14–1
Human
Heredity
14-1 Human Heredity
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Review
• What is a chromosome?
• What is one method
geneticists use to predict
the possible genotypes of
the offspring from a
specific cross?
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OBJECTIVES
• Students describe how sex is
determined.
• Students know how to determine
the genotypes of individuals using
a pedigree diagram showing
phenotypes.
• Students know how to predict the
probable mode of inheritance from
a pedigree diagram showing
phenotypes.
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14–1 Human Heredity
Human Chromosomes
Human Chromosomes
Cell biologists analyze chromosomes by looking at
karyotypes.
Cells are photographed during mitosis. Scientists
then cut out the chromosomes from the
photographs and group them together in pairs.
A karyotype is a picture of chromosomes matched
in pairs.
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14–1 Human Heredity
Human Chromosomes
Human Karyotype
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14–1 Human Heredity
Human Chromosomes
Two of the 46 human chromosomes are known as
sex chromosomes, because they determine an
individual's sex.
• Females have two copies of an X chromosome.
• Males have one X chromosome and one Y
chromosome.
The remaining 44 chromosomes are known as
autosomal chromosomes, or autosomes.
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14–1 Human Heredity
Human Chromosomes
How is sex determined?
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14–1 Human Heredity
Human Chromosomes
All human egg cells carry a single X
chromosome (23,X).
Half of all sperm cells carry an X
chromosome (23,X) and half carry a Y
chromosome (23,Y).
About half of the zygotes will be 46,XX
(female) and half will be 46,XY (male).
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14–1 Human Heredity
Human Chromosomes
Males and females are
born in a roughly 50 : 50
ratio because of the
way in which sex
chromosomes
segregate during
meiosis.
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14–1 Human Heredity
Human Traits
Human Traits
In order to apply Mendelian genetics to humans,
biologists must identify an inherited trait controlled
by a single gene.
They must establish that the trait is inherited and
not the result of environmental influences.
They have to study how the trait is passed from
one generation to the next.
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14–1 Human Heredity
Human Traits
Pedigree Charts
A pedigree chart shows the relationships within a
family.
Genetic counselors analyze pedigree charts to
infer the genotypes of family members.
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14–1 Human Heredity
Human Traits
A circle
A horizontal line represents
A shaded
circle or
square
connecting
a male
and
a female.
indicates
thatrepresents
a person a
a female
expresses
the trait.
marriage.
A square
A vertical line and a
represents
a male. bracket connect the
parents to their
children.
A circle or square
that is not
shaded indicates
that a person
does not express
the trait.
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14–1 Human Heredity
Human Traits
Genes and the Environment
Some obvious human traits are almost impossible
to associate with single genes.
Traits, such as the shape of your eyes or ears, are
polygenic, meaning they are controlled by many
genes.
Many of your personal traits are only partly
governed by genetics.
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14–1 Human Heredity
Human Genes
Human Genes
The human genome includes tens of thousands of
genes.
In 2003, the DNA sequence of the human genome
was published.
In a few cases, biologists were able to identify
genes that directly control a single human trait
such as blood type.
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14–1 Human Heredity
Mode of Inheritance
• Recessive: disorders caused by having 2
recessive alleles
• Dominant: disorders caused by having
just 1 dominant allele
• Sex Linked: disorders carried on one of
the sex chromosomes (either X or Y)
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14–1 Human Heredity
Human Genes
Recessive Alleles
The presence of a normal, functioning gene is
revealed only when an abnormal or nonfunctioning
allele affects the phenotype.
Many disorders are caused by autosomal
recessive alleles.
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14–1 Human Heredity
Human Genes
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14–1
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Continue to:
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14–1
A chromosome that is not a sex chromosome is
know as a(an)
a. autosome.
b. karyotype.
c. pedigree.
d. chromatid.
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14–1
Whether a human will be a male or a female is
determined by which
a. sex chromosome is in the egg cell.
b. autosomes are in the egg cell.
c. sex chromosome is in the sperm cell.
d. autosomes are in the sperm cell.
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14–1
Mendelian inheritance in humans is typically
studied by
a. making inferences from family pedigrees.
b. carrying out carefully controlled crosses.
c. observing the phenotypes of individual
humans.
d. observing inheritance patterns in other
animals.
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