the inheritance of single-gene differences
TRANSCRIPT
The Inheritance of Single-Gene Differences
Human pedigree analysis and organelle inheritance
Genotype and
Phenotype
What is the difference?
Review Words} Characteristics – are the category of a trait –
} Example – eye color, height, likes/dislikes
} Traits – the physical, social, and emotional qualities of an organism} Example – blue eyes, tall, hates carrots
} Dominant Trait – when a majority of an organism shows the trait. } Example – most pea plants show as tall
} Recessive Trait – when a minority of an organism shows the trait.} Example – few pea plants show as short
} Alleles – all the possible choices for a characteristic} Example – eye color – blue, brown, gray, green
Genotype
}How the genes code for a specific trait.
} If the trait is dominant it uses a capital letter} Example – Tall (T)
} If the trait is recessive it uses the same letter but lower case} Example – short (t)
}Genotypes always have two letters –one for dad and one for mom
Types of genotype
´Purebred (homozygous) dominant – the genes only have the dominant trait in its code.´Example – Dominant Tall -- TT
´Purebred (homozygous) recessive – the genes only have the recessive trait in its code.´Example – Recessive short – tt
´Hybrid (heterozygous) – the genes are mixed code for that trait.´Example – hybrid Tall -- Tt
Phenotype
´The outward appearance of the trait.
´How an organism looks´How an organism acts´How an organism feels
Tricks to remembering the difference between Genotype and Phenotype
´Genotype – deals with GENE CODE.´Phenotype – deals with looks you can take a PHOTO with.
Pedigrees analysis´Analysis of inheritance in human families´Typically small number of offspring´ -> Mendelian ratios rarely observed´Allow inferences concerning genotypes
and predictions concerning phenotypes of offspring (genetic counseling)
unaffectedmale
unaffectedfemale
affectedmale
affectedfemale
Most common signs and symbols used in pedigree
analysis
Categories of inheritance ´Autosomal recessive
´e.g., PKU, Tay-Sachs, albinism´Autosomal dominant
´e.g., Huntington’s Disease´X-linked recessive
´e.g., color-blindness, hemophilia´X-linked dominant
´e.g., hypophosphatemia´Y-linked´Organelle
Pedigree analysis: case 1
´ Two children, one of each sex, show the trait but trait was not shown in the parents
´ Conclusions:´ must be autosomal recessive trait (example: PKU)´ parents must be heterozygous (Pp)´ 2/3 chance for each child to be heterozygous (Pp)´ 1/3 chance for each child to be homozygous (PP)
P/p P/p
pppp PP or Pp PP or Pp PP or Pp
An autosomal recessive disorder is revealed by the appearance of the phenotype in both male and female progeny of unaffected individuals, who may be inferred to be heterozygous carriers
Autosomal recessive inheritance in pedigrees
Pedigrees: Case 2
A/a a/a
a/a a/aA/a A/aA/a
A/a A/a A/a
a/aa/aa/a
a/a a/a a/a a/a a/a a/a
Autosomal dominant (AD) disorders are those in which both heterozygotes and homozygousdominant individuals show the abnormal phenotype.
One copy of the mutant gene is sufficient for expression of the abnormal phenotype = haplo-insufficiency (Note: In fact, in some AD diseases the homozygous genotype is incompatible with life)
Autosomal dominant disorders
- Every individual developing the disease must have an affected parent (except in cases of de novo mutations)
- Males and females are equally likely to inherit the allele and be affected (autosomal disorder)
- Recurrence risk (the probability that a genetic disorder that is present in a patient will recur in another member of the family) for each child of an affected parent is 1⁄2. If one parent is a heterozygote for a particular gene, their offspring will either inherit the gene or they will not, with each outcome equally likely.
- Normal siblings of affected individuals cannot pass the trait on to their offspring. If an affected individual’s siblings are not affected, they do not carry the mutation and cannot pass it on to their own offspring (thus a dominant mutant allele should be lost rapidly from the population if it affects greatly the fitness of the carrier).
Characteristics of pedigrees for AD disorders:
Typical pedigree for AD disorder
A/a a/a
a/a a/aA/a A/aA/a
A/a A/a A/a
a/aa/aa/a
a/a a/a a/a a/a a/a a/a
- Half the people in the Venezuelan village of Barranquitas are affected
- A large-scale pedigree analysis was conducted including 10,000 people
- Example for one particular family:
Huntington’s disease: an example of AD disorder
Huntington’s disease: an example of AD disorder- Neurological disorder causing convulsions, paralysis, loss of memory and eventually death
- Affected people do not show symptoms until their 30s to 50s (so they may not know that they are affected before they have children)
- Caused by programmed death of brain cells
- Exact reasons why brain cells die are unknown but HD gene has been identified on Chr4. He encodes a protein of unknown function (huntingtin).
- Disease caused by extension of CAG triplet within coding sequence of HD gene, resulting in a protein with a longer stretch of glutamin residues
- It is the presence of this abnormal form, and not the absence of the normal form, that causes harm in HD. This explains why the disease is dominant and why two copies of the defective gene do not cause a more serious case than inheritance from only one parent.
Pedigrees: Case 3
´ If allele associated with trait is very rare, this pedigree is most consistent with X-linked recessive inheritance
´ A single affected female would indicate autosomal (because for X-linked trait, homozygous females can only result from parents both carrying the recessive allele so affected female are very, very rare)
XAXaXAY
XaY XAXa or
XAXA? XAY XAXa or
XAXA ?XAY
Pedigrees: Case 3 XAXaXAY
XaY Must beXAXa XAY XAXa or
XAXA XAY
XAYXAYXaY XaY XaYXAXa or
XAXA
are characterized by the following pedigree pattern:
(1) Many more males than females develop the disease (ie. show the phenotype)
(2) None of the offspring of an affected male are affected, but all of its daughters must be heterozygous carriers (half the sons born to these carrier daughters are affected)
X-linked recessive disorders
Pedigree of an X-linked recessive disorder
Son non affected
Daughter carrier
1/2 grandsons affected
Father affected
are characterized by the following pedigree pattern:
(1) Affected males pass the condition on to all their daughters but none of their sons (unlike dominant autosomal disorders where daugthers and sons have an equal probability to inherit the disease)
(2) Affected females are mostly heterozygotes. When married to unaffected males, they pass the condition on to 1/2 of their sons and 1/2 of their daughters (same pattern than for autosomal dominant disorder)
X-linked dominant disorders
Note: X-linked dominant disorder are rare traits in human (ex: hypophosphatemia: low levels of inorganic phosphate in the blood.) Diagnose is complicated by the process of X inactivation in females.
Case 4: X-linked dominant disorders
Organelle inheritance
´ Mitochondria and chloroplasts´ small number of genes on circular chromosome´ mostly inherited through maternal lineage via egg cytoplasm´ organelle DNA are circular, and furthermore DNA circles can be
seen in organelle preparations under the electron microscope.´ organelle chromosomes are present in many copies per cell, often
in the hundreds or thousands´ In human there can be from 2 -
10 mtDNA molecules/mitochondrion´ 22 human mitochondrial tRNAs are shown on the maps
´ Examples´ white green variegation in plants´ Maternal inheritance also shown to primarily occur in humans but
some controversy on possible paternal contributions as well (and possible recombination between maternal and paternal mitochondrial genomes)
THE STEPS WHEN INTERPRETING A PEDIGREE CHART
§ Determine if the pedigree chart shows an autosomal or X-linked disease.
§ If most of the males in the pedigree are affected, then the disorder is X-linked
§ If it is a 50/50 ratio between men and women the disorder is autosomal.
INTERPRETING A PEDIGREE
§
CHARTDetermine whether the disorder is dominant or recessive.
§ If the disorder is dominant, one of the parents must have the disorder.
§ If the disorder is recessive, neither parent has to have the disorder because they can be heterozygous.
Pedigree showing transmission and expression of a mitochondrial trait. Note that transmission occurs only through females.
Rules of InheritanceAutosomal Recessive
•Appears in both sexes with equal frequency••
•
Trait tend to skip generations Affected offspring are usually born to
unaffected parentsWhen both parents are hetrozygout,
approx. 1/4 of the progeny will be affected• Appears more frequently among the
children of consanguine marriages
Rules of Inheritance
•••
Autosomal DominantAppears in both sexes with equal frequency Both sexes transmit the trait to their offspring Does not skip generations
• Affected offspring must have an affected parent unless they posses a new mutation
•When one parent is affected (het.) and the other parent is unaffected, approx. 1/2 of the offspringwill be affected
• Unaffected parents do not transmit the trait
Rules of Inheritance
•
•
X-Linked DominantBoth males and females are affected; often more
females than males are affectedDoes not skip generations.
••
Affected sons must have an affected mother; affected daughters must have either an affected mother or an affected father
• Affected fathers will pass the trait on to all theirdaughters
• Affected mothers if heterozygous will pass the trait on to 1/2 of their sons and 1/2 of their
daughters
Rules of InheritanceX-Linked Recessive
´More males than females are affected Affected sons are usually born to unaffected mothers, thus the trait skips generations Approximately 1/2 of carrier mothers’ sons are affected´ It is never passed from father to son´All daughters of affected fathers arecarriers
Rules of Inheritance
••
MitochondrialTrait is inherited from mother only
All children of a mother are at risk to beaffected or carriers
•••
Y-Linked Dominant Only males are affected
It is passed from father to all sons It does not skip generations
Example 1
X-linked recessive Hemophilia
Only males are affected and sons do not share the phenotype of their father - Thus X-linked
Expression of hemophilia skips generations: RECESSIVE
Example 2
X-Linked Dominant Every Generation: Dominant
Father passes on to only daughters Mothers passes on to 1/2 of offspring
Example 3
Autosomal RecessiveAffected individual from
unaffected parents
Example 4
Autosomal recessive Expressed in both sexes at approximately
equal frequency: AUTOSOMALNot expressed Autosomal Recessive in every
Example 5
Autosomal Dominant In every generation:
DOMINANTEqual in Males and Females:
Example 6
Autosomal DominantAppears equally In both sexes so autosomal
In every generation so
Example 7
Y-LinkedOnly males are affected All sons of
affected father
Example 8
X-Linked Dominant
Every generation: DOMINANT
Daughters of affected males are affected
Example 9
Autosomal DominantIn every generation: DOMINANT
Both Male and female affected:
Example 10
Mitochondrial All children at risk
Father doesn’t pass it along to any children
Example 11
Autosomal Recessive
Consanguinity
Example 12
Mitochondrial Inheritance
Example 13
Autosomal dominantIn every generation: DOMINANT In males and females: Autosomal
Example 14
Mitochondrial InheritanceFathers don’t transmit, just mothers
Example 15
X-linked Recessive