the work of gregor mendel. essential question: describe mendel’s studies and conclusions about...
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The Work of Gregor Mendel
Essential Question:
Describe Mendel’s studies and conclusions about inheritance.
Gregor Mendel = Father of Genetics
Worked with garden pea plant
1857
Mendel’s Experiments
• Peas are a “model system.”
•
• Started with “true breeding” plants
• Trait: specific characteristic (e.g., seed color, plant height) of an individual
• Hybrid: created from cross of true-breeding individuals
• Small, easy to grow. • Makes them convenient to
study. • Help explain how organisms
inherit traits
Pollination and Fertilization in Plants
Male part
Female part
Plants have the ability to self-pollinate (self-fertilize) because they have both reproductive parts.
Cross-Pollination
Pollen
Taking pollen from one plant and using it to fertilize the eggs of another plant
Pea Characteristics of Mendel’s peas
Mendel studied these characteristics to observe patterns in traits across generations.
Mendel’s CrossesWhen Mendel crossed plants with contrasting traits, the hybrid offspring showed traits of only one parent.
Dominant and Recessive Traits
Yellow seed color is
to green seed color.
Green seed color is
to yellow seed color.
dominant
recessive
When a yellow pea plant is crossed with a green pea plant, the resulting offspring are yellow.
Principle of Dominance
• Some alleles are dominant, some recessive.
• An organism with at least one dominant allele will exhibit that trait.
• An organism with a recessive allele will exhibit the trait only in the absence of a dominant allele.
dominantrecessivedominant
Genes and AllelesTraits (seed shape, seed color) are determined by genes. •Genes: passed from one generation to the next; genetic code that determines an individual’s characteristics
Forms of traits (yellow/green, round/wrinkled) are called alleles. •Alleles: the different forms of a gene.
Characteristics such as pod color
are determined by .
Yellow and green pod colors are
determined by .
The allele for green pods is to the allele for yellow.
genes
alleles
recessive
The P1 and F1 Generation
P1 generation
F1 generation
True-breeding
Hybrids
Let’s figure out how we can predict the F1 gen from the P1 gen.
Monohybrid Cross = One Trait
“Height”
“True Breeding” = Homozygous
Tall Pea x Short PeaTT x tt
T = Dominantt = Recessive
X
“Homozygous Dominant”
“Homozygous Recessive”
Heterozygous and Homozygous
Homozygous: has two identical alleles for a gene
Heterozygous: has two different alleles for a gene
Which are:
homozygous?
heterozygous? 1 2 3
1, 4
2, 3
4
1st LAW OF HEREDITY
“Law of Segregation” = – The 2 alleles for each trait must separate when
gametes (sperm/egg) are formed.
-A parent passes on, at random, only one allele for each trait to each offspring
A a a
Segregation
Random Fertilization
The Formation of Gametes
Eggs and sperm
Fertilization
Making a Punnett Square
Combine gamete genotypes
Gametes for a parent along one side
Possible gametes
A cross of birds: Beak size (B, b)
Generations
P1 = Parent Plants
-True breed Tall x True breed Short
= First Generation (children)F1
TT x tt
What’s the probability that the offspring will be tall? ________ Short? _______
Genotype of offspring? ______________Genotype=genetic makeup of organism
Phenotype of offspring? _____________Phenotype=physical characteristics
Law of Probability“Rules of Chance”“Odds”
20:1 5%10:1 10%2:1 50%
Numerator = winnerDenominator = # horses1/12 1:12 “odd”
8% “chance”
Probability?
“Chance” of getting a tail? “Chance” of getting snake eyes on two dice?
Male: FemaleXY XX
½ 1:2 odd Boy 1:2 odd Girl50% chance
Survey Large Number1:1
“Chance” of getting a boy?
Dominant = Trait ObservedRecessive = Trait that disappeared
Tall = Dominant (T) Short = Recessive (t)
Alleles:
TT = TallTt = Talltt = Short
PhenotypeGenotype
Can’t always know an organisms genotype bylooking at itsphenotype?
TT tt
T t
Tt
Homozygous Dominant
Homozygous Recessive
Heterozygous
Tt x Tt
3 Tall and 1 ShortTT
tt
F1
F2
Tt
Combination of Letters/Alleles?
Tt
Change of colors to follow the parental alleles:
The F2 Generation – created from a cross of two F1 generation offspring.
What proportion of F2 offspring were short?
What proportion of F2
offspring were tall?
1/4
3/4
Dihybrid Crosses:Crossing for 2 characteristics/ traits
•2 TRAITS: Seed Shape and Seed Color
•Shape =Round (R) was dominant to Wrinkled (R)•Color = Yellow (Y) was dominant to Green (y)
EXAMPLE
Making a Punnett Square: Two Factors
Combine gamete genotypes A cross of pea plants: size (T, t) and pod color (G, g)
All possible gametes
All gametes for a parent along one side
Round Yellow Seeds Wrinkled Green SeedsHomozygous dominant for both traits Homozygous recessive for both traits
x
_RRYY_ ___rryy____(Genotype) 1. We need to first find all the possible
gamete combinations of
each parent
(Genotype)
Use FOIL to help you figure out the combinations
(RR)(YY)F(irst) RYO(uter) RYI(nner) RYL(ast) RY
Use FOIL to help you figure out the combinations
(rr)(yy)F(irst) ryO(uter) ryI(nner) ryL(ast) ry
gametes gametes
2. Place your gametes in a Punnett square. RY RY RY RY
ry
ry
ry
ry
RrYy RrYy RrYy RrYy
RrYy RrYy RrYy RrYy
RrYy RrYy RrYy RrYy
RrYy RrYy RrYy RrYy
RY RY RY RY
ry
ry
ry
ry
3. Fill in the Punnett square.
So the P1 cross of true breeding plants:
Round Yellow Seeds Wrinkled Green Seeds
RRYY rryy
produced
RrYy
P1
F1 All plants Round Yellow Seeds
1. We need to first find all the
possible gamete combinations of
each parent
Now let’s cross two F1 generation plants.
RrYy RrYy
Use FOIL!RYRyrYry
RYRyrYry
RrYy
RY Ry rY ry
The two pairs of alleles segregate independently of each other.
The presence of one specific
allele for one trait has no impact
on the presence of a specific
allele for the second trait.
Four classes of gametes
(RY, Ry, rY, and ry) would be
produced in equal amounts.
2. Place your gametes in a Punnett square. RY
RyrYry
RYRyrYry
RY Ry rY ry
RY
Ry
rY
ry
RrYy RrYy
Round Yellow Round Green Wrinkled Yellow Wrinkled Green
2ND LAW OF HEREDITY
• Law of Independent Assortment = genes for different traits are inherited “independently” of each other.
•RrYy = the R and r will separate as well as the Y and y.
•Alleles can recombine in 4 different ways.