Humans have 46 chromosomes - 23 from mom and 23 from dad
- 23 pairs of chromosomes
    - 22 pairs autosomes
        - 1 from mom and 1 from dad that contain the same sequence of genetic info
    - 1 pair of sex cell

Chromosomes are made up of DNA
DNA - Deoxyribonucleic Acid
    - complex double helix that looks like a twisted ladder
    - rungs of the ladder are made up of base pair
    - 4 bases - Adenine (A)
            - Thymine (T)
            - Cytosine (C)
            - Guanine (G)
    - A always matches up with T
    - C always matches up with G

From the single cell, a baby forms and develops
Cells divide and multiply through
    Mitosis (to form an identical duplicate of the original cell) and
    Meiosis (to form gametes or sex cells).
 

Mitosis
Step 1 - original cell

Step 2 - chromosomes split lengthwise and duplicate
(base pairs A-T, C-G divide and new bases add to each open pair to forming a duplicate chain

A---T     ->      A -    - T     ->     A--T    A--T
C---G     ->     C -    - G     ->     C--G   C--G
C---G     ->     C -    - G     ->     C--G   C--G
A---T     ->      A -    - T     ->     A--T    A--T
A---T     ->      A -    - T     ->     A--T    A--T
C---G     ->     C -    - G     ->     C--G   C--G

Step 3 - duplicate chromosome move to opposite ends of the cell

Step 4 - cell divides creating 2 identical cells
 

Meiosis - Generation of sex cells or gametes

Step 1 - Original Cell

Step 2 - Chromosomes divide and duplicate
        - Autosomes and sex cells pair up

Step3 - Cross Over between pairs of autosomes

Step 4 - Parent cells are formed (2 cells)
    - each parent cell contains one member of the 22 pairs of autosomes and 1 sex cell (each of these chromosomes are still duplicated)

Step 5 - Each parent cell then divides
    - duplicate information goes to opposite sides of the cell and the cell divides
    - each parent cell becomes 2 gametes
    - there are 2 parent cells so you end up with 4 gametes

If there is no cross-over, then each chromosome would either be from mom or dad and the amount of variation would not be as large.

If, during meiosis, the cell does not divide properly, then either the child will have a congenital defect or the zygote will not survive (spontaneous abortion).

For example if there are 3 chromosome 21, (trisomy 21) then the person will develop Down Syndrome. (probability increases as a function of age)
 

If the sex cell does not divide properly and there is more than 2 X chromosomes in a Female, then Fragile X Symdrome.

If there is only 1 X chromosome, woman with Turner Syndrome

If there is XXY, then you will have a male with Klinfelter's Syndrome.
 

Genetic Expression
Gene - a segment of base pairs on a chromosome
    - with autosomes, both members of the pair of autosomes contain this gene
    - 1 of the pair of autosomes comes from mom & 1 from dad

Alleles - the pair of genes
Homozygous - if the alleles contain the same information
Heterozygous - if the alleles contain different information

If the allele is Recessive, it will only be expressed in homozygous pairing in heterozygous pairings, carrier for the recessive trait
If the allele is Dominant, it will be expresses whenever it is present

Genotype - genetic makeup

Phenotype - expressed characteristic

Simple inheritance
- sex-linked
- recessive linked
- dominant linked
Co-dominance

Genetic Counseling
- examines family history to identify the probability of have a child with a genetic disorder

Prenatal Screening
- amniocentesis
    - a sample of the amniotic fluid is withdrawn from the mother's abdomen. This fluid contains cells from the fetus that can be tested for genetic disorders.
    - considered to be relatively safe
    - rate of miscarriage is approximately 1/150
    - generally completed between the 11^th and 14^th week  (3^rd month)
    - if completed before 11^th week, a large enough sample of fluid may not be obtained
    - results take approximately 2 weeks

- chorionic villus sampling
    - a sample of the chorion membrane is withdrawn from the mother's abdomen. These cells contain the genetic information of the fetus and can be tested for genetic disorders.
    - considered a to be more risky procedure
    - rate of miscarriage 1/50
    - has been linked to limb deformities
    - generally completed in the 8^th to 9^th week
    - results available within 24 hours

- ultrasound
    - uses sound waves to scan the womb
    - safe procedure
    - mom's that are having difficult pregnancies will often have multiple ultrasounds
    - can be completed at any point within the pregnancy
 

Treating Hereditary Disorders -
Phenylketonuria - PKU
- children lack the enzyme that allows them to metabolize phenylalanine
- as phenylalanine accumulates it turns into an acid that attacks the nervous system and the brain
- can lead to severe mental retardation and hyperactivity

Screening - blood test taken at least 48 hours after birth
    - if they detect the phenylpyruic acid, then the child is put on a low phenylalanine diet and the negative effect of the disorder do not manifest
    - if child goes off of the low phenylalanine diet during early or middle childhood (before the age of 10 roughly) then negative effects will begin to manifest

Moms with PKU need to make sure that phenylalanine levels are low both immediately before and during their pregnancy to reduce the risk of miscarriage and to reduce the risk of the child having mental retardation.
 

Heritability
- the amount of the variability in a trait that is attributed to hereditary factors
- selective breeding studies are done with animals
    - maze bright rats v maze dull rats
- family studies are done with humans
    - twin - compare monozygotic and dizygotic twins
    - adoption - compares adopted children to adopted andbiological parents
    - concordance rates - % of time when both members of a pair being tested displays the trait.
 

Heritability coefficient

Nonshared environmental influences (NSE)
- experiences that are unique to you

Shared environmental influences (SE)
- experiences that are similar across people
 
Genetic predisposition - your genotype possesses the gene for a given trait or disorder but the environment must provide the triggers for the disorder/trait to manifest
    - evidence for this is that with MZ twins have a higher concordance rate than DZ twins but MZ twins do not have perfect concordance

Range of Reaction - genes set upper and lower limits on the range of the phenotype
- the way the environment interacts with the genes will determine where within this range the person will fall.