg or sperm)– Haploid: Containing a single-set of chromosomes (n)
Back to Mendel’s Conclusions
Law of Segregation:
One version of each gene is inherited from each parent
Ww ww
W
w
orw
w
or
W = widow’s peak
w = no widow’s peak
P (Parent) Generation True-breeding parents (PP x pp)
F1 (1st Filial) Generation Hybrid Offspring (Pp)
F2 (2nd Filial) Generation
What is the F2 ratio?
705 plants had purple flowers
224 plants had white flowers
705:224 3:1
What happened to the recessive traits?
Dominant alleles mask recessive ones
P pPP pp
P p
Pp
PP
Pp Pp
pp
P P
p p
P Generation
F1 Generation
F2 Generation
Phenotype (Appearance) Purple Flowers White FlowersGenotype (Genetic Makeup)Gametes
Phenotype (Appearance)
Gametes
Purple FlowersGenotype (Genetic Makeup)
3: 1
What is the Genotypic Ratio of the F2 Generation?
PP:Pp:pp 1:2:1
What is the Phenotypic Ratio of the F2 Generation?
Dominant alleles mask recessive ones
Punnett Square
Genotype Phenotype
Purple
Purple
Purple
White
PP
Pp
Pp
pp
(homozygous)
(heterozygous)
(heterozygous)
(homozygous)
1
1
2
3
1
Ratio = 1:2:1 Ratio = 3:1
Dominant alleles mask recessive ones
Homozygous: same alleles
Heterozygous: different alleles
We will now play “Got Gametes?” in order to practice understanding the following
new terms: alleles, genotype, phenotype, homozygous, heterozygous
Vocabulary Practice
Vocabulary Practice
Each of you are haploid gametes carrying single alleles – for a trait.
Your single letter can be combined with another single letter (i.e., Hh) to form a genotype in a diploid organism.
You will observe a series of faces. Come to the front of the classroom if you think you have the right genotype to match the phenotype shown. Make sure to find the corresponding allele for the trait you represent!
How do you set up a Punnett square?
Example 1:
Heterozygous short hair (____) X heterozygous short hair (____)
HhHh
Hh Hh
HH
hh HH
Hh Hh
hhGenotypic Ratio HH:Hh:hh
1:2:1
Phenotypic Ratio Short hair:long hair
3:1
Now you try!
Dominate alleles mask recessive ones
Dominant phenotype, unknown genotype:
PP or Pp?
Recessive phenotype, known genotype:
pp
If PP, then all offspring purple:
If Pp, then ½ offspring purpleand ½ offspring white:
What would your hypothesis be if the genotype was PP?
P p
P p p
P p
pPp
Pp
Pp
Pp
Pp
Pp
pp
pp
What would your hypothesis be if the genotype was Pp?
This is called a test cross
If you have a dominant phenotype (like purple flowers) how would you
determine if it was homozygous (PP) or heterozygous (Pp)? What
experiment would you design?
Back to Mendel’s Conclusions
Exception to Dominant Alleles Masking Recessive Alleles
Incomplete Dominance: Pink Snapdragons
Use Root Letter “C” to designate incomplete dominance interaction
CR
CR
CW
CW
CWCW
CR CR
CRCW CRCW
CRCR
CWCW
P Generation
F1 Generation
F2 Generation
Phenotype(s): Red and White
Phenotype: Pink
Genotype(s): CRCR and CWCW
Genotype: CRCW
Gamete of Red flower: CR Gamete of White flower: CW
Gametes: CR and CW
Back to Mendel’s Conclusions
Exception to Dominant Alleles Masking Recessive Alleles:
Co-dominance: Blood Types (video)
Use root letter “I” for dominant alleles of equal strength and “i” for recessive
Back to Mendel’s Conclusions
This occurs in a process called meiosis:Specifically it is “crossing over,” which occurs very early during Prophase I of Meiosis and separation
in Anaphase I and II
Law of Segregation:
Two alleles for each trait segregate (separate) during gamete production
Back to Mendel’s Conclusions
Mendel’s Conclusions
The law of segregation followed one single trait at a time, such as seed color.
What if two traits were followed, such as seed color and seed shape?
Are these genes somehow connected (linked) and inherited together?
Following two traits:Seed shape and seed color
Hypothesis: If the genes for seed shape and color are connected in some way,
then the dominant R and Y alleles and the recessive r and y alleles will be matched sets in the gametes.
YR yr
YyRr
P
Gametes
F1
F2
YR YR
yr yr
Ova Sperm
For the traits of seed shape and color, this hypothesis is NOT supported by experimental
evidence
What did the data support instead?
YYRR yyrr
YR yr
What are the genotypes of the P Generation?
What does the P Generation pass on to F1?
F1 Phenotype?
YyRrF1 Genotype?
Gametes
What does the F1 Generation pass on to F2?
YR YR
Yr Yr
yR yR
yryr
SpermOva
YYRRCombine the sperm and ova to produce the offspring in F2
What do you expect the phenotype will be when YR and YR are combined?
What will the Genotype be when YR and YR are combined?
Now try to do it on your own!
YYRr YYRr
YyRR YyRR
YyRrYyRr YyRrYyRr
yyRryyRr
yyRRYyrr
YYrr
Yyrr
yyrr
Phenotypic Ratios
Yellow & Round
Green & Round
Yellow & Wrinkled
Green & Wrinkled
9/16
3/16
3/16
1/16
What are the phenotypic ratios?
Law of Independent Assortment
Mendel’s Conclusion!
Law of Independent Assortment:If the genes are not connected, then they
should segregate independently. The alleles are randomly packaged into different gametes during meiosis
(For example, genes for seed shape and color were not inherited together.)
YyRr YR Yr yR yr
FOIL
Significance of Mendel’s Research
Punnett Square was introduced as a tool to predict or determine the probability of an event
Now, let’s practice dihybrid crosses!
White board exercises
One-trait cross
The allele for the hairy trait is “H,” while the
allele for the hairless trait is “h.”
Make a cross between two homozygotes for each of these traits.
Punnett Square ExerciseOne-trait cross
In a population of Wisconsin fast plants, purple color is created by the pigment anthocyan. The gene that codes for this pigment is dominant (A), and without this pigment, the plant is green recessive (a).
Predict the offspring from the cross of a purple
homozygous plant and a green plant. What if the purple plant was heterozygous? How would the offspring be different?Green Purple
Purple plant: AA
Green plant: aa
A aaA Aa
AaAa
Aa100% Purple
Purple plant: Aa
Green plant: aa
A aaa Aa
Aaaa
aa
50% Purple
50% Green
Punnett Square ExerciseOne-trait cross
In populations of hamsters, brown fur is dominant (B), and white fur is recessive (b).
Predict the offspring from the cross of a white hamster and a brown hamster if the brown hamster's mother was white.
Brown hampster: Bb
White hampster: bb
B bbb Bb
Bbbb
bb
Offspring
50% brown mice
50% white mice
Punnett Square ExerciseOne-trait cross
Henry VIII divorced six of his wives (two of whom were
executed) for not bearing him any sons. Use Mendelian Genetics to prove to King Henry that it wasn’t his wives’ fault.Remember:
Females are XX and Males are XY.Go to Test Crosses
White-board exercises
Two-trait cross
Following two traits:
Kernel shape: Plump vs. withered (P and p)
and Kernel color Red vs. yellow (R and r) Predict the cross between a homozygousrecessive corn plant and a homozygous dominant corn plant.
Punnett Square Exercise
Two-trait cross
Now take the offspring from that cross, and self-pollinate that plant.
What is the phenotypicratio of this cross?
Punnett Square ExerciseTwo-trait cross
My pet guinea pigs (Joni and Chachi) are going to have little guinea pigs.
What is the possibility that their offspring will have long hair, if I know that Joni and Chachi are heterozygous for the following traits?
Black fur (B) White fur (b)
Long hair (L) Short hair (l)