Mendel

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Mendels Principle of Dominance or
Recessiveness
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The Father of Genetics

• Mendel The Gene Idea

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1. It All Began with Mendel

• Gregor Mendel was born in 1822.
• Called the Father of Genetics
• Late 1800 chromosomes and the process of meiosis
  were unknown.
• Mendels work was considered obscure and
  unimportant until 1900
• Walter Sutton proposed the Chromosome Theory and
  people began to listen to his ideas.
• Chromosome Theory specific genes are located on
  specific chromosomes

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2. Mendels Findings

1. Principle of Dominance and Recessiveness
2. One allele in a pair may mask the effect of the
   other
3. Principle of Segregation
4. The two alleles for a characteristic separate
   during the formation of eggs and sperm
5. Principle of Independent Assortment
6. The alleles for different characteristics are
   distributed to reproductive cells independently.

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3. Terminology

1. Character (heritable feature, i.e.,
   fur color)
2. Trait (variant for a character, i.e., brown)
3. True-bred (all offspring of same
   variety)
4. Hybridization
5. (crossing of 2 different true-breds)
6. P generation (parents)
7. F1 generation (first filial generation)
8. F2 generation (second filial generation)

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3. Terminology

1. Punnett square
2. Gene point on a chromosome that controls the
   trait
3. Allele an alternate form of a gene A or a
4. Homozygous identical alleles for a character
5. Heterozygous different alleles for a gene
6. Phenotype physical traits
7. Genotype genetic makeup
8. Testcross breeding of a recessive homozygote X
   dominate phenotype (but unknown genotype)

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Exceptions to Mendels Rule

• Non Mendelian Genetics

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Incomplete Dominance

• The phenotype of the heterozygote is intermediate
  between those of the two homozygotes.
• Ex) Snap Dragon Color
• Red, Pink, White

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Co-dominance

• Phenotype of both homozygotes are produced in
  heterozygotes individuals.
• Both alleles are expressed equally.
• Ex)Roan Cattle
• White-feathered birds are both homozygotes for
  both B and W alleles

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Multiple Alleles

• Ex )Blood type
• Blood type A and B are co-dominant, while O is
  recessive.
• Forms possible blood types of A, B, AB, and O.

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Blood Also Shows Codominance
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Sex-Determination

• Chromosome pairs 1-22 are autosomes
• Chromosome pair 23 are sex chromosomes
• They determine the sex of an individual
• XX female XY male

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Sex-Linked Inheritance

• Traits that are only found on the X chromosome
• Colorblindness and Hemophilia are examples of
  sex-linked traits.
• These genes are recessive and found only on the X
  chromosome.

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What about 2 Traits?

• BbLl x BbLl
• The Gametes contain one of each of the alleles.
  (BL).
• Each of the offspring contain four alleles
  exactly like the parents.(BbLl).
• Notice the number of possible offspring has
  increased.
• The phenotypic ratio is 9331

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BbLl x BbLl
BL Bl bL bl
BL BBLL BBLl BbLL BbLl
Bl BBLl BBll BbLl Bbll
bL BbLL BbLl bbLL bbLl
bl BbLl Bbll bbLl bbll
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Now on your own

• Work the Dihybrid Crosses on your own.

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Polygenic Inheritance

• Inheritance pattern of a trait that is controlled
  by two or more genes.
• Gene may be on the same chromosome or on
  different chromosomes.
• Ex) Skin color and Height