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
 Pea plants self pollinate usually 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, ¼ were 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
 1. Mendel concluded that each organism has 2 factors called
genes that control each trait
 Alleles are different gene forms that determine the different
form of the trait
 For example the gene for height has an allele for tall and another for small
 Typically we use upper and lower case letters to identify these T=tall
allele and t=short allele
 Genes can either be homozygous or heterozygous
 Homozygous has the same 2 alleles ie TT or tt
 Heterozygous has different alleles ie Tt
Mendel's findings
 2. 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=
Mendel's findings
 Noticed that alleles segregate when
forming gametes
 Also noticed alleles segregate
independently of each other – called
Independent Assortment
Monohybrid crosses illustrate Mendel's
findings
 A monohybrid cross is one that looks at only one specific
trait and how it segregates
 Easy to do this using the punnett square method
Determining Genotype and Phenotype
 Genotype refers to the allele makeup of
a gene
 For example the genotype of the punnett
square is ¼ TT, ½ Tt, and ¼ tt
 This gives a genotypic ratio of 1:2:1
 Phenotype refers to the trait the is expressed by
the organism or what it looks like
 Since both TT and Tt are tall, the phenotype of the
punnett square is ¾ tall and ¼ short
 The phenotypic ratio is 3:1
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: 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
Dihybrid crosses
 Step 2: Set up 4x4 punnett square and put possible gametes from
parents on top and side, then fill in squares for possible offspring
 All offspring are heterozygous for both seed shape and
seed color - RrYy, what is their phenotype and genotype?
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