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medical genetics > S6) Genotype, Phenotype, Inheritance > Flashcards

S6) Genotype, Phenotype, Inheritance Flashcards

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1
Q

What is a pedigree chart?

A

A pedigree is a diagram showing genetic information from a family, using standardized symbols

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2
Q

State 2 purposes of a pedigree chart

A
  • Determines whether the pattern of inheritance for a given trait
  • Discovers whether the gene in question is located on an X or Y chromosome or an autosome
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3
Q

Why is it important to establish how a trait is inherited?

State 3 reasons

A

If the pattern of inheritance can be established, it can be used to predict genetic risk in several situations, including:

  • Pregnancy outcomes
  • Adult-onset disorders
  • Recurrence risks in future off spring
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4
Q

Pedigrees use a standardised set of symbols.

How are males and females represented?

A
  • Squares represent males
  • Circles represent females
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5
Q

Pedigrees use a standardised set of symbols.

How are affected individuals represented?

A

Individuals with the phenotype in question is represented by a filled-in symbol

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6
Q

Pedigrees use a standardised set of symbols.

How are heterozygotes represented?

A

Heterozygotes, when identifiable, are indicated by a shaded dot or a half-filled symbol

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7
Q

Pedigrees use a standardised set of symbols.

How are individuals represented when their sex is unknown?

A

If the sex of a person is unknown, a diamond is used

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8
Q

Pedigrees use a standardised set of symbols.

How are romantic relationships represented?

A
  • Parents are connected by a horizontal line
  • Closely related parents (such as first cousins) are connected by a double line
  • Divorced parents are connected by a horizontal line with slanted lines
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9
Q

Pedigrees used a standardised set of symbols.

How are offspring represented?

A
  • Offspring are connected to parents by a vertical line
  • Off spring are connected by a horizontal sibship line (listed in birth order from left to right)
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10
Q

Pedigrees used a standardised set of symbols.

How are twins represented?

A
  • Non-identical twins are connected by two diagonal lines
  • Identical twins are connected by two diagonal lines as well as a horizontal sibship line
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11
Q

Pedigrees used a standardised set of symbols.

How are aborted, miscarried or stillborn offspring represented?

A

Deceased offspring are represented with a cross through their respective symbols

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12
Q

Pedigrees used a standardised set of symbols.

How are deceased individuals represented?

A

Deceased individuals are represented with a diagonal line through their respective symbol

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13
Q

Explain the numbering system in pedigree construction

A
  • Each generation is identified by a Roman numeral (I, II, III, and so on)
  • Each individual within a generation is identified by an Arabic number (1,2,3, and so on)
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14
Q

What are the five basic Mendelian patterns of inheritance for traits controlled by single genes?

A
  • Autosomal recessive inheritance
  • Autosomal dominant inheritance
  • X-linked dominant inheritance
  • X-linked recessive inheritance
  • Y-linked inheritance
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15
Q

Identify a non-Mendelian pattern of inheritance

A

Mitochondrial inheritance - traits controlled by single genes coded for by mitochondrial genes

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16
Q

Identify 3 unique features of autosomal recessive inheritance

A
  • All the children of two affected (homozygous) individuals are affected
  • The risk of an affected child from two heterozygote parents is 25%
  • For rare traits, affected individuals have unaffected parents
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17
Q

What can be expected with traits on autosomes (non-sex chrosomes)?

A
  • Expressed in both males and females (affected in roughly equal numbers)
  • Both the male and the female parent transmit the trait
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18
Q

Identify 6 different diseases with autosomal recessive inheritance

A
  • Albinism
  • Cystic fibrosis
  • Phenylketonuria
  • Sickle cell anaemia
  • Thalassemia
  • Xeroderma pigmentosum
19
Q

Briefly, describe the features of the following clinical conditions:

  • Albinism
  • Cystic fibrosis
  • Xeroderma pigmentosum
A
  • Albinism: absent pigments in skin, eyes, hair
  • Cystic fibrosis: mucous production that blocks the ducts of certain glands and lung passages
  • Xeroderma pigmentosum: lack of DNA repair enzymes leading to UV light sensitivity and skin cancer
20
Q

Briefly, describe the features of the following clinical conditions:

  • Phenylketonuria
  • Sickle cell anaemia
  • Thalassemia
A
  • Phenylketonuria: excess accumulation of phenylaline in blood leading to mental retardation
  • Sickle cell anaemia: abnormal haemoglobin leading to blood vessel blockage (crises)
  • Thalassemia: improper haemoglobin production with symptoms ranging from mild to fatal
21
Q

Identify 4 unique features of autosomal dominant inheritance

A
  • Most affected individuals are heterozygotes and still have an abnormal phenotype
  • Unaffected individuals carry two recessive alleles
  • All offspring have a 50% chance of being affected
  • Every affected individual has at least one affected parent
22
Q

How can one differentiate between homozygous and heterozygous patients for autosomal dominant disorders?

A
  • Phenotype in homozygous dominant individuals is often more severe
  • Homozygous dominant trait is often not compatible with life
23
Q

An individual with an autosomal dominant disorder has two unaffected parents. How did this disorder occur?

A
  • A mutation occured in single gene
  • Common for genes with high mutation rates
24
Q

Identify 4 different diseases with autosomal dominant inheritance

A
  • Achondroplasia
  • Ehlers-Danlos syndrome
  • Marfan syndrome
  • Huntington disease
25
Q

Briefly, describe the features of the following clinical conditions:

  • Achondroplasia
  • Marfan’s syndrome
A
  • Achondroplasia: dwarfism associated with defects in growth regions of long bones
  • Marfan syndrome: connective tissue defect due to mutation in fibrillin gene, individuals are incredible tall & lanky
26
Q

Briefly, describe the features of the following clinical conditions:

  • Ehler-Danlos syndrome
  • Huntington’s disease
A
  • Ehlers-Danlos syndrome: connective tissue disorder presenting with elastic skin and loose joints
  • Huntington disease: progressive degeneration of nervous system, resulting in dementia and early death
27
Q

What is sex-linked inheritance?

A
  • Sex chromsomes are the X and Y chromosomes
  • Genes on the X chromosome are called X-linked
  • Genes on the Y chromosome are called Y-linked
28
Q

Describe paternal inheritance - genes on the Y chromosome

A
  • Only males have Y chromosomes, hence, traits encoded by Y genes are passed directly from father ⇒ son
  • All Y-linked traits should be expressed because males are hemizygous for all genes on the Y chromosome
29
Q

What does it mean to be hemizygous for genes on sex chromosomes?

A
  • Males cannot be homozygous or heterozygous for both X-linked and Y-linked geness
  • A gene present on either the X/Y chromosome is expressed in males in both the recessive and dominant forms
30
Q

Identify 3 unique features of X-linked dominant inheritance

A
  • Affected males transmit the trait to all their daughters but none of their sons.
  • An affected heterozygous female will transmit the trait to half of her children (both genders affected equally)
  • On average, twice as many females are affected as males (females can be heterozygous or homozygous)
31
Q

Identify 4 unique features of X-linked recessive inheritance

A
  • Affected individuals are hemizygous males and females homozygous for the recessive allele
  • Affected males transmit the trait to all their daughters only
  • Daughters of affected males are usually heterozygous (unaffected)
  • Sons of heterozygous females have a 50% chance of receiving the recessive gene
32
Q

Why are far more males affected by X-linked recessive genetic disorders than are females?

A
  • Females have two X chromosomes and, hence, can be heterozygous or homozygous for any X-linked genes
  • Males only carry only one copy of the X chromosome hence are hemizygous for X-linked genes
  • Males have no dominant X allele to mask expression of the recessive X allele
33
Q

Identify 3 unique features of maternal mitochondrial inheritance

A
  • All the children of affected females are affected
  • Affected females transmit the trait to all their offspring
  • Affected males do not transmit the trait to offspring
34
Q

Why do mitochondrial genes follow a maternal inheritance?

A
  • Mitochondria are transmitted from mothers to all their children through the cytoplasm of the egg
  • Sperm do not contribute mitochondria at fertilization
35
Q

Outline the effects of mitochondrial disorder

A
  • Mutations in mitochondrial genes reduce the amount of energy available for cellular functions
  • Phenotypic effects are highly variable and generally affect tissues with the highest energy demands (muscles & nervous system)
36
Q

What is the name given to mitochondrial disorders which affect mainly muscles?

A

Mitochondrial myopathies

37
Q

What is the name given to mitochondrial disorders which affect both muscles and the nervous system?

A

Mitochondrial encephalomyopathies

38
Q

Incomplete dominance does not follow predicted ratios for a Mendelian trait.

Illustrate this using an example

A

- Incomplete dominance is the expression of a phenotype that is intermediate to those of the parents

  • This involves a distinctive phenotype in heterozygotes:
39
Q

Codominance does not violate the expectations of Mendel’s laws.

What is this phenomenon?

A
  • Codominance is when both alleles in a heterozygote are fully expressed
  • The expected Mendelian genotypic ratio of 1:2:1 is observed
40
Q

Explain the phenomenon of multiple alleles

A
  • Many genes have more than two alleles (multiple alleles)
  • An individual can carry only two alleles of a gene but members of a population can carry many different alleles of a gene
  • E.g. blood groups*
41
Q

Explain the phenomenon of epistasis

A
  • Epistasis is the interaction of two or more non-allelic genes to control a single phenotype
  • The action of one gene masks or prevents the expression of another gene e.g. Bombay phenotype
42
Q

What is the Bombay phenotype?

A
  • The Bombay phenotype is when a mutation in a the h gene prevents the expression of the A and B phenotypes
  • Individuals homozygous for a recessive allele h are blocked from expressing the A or B surface antigen
  • They are phenotypically blood type O, even though genotypically carry IA / IB alleles
43
Q

Explain mendelian inheritance by independent assortment

A
  • The inheritance of two traits in humans follows the principle of independent assortment
  • Gametes independently assort themselves during metaphase of meiosis increasing the variation in offspring
44
Q

What is a proband?

A

A proband is the first affected family member who seeks medical attention for a genetic disorder (indicated by an arrow)

medical genetics (12 decks)

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