Inheritance patterns

Question 1

autosome

Answer 1

any chromosome, other than sex chromosomes, that occurs in pairs in diploid cells

Question 2

recessive

Answer 2

manifest only in homozygotes

Question 3

allele

Answer 3

one or more alternative forms of a gene at a given location (locus)

Question 4

homozygous

Answer 4

presence of identical alleles at a given locus

homozygotes are affected

Question 5

heterozygous

Answer 5

presence of two different alleles at a given locus

heterozygotes are unaffected - carriers

Question 6

allelic heterogeneity

Answer 6

different mutations within the same gene result in the same clinical condition
individual with an autosomal recessive may be a compound heterozygote for 2 different mutations

Question 7

autosomal recessive inheritance

Answer 7

disease manifest in homozygous state (double dose of affected allele)

Question 8

typical features of autosomal recessive inheritance

Answer 8

males and females affected in equal proportions
affected individuals only in single generation
parents can be related

Question 9

cystic fibrosis

Answer 9

commonest AR condition affecting northern europe

chronic condition affection mainly lungs and gut, variable presentation

Question 10

incidence of cystic fibrosis

Answer 10

1 in 2500

Question 11

gene affected by cystic fibrosis and mutations

Answer 11

CFTR gene on 7q31.2

over 1000 mutations (mutational hetergeneity)

Question 12

commonest mutation in cystic fibrosis

Answer 12

deltaF508

Question 13

standard cf carrier testing

Answer 13

detects top 29 mutations

Question 14

diagnostic test for cf

Answer 14

sweat testing

Question 15

genotype-phenotype correlations in cf

Answer 15

F508/F508 - pancreatic insufficiency and chronic lung disease

F508/R117H -majority are pancreatic sufficient but have chronic lung disease

Question 16

consanguinity

Answer 16

reproductive union between 2 relatives

Question 17

autozygosity

Answer 17

homozygosity by descent - inheritance of same altered allele through 2 branches of same family

Question 18

autosomal dominant inheritance

Answer 18

disease manifest in the heterozygous state, i.e only one affected gene needed

Question 19

typical features of autosomal dominant inheritance

Answer 19

males and females affected in equal proportions
affected individuals in multiple generations
transmission by individuals of both sexes to both sexes

Question 20

penetrance

Answer 20

percentage of individuals with a specific genotype showing the expected phenotype

Question 21

expressivity

Answer 21

range of phenotypes expressed by a specific genotype

Question 22

recurrence risk

Answer 22

50% for transmission of mutation

depends on penetrance and expression

Question 23

new mutation example

Answer 23

neurofibromatosis type 1 - up to 50% of cases occur due to de novo mutation

Question 24

anticipation and example

Answer 24

genetic disorder affects successive generations earlier or more severely due to expansion of unstable triplet repeat sequences

myotonic dystrophy

Question 25

somatic mosaicism

Answer 25

genetic fault present in only some body tissues

Question 26

gonadal mosaicism

Answer 26

genetic fault present in gonadal tissue

Question 27

late onset

Answer 27

condition not manifest at birth

hypertrophic cardiomyopathy

Question 28

sex limited

Answer 28

condition inherited in AD pattern that seems to affect one sex more than the other

Question 29

X linked inheritance

Answer 29

genes carried on X chromosome

Question 30

typical features of X-linked inheritance

Answer 30

usually only males affected
transmitted through unaffected females
no male-to-male transmission

Question 31

lyonization

Answer 31

generally only one of two X chromosomes active in each female cell
can be skewed

Question 32

imprinting

Answer 32

epigenetic phenomenon causing genes to be expressed in a parent of origin specific manner

Question 33

where is mitochondrial DNA inherited from?

Answer 33

mother

Question 34

homoplasmy

Answer 34

eukaryotic cell whose copies of mitochondrial DNA are all identical (identically normal or identical mutations)

Question 35

heteroplasmy

Answer 35

multiple copies of mtDNA in each cell
denote mutations affecting only a proportion of molecules in a cell
level varies between cells, organs and individuals

Question 36

mitochondrial genetic disease

Answer 36

group of disorders caused by dysfunctional mitochondria

mutations in mtDNA, in nuclear genes or by acquisition e.g. drugs