Additional information

Heritability

Heritability describes how much of the variation in a trait (within a population) can be explained by genetic differences between people. It’s written as h^2 and ranges from 0 to 1.

A heritability close to 1 means most of the variability in the trait is due to genetic differences, with relatively little contribution from environmental factors.
A heritability close to 0 means most of the variability is due to environmental factors, with relatively little contribution from genetic differences.

Multifactorial disorders (such as type 2 diabetes, obesity, and heart disease) are influenced by multiple factors, including genetics, so their heritability is typically somewhere in the middle.

Heritability has historically been estimated using twin studies by comparing incidence in identical (monozygotic) versus fraternal (dizygotic) twins. A disorder with high heritability will show a higher incidence in identical twins than in fraternal twins.

Types of chromosomes

Metacentric: have a centrally located centromere
Submetacentric: have an off-centered centromere
Acrocentric: centromere at the end of one arm; seen in chromosomes 13, 14, 15, 21 and 22

Homologous and non-homologous chromosomes: Homologous chromosomes carry the same types of genes at the same loci (though the alleles may differ). Non-homologous chromosomes carry different sets of genes. For example, paternal chromosome 1 is homologous to maternal chromosome 1, but it is non-homologous to maternal chromosome 3. Homologous chromosomes pair during meiosis I; non-homologous chromosomes do not. Exchange of genetic material between homologous chromosomes occurs by recombination, while genetic exchange between non-homologous chromosomes occurs through translocation.

Williams syndrome

Congenital microdeletion of chromosome 7q
“Elfin” facies: broad forehead, wide mouth, short nose, dental malformations
Mental retardation, ADHD, anxiety, phobias
Supravalvular aortic stenosis, hypertension
Skin laxity, short stature, hypercalcemia

Two siblings have a 25% chance of being genotypically HLA identical, a 50% chance of being HLA haploidentical (sharing one haplotype), and a 25% chance of sharing no HLA haplotypes. In each cell, one haplotype is paternal and the other is maternal, so there is 50% HLA sharing with each parent.

DNA sharing between relatives

Relationship	Percent sharing
Identical twin	100
Parent	50
Sibling	50
Grandparent, uncle, aunt, grandchild, niece, nephew, half-sibling	25
1st cousin	12.5

Heritability

Measures proportion of trait variation due to genetics (h^2, 0–1 scale)
High heritability: mostly genetic; low heritability: mostly environmental
Estimated via twin studies (monozygotic vs. dizygotic comparison)
Multifactorial disorders: intermediate heritability

Types of chromosomes

Metacentric: centromere in center
Submetacentric: off-center centromere
Acrocentric: centromere near end (chromosomes 13, 14, 15, 21, 22)

Homologous and non-homologous chromosomes

Homologous: same genes/loci, pair in meiosis I, recombination occurs
Non-homologous: different genes, do not pair, exchange via translocation

Williams syndrome

Microdeletion of chromosome 7q
Features: elfin facies, intellectual disability, ADHD, anxiety, supravalvular aortic stenosis, hypertension, hypercalcemia

HLA haplotype sharing

Siblings: 25% HLA identical, 50% haploidentical, 25% no sharing
Each parent shares 50% HLA haplotype with child

DNA sharing between relatives

Identical twin: 100%
Parent/sibling: 50%
Grandparent, uncle/aunt, half-sibling: 25%
1st cousin: 12.5%

Additional information

Heritability

Heritability describes how much of the variation in a trait (within a population) can be explained by genetic differences between people. It’s written as h^2 and ranges from 0 to 1.

A heritability close to 1 means most of the variability in the trait is due to genetic differences, with relatively little contribution from environmental factors.
A heritability close to 0 means most of the variability is due to environmental factors, with relatively little contribution from genetic differences.

Multifactorial disorders (such as type 2 diabetes, obesity, and heart disease) are influenced by multiple factors, including genetics, so their heritability is typically somewhere in the middle.

Types of chromosomes

Metacentric: have a centrally located centromere
Submetacentric: have an off-centered centromere
Acrocentric: centromere at the end of one arm; seen in chromosomes 13, 14, 15, 21 and 22

Williams syndrome

Congenital microdeletion of chromosome 7q
“Elfin” facies: broad forehead, wide mouth, short nose, dental malformations
Mental retardation, ADHD, anxiety, phobias
Supravalvular aortic stenosis, hypertension
Skin laxity, short stature, hypercalcemia

DNA sharing between relatives

Relationship	Percent sharing
Identical twin	100
Parent	50
Sibling	50
Grandparent, uncle, aunt, grandchild, niece, nephew, half-sibling	25
1st cousin	12.5

Heritability

Measures proportion of trait variation due to genetics (h^2, 0–1 scale)
High heritability: mostly genetic; low heritability: mostly environmental
Estimated via twin studies (monozygotic vs. dizygotic comparison)
Multifactorial disorders: intermediate heritability

Types of chromosomes

Metacentric: centromere in center
Submetacentric: off-center centromere
Acrocentric: centromere near end (chromosomes 13, 14, 15, 21, 22)

Homologous and non-homologous chromosomes

Homologous: same genes/loci, pair in meiosis I, recombination occurs
Non-homologous: different genes, do not pair, exchange via translocation

Williams syndrome

Microdeletion of chromosome 7q
Features: elfin facies, intellectual disability, ADHD, anxiety, supravalvular aortic stenosis, hypertension, hypercalcemia

HLA haplotype sharing

Siblings: 25% HLA identical, 50% haploidentical, 25% no sharing
Each parent shares 50% HLA haplotype with child

DNA sharing between relatives

Identical twin: 100%
Parent/sibling: 50%
Grandparent, uncle/aunt, half-sibling: 25%
1st cousin: 12.5%

Additional information

Heritability

Heritability describes how much of the variation in a trait (within a population) can be explained by genetic differences between people. It’s written as h^2 and ranges from 0 to 1.

A heritability close to 1 means most of the variability in the trait is due to genetic differences, with relatively little contribution from environmental factors.
A heritability close to 0 means most of the variability is due to environmental factors, with relatively little contribution from genetic differences.

Multifactorial disorders (such as type 2 diabetes, obesity, and heart disease) are influenced by multiple factors, including genetics, so their heritability is typically somewhere in the middle.

Types of chromosomes

Metacentric: have a centrally located centromere
Submetacentric: have an off-centered centromere
Acrocentric: centromere at the end of one arm; seen in chromosomes 13, 14, 15, 21 and 22

Williams syndrome

Congenital microdeletion of chromosome 7q
“Elfin” facies: broad forehead, wide mouth, short nose, dental malformations
Mental retardation, ADHD, anxiety, phobias
Supravalvular aortic stenosis, hypertension
Skin laxity, short stature, hypercalcemia

DNA sharing between relatives

Relationship	Percent sharing
Identical twin	100
Parent	50
Sibling	50
Grandparent, uncle, aunt, grandchild, niece, nephew, half-sibling	25
1st cousin	12.5

Key points

Heritability

Measures proportion of trait variation due to genetics (h^2, 0–1 scale)
High heritability: mostly genetic; low heritability: mostly environmental
Estimated via twin studies (monozygotic vs. dizygotic comparison)
Multifactorial disorders: intermediate heritability

Types of chromosomes

Metacentric: centromere in center
Submetacentric: off-center centromere
Acrocentric: centromere near end (chromosomes 13, 14, 15, 21, 22)

Homologous and non-homologous chromosomes

Homologous: same genes/loci, pair in meiosis I, recombination occurs
Non-homologous: different genes, do not pair, exchange via translocation

Williams syndrome

Microdeletion of chromosome 7q
Features: elfin facies, intellectual disability, ADHD, anxiety, supravalvular aortic stenosis, hypertension, hypercalcemia

HLA haplotype sharing

Siblings: 25% HLA identical, 50% haploidentical, 25% no sharing
Each parent shares 50% HLA haplotype with child

DNA sharing between relatives

Identical twin: 100%
Parent/sibling: 50%
Grandparent, uncle/aunt, half-sibling: 25%
1st cousin: 12.5%