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Mendelian inheritance

Think "single gene inheritance"

Monogenic: single-gene


Law of segregation:

either tall OR dwarf


Law of independent assortment:

wrinkled/tall VS wrinkled/dwarf

One particular trait wasn't dependent upon another

Chromosome 1 is not dependent on chromosome 4 etc in division


Dystrophin and Duchenne MD

Loss of function


Oncogene and cancer

Gain of function


β-globin and sickle cell anemia

Protein alteration



An individual's genetic makeup

Gene: A, a etc
Genotype: AA or Aa



What is actually observed


Individuals w/ distinct genotypes can have a single phenotype

Ex. Cystic Fibrosis

"Allelic heterogeneity" -can affect the expressivity of the disease

Gene can be mutated in several ways leading to various outcomes

(Just know general overview of classes)

Class 1 defect: don't get a protein even though you have the gene (most severe)

Class 2: gets made and gets outside but is desposed of because cell thinks it's garbage

Class 3: Makes way to cell surface but isn't regulated, doesn't bind well to membrane or other proteins

Class 4: Makes way to membrane but doesn't transport chloride as it should

Class 5: Underproduction: you get the protein but you just don't have enough of it (not as serious)

Class 6: recognized as not doing well and gets disposed of



People w/ PKU have a defective PAH enzyme so phenylalanine builds up. Normally the PAH enzyme breaks down phenylalanine. Affects person's IQ

*Individuals with the same genotype can have multiple phenotypes



Site on chromosome: where gene is. 2 alleles at locus


Pedigrees: proband (propositus)

The 1st diagnosed person in pedigree

(Arrow --> donotes the proband)


Autosomal Dominant Inheritance

Ex. Postaxial Polydactyly

**Only 1 allele of a gene is needed for expression, 1 copy

Autosomal: all chromosomes except x/y, so there are 22 out of the 23

Affected offspring has one affected parent

Both males and females can transmit trait to both males and females-autosomal

Trait is expected in EVERY generation (i.e. vertical transmission)

Recurrent risk for having affected children of proband is 50%
(chance of infected person giving it to their kids-infected father+not infected mother) Aa x aa

Affected offspring have one affected parent


Autosomal Recessive

2 copies of a gene is needed to influence phenotype

Ex. Tyrosinase (converts tyrosine to precursor for melanin)-albinism. If enzyme is defective, you get albinism


Autosomal Dominant vs Recessive

Dominant needs 1 copy and recessive needs 2 copies


Punnet Square for Autosomal Recesive Inheritance


25% chance of getting disease (need aa)


Autosomal Recessive: Pedigree

Affected individuals have normal parents

Recurrent risk for heterozygote parents is 25%

Both males and females may be affected

Affected individuals who mate with normal individuals tend to have normal children

Occurrence is more likely among individuals who share genes, as with consanguinity (first cousin mating)


Autosomal dominant vs Autosomal Recessive inheritance

Autosomal dominant: disease expected in every generation


What does consanguinity (first cousin mating) look like on pedigree chart?

Double line connecting sexual partners





X-linked Recessive

Males only have 1 X chromosome and females have 2

Unaffected males do not transmit the trait (no carriers)

All daughters of affected males are heterozygous carriers

Ex. Duchenne Muscular Dystrophy


Allele on X-chromosome in males is termed _______



X-linked recessive for women

Females can be heterozygous or homozygous


Females passing on X-linked Recessive trait to children

Female carriers transmit the affected allele to 50% of sons and 50% of daughters


X-linked dominant

Ex. Hypophosphatemia (defect in the reabsorption of phosphate in kidney).

Very rare

No carriers

Males transmit the trait to only females; 100% of females (men give their Y to the son and not their X, unlike females getting both X's from parents)

Females transmit the trait to males and females; transmit to 50% of offspring (Xx x Xy)

Both males and females affected



Abnormally low levels of phosphorus in blood due to defective reabsorption of phosphate in kidney

Deficient reabsorption of Ca in intestines causes softening of bones (RIckets)

Vitamin D metabolism abnormal

Short stature: abnormally short limbs with onset


Reduced Penetrance

We would expect that 100% of people inheriting the genetic defect to have the problem (phenotype) but it's more like 90%

Ex. Retinoblastoma

Autosomal dominant inheritance

Phenotype occurs in 90% of individuals inheriting gene defect so 90% penetrance


Variable Expressivity

Describes the range of phenotypes that vary between individuals with a specific genotype

Ex. Neurofibromatosis
-Develop tumor-like growths called neurofibromas
-Patients have cafe-au-lait spots; pigmented areas the color of coffee with cream (spots differ in number, shape, size and position)


Locus Heterogeneity

Single disorder, trait, or pattern of traits caused by mutations in genes at different chromosomal loci

Also in PKU?

Ex. Osteogenesis Imperfecta
-Brittle-bone disease
-Mutations in collagen genes (2 loci; chromosome 7 and 17), either mutation exhibits the same phenotype



Pleiotropy occurs when one gene influences two or more seemingly unrelated phenotypic traits.

If same disease has many tissues/organ involve it is example of pleiotropy

Literally means"many turnings" -a given phenotype is carefully observed at different levels or as you said a mutation that affects multiple organs.