High School
SAT
ACT
University
MCAT
LSAT
DAT
Wize University Biology Textbook > Mendelian Genetics

Inheritance of Autosomal Traits

Intro to Pedigree AnalysisPrevious Section
Inheritance of X-Linked TraitsNext Section
Popular Courses
Find My Course
0:00 / 0:00

Inheritance of Autosomal Traits

Autosomal traits are ones that are not linked to the sex chromosomes. X-linked traits are ones linked to the X sex chromosome.

Autosomal Dominant

How to tell if a trait is dominant:
  • It is prevalent within the pedigree (i.e. more than half affected). Both males and females express the allele and can transmit it equally to sons and daughters.
  • The trait is present in almost all generations. Direct transmission through three generations is practically diagnostic of a dominant.
  • In affected families, the ratio of affected to unaffected children is almost always 1:1.
  • There are no carriers. If both parents are unaffected, all the children are unaffected.
  • That is, no two unaffected parents (white circle and square) give rise to an affected child (dark symbol).

Photo by Madibc68 / CC BY

PAGE BREAK

Autosomal Recessive

How to tell if a trait is recessive:
  • Carriers have the recessive allele but don't have the actual trait.
  • The trait is not very prevalent in the family (i.e. not many have the trait or disease, not present in all generations).
  • Two unaffected parents have a child that is affected by the trait or disease, usually in the ratio of one affected to three unaffected.
  • When both parents are affected all the children are affected.

Photo by Madibc68 / CC BY

Example: Autosomal Recessive

A patient is suspected of having an inherited disorder. Pedigree analysis shows the pattern below, which of the choices is most compatible with the patient's condition?
A) Autosomal Dominant Disease
B) Autosomal Recessive Disease
C) X-linked Dominant Disease
D) X-linked Recessive Disease
E) Mitochondrial disease


Both males and females are affected, and each of the affected individuals on this pedigree has inherited the disorder from asymptomatic parents, consistent with an autosomal recessive inheritance pattern. Therefore, the answer is b.

Example: Autosomal Dominant

In Marfan Syndrome, patients are usually tall, with long, thin limbs and fingers and dislocated eye with several other conditions. Below is a pedigree chart of a family in which Marfan syndrome is found.
Photo by Jerome Walker / CC BY

Is this condition X-linked, autosomal, dominant or recessive? Why?
Autosomal dominant.
Looks like both males and females are equally affected, it is present in every generation, and an affected parent passes it on to 1/2 of its offspring. Also no children of unaffected parents have the disease.


What is the ratio of affected to unaffected children if one heterozygous (affected) parent mates with a normal (unaffected) parent? Use B for the dominant allele, and b for the recessive.
Unrelated marriage partners therefore usually recessive homozygotes (bb) and the mating can be represented:
Bb x bb =>
The possibilities for children are: 1 Bb, 1 Bb, 1 bb, 1 bb =>
Ratio of affected:unaffected is 1: 1

What is the ratio of affected to unaffected children if one heterozygous (affected) parent mates with another heterozygous (affected) parent?
Matings between heterozygotes are the only kind that can produce homozygous offspring (remember Mendelian laws):
Bb x Bb =>
The possibilities for children are: 1 BB, 1 Bb, 1 Bb, 1 bb =>
The ratio is 1:2:1 (or 3 affected: 1 unaffected).

Extra Practice
A young couple has undergone a successful in vitro fertilization procedure. The father has cystic fibrosis and the mother has a sister with cystic fibrosis. Both of these affected individuals have ~F508 mutations. Before making the decision to conceive, the couple underwent extensive genetic counseling regarding the potential risks of having a child with cystic fibrosis. However, the mother refused prenatal screening for CF as it would not affect the decision to raise their child. The family pedigree is diagrammed below with the unborn child
marked by the red arrow.
What is the chance that this child will have cystic fibrosis?
Inheritance of traits practice problem 3
In cats, big ears are dominant over short ears, and the gene is autosomal. Another gene controls eye colour, and is X-linked. "E" gives brown eyes, "e" gives blue eyes, and Ee gives green eyes.
Brown eyed male with the homozygous genotype for big ears is mated with a female with short ears and green eyes. What kittens could be produced?
Note: X-linked traits are on the X chromosome. Females have two X chromosomes while males have one X chromosome and one Y chromosome.
Inheritance of traits practice problem 3
In cats, big ears are dominant over short ears, and the gene is autosomal. Another gene controls eye colour, and is X-linked. "E" gives brown eyes, "e" gives blue eyes, and Ee gives green eyes.
Brown eyed male with the homozygous genotype for big ears is mated with a female with short ears and green eyes. What kittens could be produced?
Note: X-linked traits are on the X chromosome. Females have two X chromosomes while males have one X chromosome and one Y chromosome.
Identify if pedigree is autosomal dominant, autosomal recessive, or X-linked:
Identify if pedigree is autosomal dominant, autosomal recessive, or X-linked:
Identify if pedigree is autosomal dominant, autosomal recessive, or X-linked:
Identify if pedigree is autosomal dominant, autosomal recessive, or X-linked:
Pedigree practice problem 2
Identify if pedigree is autosomal dominant, autosomal recessive, or X-linked:
Pedigree practice problem
Identify if pedigree is autosomal dominant, autosomal recessive, or X-linked:
Inheritance of traits
In cats, big ears are dominant over short ears, and the gene is autosomal. Another gene controls eye colour, and is X-linked. "E" gives brown eyes, "e" gives blue eyes, and Ee gives green eyes.
Brown eyed male with the homozygous genotype for big ears is mated with a female with short ears and green eyes. What kittens could be produced?
Note: X-linked traits are on the X chromosome. Females have two X chromosomes while males have one X chromosome and one Y chromosome.
Why has human evolution not eradicated the sickle cell hemoglobin allele?
Individuals with Down’s syndrome have a trisomy of chromosome 21 that arose from a single error in meiosis. Imagine a child, Bella, who has parents with normal karyotypes. Only 1 of Bella’s chromosomes carries the same SNP as is found in her father. Which of the follow stages of meiosis could this error have occurred?
Meiosis I in the mother
Meiosis I in the father
A cystic-fibrosis mutation in a certain pedigree is due to a single nucleotide-pair (SNP) change. This change destroys an EcoRI restriction site normally found in this position.
a) How would you use this information in counseling members of this family about their likelihood of being carriers? State the precise experiments needed.
b) Assume that you find that a woman in this family is a carrier, and it transpires that she is married to an unrelated man who also is a heterozygote for cystic fibrosis, but, in his case, it is a different mutation in the same gene. How would you counsel this couple about the risks of a child’s having cystic fibrosis?
Based on the pedigree below, what type of trait is this?
You are examining a family in which both parents are known to be carriers for sickle cell anemia, which is a rare autosomal recessive disease with 2% of the population carrying the mutant allele. Their first child was born with the disease, and the parents have come to you to assess whether the new fetus is also diseased, is a carrier, or is homozygous normal. DNA samples from each family member and the fetus are tested by PCR and gel electrophoresis. The following results are obtained:
What is the probability that the child who will develop from this fetus will exhibit the disease?
When this child grows up and mates, what is the probability that any one of her children will be afflicted with the disease?
A young couple has undergone a successful in vitro fertilization procedure. The father has cystic fibrosis and the mother has a sister with cystic fibrosis. Both of these affected individuals have ~F508 mutations. Before making the decision to conceive, the couple underwent extensive genetic counseling regarding the potential risks of having a child with cystic fibrosis. However, the mother refused prenatal screening for CF as it would not affect the decision to raise their child. The family pedigree is diagrammed below with the unborn child
marked by the red arrow.
What is the chance that this child will have cystic fibrosis?
Practice: Pedigree
Identify if pedigree is autosomal dominant, autosomal recessive, or X-linked:
Practice: AD, AR or X?
Identify if pedigree is autosomal dominant, autosomal recessive, or X-linked: