Genetics

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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

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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

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

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ex. baldness depends on level of testosterone

Klinefelter Syndrome

= 1+ extra X chromosome(s)

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**XXY or XXXY**

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* 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

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**XO**

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* 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

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**XXX**

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* 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

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Heterozygote Advantage: protects against malaria

Cystic Fibrosis

= recessive disease

* extra thick mucous in lungs/digestive system

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

Tay Sachs

= neurodegenerative recessive disease

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* almost all die before 5 yrs
* more prominent in Eastern European Jews

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Heterozygote Advantage: protects carriers from tuberculosis

Huntington’s Disease

= dominant neurodegenerative disease

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* 100% fatal, no cure
* BUT not fatal until 50-60s (after having kids, that’s why it persists in population)

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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

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= identical twins

* share same genes

Dizygotic

2 eggs → 2 babies

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= 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

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* 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

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* 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?

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* 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

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ex.

person A has Senioritis

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

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

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Sibling = 15% concordance rate

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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)

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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

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Genetic Predisposition

= the probability of developing a disease

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* __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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