Mendelian Inheritance I

Question 1

what is the “locus”?

Answer 1

position of a gene on a chromosome

Question 2

what are “alleles”?

Answer 2

different forms of a gene that can occupy a single locus

Question 3

what is the “wild type”?

Answer 3

m/c form

Question 4

what is a “mutation”?

Answer 4

form that differs from wild type, can be normal variation or disease causing

Question 5

what is “polymorphism”?

Answer 5

2 or more alleles for a given locus, each present is a populate at a frequency of at least greater than 1%

Question 6

what is “genotype”?

Answer 6

alleles present at a given locus

Question 7

what is “phenotype”?

Answer 7

observable expression of a genotype as a morphological, biochemical or cellular trait

Question 8

what is a compound heterozygote?

Answer 8

genotype with 2 different mutant alleles at the same locus, they are both different and both defective

Question 9

what are the mendelian principles? (3)

Answer 9

principle of dominance
principle of segregation
principle of independent assortment

Question 10

what is the principle of dominance?

Answer 10

genes come in pairs, one copy from each parent

individual genes have different alleles, some are expressed over others

Question 11

what is the principle of segregation?

Answer 11

organisms inherit 2 alleles for each trait and during gamete formation, the pairs separate thus that each gamete has a single allele for every trait
this allows for the ability of a single trait to be traced through generations

Question 12

what is the principle of independent assortment?

Answer 12

genes located at different loci are transmitted independently
allele pairs are randomly reunited at fertilization
an allele at one locus has no effect on an allele at a different locus

Question 13

explain “dominance”

Answer 13

a property of the relationship between a pair of alleles- considered dominant if expressed in heterozygote (only one copy required for expression)

Question 14

what are the characteristics of autosomal dominant inheritance? (5)

Answer 14

phenotype present in every generation 
vertical transmission (each affected offspring with an affected parent)
offspring of affected parent has 50% chance of inheriting 
phenotypically normal parents usually do not transmit
male-to-male transmission can occur

Question 15

in AD disorders, will unaffected parents have affected offspring?

Answer 15

no

Question 16

what is incomplete dominance?

Answer 16

intermediate phenotype exists between 2 alleles

AB is an intermediate between AB and BB

Question 17

what is an example of a condition with incomplete dominance?

Answer 17

achondroplasia

Question 18

what are the 3 allelic possibilities for achondroplasia?

Answer 18

dd- normal
Dd- achondroplasia
DD- lethal severe achondroplasia

Question 19

what is codominanace?

Answer 19

phenotype has features of both alleles

Question 20

what is an example of codominance?

Answer 20

ABO blood typing

A+B are codominant, O is recessive

Question 21

what is a “de novo” mutation?

Answer 21

child born with a genetic d/o when there is no family hx of the disease due to spontaneous mutation at the time of conception
recurrence risk - for the parents to have another child with the same- less than 1%, for the affected offspring to have a child with the same- 50%

Question 22

what is achondroplasia type II?

Answer 22

rare skeletal d/o characterized by mutation in collagen II, autosomal dominant, typically from de novo mutation bc survival past puberty is rare

Question 23

what is germ line mosaicism?

Answer 23

presence of more than one genetically distinct cell line in the germ cells of one of the parents, cannot test for

Question 24

when in germ line mosaicism suspected?

Answer 24

when there are 2 or more children with an autosomal dominant d/o and no family hx- like hemophilia A, DMD, OI

Question 25

what is somatic mosaicism?

Answer 25

presence of more than 1 somatic cell line in individuals

Question 26

what is tissue mosaicism?

Answer 26

occurs after conception, confined to one part of the body

Question 27

what are some AD disorders that present in adulthood?

Answer 27

huntington’s disease and breast CA

Question 28

what are the implications of AD d/o that present later in life?

Answer 28

may present after patient has had children

Question 29

what is penetrance? what are the 2 senarios?

Answer 29

the probability that a gene will be expressed
1- all or nothing
2- reduces/variable- hard to determine

Question 30

what are some examples of diseases with variable penetrance?

Answer 30

HNPCC- 80%
split-hand deformity- 70%
unable to determine who will be affected phenotypically

Question 31

what is variable expression?

Answer 31

differing severity of disease among people with the same genotype, exhibits great variability

Question 32

what is pleiotropy?

Answer 32

one gene that produces multiple different effects

Question 33

what is locus heterogeneity?

Answer 33

single d/o that is caused by mutations in genes at different loci, ex- hereditary breast CA (BRCA1/2)

Question 34

what is allelic heterogeneity?

Answer 34

a single disorder caused by different mutations in the same gene (like CF)

Question 35

what is phenotypic heterogeneity?

Answer 35

different mutations in the same gene give rise to different phenotypes

Question 36

what is an example of phenotypic heterogeneity?

Answer 36

craniosynostosis syndromes- FGFR2 gene
Pfieffer synd-
Apert synd- dev delay
Crouzon synd-

Question 37

characteristics of autosomal recessive inheritance

Answer 37

clinical manifestations only in homozygous for mutant
not typically in every generation
equal male/female affectedness
risk is 1/4 for each pregnancy when both parents carriers

Question 38

what is consanguinity?

Answer 38

parents share a common ancestor

Question 39

what increases risk for AR disorders?

Answer 39

parents both from the same ethnic group

Question 40

what is heterozygote advantage?

Answer 40

persistence of a carrier state bc it provides survival advantage