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Grade 11 University Biology - Unit 2 Genetics
Beyond Mendel
Chapter 6.1 – Pages 242-250
Genetic inheritance is very complicated….more so than Mendel imagined or realized. The patterns of
inheritance can be unique, result in phenotypes that are seemingly an equal mix of dominant and
recessive, or any combination between.
Recall, Mendel observed traits must be passed on in discrete heredity units. Moreover, Mendel
hypothesized that a trait can still be passed on but not be expressed. The trait expressed in all F1
generations is called the “dominant factor”. The hidden trait that is expressed in the F 2 generation is the
“recessive factor”. Finally, Mendel observed a 3:1 ratio of expression in the F2 generation. Thus,
Mendel’s law of segregation stated (1) an organism carries two genes – one from each parent and (2)
parents donate only one copy of each gene in their gametes (i.e., during meiosis, two copies of each
gene separate – or segregate).
Now recall, heterozygous describes an individual carrying two different alleles for a given trait. What
does this mean? Let’s assume the gene for plant height results in tall or short plants. If the allele for tall
is dominant, it is written as T. The allele for short is recessive, and it is represented as t. The phenotype
for a homozygous dominant plant is TT. A homozygous recessive plant is tt. The heterozygous
phenotype is Tt. Any plant with at least one dominant T allele is tall (i.e., TT or Tt). Only plants with tt are
short.
In heterozygous individuals, what allele is expressed? A dominant allele expresses the trait whenever
the allele is present. A recessive allele is expressed only when both alleles are of the recessive form.
Using the example, the heterozygous plant only expresses the dominant T allele (i.e., the plant is tall).
In some cases, a cross between two organisms with two different phenotypes produced offspring with a
third genotype.
Mendel’s work explained why traits are not blended. Yet, blended inheritance is common.
Recall – An allele is a form of a gene which codes for one possible outcome of a phenotype.
Incomplete Dominance
 A condition in which neither allele for a gene completely conceals the presence of the other, and
it results in an intermediate expression of a trait. In other words, the phenotype is somewhere
between a dominant
phenotype and a
recessive phenotype.
 In this inheritance,
neither allele is
dominant. In a common
inheritance, lower-case
letters would be used to
represent a recessive
allele. Thus, the alleles
are represented by
“superscript” lettering
(e.g., Colour is C and red
is R written as CR).
 See Figure 6.1 on Page
242.
 The example is snapdragon plants. The gene that controls colour is C. The allele for red is C R
while the allele for white is CW. A homozygous CRCR will produce a red flower. A homozygous
CWCW will make a white flower. The heterozygous CRCW will produce a pink flower (....a blend of
red and white). Recall, neither allele CR and CW is dominant over the other allele (i.e., red cannot
mask white and white cannot mask red). The blending of phenotype expression is Incomplete
Dominance (see Punnett square below).

CR
CW
CR
CRCR or red flower
CRCW or pink flower
CW
CRCW or pink flower
CWCW or white flower
NOTE: The Punnett square predicts all three phenotypes in a ratio of 1:2:1
Codominance
 A condition in which both alleles for a trait are expressed equally in a heterozygote (i.e., both
alleles are dominant). The expression produces offspring with a third phenotype...a MIXED
phenotype.
 Heterozygous Advantage is a survival benefit for individuals who inherit two different alleles for
the same trait
 In human blood, blood types are determined by alleles that are codominant (I A and IB) and the
allele i which is recessive to IA and IB. A mother has type AB blood (genotype IAIB) and a father
has type B blood (IBi). Using a Punnett square, how many offspring will have type O blood (ii)?
The cross is IAIB X IBi.


Parental Gametes
IA
IB
IB
IAIB
IBIB
i
IAi
IBi
None of the offspring have genotype ii. Thus, no offspring are type O blood.
The codominance is type AB blood. In codominance, both alleles are expressed equally in the
heterozygote.
NOTE: The Punnett square predicts a phenotype ratio of 1:2:1
Multiple Alleles
 What if a gene has more than two alleles? Traits can be the result of the interactions of MORE
THAN TWO ALLELES for one gene. This is called Multiple Alleles.
 If there are four or more possible phenotypes for a particular trait, then more than two alleles for
that trait must exist in the population. There may be multiple alleles within the population, but
individuals have only two of those alleles. Why? Individuals have only two biological parents.
Half the gene (alleles) are inherited from the female and the other half from the male. As such,
there are two alleles for every trait in the phenotype.
 Human blood type is an example. Blood types are controlled by three alleles: A, B, O
(represented as i). A and B are co-dominants and O is recessive to both A and B. Moreover,
there are four possible phenotypes.
 Type A blood can have genotypes IAIA and IAi
 Type B blood can have genotypes IBIB and IBi
 Type AB (universal recipient) is genotype IAIB
 Type O (universal donor) genotype ii

Try to answer the following questions. HINT: Use a Punnett square.
1. A woman homozygous for blood type B marries a man that is heterozygous for blood type A.
State the possible phenotypic ratios of the offspring.
Solution #1
The genotype cross is IBIB X IAi. The gametes are IB and i, and gametes for LA and i
The resulting phenotype ratio: 50% Type AB blood and 50% Type B blood. The resulting
genotype ratio is 50% IAIB and 50% IBi
2. A man with blood type O marries a woman with blood type AB. State the possible phenotypic
ratios of the offspring.
3. A type B woman whose mother was type O marries a type O man. What are the possible
phenotypic ratios of their offspring?
4. A type A woman whose father was type B marries a type B man whose mother was was type
A. What are the possible phenotypes of their offspring?
5. What is the probability that a couple whose blood types are AB and O will have a type A
child?
The same questions BUT not a word questions....complete Punnett square for each scenario and
determine from your answer, the percent of each blood type.
1. Father is Type O and Mother is Type O
_____________% O
_____________% A
_____________% B
_____________% AB
2. Father is Type A, homozygous and Mother is homozygous Type B
_____________% O
_____________% A
_____________% B
_____________% AB
3. Father is heterozygous Type A and Mother is heterozygous Type B
_____________% O
_____________% A
_____________% B
_____________% AB
4. Father is Type O and Mother is Type AB
_____________% O
_____________% A
_____________% B
_____________% AB
5. Father and Mother are both Type AB
_____________% O
_____________% A
_____________% B
_____________% AB
Environmental Effects on Complex Patterns of Inheritance
 Environmental conditions can impact the expression of traits.
 The phenotype of an individual is not only the result of inheriting a particular set of parental
genes. The specific environmental characteristics of the uterus in which a fertilized egg is
implanted and the health of the mother can have major impacts on the phenotype of the future
child. For instance, oxygen deprivation, alcohol exposure or inappropriate hormone levels can
cause lifelong, devastating effects.
 Likewise, accidents, poor nutrition, drinking and drug use, and other environmental influences
throughout life can alter an individual's phenotype.
 In some cold-blooded vertebrates, some fishes, many reptiles (e.g. certain snakes, lizards,
turtles, and all crocodiles and alligators) as well as in some invertebrates (e.g. certain
crustaceans), sex is determined after fertilization — not by sex chromosomes deposited in the
egg. The choice is usually determined by the temperature at which early embryonic
development takes place. In some cases (e.g. many turtles and lizards), a higher temperature
during incubation produces more females, while in other cases (e.g., alligators), a higher
temperature favours the production of males.
Polygenic Inheritance
 Continuous Variation is a range of variation in one trait resulting from the activity of many genes
 Polygenic Trait is a trait that is controlled by more than one gene. A group of genes that all
contribute to the same trait is called a polygene. Each dominant alleles contribute to the trait.
The recessive alleles do not contribute to the trait. The MORE dominant alleles the organism
inherits, the more apparent the trait.

See Figure 6.9 on Page 248
Questions
 Practice Problems 1-10, Page 247
 Activity 6.1 “Identifying a Polygenic Trait” Page 248
 Quirks and Quarks “Selecting for Genetic Defects” Page 249, Questions 1-2
 Review Questions 1, 3-8, 11 Page 250


Additional Practice at http://www.hobart.k12.in.us/jkousen/Biology/multalle.htm
Assignment – Who gets the money?
Mr. and Mrs. Jones died in a tragic farm accident. The tractor they were riding on rolled over in a ditch.
During the investigation into the deaths, authorities found $15 million hidden in the feed bin of the chicken
coop. The couple is known to have a son from whom they are estranged. Many years prior, the son left
the farm and moved to the city. He has not been heard from in many years. The son is the only heir to
the money. Five men show up, and each man claims to be the long lost son. As a genetic expert, you
must determine which man is the rightful heir.
Phenotypes are provided in Table 1.
Note the following information
 Brown eyes are dominant over blue eyes
 Free ear lobes are dominant over attached ear lobes
 Hair texture is Incomplete Dominance (HCHC is curly hair, HSHS is straight, HSHC is wavy)
 A widow’s peak (PP or Pp) is dominant over a straight hair line (pp)
Table 1. Phenotypes for the characters of the story
Person
Phenotype
Mr. Jones
Colour-blind
Heterozygous for
free ear lobes
Wavy Hair
Heterozygous
for widow’s peak
Homozygous
for brown eyes
Mrs. Jones
Normal vision
(homozygous)
Heterozygous for
free ear lobes
Straight Hair
Straight hair line
Heterozygous
for brown eyes
Carl
Colour blind
Attached ear
lobes
Curly Hair
Straight hair line
Blue eyes
Ray
Normal vision
Homozygous for
free ear lobes
Wavy Hair
Homozygous
widow’s peak
Blue eyes
David
Normal vision
Heterozygous for
free ear lobes
Straight Hair
Straight hair line
Homozygous
for brown eyes
Earl
Colour-blind
Heterozygous for
free ear lobes
Wavy Hair
Straight hair line
Heterozygous
for brown eyes
Peter
Normal vision
Homozygous for
free ear lobes
Curly Hair
Heterozygous
for widow’s peak
Brown eyes
Questions
1. In Table 2, write each person’s genotype
2. On a next page of paper, cross Mr. and Mrs. Jones for each trait (i.e., make a Punnett square
for each trait
3. Using the information you create, decide which man is the biological son. Explain your
decision...both the son and reasons for rejections. Be sure to include the genotypes in your
explanation
Table 2. Genotypes by Person
Person
Vision
Mr. Jones
Mrs. Jones
Carl
Ray
David
Earl
Peter
Your answers and rationale.....
Ear Lobes
Hair Texture
Widow’s Peak
or Straight
Eye Colour
Solutions – Multiple Alleles
2. A man with blood type O marries a woman with blood type AB.
ii X IAIB, gametes for i and i, gametes IA and IB
Resulting phenotype ratio: 50% A / 50% B
Resulting genotype ratio: 50% IAi / 50% IBi
3. A type B woman whose mother was type O marries a type O man.
The mother must be LBLO because her mother could only give her the LO allele.
IBi X ii, gametes for IB and i, gametes i and i
Resulting phenotype ratio: 50% B/ 50% O, resulting genotype ratio: 50% IBi / 50% LOLO
4. A type A woman whose father was type B marries a type B man whose mother was type A.
The woman must be IAi because her father could only give her the i allele (he must be IBi).
The man must be IBi because his mother could only give him the i allele (she must be IAi).
IAi X IBi, gametes for IA and i, gametes for IB and i
Resulting phenotype ratio: 25% AB / 25% B / 25% A / 25% O,
Resulting genotype ratio: 25% IAIB / 25% IBi / 25% IAi/ 25% ii
5. What is the probability that a couple whose blood types are AB and O will have a type A
child?
There is no chance of this couple having a type O child.
IAIB X ii, gametes for IA amd IB, gametes for i and i
Resulting phenotype ratio: 50% B / 50% A
Resulting genotype ratio: 50% IBi / 50% IAi