Post on 30-Dec-2015
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Genetics:
Mendel & The Gene Idea
Mendelian genetics
Character (heritable feature, i.e., fur color)
Trait (variant for a character, i.e., brown)
True-breed (all offspring of same variety)
Hybridization – (crossing of 2 different true-breds)
P generation (parents) F1 generation (first filial generation) F2- the next generation, etc….
Leading to the Law of Segregation
Alternative versions of genes (alleles) account for variations in inherited characteristics
For each character, an organism inherits 2 alleles, one from each parent
If the two alleles differ, then one, the dominant allele, is fully expressed in the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance
The alleles for each character segregate (separate) during gamete production (meiosis).
Genetic vocabulary……. Homozygous: pair of identical
alleles for a character.– Example: PP or pp– They are “true-breeding”
Heterozygous: two different alleles for a gene– Example- Pp
Phenotype: an organism’s traits– It describes what you “see”
Genotype: an organism’s genetic makeup (PP, Pp, or pp)
Testcross: breeding of a recessive homozygote X dominant phenotype (but unknown genotype)
The Law of Independent Assortment
Law of Segregation involves 1 character. What about 2 (or more) characters?
Monohybrid cross vs. dihybrid cross
The two pairs of alleles segregate independently of each other.
Mendel’s Law of Independent Assortment
Now you can do more predicting… Dihybrid Cross- A mating between 2 parents that are both
heterozygous for the same traits.
Ex. RrYy x RrYy– Always should give a 9:3:3:1 phenotypic ratio
In fact if we know the genotypes, we can use laws of probability:
Rule Of Multiplication- probability that independent events will occur simultaneously is a product of their individual probabilities
Rule Of Addition- probability of an event occurring in 2 or more independent ways is the sum of their individual probabilities.
Of course, Genetics is not purely Mendelian...
Incomplete dominance: results in a phenotype which is intermediate between the dominant and recessive trait. – Ex: pink snapdragons
Codominance: two alleles are equally expressed and affect the phenotype in separate, distinguishable ways. – Ex: Variegated flowers
but sometimes, even “Complete Dominance” really isn’t:
It’s like a continuum: example AA, Aa, aa
Complete Dominance
Co-dominanceIncomplete Dominance
AA andAa have same phenotype
“A” phenotypeand “a” phenotypeexpressed equally
Aa shows intermediatephenotype
Example: Tay-Sacs Disease Tay-sacs is a disease in which brain cells lack a
lipid metabolizing enzyme. It all depends on the level at which the phenotype
is studied: Organismal level- “complete dominance”
– Heterozygote is symptom-free because half the number of functional enzymes is enough to do the job.
Biochemical Level- “Incomplete Dominance” – Lab Enzyme reaction rates are intermediate for carriers.
Molecular Level- “Codominance” – Both functional and dysfunctional versions of the
enzyme are produced.
More non-mendalian…. Pleiotropy: ability of a single gene to
have multiple phenotypic effects. – Ex: sickle-cell anemia & malaria
resistance Epistasis: a gene at one locus affects
the phenotypic expression of a gene at a second locus. – Dihybrid cross deviates from 9:3:3:1– Ex: mice coat color
Polygenic Inheritance: an additive effect of two or more genes on a single phenotypic character – Ex: human skin pigment and height
Multiple alleles: more than 2 possible alleles for a gene. – Ex: human blood types– A, B, AB, O(A&B codominant, both dominant to O)
Human disorders
The family pedigree Recessive disorders:
•Cystic fibrosis•Tay-Sachs•Sickle-cell
Dominant disorders:•Huntington’s
Testing:•amniocentesis•chorionic villus sampling (CVS)