Typical Mendelian Inheritance Flashcards
What are mandelian disorders?
Single gene disorders
What can cause mandelian disorders?
- mutations of one or both members of a pair of autosomal genes
- mutations in genes on the X or Y chromosomes
Define homozygous
both alleles are identical
Define heterozygous
Alleles are different
Define trait
Gene-determined characteristic
What is the pattern if a trait is expressed in a heterozygote?
Dominant
What is the pattern if a trait is expressed in a homozygote?
recessive
What are the typical features of autosomal dominant inheritance?
- both male and females affected ~ equally
- people affected in each generation
- both males and females can pass on the trait
- unaffected individuals do not pass on the trait
What are the chances for a child of a heterozygote parent will be affected?
1 in 2 (50%)
Define non-penetrance
An individual may have the mutant gene and yet a normal phenotype. They are able to pass the condition on to descendants producing a skipped generation
What can affect penetrance?
May be age dependent (e.g. Huntington disease)
What are the chances for the offspring of two heterozygotes (carriers) for a mutant gene? (AR)
- 1/4 homozygous normal
- 1/2 heterozygous
- 1/4 homozygous normal
If an affected person has a child with a heterzygote, what are the chances the offspring are affected?
1 in 2
Describe the inheritance pattern of autosomal recessive
- usually horizontal
- both males and females affected
- may be consanguinity in the family
What are examples of AR conditions?
- cystic fibrosis
- phenylketonuria (PKU)
- spinal muscular atrophy (SMA)
- congenital adrenal hyperplasia
Define compound heterozygote
an individual with two different mutant alleles at a locus
What are the typical features of X-linked recessive inheritance?
- Males > females
- females typically carriers
- ‘Knight’s move’ pattern
What are the expected proportions of inheritance in X-linked recessive diseases
- 25% affected boy
- 25 % normal boy
- 25% carrier girl
- 25% normal girl
Who can be defined as an obligate carrier?
A woman with an affected child and an affected brother OR more than one affected child
How can females be affected by X-linked recessive diseases
- Skewed X-inactivation
- Turners disease
What are the features of X-linked dominant inheritance?
Both males and females affected
N.b - males more uniformly sever, females more variable due to X inactivation
What is the pedigree pattern of X-linked autosomal dominant inheritance?
resembles autosomal dominant but with a lack of male to male transmission
What are examples if X-linked autosomal dominant conditions?
- Vitamin D resistant rickets
- Incontentia pigmenti (male lethality)
- Rett syndrome (usually has male lethality)
