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Flashcards in Inheritance patterns Deck (42)
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1
Q

On a pedigree drawing how are males and females represented?

A
  • Males are represented by squares
  • Females represented by circles
2
Q

On a pedigree diagram how are partners represented?

A
  • Partners represented by a line between the two of them
3
Q

On a pedigree diagram how are siblings represented?

A
  • Siblings are represented by a line above them
4
Q

On a pedigree diagram how are carriers and the affected represented?

A
  • Affected people are completely shaded
  • Carriers can either be unshaded or have a dot in them
5
Q

On a pedgree diagram how are children represented?

A
  • Children are represented by a line running down from the parents
6
Q

What does this symbol represent on a pedigree diagram

A
  • Represents a consanguineous partenership (Partners are related to each other)
7
Q

What does this symbol represent on a pedigree diagram?

A
  • Indicates that the person is deceased
8
Q

What do these 2 symbols represent on a pedigree diagram?

A
  • Triangle represents spontaneous abortion
  • Triangle with the line through it represents therapeutic abortion
9
Q

What do these 2 symbols represent on a pedigree diagram?

A
  • Angular lines, with horizontal line connecting them, running down to siblings of same sex indicates monozygotic twins (identical twins)
  • Angular lines, without horizontal line connecting them, which run down to siblings of different sex indicates dizygotic twins (Non-identical twins)
10
Q

What does this symbol represent on a pedigree diagram?

A
  • Represents pregnancy
11
Q

What is autosomal dominant inheritance?

A
  • A form of inheritance in which an individual only needs to have one copy of the mutant allele in order to be affected (herteozygous for the mutant allele).
12
Q

For autosomal dominant inheritance why don’t you usually see people with a homozygous genotype?

A
  • Because usually if a person has both copies of the mutant allele (homozygous for the mutant allele) is in-utero lethal which means the embryo won’t survive the pregnancy
13
Q

What are some of the features of a pedigree diagram showing autosomal dominant inheritance

A
  • Multiple generations affected
  • Both sexes affected equally
  • Male to female & female to male transmission
14
Q

In autosomal dominant inheritance what is the risk of offspring being affeceted?

A

50%

15
Q

Most people who are affected by an autosomal dominant disorder will have a parent that is affceted. What are the two exceptions to this rule?

A
  • De novo mutations - New mutation within a persons genome may mean that that person develops an autosomal dominant disorder without either parent being affected
  • Incomplete penetrance - Results in offspring not showing any symptoms of the disorder (not showing the phenotypes) despite having inherited the mutant from their parents (they have the genotype).
16
Q

Define the term “penetrance”

A

The percentage/proportion of individuals who carry the mutation and develop symptoms of the disorder

17
Q

What is age-dependent penetrance?

A

When the penetrance for a particular disorder increases as individuals get older.

18
Q

What type of penetrance do most autosomal dominant disorder show?

A

Reduced penetrance

19
Q

What is reduced penetrance?

A

When individuals who must possess the mutant allele for a particular disorder show absolutely no manifestations/symptoms of that disorder.

20
Q

Apart from reduced penetrance what are some of the other characteristics of autosomal dominant inheritance?

A
  • Variable expressivity - Idea that the severity of symptoms/symptoms themselves can vary between individuals with the same mutation
  • New mutation rate - New muation rate (de novo mutation rate) varies between different autosomal dominant disorders
  • Somatic mosaicism
  • Germ-line mosaicism
  • Anticipation - Worsening of disease severity in successive generations
21
Q

Explain the idea of Somatic mosaicism

A
  • Refers to the presence of 2 or more populations of cells with different genotypes within the somatic cells of an individual due to the prescence or absence of a new mutation.
  • The new mutation developes at an early stage of embryogenesis.
  • Mutation only affects some of the somatic cells so some tissues/cells will reamin unaffected
22
Q

Explain the idea of germline mosaicism (Gonadal mosaicism)

A
  • Refers to the presence of 2 or more populations of cells with different genotypes within the gametes of an individual due to a new mutation
  • New mutation develops during oogenesis or spermatogenesis
  • Mutation can be passed down to offspring depending on whether the gametes responsible for fertilisation have the mutation or not
23
Q

What is autosomal recessive inheritance?

A
  • A form of inheritance in which an individual needs to have both copies of a mutated allele in order to be affected
  • Person can either be homozygous for the mutant allele or be a compound heterozygote
24
Q

What is meant by the term compound heterozygote and how does it compare to being a homozygote

A
  • A person is a compound heterozygote when they have 2 mutations within the same gene but these 2 mutations are different so they have 2 different recessive alleles
  • A person is a homozygote when they have 2 mutations within the same gene but these 2 mutations are exactly the same so they have 2 of the same recessive allele
25
Q

What are some of the features of a pedigree diagram showing autosomal recessive inheritance?

A
  • Carriers (heterozygote) not affected
  • Both sexes affected
  • Male to female and female to male transmission
  • Usually one generation affected
26
Q

What are some features of autosomal recessive inheritance?

A
  • Trait often found in clusters of siblings but not in parents & offspring.
  • Biological parents of an affected child are usually both heterozygous
  • Recurrence risk = 1/4 for each sibling of affected person.
  • Carrier probability =2/3 for unaffected siblings of affected person.
  • All offspring of affected person are obligate carriers.
27
Q

What is meant by the term obligate carrier?

A

Somebody who may be unaffected but must carry a copy of the abnormal gene based on analysis of family history.

28
Q

In autosomal recessive inheritance how can a child still be affected if only one of their biological parents are heterozygous for the mutant allele?

A
  • A new mutation occurs on the gamete inherited from the other parent
  • Uniparental isodisomy occurs resulting in the offspring inheriting two copies of the heterozygous parent’s mutated allele
29
Q

What is x-linked inheritance?

A
  • A form of inheritance in which a mutation within a gene on the X-chromosome (X-linked gene) leads to a male developing a disorder
30
Q

Why can females be homozygous/heterozygous for X-linked genes while males can only be Hemizygous?

A
  • Females can be either homozygous or heterozygous because they have two X chromosomes and therefore have two copies of X-linked genes
  • Males can only be hemizygous because they have one X and a Y chromosome and so only have a single copy of X-linked genes
31
Q

X-linked inheritance can be dominant or recessive. What are some features of both types of X-linked inheritance?

A

Dominant

  • Women are affected
  • Males more severely affected/lethal

Recessive

  • Women are carriers and unaffected
  • Men are always affected
  • No male to male transmission
32
Q

What are some general features of X-linked inheritance?

A
  • X-linked genes never passed from father to son.
  • All daughters of affected males are obligate carriers.
  • Children of carrier females have a 50% chance of inheriting the mutant allele.
33
Q

What is skewed X-inactivation?

A
  • This occurs when all or most of the genes present on one of the two X chromosomes are silenced.
  • The choice of which X chromosome is silenced is generally random but 10% of women have uneven or skewed X-inactivation - when inactivation of one X chromosome is favoured over the other.
34
Q

Explain the concept of manifesting carriers

A
  • When some women have some symptoms in X-linked recessive conditions
  • Occurs as a result of X-inactivation silencing the genes on a healthy X-chromosome which would leave all the genes on the unhealthy X-chromosome to be expressed
35
Q

What is Y-linked inheritance?

A
  • When a trait/disorder that is produced by genes located on the Y chromosome are passed down to offspring
36
Q

What are some features of Y-linked inheritance?

A
  • Appears in all sons of males who exhibit that trait
  • Is absent from daughters of trait carriers
37
Q

What is the difference between a mutation and a pathogenic mutation?

A
  • Mutation - A change in the structure of a gene
  • Pathogenic mutation - A change in the structure of a gene that results in an alteration of the function of the gene product and can cause a disease phenotype
38
Q

What are the different types of mutation?

A
  • Substitutions
  • Deletions
  • Insertions
39
Q

What are the different types of substitution mutation?

A
  • Synonymous substitution - occurs when a change in a single base doesn’t change the amino acid sequence

Missense substitution - occurs when a change in a single base does change the amino acid sequence therefore changing the protein produced

Nonsense substitution - occurs when a premature stop codon is seen within a DNA sequence resulting in the production of an incomplete protein

40
Q

What is an insertion mutation?

A

Insertion of a single nucleotide into a DNA sequence

41
Q

What is a deletion mutation?

A

Deletion of a single nucleotide from a DNA sequence

42
Q

What is meant when it is said that insertions and deletions can occur “in frame” or “in frameshift?”

A
  • In frame - Insertion/deletion of bases in multiples of 3
  • Frameshift - Insertion/deletion of bases not in multiples of 3 e.g. deletion of only 2 bases