7th Grade Science Notes
April 30, 2007
What are traits?
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Traits are all the different physical characteristics of organisms.
Traits are the visible signs of the genetic code on an organism’s DNA.
During asexual reproduction (mitosis), a mother cell passes on its genetic code to each
daughter cell. Unless there is a mutation, there is no variation in the code. Each
daughter cell is identical to the mother cell.
During sexual reproduction two gametes, or sex cells, are formed with ½ of the genetic
code needed by the new organism to grow. This is called meiosis.
Eggs and sperm are examples of gametes.
Fertilization occurs when two gametes combine and producing a zygote that will have a
complete set of chromosomes, half from each parent.
Only identical twins have the same genetic code on their DNA. Fraternal twins and
every other person on earth has a unique genetic code.
Gregor Mendel:
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Gregor Mendel is called the Father of Genetics. He was a monk in Austria who noticed
that the pea plants in his garden showed different traits. He showed how those traits
were inherited from one generation to the next. In 1865 Mendel published a paper with
his findings that showed the world that something other than chance was involved with
the inheritance of traits, or heredity.
Mendel had 3 theories of heredity:
1. Traits are produced by pairs of factors.
2. Each parent contributes one half of the pair of factors needed for a trait.
3. The factors are distributed independently in the parent’s gametes, or sex cells.
Why peas:
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Mendel studied peas because they had several traits that were clearly opposites. Some
traits he studied were:
o Plant height - tall or short
o Seed shape - round or wrinkled
o Seed color - green or yellow
o Pod shape – smooth or pinched
o Pod color – green or yellow
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Peas are able to self-pollinate. The female part of the flower (pistil) produces the eggs
and the male part of the flower (stamen) produces the sperm or pollen that fertilizes the
eggs.
Because they were self-fertilized, Mendel’s pea plants were “purebreds” meaning that
they always produced the same traits in each new plant. In his experiments, Mendel cut
off the stamen from purebred pea plants and cross-fertilized them with pollen from a
different, purebred pea plant. This generation was called the P or “parent” generation.
The new plants that came from the parents were called the “filial” or F1 generation.
Filial means “son” in latin.
The next generation of plants were called the F2 generation and so on.
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Each time he crossed pea plants with different characteristics, he got a mixture of
offspring with each type of trait. These offspring were no longer purebred peas,
because they now contained a mixture of traits, so he called them all hybrids.
What did Mendel’s work show:
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Mendel’s research showed that factors called genes control traits.
Each version of a trait is called an allele.
Both parents contribute one version, or allele, to their offspring.
Sometime one allele is more powerful than it’s other half and overpowers it in the gene.
These are called dominant alleles.
An allele that is less powerful can only show up if there are two of them in a gene so
they are called recessive alleles.
Geneticists use capital letters to show dominant alleles and lower case letters to show
recessive alleles.
Mendel’s work was forgotten for 34 years. In 1900, several scientists rediscovered his
paper and connected his ideas to the new discovery of chromosomes to form the
science of genetics.