1. Mendel’s Law of Dominance: The dominant (S, A, etc.) allele, WHEN PRESENT, always covers (masks) completely expression of the recessive allele.
(One exception: Incomplete Dominance)
**Recessive alleles CANNOT display (show through) their Phenotype unless TWO are present (2 recessive alleles) and NO dominant alleles are present.
2. Mendel’s Law of Segregation:
BOTH alleles of a gene SEPARATE AT ANAPHASE 2.
This helps to form unique gametes.
This separation at Meiosis 2 allows for them to then RANDOMLY UNITE at Fertilization (Random Fertilization).
3.Mendel’s Law of Independent Assortment
Each homologous Pair aligns independently (of all other homologous pairs) along the metaphase I Plate.
When: Metaphase I of Meiosis I
EACH GENE PAIR IS INDEPENDENT OF ALL OTHER GENE PAIRS.
EX: Pea color alleles (Y,y) are independently separated from Pea Shape alleles (R,r). There is no relation between the two.
MEIOSIS=”REDUCTION DIVISION’ (Why? What do these terms mean?)
RESULTS: In RECOMBINATION of genes; GREATER DIVERSITY OF ALLELES for natural selection to work upon (the ‘gene pool’ of a population)
_________>>>>SUPER IMPORTANT FOR BIOLOGY 2 AND THE HARDY-WEINBERG PRINCIPLE
3 ways that achieve Recombination:
1. Crossing Over (Prophase I at synapsis)
2. Independent Alignment of homologous Pairs (at Metaphase I)
3. Random Fertilization (different gamete gene recombinations present a ‘choice’ or selection for the egg gamete)
GENETIC DISORDERS:
CAUSE: Mutation (a change in nucleotide sequence within the DNA of that gene)
I. Autosomal
A. Recessive (MOST genetic disorders are these)
Cystic Fibrosis: C=healthy allele c=disease allele
Sickle Cell Anemia
Where S=healthy allele s=sickle allele
SS is normal; Ss is Sickle Cell Trait which protects against ________________;
ss is the genotype for the severe disease, Sickle Cell Anemia
B. Dominant
Huntington’s Chorea (any H dominant allele presence means you have the disease)
Hh=healthy and normal; Hh and HH both have the Disease
Achondroplasia (dwarfism; many forms):
D=disease allele
II. Sex (X)-linked Genetic Disorders
Color blindness (many forms); hemophilia (Xh is the allele, XH is healthy)
DMD (Duchennes Muscular Dystrophy)
CHROMOSOMAL DISORDERS
CAUSE: NONDISJUNCTION
(Hint: Review Karyotype)
*can occur at ANY chromosome (1-23)
Segments and arms of chromosomes or ENTIRE xsomes are deleted, missing, or in extra (abnormal) numbers
[IGNORE: aneuploidy, polyploidy etc. terms]
X= genetic female missing one X=
Trisomy 21=extra chromosome at homologous pair 21=
XYY=genetic male with an extra Y=
XXY=genetic male with extra X=
