Biology, Answering the Big Questions of Life/genetics2

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Mendelian Genetics[edit]

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.

EXERCISE • genetics2 • Problems

1. A true breeding plant with yellow seeds (YY) is crossed to a plant with green seeds (genotype yy). Yellow is dominant to green. Draw the first filial (F1) and second filial (F2)cross. What is the genotypic and phenotypic ratio of the offspring?

2. A true breeding cat with a striped coat (SS) is crossed to a cat with a solid colorcoat(ss). Show the F1 and F2 generations (show gametes for each cross).

SOLUTION • Biology, Answering the Big Questions of Life/genetics2 • Problems

The Monohybrid cross

1. Yellow and Green seeded plant cross

P1 (parents)

Phenotype: Yellow Seeds X Green Seeds Genotype: YY X yy


Gametes: Y y



F1 cross (First Filial)

Phenotype: All Yellow seeds


Genotype: Yy




F2 (Second Filial) cross the F1 to the F1


Phenotype: Yellow Seeds X Yellow Seeds

Genotype: Yy X Yy


F1 Gametes: Y, Y and Y, y


Offspring genotypic ratio: 1/4 LL, 1/4 Ll, 1/4 lL, 1/4 ll

Phenotypic ratio: 3 Long Stemmed: 1 short stemmed


The Yellow seed coat trait is DOMINANT to the green seeded trait. The Dominant gene is drawn with a capital letter, and the recessive gene has the same letter as the dominant gene, only in the lower case.

2. Striped and Solid plant cross

P1 (parents)

Phenotype: Striped X solid Genotype: SS X ss


Gametes: S s



F1 cross (First Filial)

Phenotype: All Striped Cats


Genotype: Ss




F2 (Second Filial) cross the F1 to the F1


Phenotype: Striped cat X Striped cat

Genotype: Ss X Ss


F1 Gametes: S, s and S, s


Offspring genotypic ratio: 1/4 SS, 2/4 Ss, 1/4 ss

Phenotypic ratio: 3 Striped cats: 1 solid cat



Exceptions to Mendel[edit]

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)
EXERCISE • genetics2 • Problems

1. A light Blue flower (Bb) is crossed with another light blue flower (Bb). The offspring have three different colors 1/4 Dark Blue, 1/2Light blue, and 1/4 white.

What type of dominance is shown here? Show the cross and the genotypes of the offspring.

SOLUTION • Biology, Answering the Big Questions of Life/genetics2 • Problems


Incomplete dominance - The heterozygote has a visibly different phenotype.

Phenotype: Light blue(Bb) X Light blue (Bb)

Genotype: Bb X Bb


F1 Gametes: B, b and B, b


Offspring genotypic ratio: 1/4 BB (dark blue phenotype), 2/4 Bb (Light blue phenotype), 1/4 bb (white phenotype)

References[edit]

Experiments in Plant Hybridization (1865) [1] by Gregor Mendel