Autosomal Dominant And Recessive Diseases

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Autosomal Trait

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comes from a gene localized on chromosomes 1-22 (autosomes) rather than sex linked or mitochondrial chromosomes

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Gene

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a DNA sequence that codes for the amino acid sequence

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Locus

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location in the chromosome of a particular gene

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Allele

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one or more alternative forms that a gene may have in the population

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Principle of Segregation (Mendel's First Law)

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genes are possessed in pairs and only one of each pair is transmitted to offspring

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Principle of Independent Assortment (Mendel's Second Law)

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genes that reside at different loci are transmitted independently

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Multiplication Rule

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probability of event 1 AND event 2

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Addition Rule

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probability of event 1 OR event 2

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Gene Frequency

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proportion of chromosomes that contain a specific gene in a population (i.g., gene s)

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Genotype Frequency

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how often the genotype occurs in the population (i.g., genotype ss)

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Conditions necessary for Hardy-Weinberg

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very large population, random mating, no mutations, no migration, no natural selection

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Proband

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the first person in a pedigree to be identified as having the disease

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First Degree Relatives

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parents and siblings (of proband)

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Second Degree Relatives

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grandparents, aunts/uncles, nephews/nieces (of proband)

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Third Degree Relatives

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great grandchildren, great grandparents, first cousins (of proband)

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Characteristics of Autosomal Dominant Diseases

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1 - both sexes may transmit disease
2 - no skipping of generations
3 - father to son transmission
4 - disease transmits to 1/2 of offspring

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Huntington Disease

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autosomal dominant disease
trinucleotide repeat expansion
HD gene
characterized by progressive dementia

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Achondroplasia

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autosomal dominant disease
missense point mutation
FGFR-3 gene
characterized by short-limbed dwarfism
homozygotes are more severe
90% from new mutation

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Neurofibromatosis type 1 (NF1)

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autosomal dominant disease
nonsense mutation
NF1 gene
characterized by variable cafe-au-lait spots and neurofibromas
50% from new mutations

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Familial Hypercholesterolemia

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autosomal dominant disease
haploinsufficiency
LDL receptor gene
characterized by high plasma LDL, coronary heart disease, infarction

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Marfan Syndrome

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autosomal dominant disease
25% from new mutation
FBN1 gene
characterized by arachnodactyly, disproportionate stature, CVS problems

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What do genes of autosomal dominant diseases encode (usually)?

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structural proteins

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What do genes of autosomal recessive diseases encode (usually)?

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enzymes

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Characteristics of Autosomal Recessive Diseases

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1 - seen in one or more siblings, but not in early generations
2 - males and females equally affected
3 - 1/4 of offspring will be affected
4 - may be result of consanguinity

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Hurler Syndrome

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autosomal recessive disease
(alpha)-L-iduronidase gene
characterized by buildup of GAGs in lysosomes, leading to short stature, retardation

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Hereditary Hemochromatosis

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autosomal recessive disease
missense point mutation
HFE gene
characterized by iron accumulation in liver, heart, pancreas, skin, joints, endocrine glands

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Germline Mosaicism

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mutation of all or some germ cells, but not enough somatic tissue is affected to produce phenotype; when two of more offspring present with an autosomal dominant disease without a family history of the disease, this is a possibility (osteogenesis imperfecti)

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Penetrance

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proportion of individuals carrying a genotype that also expresses the phenotype

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Reduced Penetrance

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an individual has the genotype, but does not exhibit the disease (retinoblastoma has 90% penetrance)

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Variable Expression

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severity of the disease independent of penetrance (NF1 is 100% penetrant with variable expressivity)

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Pleiotropy

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a single gene exerts its effects on multiple aspects of physiology or anatomy (e.g., cystic fibrosis, Marfan syndrome, von Gierke, diabetes)

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Heterogenous Disease

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mutations at different loci produce same phenotype (e.g., osteogenesis imperfecti)

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Genomic Imprinting

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differential activation of genes, depending upon the parent from which they are inherited (inactive genes are imprinted)

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Angelman Syndrome

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transmission of gene (UBE3A) deletion from mother, father's gene is imprinted

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Prader-Willi Syndrome

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transmission of gene (SNRPN) deletion from father, mother's gene is imprinted

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Anticipation

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disease displays an early onset and/or more severe expression in more recent generations

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Myotonic Dystrophy

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caused by trinucleotide repeat expansion in DMPK gene
exhibits anticipation
characterized by progressive muscular deterioration