Terms/Definitions

Question 1

Codominance

Definition + example

Answer 1

Both alleles contribute to phenotype of heterozygote

ex. AB blood type
- alpha-1-antitrypsin deficiency

Question 2

Variable expressivity

Answer 2

Pt. w/ same genotype, but varying phenotype

Ex. 2 patients with neurofibromatosis type 1 (NF1) can have varying disease severity

Question 3

Incomplete penetrance

Answer 3

Not all individuals with mutant genotype show the mutant phenotype
ex. BRCA1 mutation; doesn’t always cause breast or ovarian cancer

Question 4

Pleiotropy

Answer 4

One gene contributes to multiple phenotypic effects

ex. untreated PKU –> light skin, intellectual disability, musty body odour
ex. sickle cell disease
ex. cystic fibrosis

Question 5

Anticipation

Answer 5

Increased severity of earlier onset of disease in succeeding generations
ex. Huntingtons, trinucleotide repeat diseases

Question 6

Loss of heterozygosity

Answer 6

Loss of a normal allele from region of one chromosome of a pair that allows a defective allele on the homologous chromosome to be clinically manifested

• -> Pt. may inherit/develop mutation in tumor suppressor gene (TSG), but the remaining allele is still functioning on the other gene of the chromosome pair. But if there is a loss of heterozygosity with the “normal” allele, normal function is lost (i.e. no TSG) and can lead to development of cancer.
  ex. retinoblastoma, lynch syndrome (HNPCC), Li-Fraumeni syndrome, breast cancer

Question 7

Mosaicism

Answer 7

Presence of genetically distinct cell lines in same individual
ex. McCune-Albright syndrome

Question 8

Locus heterogeneity

Answer 8

Mutations at different loci can produce similar phenotype

ex. albinism, waardenburg syndrome , deafness, retinitis pigmentosa

Question 9

Allelic heterogeneity

Answer 9

Different mutations in same locus produce same phenotype

ex. beta-thalassemia, achondroplasia, familial hypercholesterolemia, polydadctyly

Question 10

Heteroplasmy

Answer 10

Presence of more than one kind of mitochondrial DNA in mitochondria of a single individual

Question 11

What is the Hardy-Weinberg law?

Answer 11

Law assumptions about population:

• no mutation occurs at locus
• natural selection is not occurring
• completely random mating
• no net migration

Question 12

What is uniparental disomy? (UDP)

Answer 12

Offspring receive 2 copies of a chromosome from one parent and NO copies from other parent.
HeterodIsomy (heterozyg) –> indicates meiosis I problem
Isodisomy (Homozyg) –> indicates meiosis II error or post zygotic chromosomal duplication and loss of original
** They still have correct # of chromosomes; euploid (NOT aneuploid!)
- usually normal phenotype

Question 13

Imprinting

Answer 13

Phenomenon of different expression of alleles depending on parent of origin.
At some loci, only one allele is active. The other is inactive (via imprinting/inactivation by methylation)
- if the functioning allele is deleted, it will cause a disease
ex. - Angelman syndrome
- Prader-Willi syndrome
(both chromosome 15 deletion/mutation)

Question 14

Describe type of genetic problem and which type for these syndromes:

• Prader-Willi syndrome
• Angelman syndrome

Answer 14

Prader-willi syndrome –> MATERNAL imprinting. Maternal gene is silent, the paternal gene is deleted/mutated.

Angelman syndrome –> PATERNAL imprinting. Paternal gene is silent, maternal gene silent, paternal gene is mutated/deleted.

Question 15

Haplotype

Answer 15

Set/cluster of alleles at a loci, DNA variations or polymorphisms, that tend to be inherited together as

Question 16

Linkage disequilibrium

Answer 16

Restriction in diversity of haplotypes causing them to be rare in population
–> 2 alleles are said to be in linkage diseq. if they inherit together more (or less) frequently than what is expected

Question 17

Odds ratio

Answer 17

Comparison of the odds of individuals who shared a particular factor (i.e. genotype) that will express the disease/trait vs the odds of individuals without the factor

Question 18

Lyonization

Answer 18

Phenomenon of X inactivation

Question 19

Polyploidy

Answer 19

Any multiple of the haploid chromosome number that is other than the diploid number i.e. 3n, 4n etc. thus you have increase in complete extra set of chromosomes

Question 20

Hemizygous

Answer 20

When one copy is present of a chromosome

ex. sex-linked traits for males are hemizygous

Question 21

Haploinsufficiency

Answer 21

Inability to carry out a function due to only one (half) copy being present instead of two.
–> Can happen if there has been a deletion

Question 22

What are homologous chromosomes?

Answer 22

They are chromosomes that contain DNA that code for the same gene
- BUT, homologous chromosomes are NOT replicas of each other (sister chromatids are exact replicas)

Question 23

Multiple allelism

Answer 23

for many traits/diseases there are more than one mutant alleles @ locus (e.g. cystic fibrosis)
Ex. PKU, marfan syndrome, hemophilia A

Question 24

Genetic compound

Answer 24

individual who has 2 mutant alleles @ locus (complex heterozygosity)

Question 25

Genetic heterogeneity

Answer 25

Many conditions have different genetic causes

Question 26

Define epistasis

Answer 26

Gene interactions can affect phenotypes; when 2 different genes contribute to a single phenotype those genes are epistatic. One gene masks the effect of another

ex. albino colour fur group of labs, if you have “ee” doesn’t matter what your B genotypes are, it won’t be black or brown with homo e’s
ex. bombay blood group

Question 27

Phenocopy

Answer 27

Mimicking of a phenotype that is usually determined by a specific genotype, but instead is caused by interaction of environmental factors

Question 28

What is epigenetic?

Answer 28

Any factor that can change function of a gene without changing the genotype
(Examples of mechanisms: DNA methylation, histone modification, transcription factor binding)

Question 29

Hemizygote

Answer 29

Diploid organism that has one copy of a chromosome

ex. recessive traits are expressed in hemizygotes, can be ex. males for x-linked traits

Question 30

Sex-linked

Answer 30

old term for “x-linked”
- distinctive inheritance pattern of alleles at loci on X chromosome that do not undergo recombination (crossing over) during male meiosis

Question 31

Sex-limited

Answer 31

Trait expressed in only one sex, but the gene that determines the trait is NOT x-linked

Question 32

Sex-influenced

Answer 32

Trait that is not X-linked in its pattern of inheritance but is expressed differently either in degree or frequency in males and females