Genetics

46

46

How many chromosomes do humans have?

23

How many chromosomes does one get from each parent?

Autosomal genes

Chromosomes 1-22

Sex-linked Genes

chromosome 23

either XY or XX

Sex-Linked Traits

= traits on only either X or Y chromosome

• most are carried on the X

• if a defect is carried on the X, then the males are more likely to get a defect (since no other X chromosome is there to dominate/cover the recessive gene)

Sex-limited Traits

= traits limited by hormone level

• gene is on both X and Y chromosomes, BUT whether it is expressed or not depends on reproductive hormone levels

ex. baldness depends on level of testosterone

Klinefelter Syndrome

= 1+ extra X chromosome(s)

XXY or XXXY

• only in males

• slow motor/speech development

• puberty delayed or absent

• low testosterone levels

• breast development

• taller than average

• usually infertile

• gay/bisexual

Turner Syndrome

= 1 X and no Y

XO

• results in child being a girl

• short

• broad chest/neck

• no puberty → infertile

XYY Syndrome

= an extra Y chromosome

(no special name for this syndrome)

• male

• genital anomalies

• cerebral cortex atypical development

  • low intelligence/autism-related symptoms

• does NOT cause extreme masculinity

• low fertility

Triple-X Syndrome

= 3 Xs

XXX

• girl

• mild cognitive defects

• usually go undiagnosed

• low fertility

Androgen Insensitivity Syndrome (AIS)

= in XY people, androgen receptors (for male sex hormones like testosterone) are defected/absent

• so embryos develop as females

• infertile

Phenylketonuria (PKU)

• recessive metabolic disorder (AKA missing a gene that codes for an enzyme that digests parts of what you eat (in this case, ketons))

• if they don’t get broken down, they gather in brain and damage brain cells

  • → significant intellectual disabilities

Sickle Cell Disease

= recessive disease with malformed red blood cells

lot of sickle cell = body deprived of oxygen = pain

Heterozygote Advantage: protects against malaria

Cystic Fibrosis

= recessive disease

• extra thick mucous in lungs/digestive system

Heterozygote Advantage: carriers have an extra mucous which protects them better from diarrhea

Tay Sachs

= neurodegenerative recessive disease

• almost all die before 5 yrs

• more prominent in Eastern European Jews

Heterozygote Advantage: protects carriers from tuberculosis

Huntington’s Disease

= dominant neurodegenerative disease

• 100% fatal, no cure

• BUT not fatal until 50-60s (after having kids, that’s why it persists in population)

Down’s Syndrome

= 3 copies of chromosome 21 (AKA trisomy-21)

• wide range of symptoms (autism spectrum: mild to severe)

• random mutation/non-heritable

Monozygotic

1 egg → 2 babies

= identical twins

• share same genes

Dizygotic

2 eggs → 2 babies

= fraternal twins

• don’t share same genes

• but nurture is going to be more similar

Same nature, different nurture

a type of twin study

= identical twins separated at birth

• useful in determining the effect of nurture (since nature is controlled for)

• more useful in showing differences in physical stuff like blood pressure

Different nature, same nurture

a type of twin study

= fraternal twins raised together

• same-sex fraternal twisn compared to each other AND a different aged sibling (also of the same sex)

  • → then, compare who is more similar, twins to each other, or each twin compared to other sibling?

• these studies more useful in showing more sensitive stuff (personality, interests)

Concordance Rate

= probability that 2 people have the same disorder based on their relationship to each other

ex.

person A has Senioritis

person B’s relationship to person A: | Concordance Rate for person B:

---------------------------------------------------------------------------------------

Identical Twin = 46% concordance rate (46 chance that person B has it)

Sibling = 15% concordance rate

Unrelated person = 6% concordance rate (since this person is unrelated, this rate is applicable to the entire population, AKA 6% would be the prevalence rate)

Heritability

= the % of “nature” in the nature-nurture debate

(AKA how much contribution genetics has to a trait/disorder)

ex. alcoholism has a 60% heritability rate

• 60% is explained by genetics, so 40% is due to the environment

• does NOT mean that there’s a 60% chance for a child of an alcoholic to become one

Genetic Predisposition

= the probability of developing a disease

• not to be confused with heritability

Heterozygous vs Homozygous Genes

Individuals carrying two identical alleles (RR or rr)

vs.

different alleles (Rr)

Phenotype vs Genotype

unique sequence of DNA. (the two alleles a person has inherited for a particular gene)

vs.

the detectable expression of this genotype – a patient's presentation/appearance

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