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# ___ Notes – Patterns of Heredity and Human Genetics (Chapter 12)
Section 12.1
Pedigree – a family tree that shows patterns of inheritance. Refer to p 309-310 to complete this key.
 male/female
 known heterozygous (carrier) male/female
 male/female
 known heterozygous (carrier) male/female
 affected male (shows the trait in his phenotype/genotype)
 affected female (shows the trait in her phenotype/genotype)

 parents (the line between them represents _________________,
the line going down represents ________________)
 siblings (both are male/female in this case)
Section 12.3
Human Genetics.
Previously, we learned about genetics in peas, guinea pigs, and other organisms. Now we’ll learn more
about human genetics. A person may be homozygous dominant, heterozygous, or homozygous
___________________ for a certain trait. Given the allele f for earlobe attachment, a homozygous dominant
individual will have the genotype
. A heterozygous individual will have the genotype
and a
homozygous recessive individual will have the genotype
.
While genotype will influence phenotype, remember that environmental factors can also influence
how/whether a gene is expressed. (See pp. 321-322.)
Remember that humans have
chromosomes in every cell. Of these,
pairs are
autosomes. An autosome is any chromosome except the sex chromosomes. Each person has one pair of sex
chromosomes. Females have 2 X chromosomes while males have
.
Autosomal heredity just means that the allele for the trait being discussed is located on an autosome. If a
trait is autosomal
, this means that an individual will only need one dominant
allele for the trait to be expressed in its phenotype. If a trait is autosomal
, a person
must have both recessive alleles before this trait is expressed. This is exactly the same as the genetic rules we
were following in Chapter 10.
A trait is considered sex-linked if its allele is located on a sex chromosome (usually the
chromosome). We’ll learn more about this later.
Punnett Problems (Part 1):
1. The allele for thick lips is dominant to the allele for thin lips. If a woman with thick lips marries a man with
thin lips, how likely are they to have a child with thin lips?
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2.
Two people with thin lips get married. How likely are they to have a child with thick lips? _____
3. Polydactyly is an autosomal dominant disorder. A woman who is heterozygous for polydactyly marries a
man without polydactyly.
a. What is the woman’s phenotype?
b. What is the man’s phenotype?
c. How likely are they to have a child with polydactyly?
Section 12.2
1. complete dominance – what we learned earlier in Chapter 10. If one dominant allele is present in the
genotype, it shows in the
2. incomplete dominance –
-
example – flower color in snapdragon plants (3 phenotypes: red, white and pink); pink is the phenotype of
heterozygote
-
heterozygous phenotype shows
3. codominance -
examples -
-
heterozygous phenotype shows
4. polygenic inheritance – a trait that is controlled by the inheritance of
-
in these cases, remember that a capital letter does not represent a dominant allele. All heterozygotes
are intermediate in phenotype. What does this mean? (Read page 320 for a more thorough
explanation.)
-
examples -
5. multiple phenotypes from multiple alleles – more than one allele contributes to the phenotypes of these traits
-
examples are
color in pigeons (see Figure 12.9), HLA (human leukocyte antigen)
system that recognizes foreign tissue in humans – because many alleles contribute to the phenotypes,
there are many possible phenotypes (30 million for the HLA system!). Keep on reading for another
example in humans.
Traits Controlled by Multiple Alleles (pp. 324--325)
You are probably used to seeing just two options for alleles, the dominant and the recessive (capital letter
and small letter), right? However, some traits are determined by more than two alleles. One classic example is
blood type. Blood type has
different alleles, often written as
,
and
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, which results in 4 different phenotypes (from 6 genotypes). Note: Blood type is not just multipleallelic. The A and B alleles are codominant and the O allele is recessive. (What does this mean?)
Blood
Type
Possible genotypes
Can accept what
blood type?
Can donate to
what blood type?
A
B
AB
O
Is it possible for a type A mother and a type B father have a type O child? Explain your answer.
Can a type AB mother and a type B father have a type O child? Explain your answer.
Problem-Solving Lab 12.2: Read the introduction on page 318 for another example of multiple alleles.
Coat Color in Rabbits
Phenotype
Dark grey coat C
Chinchilla
Himalayan
White
Allele
C
cch
ch
c
Pattern of Inheritance
Dominant to all other alleles
Dominant to Himalayan and to white
Dominant to white
Recessive
1. Complete the chart below.
Phenotype
All Possible Genotypes
Dark grey-coated rabbit 4 possible:
Chinchilla
3 possible:
Himalayan
2 possible:
White
1 possible:
2. Can a white rabbit and a chinchilla rabbit have white offspring? Explain your answer.
3. Can a Himalayan rabbit and a white rabbit have chinchilla offspring? Explain your answer.
X-Linked Inheritance
A trait is considered sex-linked if its allele is located on a sex chromosome (usually the
chromosome). The sex chromosomes contain different genes. Genes found on the X chromosome are called
-l
genes. Genes on the Y chromosome are called -l
g
. Sex-linked
traits are not autosomal. (Of course not. Why?) Examples of human traits/conditions that are sex-linked are
and
.
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Punnett Problems (Part 2): Write the allele as a subscript on the sex chromosome.
1. Hemophilia is an X-linked recessive trait. If a man with hemophilia marries a carrier, how likely (%
probability) are they to have a son with hemophilia?
Phenotype of mother: not affected with hemophilia (because the allele is recessive)
Genotype of mother: XHXh
Phenotype of father: affected (and NOT homozygous recessive, because only the X chromosome
carries the allele)
Genotype of father: XhY
Likelihood of having an affected son?
2. Colorblindness is also an X-linked recessive trait. Can a colorblind man who marries a non-colorblind
woman have a son who is colorblind? Why/Why not?
3. Can a colorblind man have a colorblind daughter? Why/Why not?
Common genetic conditions:
Name of
condition
What is the defect (at the cellular
or molecular level)
Tay-Sachs
disease
Missing _________________ that
breaks down _____________ in the
central nervous system
Cystic fibrosis
A defective _________________ in
cell membranes cannot remove
_______________ in the digestive
tract and lungs
Sickle-cell
anemia
_________________________ are
shaped like crescents - can slow
blood flow, block _______________
______________, and damage
tissues; causes pain
Huntington’s
disease
Parts of the ______________ break
down
Phenylketonuria
Can’t change the amino acid ______
______________ to ____________,
which damages the central nervous
system
What is the phenotype?
Male-pattern
baldness
What is this condition’s
pattern of inheritance?
Is it more common in
some ethnic groups? If
so, which one(s)?
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