Ch.14 Mendel and the Gene Idea

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Ch.14 Mendel and the Gene Idea

• Gregor Mendel

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Mendels discoveries

• Blending- Hereditary material- Both parents
  contribute genetic material.
• Inheritable factors, genes are passed from
  generation to generation

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Pea experiments

• Worked with 7 traits
• Used purebreeding plants first then crossed them.

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Mendels Pea Crosses
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Law of independent assortment.

• homologous chromosomes move independently.
• Ex. Chromosome with A or a will go separate from
  chromosome with B or b.

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Law of Segregation

• Homologous pairs segregate during gamete
  formation(meiosis) and pair again after
  fertilization
• Ex. If a pea is Pp then half the gametes will
  receive a P and half a p.

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Know the vocabulary

• Homozygous
• Heterozygous
• Phenotype
• Genotype

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Testcross
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Dihybrid Cross
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Incomplete dominance

• Incomplete dominance leads to a blending of
  traits, red white yields pink

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Codominance
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Codominance

• Codominance - A and B are both dominant to O but
  not to each other, giving the AB blood type

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Epistasis

• One gene has an effect on another. C leads to
  deposition of color while B or b leads to color
  BBcc would be white even though the genes code
  for black color.

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Polygenic inheritance

• Combinations of genes can yield cumulative
  effects. The more dominant alleles the more
  color.

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Environmental Effects

• Sometimes the environment can change the way a
  gene is expressed, like these hydrangia, and the
  color based on pH of the soil.

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Pleiotropy

• One gene can have an effect on other parts of the
  body. The sickle cell gene which causes problems
  with blood cells has far reaching affects on the
  body.

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Sample problems
Mendel crossed short and tall pea plants(purebred
cross) TT x tt P gen. t t T Tt Tt
All offspring are heterozygous T Tt Tt All
are tall (Tt)
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Mendel crossed round, yellow pea plants to
wrinkled, green pea plants (RRYY x rryy) The
only possible gametes are RY from parent 1 and ry
from parent 2 so ry ry RY RrYy RrYy
All offspring are round RY RrYy RrYy and
yellow (RrYy)
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Monohybrid Cross -two heterozygotes mated Tt xTt
T t T TT Tt 1TT2Tt1tt t Tt
tt 3 tall 1 short
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Dihybrid cross Heterozygotes for 2 traits are
crossed RrYy x RrYy You can make the folowing
gametes for each parent RY,Ry,rY,ry, this means a
four by four box as in the next slide
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Dihybrid Cross
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Dihybrid crosses can also be done like this
R r Y y R RR
Rr Y YY Yy r Rr rr
y Yy yy Now calculate the chances
of RRYY (1/4 x 1/4) RRYy (1/4 x2/4) RrYY
(2/4 x 1/4) RRyy (1/4 x 1/4) RrYy ( 2/4 x
2/4) Rryy (2/4 x 1/4) rrYY (1/4
x/1/4) rrYy (1/4 x 2/4) rryy (1/4 x 1/4)