Mendelian Genetics
Learning Goal: How are traits passed down
from parents to offspring?
Concept 1: Analyzing the effects of classic Mendelian genetic
crosses such as monohybrid, dihybrid, testcross, and applying to
pedigree diagrams.
PART 2
Try This!
 An organism with the genotype AaBb is crossed with an
organism that has the genotype AABb.
 What are all the possible gametes of each individual parent?
 What is the probability that any of the offspring has the AABB
genotype?
 If A and B exhibit complete dominance, what is the probability
of the offspring exhibiting both the A and B phenotypes?
Try This!
 An organism with the genotype AaBb is crossed with an
organism that has the genotype AABb.
 What are all the possible gametes of each individual parent?
AaBb x AABb
A
A
B
b
AB
AB
Ab
Ab
A
AA
AA
B
BB
Bb
aB
a
Aa
Aa
b
Bb
bb
ab
 What is the probability that any of the offspring has the AABB
genotype?
AABB – 2 punnet squares!
AA: ½ BB: ¼ ½ x ¼ = 1/8
 If A and B exhibit complete dominance, what is the probability of
the offspring exhibiting both the A and B phenotypes?
What genotypes give dominance for A and B?
A_B_
A_: 1 B_: ¾ 1 x ¾ = 3/4
Concept 1: Analyzing the effects of classic
Mendelian genetic crosses such as
monohybrid, dihybrid, testcross, and
applying to pedigree diagrams.
 Terms associated with genetics problems: P, F1, F2,
dominant, recessive, homozygous, heterozygous,
phenotypic, and genotypic
 How to derive the proper gametes, genotypes, and
phenotypes, when working a genetics problem
(monohybrid, dihybrid, testcross)
 The difference between an allele and a gene
 How to read a pedigree (monohybrid, dihybrid, testcross)
Make Math Work for YOU!
 Rule of Multiplication:
 Independent events happening at the same time
 “this and that together”
 Example: three coins are flipped. The probability that all
three coins show tails is
 ½ x ½ x ½ = 1/8
Make Math Work
for YOU!
 Rule of Addition
 Mutually exclusive events
 “this or that” “different ways of getting the same thing”
 Example: three coins are flipped.
What is the probability of getting one tail and two heads?
 - tail, head, head 1/8 (½ x ½ x ½ rule of multiplication)
 - head, head, tail 1/8
 - head, tail, head 1/8
 The probability that two coins are heads and one is tail is:
1/8 + 1/8 + 1/8 = 3/8
Tips for working through
genetics problems:
 THERE IS NO MAGICAL FORMULA!!!!
 What do you HAVE?
 What do you WANT?
 Plan a way to get there using a combination of:
 punnet squares
 probabilities
 logic
Try This!
 In peas, yellow seed colour is dominant over green
seed colour, and round seed shape is dominant over
wrinkled seed shape.
 A round-yellow seeded plant was crossed with a
green-wrinkled seeded plant producing 20 plants: 11
of these plants has yellow-round seeds, and 9 of
these plants had green-round seeds.
 What was the probable genotypes of the parent
plants?
Tips for working through
genetics problems:
 Write down the allele symbols using the same letter for a
gene:
 upper case for dominant (ex: Yellow – Y)
 lower case for recessive (ex: green – y).
 Write down the phenotypes for each individual.
Tips for working through
genetics problems:
 Write down possible genotypes for each individual.
 If the phenotype is recessive, genotype must be
homozygous recessive (green = yy)
 If the phenotype is dominant, genotype is either
homozygous dominant or heterozygous. (yellow = YY or Yy,
can write Y_)
 If the phenotype is “true-breeding,” the genotype is
homozygous for that trait.
Tips for working through
genetics problems:
 Write down what you are looking for in the question.
 If you need to find genotype/phenotypes of offspring (or
“grandchildren”), complete the cross (Yy x yy) and use the
probability rules or the Punnet square.
 If you need to find genotypes/phenotypes of parents or (or
“grandparents”), deduce by working backwards and
reasoning.
(Note: numbers of offspring may be actual rather than
predicted by ratios… 1:1 ratio may be 13 individuals:15
individuals)
Shortcuts!
 3:1 ratio indicates a monohybrid cross (Aa x Aa)
 1:1 ratio indicates a cross of a heterozygote with a
homozygous recessive (Aa x aa)
 All offspring identical to parents suggests a cross of parents
that are homozygous for the same trait, or homozygous
dominant with heterozygous (AA x AA or aa x aa or AA x
Aa)
 9:3:3:1 ratio indicates a dihybrid cross (AaBb x AaBb)
 1:1:1:1 ratio indicates a cross of a dihybrid with a double
homozygous recessive (AaBb x aabb)
We just need to practice…
 Try… On Worksheets:
Multiple Choice: 1, 2, 3, 5, 7, 8, 10, 13
Genetics Problems: 1, 2, 6, 12, 13
Pedigrees…