Biology, Answering the Big Questions of Life/genetics2

Mendelian Genetics
Definitions:

phenotype: The trait that that you can see. genotype: What traits are carried in the genes on the chromosome. Dominant: The trait that is visibly expressed. (e.g. Y yellow seed coat) Recessive: The trait that is hidden. (e.g. y green seed coat) Homozygous: having two identical genes. (e.g. yy or YY) Heterozygous: Having two different genes. (e.g. Hh ) Gametes: The haploid sex cells. Truebreeding: The parents are homozygous for a trait (e.g. YY)

In Classical Mendelian Genetics, each trait can be either dominant or recessive. When two true breeding parents are crossed, the recessive trait is hidden in the offspring, only to reveal itself later in the grandchildren.

The classic ratio for a monohybrid cross with straight dominance is 3:1 Dominant to recessive in the F2.

The classic ratio for a dihybrid cross with straight dominance is 9:3:3:1 in the F2.

Exceptions to Mendel
I. Exceptions to straight dominance A. Incomplete Dominance - offspring have intermediate phenotype (RR=red, rr=white, Rr=pink flowers) B. Codominance ( ABO blood group)(IA, IB, i)

II. Exceptions to the second law A. Linkage B. Sex-linkage (Hemophilia)

III. Other A. Epistasis - one gene effects the expression of another. (spot color in black cats) B. Pleiotropy - one gene has many effects (albinism, sickle-cell anemia) C. polygenic inheritance (human height and intelligence)