In the 19th Century, monk called
Gregor Mendel lived in a monastery in what
is now the
Czech Republic. His work in the monastery gardens led him to
observe how characteristics were passed from one generation of plants to
another. In 1867 he published his observations, and called them his
First Law--of
Segregation and
Characteristics.
Mendel said that each characteristic had severl forms of Alleles. When 2
different alleles occured in the same individual, only one, the dominant one
whould be expressed and the other would remain hidden, or recessive.
Thus the first generations offspring (F1) of pure breeding parents
for different alleles would be hybrids, looking like the dominant allele if
these then bred together. Half their offspring will be hybrids, but the other
half will be like the grandparents. In other words, the original parental
characteristics segregate out in the second generations (F2).
Example: yellow seeds vs. green
Let green = G and yellow = g
(Green is dominant)
Green plant can be GG or Gg and yellow only gg
First generation
Parents (p): GG x gg
Gametes (g): G, g
F1: Gg
Second generation
p: Gg, Gg
g: G1,
g1:
G2,
g2
F2:
G1G2,
G1g2,
G2g1,
g1g2
This shows that there is the probability for at least one pure Green
(GG), some Green hybrids (Gg, looking Green), and one Yellow
(gg).
We did an experiement of a random selection of seeds, and got the ratio of
2:57:1 for Green vs. Hybrids vs. Yellows. We found that if one kills the
recessive pairs, by the fifth generation, F2 all the double
recessives will be dead, but that one hybrid will remain, proving that
recessive characteristics can never be completely removed
Example 2: Tongue Rolling
Tongue rolling in humans is controlled by 2 alleles, the dominant Rolling
(R) and recessive, non-rolling (r).
Parents (p): pRR (pure breeding roller) x prr (pure breeding non-roller)
Spermatozoa & Ova (g): gR x gr
F1: pRr x pRr
Gametes (g2): ;gr, gr
F2:
G1G2 (pure roller),
G1g2 (hybrid roller),
G2g1 (hybrid roller),
g1g2 (pure non-roller)
Question: Two blue eyed girls have a brown eyed mother and a blue-eyed father.
What percentage chance is there of their expected sibling of having brown
eyes? (Brown is dominant)
Answer: 50%.
m(Bb) x f(bb) yields Bb (hybrid brown),
Bb (do.), bb (pure blue), bb (do.).
Node your homework: 14.5.96: 15 yrs:
Anthropolgy