Pedigree/Mendelian Inheritance

Question 1

The affected member through whom a family with a genetic disorder is brought to attention is
the__.

Answer 1

proband (‘propositus’ or ‘index case’)

Question 2

Genes can come in different versions, called ‘_____’, which can result in different observed
phenotypes.

Answer 2

alleles

Question 3

Mendel’s First Law:

Answer 3

The Law of Segregation. At meiosis, alleles separate (or segregate) from each other such that each gamete (egg or sperm) receives one copy from each allele pair.

Question 4

Mendel’s Second Law:

Answer 4

The Law of Independent Assortment. At meiosis, the segregation of each pair of alleles is independent. [Note: genes physically near each other (‘linked’) on the same
chromosome violate this law]

Question 5

Traits are_____ if both traits (alleles) are expressed in the heterozygous state

Answer 5

codominant

Question 6

When the heterozygous phenotype is intermediate between the two homozygous phenotypes, the
terms _______ or ______ are appropriate.

Answer 6

semi-dominant; incompletely dominant

Question 7

Mitochondrial Inheritance:

Answer 7

This is a form of non-Mendelian inheritance, where recognizable patterns can be found. The inheritance pattern is matrilineal (maternally-inherited from mothers to her
offspring) and is discussed in your text and in a future lecture.

Question 8

Penetrance:

Answer 8

The fraction of individuals with a trait (disease) genotype who show manifestations of the disease. If all persons carrying a mutation have the trait, the condition is 100% penetrant. If some mutation carriers do not show signs of the trait, penetrance is incomplete (<100%). Incomplete penetrance is more commonly encountered. Penetrance is analogous to a light switch (can be ‘on’ or ‘off’).

Question 9

Expressivity:

Answer 9

The degree to which a trait is expressed in an individual (is a measure of severity). Expressivity is analogous to a light dimmer (the light is ‘on’ but the brightness (expressivity) exists along a spectrum (of severity)). This variation is explained (in part) by sex influence, environmental factors, stochastic effects, and modifier genes.

Question 10

Pleiotropy:

Answer 10

Used to describe multiple different phenotypic effects due to mutation(s) in a single gene. Often used, when the phenotypes are seemingly unrelated and/or in multiple different tissues. Example: Neurofibromatosis Type I leads to: café au lait spots, neurofibromas, hammartomas in the eyes, abnormal freckling, and learning difficulties.

Question 11

Phenocopies:

Answer 11

Diseases (traits) which are due to non-genetic factors. Example: A thyroid cancer due to radiation exposure cannot always be distinguished from a thyroid cancer due to mutations in RET gene.

Question 12

Hemizygous:

Answer 12

Single copy of a gene (ie men are hemizygous for x)