Download Human inheritance

4 Human inheritance
(2015)
Human inheritance
Lesson overview
In this lesson we will describe the inheritance of traits
due to dominant and recessive genes, located on
autosomes and sex-chromosomes.
We will learn how to determine the mode of inheritance
of a given genetic disorder using pedigree analysis.
Human inheritance
Mendelian Inheritance in Humans
Some human traits have simple inheritance
patterns like the traits that Gregor Mendel
studied in pea plants. Other human traits have
more complex inheritance patterns.
How Mendelian traits are inherited depends on
whether the traits are controlled by genes on
autosomes or the X and Y chromosomes.
Human inheritance
Human chromosomes
22 pairs of autosomes
2 sex chromosomes
Human inheritance
Genetic determination of sex
Mammals: female has
two X chromosomes,
male has one X and
one Y.
Birds: female has one Z
and one W, male has
two Z chromosomes.
Human inheritance
Autosomic traits
Capacity to roll the edges of the tongue
Human inheritance
Autosomic traits
Human inheritance
X-linked traits
Color blindness (Daltonism) is a x-linked recessive trait
Human inheritance
Genetic disorders
Many genetic disorders are caused by mutations in one or a
few genes. Some of the disorders are caused by
mutations in autosomal genes, others by mutations in Xlinked genes. In both cases, the allele responsible for the
disease can be either recessive or dominant.
Sue and John
Sue and John are planning to start a family. They visit a genetic counselor
seeking advice about a genetic disease that both John’s and Sue’s families
are suffering from. There is no genetic test for this trait. Its onset, which can be
gradual, occurs after age 40. Both John and Sue are in their early 30’s.
They would like to determine the chances of their children inheriting the
disease.
1. What are the possible genotypes of John and Sue?
2. If you are John and Sue’s genetic counselor, how
would you explain the chances of their children
inheriting the disease?
Human inheritance
In a pedigree analysis, information about family members is summarized
in a special kind of diagram. Each individual is represented by a symbol, a
square for a male and a circle for a female. Individuals who have the
trait of interest are represented by black squares or circles.
Human inheritance
Pedigree analysis and inheritance (part 1)
Huntington desease
Woody Guthrie
Pattern of autosomal dominant trait
•Males and females are equally likely to have the trait.
•There is male to male transmission.
•Traits do not skip generations (generally). If the trait is displayed in
offspring, at least one parent must show the trait.
•If parents don't have the trait, their children should not have the trait
(except for situations of gene amplification).
•The trait is present whenever the corresponding gene is present
(generally). If both parents possess the trait, but it is absent in any of
their offspring, then the parents are both heterozygous ("carriers") of
the recessive allele.
•Homozygotes for the dominant condition have a more severe form of
the condition.
Human inheritance
Pedigree analysis and inheritance (part 2)
Albinism
Pattern of autosomal recessive trait
•Males and females have the same chance of expressing
the trait
•You can only express the trait if you are homozygous
•If both parents express the trait, then all their offspring
should also express the trait
•If the offspring express the trait but their parents don't,
then both parents are heterozygous
•If one parent expresses the trait, then their offspring who
don't express it are heterozygous
•The trait can skip generations
Human inheritance
Hemophilia is a x-linked recessive trait
Pattern of x-linked recessive trait
•The trait is more common in males than in females.
•If a mother has the trait, all of her sons should also
have it.
•There is no male to male transmission.
•It has the same inheritance patterns as autosomal
recessive for human females.
•The son of a female carrier has a 50 percent chance of
having the trait.
•Mothers of males who have the trait are either
heterozygous carriers or homozygous and express the
trait.
Sue pedigree
John pedigree
1. What are the possible genotypes of John and Sue?
2. If you are John and Sue’s genetic counselor, how
would you explain the chances of their children
inheriting the disease?