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Instructor: Prof. Dr. Fadel A. Sharif

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1 Instructor: Prof. Dr. Fadel A. Sharif
Medical Genetics Instructor: Prof. Dr. Fadel A. Sharif

2 Contact Medical Laboratory Sciences
University Clinic-Genetics Diagnosis lab

3 Course Syllabus - Text: Thompson & Thompson Genetics in Medicine, 7th Ed., Nussbaum, McInnes, Willard, Saunders (Required) - References: Emery’s Elements of Medical Genetics, 13 Ed., Turnpenny & Ellard Churchill Livingstone - Jorde, Carey, Bamshad. Medical Genetics, 4th edition. Mosby Elsevier, 2010. Grades Quizzes: 10% Presentation: 10% One Midterm exam worth 30% Final exam: 50%

4 Topics Patterns of Single-Gene Inheritance
Genetics of Common Disorders with Complex Inheritance Principles of (Clinical) Cytogenetics Clinical Cytogenetics: Disorders of the Autosomes and the Sex Chromosomes Genetic Variation in Individuals and Populations: Mutation and Polymorphism Human Gene Mapping and Disease Gene Identification Treatment of Genetic Disease Cancer Genetics

5 Patterns of Single-Gene Inheritance
Lecture 1 Patterns of Single-Gene Inheritance

6 Occur on average in fixed proportions among the offspring of specific types of matings
The 2015 online version of Mendelian Inheritance in Man (OMIM) lists 3,341 genes with phenotype-causing mutation. (2468 autosomal; 820 X-linkked; 53 Y-linked). The 2015 on-line edition of McKusick’s Mendelian Inheritance in Man ( lists 22,783 known single-gene, or monogenic, traits defined thus far in humans. Of these, 21,433 are located on autosomes, 1226 are located on the X chromosome, 59 are located on the Y chromosome and 65 are mitochondrial. number of diseases with known genetic causes and the number of genes in which mutations can cause disease are not the same because different mutations in the same gene can cause different diseases, and mutations in different genes can cause similar or indistinguishable diseases

7 Primarily disorders of the pediatric age range; less than 10% manifest after puberty, and only 1% occur after the end of the reproductive period In a study of more than 1 million live births, the incidence of serious single-gene disorders was estimated to be 0.36%; among hospitalized children, ~6% to 8%

8 A survey of OMIM for mendelian forms of 17 of the most common adult diseases, such as heart disease, stroke, cancer, and diabetes, revealed nearly 200 mendelian disorders whose phenotypes included these common adult illnesses. These mendelian forms are important in individual patients because of their significance for the health of other family members and because of the availability of genetic testing and detailed management options for many of them.

9 OVERVIEW AND CONCEPTS Variation in genes:
For many genes, there is a single prevailing allele, the wild-type or common allele. The other versions of the gene are variant or mutant alleles A given set of alleles at a locus or cluster of loci on a chromosome is referred to as a haplotype. If there are at least two relatively common alleles at the locus in the population, the locus is said to exhibit polymorphism

10 Genotype and Phenotype
Genotype: set of alleles that make genetic constitution Phenotype: observable expression of a genotype as morphological, clinical, cellular, biochemical trait Pleiotropy: a single abnormal gene or gene pair producing multiple diverse phenotypic effects

11 Compound heterozygous: two different mutant alleles of the same gene
Hemizygous: male with respect to X-linked genes.

12 Pedigrees Single-gene disorders are characterized by their patterns of transmission in families Pedigree: graphical representation of family tree using standard symbols The extended family depicted in such a pedigree is a kindred Proband (propositus, index case): affected member through whom a family with genetic disease is brought to attention Consultand: who brings family to attention by consulting a geneticist. May be an affected individual or unaffected relative of proband

13 Figure 7-1 Symbols commonly used in pedigree charts
Figure 7-1 Symbols commonly used in pedigree charts. Although there is no uniform system of pedigree notation, the symbols used here are according to recent recommendations made by professionals in the field of genetic counseling.

14 The proband, III-5, represents an isolated case of a genetic disorder
The proband, III-5, represents an isolated case of a genetic disorder. She has four siblings. Her partner/spouse is III-6, and they have three children. The proband has nine first-degree relatives, nine second-degree relatives, two third-degree relatives (first cousins), and four fourth-degree relatives (first cousins once removed). IV-3, IV-5, and IV-6 are second cousins of IV-1 and IV-2. IV-7 and IV-8, whose parents are consanguineous, are doubly related to the proband.

15 Consanguineous: couples who have one or more ancestors in common
Isolated case: only one affected member in a family Sporadic case: if due to new mutation Fitness: a measure of impact of condition on reproduction. Defined as: number of offspring (who survive to reproductive age) affected individuals can have, as compared to an appropriate control Patterns of single-gene inheritance: autosomal recessive/dominant, X-inked recessive/dominant

16 Dominant and Recessive Inheritance
Recessive: a phenotype expressed only in homozygotes (for X-linked, male hemizygotes) and not in heterozygotes Most recessive disorders are due to mutations that reduce or eliminate function of a gene (i.e., loss-of-function mutations)

17 Dominant: a phenotype expressed in both homozygotes and heterozygotes for a mutant allele.
Pure dominant disease: homozygotes and heterozygotes for mutant allele are equally affected. Pure dominant disorders rarely if ever exist in medical genetics Codominant: phenotypic expression of two different alleles for a locus Dominant disorders are usually more severe in homozygotes. Thus in heterozygotes, disease is incompletely dominant (or semidominant).

18 Factors affecting pedigree patterns
Penetrance and expressivity Some disorders are not expressed at all in an individual despite having same genotype that causes disorder in others in his family. In others, same disorder may have extremely variable expression in terms of clinical severity/range of symptoms/onset age Phenotypic expression can be modified by effects of aging/other genetic loci (modifier loci)/effects of environment/ allelic heterogeneity environmental effects, modifier loci, or allelic heterogeneity

19 Penetrance: probability that a gene will have any phenotypic expression at all. It is the percentage of people with a predisposing genotype who are actually affected. When some completely fail to produce phenotype, the gene is said to show reduced penetrance.

20 Expressivity: severity of expression of phenotype among individuals with same disease causing genotype. When severity differs, phenotype is said to have variable expressivity

21 Neurofibromatosis-I (NF1)
A common AD disorder of the NS, eye, and skin (1/3500 births) Characterized by: Growth of multiple benign fleshy tumors (neurofibromas) in skin Multiple flat, irregular pigmented skin lesions (café au lait spots) Small benign tumors (hamartomas) called Lisch nodules on iris of the eye Less frequently: MR, CNS tumors, diffuse plexiform neurofibromas, and development of cancer of the NS and muscle. Thus, the condition has a pleiotropic phenotype.

22 NF1 Adult heterozygotes almost always show some sign (i.e., penetrance is 100% in adults) Some may have café au lait spots, freckles on axillary skin, and Lisch nodules, others may have life-threatening benign tumors in spinal cord or malignant sarcomas of an extremity (i.e., variable expressivity) axillary skin: visible surface of armpit

23 Neurofibromatosis-1: café au lait spots, hyperpigmented spots on the skin, are a useful diagnostic sign in family members who otherwise may appear unaffected. Most patients have six or more spots at least 15 mm in diameter, usually on the trunk.

24 Diagnosis of NF1 is further complicated in children because the signs develop gradually during childhood. e.g., in the newborn period, less than half of all affected newborns show even the most subtle sign of the disease, an increased incidence of café au lait spots. Penetrance, therefore, is age dependent. Many different mutations have been found in the NF1 gene, all of which appear to cause loss of function of its gene product, neurofibromin. Approximately half the cases of NF1 result from a new rather than an inherited mutation

25 The chief genetic problem in counseling families of patients with NF1 is to decide between two possibilities: Is the disease in the proband sporadic, that is, due to new mutation, or has the patient inherited a clinically significant form of the disorder from a parent in whom the gene is present but only mildly expressed? If the proband has inherited the defect, the risk that any of his or her sibs will also inherit it is 50%; but if the proband has a new mutant gene, there is very little risk that any sib will be affected.

26 The disorder can be detected presymptomatically and even prenatally by molecular genetic analysis.
Unfortunately, molecular testing can generally only answer whether the condition will occur and not how severe it will be. Except for the association of complete gene deletions with dysmorphic features, mental retardation, and increased number of neurofibromas at an early age, there is no correlation between severity of the phenotype and particular mutant NF1 alleles.

27 Figure 7-4 Pedigree of family with NF-1, apparently originating as a new mutation in the proband of generation III (arrow). This individual appears to have a new mutant allele of NF1 because his parents and their parents are all unaffected.

28 Split-hand deformity Another example of an autosomal dominant malformation with reduced penetrance is the split-hand deformity, a type of ectrodactyly. The malformation originates in the sixth or seventh week of development, when the hands and feet are forming. The disorder demonstrates locus heterogeneity, with at least five loci recognized, although the actual gene responsible has been identified in only a few. Failure of penetrance in pedigrees of split-hand malformation can lead to apparent skipping of generations, and this complicates genetic counseling because an at-risk person with normal hands may nevertheless carry the gene for the condition and thus be capable of having children who are affected. Shfm loci

29 Figure 7-5 Split-hand deformity, an AD trait involving the hands and feet, in a 3-month-old boy. A, Upper part of body. B, Lower part of body.

30 In the pedigree, the consultand’s mother is a nonpenetrant carrier of the split-hand mutation.
Review of the literature on split-hand deformity suggests that there is reduced penetrance of about 70%. Figure 7-6 Pedigree of split-hand deformity demonstrating failure of penetrance in the mother of the consultand. Reduced penetrance must be taken into account in genetic counseling.

31 Age of Onset Genetic disorders can appear at any time in the lifetime of an individual, ranging from early in intrauterine development all the way to the postreproductive years, and all ages in between. Some may be lethal prenatally, whereas others may interfere with normal fetal development and can be recognized prenatally (e.g., by ultrasonography) but are consistent with a full-live-born infant;

32 Still others may be recognized only at birth (congenital)
Still others may be recognized only at birth (congenital). Thus, in a pedigree of a family with a lethal disorder affecting a fetus early in pregnancy, the pattern of disease occurrence may be obscure because all that one observes are multiple miscarriages and fetal losses or apparently reduced fertility, rather than recurrence of the prenatal disease itself. Conversely, in a family with a late-onset dominant disorder, an affected individual may have parents and children reportedly free of disease because the carrier parent died of unrelated causes before the disease could develop, and the children at risk have not reached the age at which the mutant gene reveals itself in a disease phenotype.

33 Other Factors Affecting Pedigree Patterns
The inheritance pattern of an individual pedigree may be obscured by a number of other factors that may make the mode of inheritance difficult to interpret. Diagnostic difficulties may be due to reduced penetrance or variable expressivity of the disease; other genes and environmental factors may affect gene expression; persons of some genotypes may fail to survive to the time of birth;

34 accurate information about the presence of the disorder in relatives or about family relationships may be lacking; the occurrence of new mutations can contribute to the occurrence of dominant and X-linked disease; and finally, with the small family size typical of most developed countries today, the patient may by chance alone be the only affected family member, making determination of any inheritance pattern very difficult.

An important component of medical genetics is identifying and characterizing the genotypes responsible for particular disease phenotypes. When a genetic disorder that appears to be inherited as a single-gene disorder is thoroughly analyzed, it is frequently found to be genetically heterogeneous; i.e., it includes a number of phenotypes that are similar but are actually determined by different genotypes at different loci. Genetic heterogeneity may be the result of different mutations at the same locus (allelic heterogeneity), mutations at different loci (locus heterogeneity), or both.

36 Recognition of genetic heterogeneity is an important aspect of clinical diagnosis and genetic counseling. Moreover, distinct phenotypes inherited in different families can result from different mutant alleles in the same gene. This phenomenon, known as clinical or phenotypic heterogeneity, is well known and must be taken into account in correlating genotype and phenotype.

37 Allelic Heterogeneity
Allelic heterogeneity is an important cause of clinical variation. Many loci possess more than one mutant allele; in fact, at a given locus, there may be several or many mutations. E.g., nearly 1400 different mutations have been found worldwide in the cystic fibrosis transmembrane conductance regulator (CFTR) among patients with CF. Sometimes, these different mutations result in clinically indistinguishable disorders. In other cases, different mutant alleles at the same locus produce a similar phenotype but along a continuum of severity; for example, some CFTR mutations cause patients to have classic CF with pancreatic insufficiency, severe progressive lung disease, and congenital absence of the vas deferens in males, whereas patients carrying other mutant alleles have lung disease but normal pancreatic function, and still others have only the abnormality of the male reproductive tract. CFTR severity is also modified by variants in several genes e.g., Mbl2 (mannose binding lectin), TGFalpha1 and TGFbeta1

38 Since any particular mutant allele is generally uncommon in the population, most people with rare autosomal recessive disorders are compound heterozygotes rather than true homozygotes. Because different allelic combinations may have somewhat different clinical consequences, clinicians must be aware of allelic heterogeneity as one possible explanation for variability among patients considered to have the same disease. Environment and modifier genes are other explanations

39 There are, however, some well-recognized exceptions to the observation that compound heterozygotes are more common than true homozygotes: The first is when the affected individuals inherited the same mutant allele from consanguineous parents, who both carry the same mutant allele they inherited from a common ancestor. Second, one mutant allele may be responsible for a large proportion of the cases of an autosomal recessive condition in a particular ethnic group, and so many patients from that group will be homozygous for this allele. The third is when the disorder normally has little if any allelic heterogeneity because the disease phenotype caused by a particular mutation is specific to that mutation (e.g., sickle cell disease).

40 Locus Heterogeneity For many phenotypes, pedigree analysis alone has been sufficient to demonstrate locus heterogeneity. For example, retinitis pigmentosa, a common cause of visual impairment due to photoreceptor degeneration, has long been known to occur in autosomal dominant, autosomal recessive, and X-linked forms. In recent years, the heterogeneity has been shown to be even more extensive; pedigree analysis combined with gene mapping has demonstrated that there are at least 43 loci responsible for 5 X-linked forms, 14 autosomal dominant forms, and 24 autosomal recessive forms of retinitis pigmentosa that are not associated with other phenotypic abnormalities. If one includes disorders in which retinitis pigmentosa is found in conjunction with other defects such as MR or deafness, there are nearly 70 different genetic diseases manifesting retinitis pigmentosa.

41 Phenotypic Heterogeneity
Different mutations in the same gene can sometimes give rise to strikingly different phenotypes. For example, certain loss-of-function mutations in the RET gene, which encodes a receptor tyrosine kinase, can cause dominantly inherited failure of development of colonic ganglia, leading to defective colonic motility and severe chronic constipation (Hirschsprung disease). Other mutations in the same gene result in unregulated hyperfunction of the kinase, leading to dominantly inherited cancer of the thyroid and adrenal glands (multiple endocrine neoplasia type 2A and 2B). A third group of mutations in RET causes both Hirschsprung disease and multiple endocrine neoplasia in the same individuals.

42 A comparable situation occurs with the LMNA gene, which encodes lamin A/C, a nuclear membrane protein. Different LMNA mutations have been associated with half a dozen phenotypically distinct disorders, including Emery-Dreifuss muscular dystrophy, one form of hereditary dilated cardiomyopathy, one form of the Charcot-Marie-Tooth peripheral neuropathy, a disorder of normal adipose tissue called lipodystrophy, and the premature aging syndrome known as Hutchinson-Gilford progeria.

Autosomal recessive disease occurs only in homozygotes or compound heterozygotes. Homozygotes (affected) must have inherited a mutant allele from each parent (barring uniparental disomy or new mutation, which is rare in autosomal recessive disorders). Three types of matings can lead to homozygous affected offspring. The mutant recessive allele is symbolized as r and its normal dominant allele as R. Although any mating in which each parent has at least one recessive allele can produce homozygous affected offspring, the most common mating by far is between two unaffected heterozygotes. Barring=with the exception of

44 Risk of Disease Offspring Parental Mating 3/4 unaffected, 1/4 affected 1/4 R/R, 1/2 R/r, 1/4 r/r Carrier by carrier R/r ×R/r 1/2 unaffected, 1/2 affected 1/2 R/r, 1/2 r/r Carrier by affected R/r ×r/r All affected r/r only Affected by affected r/r ×r/r

45 The proband may be the only affected family member, but if any others are affected, they are usually in the same sibship and not elsewhere in the kindred. Figure 7-7 Typical pedigree showing autosomal recessive inheritance

46 Sex-Influenced Disorders
Autosomal recessive disorders generally show the same frequency and severity in males and females. Some autosomal recessive phenotypes, however, are sex-influenced, i.e, expressed in both sexes but with different frequencies or severity. Among autosomal disorders, hemochromatosis is an example of a phenotype more common in males. This autosomal recessive disorder of iron metabolism occurs most commonly in the approximately 0.5% of individuals of northern European extraction that are homozygous for a missense mutation replacing cysteine at position 282 with a tyrosine (Cys282Tyr) in the HFE gene. HFE: High Iron Fe

47 Cys282Tyr homozygotes have enhanced absorption of dietary iron and often demonstrate laboratory abnormalities suggestive of excessive body stores of iron, although the condition only rarely leads to iron overload and serious damage to the heart, liver, and pancreas. The lower incidence of the clinical disorder in females (one fifth to one tenth that of males) is believed to be related, among other factors, to lower dietary intake of iron, lower alcohol usage, and increased iron loss through menstruation among females.

48 Gene Frequency and Carrier Frequency
The mutant alleles responsible for a recessive disorder are generally rare, and so most people will not have even one copy of the mutant allele. Among individuals with at least one copy of the mutant allele, however, the frequency of clinically unaffected heterozygotes with one normal allele and one mutant allele is always much greater than the frequency of affected individuals with two rare mutant alleles. Because an autosomal recessive disorder must be inherited through both parents, the risk that any carrier will have an affected child depends partly on the chance that his or her mate is also a carrier of a mutant allele for the condition. Thus, knowledge of the carrier frequency of a disease is clinically important for genetic counseling.

49 The most common autosomal recessive disorder in white children is cystic fibrosis (CF), caused by mutations in the CFTR gene. CF is virtually unknown in Asian populations and is relatively rare in African American populations, but in white populations, about 1 child in 2000 has two mutant CFTR alleles and has the disease. The frequency of carriers for one of the hundreds of possible mutant CFTR alleles can be calculated to be approximately 1/29.

50 In a population of 3247 white individuals, therefore, you can expect 1 CF patient, 112 unaffected carriers of a CFTR mutation, and 3134 normal homozygotes. Because a patient has two mutant CFTR alleles and a carrier has only one, (112 ×1)/(112 ×1+1×2)=112/114 (about 98%) of all mutant CFTR alleles in this population of 3247 individuals are hidden in carriers (who usually are unaware that they are carriers), and only 2% are in patients. #of mutant alleles in carriers/total number of mutant alleles

51 Consanguinity Because the majority of the mutant alleles responsible for autosomal recessive disorders are in carriers rather than in homozygotes, mutant alleles can be handed down in families for numerous generations without ever appearing in the homozygous state and causing overt disease. The presence of such hidden recessive genes is not revealed unless the carrier happens to mate with someone who also carries a mutant allele at the same locus and the two deleterious alleles are both inherited by a child.

52 It is believed that everyone carries few (1 to 5) mutant alleles, of which perhaps half are lethal in homozygotes before birth. The remainder cause well-known, easily recognizable autosomal recessive disorders in homozygotes. This is, however, a minimal estimate that does not take into account mutant alleles that exert their effect by interacting with mutant alleles at other loci (multifactorial inheritance). at least 8 to 10

53 The chance that both parents are carriers of a mutant allele at the same locus is increased substantially if the parents are related and could each have inherited the mutant allele from a single common ancestor, a situation called consanguinity. Consanguinity is defined arbitrarily as a union of individuals related to each other as close as or closer than second cousins. Consanguinity of the parents of a patient with a genetic disorder is strong evidence (although not proof) for the autosomal recessive inheritance of that condition.

54 Figure 7-8 Pedigree in which parental consanguinity suggests autosomal recessive inheritance.

55 The genetic risk to the offspring of marriages between related people is not as great as is sometimes imagined. For marriages between first cousins, the absolute risks of abnormal offspring, including not only known autosomal recessive diseases but also stillbirth, neonatal death, and congenital malformations, is 3% to 5%, about double the overall background risk of 2% to 3% for offspring born to any unrelated couple. Consanguinity at the level of third cousins or more remote relationships is not considered to be genetically significant, and the increased risk of abnormal offspring is negligible in such cases.

56 Consanguinity is not the most common explanation for an autosomal recessive trait. The mating of unrelated persons, each of whom happens by chance to be a carrier, accounts for most cases of autosomal recessive disease, particularly if a recessive trait has a high frequency in the population. Thus, most affected persons with a relatively common disorder, such as CF, are not the result of consanguinity, because the mutant allele is so common in the general population. However, consanguinity is more frequently found in the background of patients with very rare conditions. For example, in xeroderma pigmentosum, a very rare autosomal recessive condition of DNA repair, more than 20% of cases occur among the offspring of marriages between first cousins.

57 The Measurement of Consanguinity
The measurement of consanguinity is relevant in medical genetics because the risk of a child's being homozygous for a rare recessive allele is proportional to how related the parents are. Some types of consanguineous mating carry an increased risk.

58 Figure 7-9 Types of consanguineous mating
Figure 7-9 Types of consanguineous mating. The probability that the offspring in each of these matings is homozygous by descent at any one locus is equal to the coefficient of inbreeding, F.

59 Consanguinity is measured by the coefficient of inbreeding (F), the probability that a homozygote has received both alleles at a locus from the same ancestral source. In Figure 7-10, individual IV-1 is the offspring of a first-cousin mating. Each of the four alleles at locus A (A1, A2, A3, and A4) in generation I has a 1/8 × 1/8 =1/64 chance of being homozygous in IV-1; thus, the probability that IV-1 is homozygous for any one of the four alleles is 4 ×1/64 =1/16. F: ; it is also the proportion of loci at which a person is homozygous for an allele from the same ancestral source, a situation referred to as identity by descent.

60 Figure 7-10 A cousin marriage, used in the text to demonstrate how to calculate the coefficient of inbreeding, F, of the child IV-1.

61 Table 7-1 shows the coefficients of inbreeding for the offspring of a number of consanguineous matings. If a person is inbred through more than one line of descent, the separate coefficients are summed to find his or her total coefficient of inbreeding.

62 Consanguineous Matings
Type Degree of relationship Proportion of Genes in Common Coefficient of Inbreeding of Child (F) Monozygotic twins NA 1 Parent-child 1st 1/2 1/4 Brother-sister (including dizygotic twins) Brother-half sister 2nd 1/8 Uncle-niece or aunt-nephew Half uncle-niece 3rd 1/16 First cousins Double first cousins Half first cousins 4th 1/32 First cousins once removed Second cousins 5th 1/64 percentage of genes are shared by two related individuals. The coefficient of relationship is a measure of this percentage. Coefficients of inbreeding for the offspring of a number of consanguineous matings. If a person is inbred through more than one line of descent, the separate coefficients are summed to find his or her total coefficient of inbreeding.

63 Inbreeding Inbreeding is closely related to consanguinity. Inbreeding describes the situation in which individuals from a small population tend to choose their mates from within the same population for cultural, geographical, or religious reasons. In this situation, the parents may consider themselves unrelated but still may have common ancestry within the past few generations. Just as with consanguinity, inbreeding increases the chance that individuals will be homozygous for an allele inherited from a common ancestor.

64 Thus, in taking a family history, it is important to ask not only about consanguinity but also about the geographical origins of ancestors, especially if a couple seeking counseling is of similar ethnic or geographical origin. Although we make a distinction between consanguinity occurring within a family and inbreeding, which occurs between unrelated individuals from the same small ethnic group, an increased risk for mating between heterozygous carriers of autosomal recessive disorders exists in both situations. As with consanguineous matings, it is possible to estimate a coefficient of inbreeding for individuals in a population even if they are not known to be related to each other.

65 Rare Recessive Disorders in Genetic Isolates
There are many small groups in which the frequency of certain rare recessive genes is greater than that in the general population. Such groups, genetic isolates, may have become separated from their neighbors by geographical, religious, or linguistic barriers. Although such populations are not consanguineous, the chance of mating with another carrier of a particular recessive condition may be as high as that observed in cousin marriages.

66 Tay-Sachs disease (GM2 gangliosidosis)
is an example of AR disease with increased frequency in certain genetic isolates. The disease is a neurological degenerative disorder that develops when a child is about 6 months old. Affected children become blind and regress mentally and physically. The disease is fatal in early childhood. Among Ashkenazi Jews in North America, e.g., Tay-Sachs disease is 100 times more frequent (1 in 3600) than in other groups of European ancestry. This increased disease frequency is because the Tay-Sachs carrier frequency among Ashkenazi Jews, approximately 1 in 30, is 10-fold higher than in similar non-Ashkenazi European populations.

67 When the mutant alleles causing a recessive disease are relatively frequent in a particular population, unrelated spouses have a reasonable chance of both being heterozygous, and therefore consanguinity is generally not a striking feature in the families with affected children. For example, among Ashkenazi Jews, the parents of children with Tay-Sachs disease are usually not closely related. When the mutant allele is rare, however, the frequency of carriers is very low and consanguinity is often the explanation for how both members of a couple came to be heterozygotes. For example, consanguinity is often present in the parents of Tay-Sachs patients in the population of French ancestry in Quebec, Canada, where mutant alleles for Tay-Sachs disease are rare.

68 Characteristics of Autosomal Recessive Inheritance
An autosomal recessive phenotype, if it appears in more than one member of a kindred, typically is seen only in the sibship of the proband, not in parents, offspring, or other relatives. For most autosomal recessive diseases, males and females are equally likely to be affected. Parents of an affected child are asymptomatic carriers of mutant alleles. The parents of the affected person may in some cases be consanguineous. This is especially likely if the gene responsible for the condition is rare in the population. The recurrence risk for each sib of the proband is 1 in 4.

69 New Mutation in AR Disorders
When a child is affected with an AR condition, the assumption is generally made that both parents are heterozygous carriers for the condition. Yet, new mutations occur all the time during the generation of gametes. Might not an individual have two mutant alleles for an autosomal recessive condition by virtue of inheriting one mutant allele from a carrier parent while the other mutant allele arose de novo in a gamete that came from a parent who was not a carrier? Such a situation is, of course, not impossible but is relatively unlikely compared with the situation in which both parents are heterozygous carriers.

70 This is because the chance that the gamete from a non-carrier parent had acquired a mutant allele by spontaneous mutation ranges from 1 in 105 to 1 in 106, which is thousands of times less likely than the typical 1 in 20 to 1 in 1000 chance that the gamete contained the mutant allele because the parent is a heterozygous carrier. The relative unimportance of new mutations in autosomal recessive disease is in stark contrast to the situation with dominant and X-linked disorders, as will be discussed later

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