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Unit 6: Inheritance
Part 1: Simple (Mendelian) Inheritance
Vocab review
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Homologous chromosome pair
Sister chromatid
Diploid, haploid
Independent assortment
Gamete
Gene
Allele
Gregor Mendel
Mendel's experiments
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Characters and traits
Self-fertilization
True-breeding
Cross-fertilization
(monohybrid cross)
Mendel's results: flower color
P generation
F1 generation, hybrid
F2 generation
genotype
phenotype
Mendel's Results
What did Mendel’s findings mean?
• Some traits mask others
– purple & white flower colors are
separate traits that do not blend
• purple x white ≠ light purple
• purple masked white
– dominant allele
• masks other alleles
– recessive allele
• no noticeable effect when
inherited with the dominant
allele
I’ll speak for
both of us!
homologous
chromosomes
Punnett squares
male
1st
Pp x Pp
female
phenotype & genotype
can have different
ratios
generation
(hybrids)
%
genotype
male / sperm
female / eggs
P
p
PP
25%
75%
Pp
P
p
PP
Pp
Pp
pp
%
phenotype
50%
Pp
pp
25% 25%
1:2:1
3:1
Mendel's hypotheses
• There are alternate versions of genes (alleles) that result in
variation in heritable characteristics.
• For each character, an organism inherits two alleles (one from
each parent).
– Homozygous
– Heterozygous
• If the two alleles differ, one is dominant over the other
(determines the organism's appearance). The other has no
effect and is called recessive.
– Complete dominance
• A sperm or egg carries only one allele because alleles
segregate (separate) from each other during gamete production
– Law of Segregation
– What phase of meiosis does this occur during?
PRACTICE!
Monohybrid crosses
Allele key
Genotype key
Packet p. 2-3
Vocab Check
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P, F1, F2 generations
dominant allele
recessive allele
genotype
phenotype
homozygous
heterozygous
complete dominance
Genotype mystery
• Sir Snowy Penguini Shackleton is a three-time “Best in Show”
winner at the Annual Antarctic International Penguin Show
• His owner would like to capitalize on this success and breed Sir
Snowy to make more orange-footed penguins.
• The problem is that he doesn’t know Sir Snowy’s genotype. In show
penguins, the dominant allele for foot color, O, results in orange feet,
but the recessive allele, o, codes for green feet, which disqualifies a
penguin from shows.
• Sir Snowy will earn lots more money if his owner can prove that Sir
Snowy’s offspring will have only orange feet.
• Questions:
– What genotype does Sir Snowy have to be to guarantee all
orange-footed offspring?
– How can his owner prove that Sir Snowy is that genotype?
Testcross example
Sir Snowy's genotype could be OO or Oo. To know for sure, Sir
Snowy will be bred with a female that has green extremities.
O?
oo
X
If the offspring all have
orange feet, what genotype
is Sir Snowy?
o
o
O
Oo
Oo
O
Oo
Oo
Gametes will include only a little o allele since
the female is homozygous recessive.
What if some offspring have
o
o
green feet?
100%
orange
O
o
Oo
Oo
oo
oo
50%
orange
50%
green
Testcross: genotype detective tool
• Crossing an individual of unknown genotype
with a homozygous recessive individual to
determine the genotype.
Organisms have more than 1 trait!
• After monohybrid crosses, Mendel looked at seed shape
and color together.
• Found that they were inherited separately (not
dependent on each other).
• This led to the Law of Independent Assortment.
Law of Independent Assortment
RRYY x rryy
Possible
gametes:
RY and ry
RrYy x RrYy
Possible
gametes:
RY
rY
Ry
ry
PRACTICE!
Dihybrid crosses
Using probability to solve genetic problems
• Rule of multiplication: probability
of 2 or more events both
happening.
– Each heterozygous parent will
have half her/his gametes with B
and half with b.
– The probability of B from the
mother combining with B from the
father is ½ x ½ = ¼
– Multiply individual probabilities
– What is the probability that the
couple will have a girl with the
genotype bb?
– ½ x ¼ = 1/8
Using probability to solve genetic problems
• Rule of addition: probability of an
event happening via two different
ways.
– What is the probability that an
offspring will be heterozygous if
both of the parents are
heterozygous?
– Two combos of gametes will yield
a heterozygote.
– ¼+¼=½
• You will probably use this less
frequently than the Rule of
Multiplication (mostly for working
with two heterozygous parents as
in the above example).
Practice!
Complete the problems on page 6 in your packet.
Practice
On scrap paper
In dragons, the allele for fire breathing (F) is dominant to the allele for
non-fire breathing (f) and the allele for wings (W) is dominant to the
allele for a wingless body (w).
1. If a homozygous dominant dragon mates with a homozygous
recessive dragon, what will the genotype(s) and the phenotype(s)
of the offspring be? FfWw
2. If two of the offspring from the F1 generation are crossed, what are
the potential combinations of alleles in the gametes? FW, Fw, fW, fw
3. Make a Punnett square to show the potential genotypes of the F2
generation.
4. How many potential genotypes are there for the F2 generation? 9
5. What is the ratio of possible phenotypes for the F2 generation?
9:3:3:1 (fire & wings: no fire & wings: fire & no wings: no fire & no wings)