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Section 10.1
Mendel’s Laws of Heredity pg. 253
WHY MENDEL SUCCEEDEDGregor Mendel (1822-1884)Austrian Monk known as the “father of modern genetics”Found that inheritance follows certain laws later known as Mendel’s Laws of InheritanceHeredity is the passing on of characteristics from parent to offspring
Inherited characteristics are called traits (factors)
The branch of biology that studies heredity is called genetics – which was founded upon the rediscovery of his work.
WHY MENDEL SUCCEEDEDMendel’s pea plant collection contained around 28,000 plants
Pea plants reproduce sexually by producing male and female sex cells (gametes)
Male gamete forms in pollen
Female gamete formed in ovary
Fertilization occurs when the male gamete unites with the female gamete
Pollination is the transfer of pollen to ovary in a plant – normal reproduction. Remove male parts
Snipping the stamen could prevent self-pollination
Flower parts
Quickly sketch and label this diagram
A similar figure is in the book on pg. 642
WHY MENDEL SUCCEEDEDCross-pollination is transferring pollen of one plant to the ovary of another plant• Mendel did this to get certain plants to breed with others to be sure of the parents• Mendel was very careful with all of his work
Figure 10.1
Mendel was able to create tall plants and short plants (purebreds)• He referred to the offspring of a purebred tall and a purebred short as a hybrid• Crossing a 6’ tall plant with a 2’ tall (short) plant resulted in all 6’ tall plants• Crossing the hybrid offspring resulted in 75% tall and 25% short
• P1 refers to the “Parental generation”
• F1 (“Filial”) refers to the offspring
Tall pea plant
All tall pea plants
3 tall: 1 short
P1
MENDEL’S MONOHYBRID CROSSES
F1
F2
Figure 10.2
Short pea plant
MENDEL’S MONOHYBRID CROSSESSo what does MONOHYBRID refer to?Referring to figure 10.3:• When Mendel crossed a purebred tall with a purebred short he got all tall plants• When he crossed a purebred purple flower with a purebred white flower he got all purple flowers• He referred to the trait that was observed in these cases as dominant• The trait that seemed to “disappear” he called recessive•Law of Dominance• Mendel concluded that these plants have “factors” that control each of the traits (color, shape, height)• We call these factors genes (parts of DNA)• Alternative forms of genes (tall vs. short or yellow vs. green) are known as alleles.
Recessive trait
Dominant trait
Seed shape
Seed color
Flower color
Flower position
Pod color
Pod shape
Plant height
round yellow purpleaxial (side) green inflated tall
wrinkled green whiteterminal
(tips) yellow constricted short
Figure 10.3
Below is a list of human traits, write them down.
Widow’s peak Tongue Rolling
Dimples Mid-digit Hair
Free Ear Lobes Straight Thumb
Freckles Bent Pinky Finger
Are they dominant or recessive? (Take a guess)
HUMAN TRAITS
TT
MENDEL’S MONOHYBRID CROSSES
These two alleles for each trait can be expressed as a single letter
For plant height we can use the letters “T” & “t”
Dominant allele is “T”
Recessive is “t”
Mendel’s purebred tall plants were “TT”
His purebred recessive plants were “tt”
Fill in the blanks in the figure to the right which t goes where?
Tall plant Short plant
All tall plants
F1
TT t t tT
P1
TT
MENDEL’S MONOHYBRID CROSSES
Mendel concluded that the allele (gene form) of tall plants was dominant to the allele for short plants
Confirming that the plants had two alleles for each trait (TT = Tall, Tt = Tall, or tt = short)
Knowing that traits are inherited from parents, he also concluded that these alleles are inherited
However a plant can only get one allele from each parent
The gametes (sex cells) contained either one or the other form of the gene (T or t)
The Law of Segregation states that every individual has two alleles of each gene and when gametes are produced, each gamete receives one of these alleles.
PHENOTYPES AND GENOTYPES
Phenotype refers to the organism’s physical characteristic (what you can see) Ex: Tall
Genotype refers to the organism’s genetic makeup (what you can’t see) Ex: TT or Tt
Homozygous represents two alleles that are the same (TT or tt)
Heterozygous organisms have different alleles (Tt)
How are we going to distinguish Homo and Hetero?
PUNNETT SQUARES1905 - Reginald Punnett devised an easy way to find expected genotype proportions of offspring from known parent genotypes based off Mendel’s laws
Heterozygous tall parent
T
T t
T
t
t
TT Tt
Tt tt
Heterozygous tall parent
tT
t
t tT
T
T
G= 50% Tt : 25% TT : 25% tt
P= 3 Tall : 1 short
Monohybrid Cross (one trait)
PUNNETT SQUARESMonohybrid crosses are easy to separate alleles according to Mendel’s Law of Segregation
If we have heterozygous parents (Tt X Tt) we can just separate the T from the t
For Dihybrid crosses, the gamete separation is a little tricky
If we have two parents that are heterozygous for seed shape (Rr) and seed color (Yy) their genotype is RrYy
To separate alleles into gametes we use the FOIL method from algebra
RrYy makes four different gametes
Using the FOIL method we get…RY Ry rY ry
Round Yellow
Round green
wrinkledYellow
wrinkledgreen
MENDEL’S DIHYBRID CROSSESMendel also crossed plants with two different traitsRound=R, wrinkled=r & Yellow=Y, green=yWhat is the genotype of a purebred (homozygous) plant with Round Yellow seeds?What is the genotype of a purebred (homozygous) plant with wrinkled green seeds? Purebred (homozygous) Round Yellow seeds X Purebred (homozygous) wrinkled green seeds
Result of F1…All plants had Round Yellow seeds
However crossing the F1 gives a 9:3:3:1 ratioWhich leads us to Mendel’s second law…The Law of Independent Assortment states that genes for different traits are inherited independently of each other.
RRYY
rryy
Round Yellow (RRYY) X wrinkled green (rryy)
Round Yellow wrinkled green
All Round Yellow
Round Yellow Round green wrinkled Yellow wrinkled green
9 3 3 1
P1
F1
F2
R_Y_ R_yy rrY_rryy
Gametes from RrYy parent
RY Ry ryrY
RY
Ry
rY
ry
RRYY
RRYy
RrYY RrYyRRYy
Rryy
RrYY
Rryy
RrYy
RrYy rrYY rrYy
RrYy
RRyy
rrYy rryy
Gametes from RrYy parent
Starting here what are the gametes?
PROBABILITYKnowing the parents genotype we can predict the probable offspring genotype and phenotype
What is the probability of having Rr offspring?
What is the probability of having Round offspring?
R r
R
r
RR Rr
Rr rr
75%
50%
PROBABILITYGiven the parents genotype and number of offspring, you should be able to predict the number of each genotype and phenotype.
PROBABILITY PROBLEMR=Round seeds & r=wrinkled seeds
1. P1 genotype: RR X rr
2. All of the F1 offspring will be ______.
3. Assume 140 F2 offspring are created from F1.
4. ________ will have their parents (F1) genotype.5. ________ will have Round seeds.6. ________ will have wrinkled seeds.
7. ________ will have the same genotype as the P1.
PROBABILITY PROBLEM (#2)
Rr (Round)
2. All of the F1 offspring will be ______.
P1=RR X rr (always put first parent on top of square)R R
r
r
Rr Rr
Rr Rr
2. All of the F1 offspring will be ___________.
PROBABILITY PROBLEM (#4)
70
4. ________ will have their parents (F1) genotype.
F1= All RrR r
R
r
RR Rr
Rr rr
4. ________ will have their parents (F1) genotype.
# of F2 Offspring = _____
Expected % of genotype (Rr) that is same as parents= ______
50% (2/4) of 140 = _____
140 X .5 = 70
140
50%70
PROBABILITY PROBLEM (#5)
105
5. ________ will have Round seeds.
R r
R
r
RR Rr
Rr rr
5. ________ will have Round seeds.
# of F2 Offspring = _____
Expected % of Round phenotype = ______
75% (3/4) of 140 = _____
140 X .75 = 105
140
75%105
PROBABILITY PROBLEM (#6)
35
6. ________ will have wrinkled seeds.
R r
R
r
RR Rr
Rr rr
6. ________ will have wrinkled seeds.
# of F2 Offspring = _____
Expected % of wrinkled phenotype = ______
25% (1/4) of 140 = _____
140 X .25 = 35
140
25%35
PROBABILITY PROBLEM (#7)
70
7. ________ will have the same genotype as the P1 (RR or rr).
R r
R
r
RR Rr
Rr rr
7. ________ will have the same genotype as the P1.
# of F2 Offspring = _____
Expected % of RR or rr genotype = ______
50% (2/4) of 140 = _____
140
50%70
Actual Results of Mendel’s Work
