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Biology // CH 14: Human Genetics
14.1 Human Heredity
Each human chromosome consists of a single DNA molecule. Humans have 23 pairs of
chromosomes, for a total of __________.
Chromosomes are numbered 1 through 23. Chromosome pairs 1 through 22 are called
___________________________. Human males and females share these chromosomes, and the
genes they contain, in common.
Chromosome pair number 23 are the sex chromosomes. Males have an __________, females
have ____________.
A _____________________ is a picture of all the chromosomes in the nucleus paired up and in
order from 1 to 23. It can be used to diagnose a genetic disorder.
Mendel’s laws and other laws of inheritance all apply to humans. However, since researchers
cannot control human matings, they must analyze patterns of inheritance within families using
pedigrees.
A pedigree is a family tree that records and traces a ______________ trait through several
generations. The traits examined in human pedigrees are generally diseases or disorders caused
by either recessive or dominant alleles. Ordinary non-disease traits such as earlobe shape, cleft
chins, widow’s peak, and others can also be traced to illustrate how alleles get inherited.
Symbols:
male
parents with children
female
parents
female
male showing trait
male carrier (of a recessive trait)
female showing trait
female carrier (of a recessive trait)
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Types of human inheritance:
Autosomal traits are those controlled by genes located on the autosomes. Autosomal dominant
traits are inherited by receiving two dominant alleles or one dominant and one recessive allele.
Autosomal recessive traits are inherited by receiving two recessive alleles.
Sex-linked traits are mainly inherited on the X chromosomes. Although sex-linked traits can be
dominant, the sex-linked disorders discussed in class are sex-linked recessive.
Autosomal
recessive trait
Genes on which
chromosomes?
Chromosomes # 1 -22
Genotype to show trait
(use A, a)
Who can inherit the
trait?
How do they inherit it?
Autosomal
dominant trait
X chromosome
AA, Aa
Males and females
equally
Sex-linked trait
(recessive on X
chromosome)
XaXa, XaY
Males and females
equally
Males inherit
recessive allele from
mother only; females
inherit a recessive
allele from both
parents
Recessive allele from
each parent
Are there carriers? Who
can be a carrier?
Yes, males or females
No carriers possible
since expression of
trait is dominant
Can males pass allele for
trait to sons? daughters?
Can pass to both
Can pass to both
Can pass to daughters,
but not to sons
Can females pass allele
for trait to sons?
daughters?
Can pass to both
Can pass to both
Can pass to both
Examples
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Example 1: Autosomal inheritance. The pedigree below shows the inheritance of tongue rolling in
a family. The individuals shown by the shaded symbols cannot roll their tongues. The half-shaded
symbols are carriers. Carriers have inherited one allele for a trait but do not show it. They can
pass the trait to their offspring, as seen in this pedigree.
I
II
III
Is no tongue rolling dominant or recessive? What are the genotypes for each family member?
If the married couple in the second generation have another child, how probable is it that child
could not roll its tongue? What chance is there the child could roll its tongue but be a carrier for no
tongue rolling? (Hint: put the parents in a Punnett square and solve.)
Example 2. The pedigree below shows another type of autosomal inheritance. The shaded
symbols show persons with Huntington’s disease, a serious neurological disorder.
Is Huntington’s disease a dominant or recessive trait? Explain.
Write in the genotypes for each family member on the chart.
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Example 3: The pedigree below is for a recessive sex-linked trait such as hemophilia, colorblindness, or Duchenne muscular dystrophy. Determine the carriers and label all the genotypes.
If couple C D have another child what chance is there that the child will have hemophilia?
If the child is male, what is the chance he will have hemophilia?
If the child is female, what is the chance she will have hemophilia?
Chromosomal Disorders:
Chromosomal disorders lack an inheritance pattern because they are caused by “mistakes” that
occur during meiosis. Sometimes a pair of homologous chromosomes can fail to separate
resulting in gametes with the wrong number of chromosomes. This is called nondisjunction.
Examples of
Chromosomal
Disorders:
What causes the
inheritance of the
disorder?
Down Syndrome
Turner Syndrome
Kleinfelter’s Syndrome
Blood Typing: (Study the chart!!)
What blood types are possible if mom has A+ blood and dad has B+ blood?
If mom has B- and the baby has A+ blood, what are the possible blood types for dad?
What blood types can be given to a man in need of a blood transfusion if he has B- blood?
A person with O+ blood can donate blood to which blood types?
If blood clumps in Anti-A serum but not in Anti-B serum or Anti-Rh serum, what is the blood type?
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