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LESSON TOPIC
COMPLEX PATTERNS OF INHERITANCE
MAIN QUESTION:
How multiple alleles, incomplete dominance, co dominance is affecting the living world?
WHEN HEREDITY FOLLOWS DIFFERENT RULES.
1. INCOMPLETE DOMINANCE – APPEAREANCE OF A THIRD PHENOTYPE.
- The phenotype is intermediate between two homozygous parents.
Ex. Capital letters used for both alleles Snap dragons- red = RR, white= WW, pink
= RW
Incomplete dominance
2. Co dominance -expression of both alleles
Occurs when alternative alleles are present in the genotype and fully observed in the phenotype.
See.fig 12.8, pg.323
Codominance in Humans
Sickle-cell anemia – sickle red blood cells slow blood flow, produce tissue damage and pain.
3. Multiple phenotypes from multiple alleles- traits controlled by more than two alleles.
Ex. Pigeon feathers color
Multiple Alleles in Humans
Multiple alleles govern blood type Ex. ABO blood grouping system, where a
single gene locus features multiple alleles- IA, IB, and i. Individuals carrying alleles for both A and B express both in the phenotype AB.
Genotype Phenotype (blood group)
IA IA or IAi A
IB IB, or IBi B
IAIB AB
ii O
Multiple Alleles in Humans
THE BLOOD TYPE
Sex determination:
……white board
4. Sex - linked inheritance
1919 – T. Morgan – research on Drosophila – fruit flies;
Traits controlled by genes located on sex chromosomes – SEX-LINKED TRAITS.
(X –linked trait, Y-link trait) EX. The allele for eye colors are carried on
the X chromosome.
Sex-Linked Traits in Humans Red-green color blindness - caused by two
recessive alleles from X chromosome. Hemophilia – X linked disorder.
5. Polygenic inheritance
The inheritance pattern of a trait that is controlled by two more genes.
Ex. Skin color, height in humans, cob length in corn.
6. Changes in Chromosome Numbers
It can affect autosomes – Down syndrome (Trisomy 21)
It can affect sex chromosomes – XYY or XXX or XXY
