lecture 5 Flashcards
1
Q
family pedigrees
A
- diagrammatic representation of members of a family for many generations
- helps to understand the past and determine the future
- to help assess the risk of a disease reemerging in future generations
2
Q
autosomal dominant
A
- one copy of mutated allele -> dominant pathological gene -> disease
- vertical inheritance : appears in every generation
- affected people have at least one affected parent
- child of an affected parent has 50% chance to inherit
3
Q
autosomal recessive
A
- two copies of mutant alleles leads to disease
- low frequency in population
- horizontal inheritance: not in every generation with one or more siblings having the disorder
- parents are usually healthy carriers
- 50% of offspring are carriers and 25% have the disease
- consanguinity increases the frequency of affected individuals
- healthy children can be born from affected parents -> genetic heterogeneity ( different genes cause similar diseases, parents are not homozygous for the same gene)
4
Q
genetic heterogeneity
A
mutations in different genes (non complementation) can cause the same disease
- when two different mutations appear in the family, but they compliment each other to result in a normal phenotype
- two homozygous recessive will result in all offspring being homozygous recessive in one allele -> affected offspring
5
Q
incomplete dominant inheritance
A
- phenotype of heterozygotes is an intermediate between homozygous that are normal and homozygous that are affected
- mix of traits
- alleles do not predominate against each other but are both partially expressed -> intermediate phenotype -> incomplete dominant genes
- eg sickle cell anaemia (autosomal recessive)
carriers occasionally express pathological phenotype when exposed to unfavorable conditions
6
Q
codominant inheritance
A
- heterozygote expresses both alleles -> both affect the phenotype in separate ways
- alleles dont predominate each other, but are expressed equally -> combined phenotype -> codominant genes
- eg blood types
7
Q
sex linked inheritance
A
- X has more genes than Y
- Y : 50 genes, none necessary for life -> associated with reproductive function of males
- x-linked recessive characteristics appears in female only when homozygous
-> male always shows the character (hemizygous)
8
Q
x linked dominant inheritance
A
- affected men will 100% pass to their daughters
- affected women will transmit 50% of both sons and daughters
- vertical inheritance
- incidence is twice as high in females, but show milder symptoms
9
Q
x linked recessive inheritance
A
- males always affected
- females always carriers
- female carrier: 50% to their sons(affected), 50% to their daughters (carriers)
- male: 100% to daughters (carriers)
- higher in males than females
- may occasionally occur in females due to
– random inactivation of X
– real homozygosity (mother carrier and father is affected, or de novo mutation in one of the two parents)
– turner syndrome (X0)
– translocation between X and autosomal chromosome
10
Q
y linked inheritance
A
- only males affected
- vertical inheritance
- all sons affected when father affected
11
Q
multiple alleles
A
- 3 or more alleles at the same locus that control the same characteristic in a different way
- usually co-dominant
- co-dominant or isoalleles ( alleles that in a heterozygous state are both expressed giving a third phenotype - mix of traits)
- eg AB0 blood group
– controlled by 4 alleles for membrane proteins on red blood cells (A1,A2 and B1 are codominant, 0 is recessive)
– individual carries only 2 of the 4 alleles - rhesus factor
– at least 8 isoalleles, some dominant, some recessive
12
Q
penetrance
A
- when a genotype does not display the corresponding expected phenotype
- mainly autosomal dominant genes
- full penetrance: genotype is always expressed (heterozygotes manifest the phenotype)
- incomplete penetrance: genotype not always expressed
(some heterozygotes express the phenotype, some dont) , can skip a generation
– may be due to: interaction with normal allele, interaction with other genes, interaction with environment, random event
13
Q
expressivity
A
- when the phenotype of a character varies from person to person
- phenotype with different degrees of expression in different individuals -> varied expressivity
- high, intermediate, low or 0% expressivity
- may be due to interaction with normal allele/ other genes/ environment or random event
- observed in all types of inheritance
14
Q
Anticipation
A
- phenotype appears earlier and is more severe in subsequent successive generations
- due to nucleotide or whole chromosome segment repetitions
- instability in generations: repetition changes in size from parents to offspring
- gender of parent bearing and transmitting to offspring plays a main role
– eg disease appears earlier when mutations come from mother
15
Q
partially sex-linked inheritance
A
- genes located in homologous regions of p-arm of X and Y chromosomes -> pseudo-autosomal region (homologous regions between X and Y chromosome)
- gene exchange in crossing over during meiosis
-> explains why certain diseases appear as x-linked in some families and y-linked in others
