Transcript Heredity and Mendel

Heredity and Mendel
Heredity
 The passing of parents characteristics onto the offspring
 Genetics is the branch of biology that studies heredity
Gregor Mendel
 Considered the father of genetics
 Was an Austrian monk that used pea plants to study heredity
 Peas have easy traits to observe
 Pea plants can self pollinate so their genetic information can be
controlled
 By carefully recording his data he made some great findings
Mendel Crossed Plants with different
variations
 Mendel crossed a tall pea plant with a short pea plant to see
what would happen
•
x
 The result was that the offspring (first generation) were all
tall
 However when crossed the second generation,
3 Tall, 1 Short
Mendel repeated these crosses for all
the different variations
 He discovered that in the 2nd generation (F2)that the trait
that had disappeared in the first generation (F1) reappeared
again, always.
 Genetic generation abbreviations
 P1= parent generation
 F1= first generation
 F2=second generation
Mendel's findings
 Mendel concluded that each organism has
2 factors, which
he called genes, that control each trait
 Alleles are different gene forms that determine the different
form of the trait, one allele for a trait comes from the mother
and the other from the father
 We use capital and lower case letters to represent alleles
 Genes can either be homozygous or heterozygous
 Homozygous has the same 2 alleles ie TT or tt
 Heterozygous has different alleles ie Tt
 GENES AND ALLELES ARE NOT THE SAME!!!
Mendel's findings
 Principle of Dominance = some alleles are
dominant and will mask a recessive allele
 So TT or Tt will both be tall, whereas tt will be short
 Dominant genes are represented by upper case letters and
recessive (non-dominant) by lower case
 TT=
Tt=
tt=
Genotype and Phenotype
 Genotype refers to the allele makeup of a gene
 Possibilities: Homozygous dominant (TT), Homozygous
recessive (tt), or Heterozygous (Tt)
 Phenotype refers to the trait the is expressed by the
organism or what it looks like
 Possibilities: Endless – it depends on the trait you are studying.
IE Tall, short, pink, white, etc…
Mendel's findings
 Law of Segregation - alleles segregate
when forming gametes
Mendel’s findings
 Law of Independent Assortment -
alleles segregate independently of each
other
Summary of his findings
Monohybrid crosses illustrate Mendel's
findings
 A monohybrid cross is one that looks at only one specific
trait and how it segregates (predicts probabilities)
 Easy to do this using the punnett square method
Baby Steps Through Punnett Squares
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7.
Determine the genotypes of the parents (usually given to
you)
Figure out possible gametes (Law of Segregation)
Draw your Punnett square
Put gametes on the outside of the square – mom on one
side, dad on the other
Determine possible offspring genotypes by filling in square
(Law of Independent Assortment)
Summarize your results – genotype and phenotype ratios
Feel smart 
Let’s Try One!
 Spongebob Squarepants recently met Spongesusie
Roundpants at a dance. Spongebob is heterozygous for his
square shape and Spongesusie is round (square is dominant).
If Spongebob and Spongesusie had children…
 What are the chances of a child with a square shape?
 What are the chances of a child with a round shape?
Dihybrid crosses
 Dihybrid crosses look at 2 different traits passed on from
parents
 Ex: In peas, seed shape & seed color
 R = round
 r = wrinkled
Y = yellow
y = green
Dihybrid Crosses
 One Parent is RRYY (homozygous dominant)
 The other is rryy (homozygous recessive)
• So RRYY X rryy (step 1)
 Step 2: find all possible gamete combinations by using
FOIL (First Outer Inner Last)
 There will always be 4
different combinations in
dihybrid Crosses
 Possible gametes: RY RY RY RY x ry ry ry ry
Dihybrid crosses
 Step 3: Set up 4x4 punnett square
 Step 4: Put possible gametes from parents on top and side, then fill
in squares for possible offspring (Step 5)
 Step 6: Summarize the results (Genotype and Phenotype)
Dihybrid crosses
 Now cross two F1 plants.
 RrYy x RrYy
 Possible gametes: RY Ry rY ry x
RY Ry rY ry
 Phenotypic Ratio for a Dihybrid Cross:
 For this cross, find the number of each - round yellow :
round green : wrinkled yellow : wrinkled green
 9/16 round yellow
(R_Y_)
 3/16 round green
(R_yy)
 3/16 wrinkled yellow (rrY_)
 1/16 wrinkled green
(rryy)
 Phenotypic Ratio = 9 : 3 : 3 : 1