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Heredity
"The elementary
particles which form the embryo are each drawn from the
corresponding structure in the parent... so that in the
offspring they will reflect and reproduce a resemblance
to the parents"
Peter de
Mauperuis, 1751
 The
genes for certain traits are passed down in families
from parents to children. This has been known for
thousands of years--even in Biblical times--and has
allowed farmers to breed better crops and animals.
For example, parents with black hair will likely give
birth to children with black hair, just as parents
with long noses will have kids with long noses. Once
in awhile, though, this doesn’t work and parents with
black hair will give birth to a blond. This discrepancy
can be explained by the principle of segregation,
first noted by Austrian monk Gregor Mendel over 100
years ago. The principle has three parts: |
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1.
Hereditary traits are determined by specific genes.
2.
Individuals carry two genes for each trait, one
from the mother’s egg and one from the father’s
sperm.
3.
When an individual reproduces, the two genes split
up (segregate) and end up in separate gametes.
The principle of segregation applies to all organisms,
including humans.
1.
Hereditary traits are determined by specific
genes. Within the DNA molecule, genes exist that
specify a certain, single characteristic; there
is a gene for height, a gene for weight, and a gene
for eye color, etc. Variations of the gene relating
to the same trait are called alleles.
2.
Individuals carry two genes for each trait, one
from the mother’s egg and one from the father’s
sperm. One of these two genes is dominant over the
other. The dominant allele will mask the other,
called the recessive allele. For example, if the
father gives a tall allele of the height gene, and
the mother gives a short allele, the offspring will
be tall. This is because tall is dominant and short
is recessive.
The
British mathematician/biologist R.C. Punnett devised
a method of picturing this concept on a graph called
a Punnett Square. Punnett Squares graph the father’s
genotype (the genetic information concerned with
a specific trait: for example, two alleles for tall,
or two for short, or one for each) crossed with
the mother’s. Punnett Squares show the probability
of having children who have a certain trait.
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- Dominant alleles are shown by a capital letter.
- Recessive alleles are shown by the lowercase of the
same letter.
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This
graph is a cross between a mother who is a hybrid
or heterozygous for tall (meaning she has one allele
(T) for tallness and one (t) for shortness). Physically
she is tall because T is dominant and masks the
shortness genes from the father. Half of their offspring
will therefore be short (tt) and half will be tall
hybrids (Tt; a pure tall offspring would be TT).
This means that the parents have a 2/4 or 50% chance
of having tall children and a 2/4 or 50% chance
of having short children. This is a 1:1 ratio. |
More examples:
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All
kids will be tall: 4:0 ratio
- 50% will be pure
- 50% will be hybrids
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3
of 4 kids will be tall: 3:1 ratio
- 75% chance of being tall
- 25% chance of being pure tall
- 50% chance of being hybrid tall
- 25% chance of being short
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When
only one trait is a Punnett Square is graphed, it is called
a monohybrid cross. But when two or more traits are graphed,
it’s called a dihybrid cross. This illustrates the law
of Independent Assortment, meaning that one trait doesn’t
affect another. In other words, having red hair has nothing
to do with also having bad eyesight. The genes are independent
of each other.
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TB |
Tb |
tB |
tb |
| TB |
TTBB |
TTBb |
TtBB |
TtBb |
| Tb |
TTBb |
Ttbb |
TtBb |
Ttbb |
| tB |
TtBB |
TtBb |
ttBB |
ttBb |
| tb |
TtBb |
Ttbb |
ttBb |
ttbb |
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Sometimes
two genes will be co-dominant--that is, neither masks
the other. In this case, both genes will show. An example
is skin color: the child of dark-skinned and fair-skinned
parents will be a mixture of the two. Breeding red geraniums
with white geraniums gives you pink flowers.
3.
When an individual reproduces, the two
genes split up (segregate) and end up in different gametes.
This is explained by the process called meiosis. Meiosis
is like mitosis (normal cell division), but instead produces
sex cells (gametes: sperm and egg). Sex cells have only
23 chromosomes (called a haploid, meaning “one set”),
instead of 46 (called a diploid, meaning “two sets”) so
that when fertilization occurs, a new cell with 46 chromosomes
will form. For example, when a sperm with 23 chromosomes
unites with an egg with 23 chromosomes, the cell they
form will have 46.
In
meiosis, the cell divides normally (as in mitosis) after
copying its chromosomes. The chromosomes also undergo
crossing-over. When the chromosomes pair up, sometimes
they will switch genetic data. This ensures that the genes
from both parents will be present. Immediately after dividing,
it divides again, this time without copying the chromosomes.
This creates four sex cells, where only one existed before,
each with only 23 chromosomes.
Genotype vs. Phenotype Analogy
1
Genotype vs. Phenotype Analogy
2
 
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