1. Single Gene Mutations and Inheritance II April 4, 2008
Lisa Schimmenti, M.D.

2. Objectives Understand autosomal recessive inheritance.
Know the concept of Lyonization.
Learn and understand the various forms of X linked inheritance.
Know how to calculate the risk of affected offspring given a family history and some facts about carrier frequency using the Hardy-Weinberg Calculation.

3. Autosomal Recessive pattern of inheritance
Males and females equally affected
Condition seen in sibs but not usually in other relatives
Risk of recurrence in sibs 25%
2/3 of unaffected sibs are carriers
Consanguinity increases risk

4. Autosomal Recessive Probabilities
Mother A a AA A Aa
Father
25% affected
2/3 of unaffected are carriers a Aa aa

5. Loss of function alleles
Likely when point mutations give the same phenotype as deletion of an allele
Inherited as recessive traits when 50% of expression is sufficient for normal phenotype
Loss of function and gain of function changes in the same gene will cause different diseases

6. Effect of consanguinityDD Dd DD Dd Dd DD DD Dd Dd DD Dd DD dd
Consanguinity increases the risk of sharing a common ancestral mutation

7. Sweeping generalizations about genes inherited as AR traits
Transcription from one allele is sufficient
Severity dependent on nature and location of mutation
What we describe in inheritance pattern as homozygosity is USUALLY compound heterozygosity at molecular level. Allelic heterogeneity is COMMON

8. A common type of AR trait Mutation in enzyme gene
Example:
degradation of mucopolysaccharides requires a series of lysosomal enzymes.
abnormalities in any of these enzymes can cause a similar phenotype:
coarse facies, enlarged organs, skeletal abnormalities

9. One MPS disease: a-L-iduronidase deficiency
Severe mutations = Hurler
Milder mutations = Scheie
Phenotype is dependent on nature of mutation in alleles

11. How recessive conditions occur in population groups.
Genes in populations
How recessive conditions occur in population groups.

12. Deafness 1 in 1000 infants is born deaf.
Infants are screened for deafness/hearing loss at birth by automated hearing screening methods.
There are over 400 genes that cause hearing loss
Autosomal recessive mutations in GJB2, encoding Connexin 26, will be causative in nearly half of all deaf individuals.
Photo credit: Josie Helmbrecht

13. Hearing loss is the most common condition found at birth

14. Mutations in Connexin26 are a common cause of hearing loss
Exon 1
Exon2
158 bp
681 bp
Start codon
Gene
Protein
Six connexons form a hemichannel in one cell membrane.
When two cell membranes meet, a gap junction is formed.

15. Connexin 26 Common deafness alleles 35delG
One method for determining carrier frequency
Screen hearing individuals for the presence of the gene mutation.
Remember, we have two of each gene
Carriers are heterozygotes
one wildtype and one mutant allele
35delG
This is the most common allele in European populations.
How do you determine the carrier frequency without testing hundreds of individuals?

16. Hardy-Weinberg Law
A mathematical model for calculating allele and gene frequencies in a population
Assumes
Large population with random mating
Allele frequencies remain constant
no new mutations
no reproductive selection bias
no significant immigration for populations with different allele frequencies

17. The sum of all alleles for a population is 1
Allele frequencies
p = the frequency of allele 1
q = the frequency of allele 2
The sum of all alleles for a population is 1
p + q =1

18. Hardy Weinberg Calculation
Binomial expansion
(p+q)2 = p2 + 2pq +q2
p2 = the number of homozygous wildtype individuals in the population
2pq = the number heterozygous carriers in the population
q2 = the number of homozygous affected individuals in the population

19. Using the Hardy Weinberg calculation to determine the carrier frequency in a population
q2 = affected individuals in the population
for Connexin 26 related deafness:
q2 =1 in 2000
(an approximated number for this exercise) 1
Solve for 2pq

20. Using the Hardy Weinberg calculation to determine the carrier frequency in a population
q2 =1 in 2000 =
0.02
2pq = 0.04
The carrier frequency is 0.04 or 1 in 25.

21. Hardy Weinberg calculations are used in clinic every day
Clinical situation:
A 23 year old hearing woman named Marie comes to your genetics clinic. Her family is of European descent Marie has a deaf sister, Sally, who has deafness caused by mutations in the gene encoding Connexin 26. Her sister's genotype is 35delG/35delG.
Marie would like to know her chance of having deaf children caused by mutations in Connexin 26 if the father of the child is of European descent.

22. How do you answer Marie's question?
What is Marie's chance of being a carrier of 35delG?
What is the chance that the father of Marie's child will be a carrier of 35delG?
What is the chance that Marie will have a child with hearing loss?

23. Draw a pedigree
Sally
Marie

24. What is Marie's chance of being a carrier of 35delG?
Marie's parents are obligate carriers.
Marie is unaffected.
The chance of being a carrier
if unaffected is 2/3.
Use a Punnett square if in doubt.
Marie
Sally
Wt/35delG
Wt/35delG
35delG/35delG

25. What is the chance that the father of Marie's child will be a carrier of 35delG?
q2 =1 in 2000 =
0.02
2pq = 0.04
The carrier frequency is 0.04 or 1 in 25.
The father's chance of being a carrier is 1/25.

26. What is the chance that Marie will have a child with hearing loss?
The chance that an affected child will be born to a carrier couple is 1 in 4 with each pregnancy.
Calculation:
2/3 x 1/25 x 1/4 = 1/150
Father of baby's chance of being a carrier
Marie's chance of being a carrier

27. Inheritance patterns and genes
Single allele change gives disease
Genes on X (X - linked)
Genes on autosome (AD)
Mutation in both alleles required for disease
Genes on autosome (AR)
(rare) female homozygote for XL

28. X-linked patterns of inheritance
X-linked recessive
Primarily males affected
Females typically unaffected, but there are exceptions
X-linked dominant
Male surviving
Either sex affected, males more severely than females
Male lethal
Only females seen with disease, mosaic patterns of expression

29. X-linkage: already exceptions to the “rules”
The lines of dominant and recessive are blurred
X-linked conditions are sometimes called “semi-dominant” in women
Need dosage compensation

30. Lyonization
Only one X is active in each cell. All others are inactivated.
X inactivation is random
X inactivation is fixed
X inactivation occurs early in development (late blastocyst) XX

31. Lyonization XX XI IX
X = active X
I = inactive X
X Inactivation

32. Consequences of “Lyonization”
Females are mosaic for their X-linked gene manifestations
should be roughly equal
All are functionally hemizygous
dosage compensation: have the same number of copies as males
Inactive X may be evident in cells
The Barr body
Shifting from random distribution may result in manifestation of disease in females
skewed lyonization

33. X-linked recessive “rules”
Affects mainly males
Affected males are usually born to unaffected parents
Females can be affected if born to an affected father and carrier mother
Females can be affected if they have very skewed lyonization
No male to male transmission
Males born to carrier mother have 50% risk of inheriting altered gene

34. X-linked recessive What does the pattern look like?
Carrier female
Affected male
Normal male
Normal female

35. X-linked recessive recurrence risks
Mother X1 X2
Daughters
50% normal
50% carriers
X1X1 X1
X1X2
Father
Sons
50% normal
50% affected Y
X1Y
X2Y

36. Duchenne/Becker Muscular Dystrophy
Mutations in dystrophin gene
Duchenne
Typically del/dup but frameshift
Severe
No protein product
Becker
Also del/dup in frame so milder
Shortened protein

37. Clinical manifestations of Duchenne MD
Onset before age 5
Calf pseudohypertrophy
1/3500 males
Elevated creatine kinase
Severe muscle wasting
Affects resp and cardiac mm
Females 8-10% some weakness
CK > 95%tile in 2/3 of carriers

38. Dystrophin gene and molecule
Huge gene (2.3 Mb)
14kb transcript
3685 AAs
High mutation rate (10-4)

39. Using dystrophin antibody
normal
Becker
Duchenne B B n D D

40. X-linked dominant “rules”
Males and females both affected , but typically males are worse than females.
If male lethal, only affected females observed.
Affected males will only have affected daughters and unaffected sons.
No male -> male transmission seen

41. X-linked dominant What does the pattern look like?
Affected female
Affected male
Normal male
Normal female
Example with male survival

42. X-linked dominant, male lethal What does the pattern look like?
Affected female
Normal male
Normal female

43. X-linked dominant recurrence risks
Mother X1 X2
Daughters
50% normal
50% affected
X1X1 X1
X1X2
Father
Sons
50% normal
50% affected Y
X1Y
X2Y

44. Incontinentia pigmenti X-linked dominant (male lethal condition)
Linear blisters in newborn girls
“Crops” followed by scarring
Small teeth
Eye abnormalities
Patchy hair loss
Only girls are affected
Males are typically not affected

45. Linear erosions on soles
Infant with IP
Linear blisters on leg
Linear erosions on soles

46. Potentially confusing in X-linked pedigree analysis
Male lethal X-linked conditions
New mutations may be hard to recognize as X-linked
Sex-limited conditions may look X-linked
With carrier mother and affected father can see “male to male” inheritance