Mendel developed 3 principles of inheritance
based on his experiments with pea plants.
Our understanding of how inherited traits are
passed between generations comes from principles first proposed by Gregor
Mendel in 1866.
Mendel followed the
inheritance of 7 traits in pea plants (Pisum sativum). He chose traits
that had 2 forms:
·
Pea shape (round or wrinkled)
·
Pea colour (yellow or green)
·
Flower colour (purple or white)
·
Flower position (terminal or axial)
·
Plant height (tall or short)
·
Pod shape (inflated or constricted)
·
Pod colour (yellow or green).
Mendel began with pure-breeding pea plants because they always produced progeny with the same characteristics as the parent plant.
1.
Fundamental theory of heredity
Inheritance
involves the passing of discrete units of inheritance, or genes, from parents
to offspring.
Mendel found that paired pea traits were either dominant or recessive. When pure-bred
parent plants were cross-bred, dominant traits were always seen in the progeny,
whereas recessive traits were hidden until the first-generation (F1) hybrid plants were left to self-pollinate. Mendel counted the number
of second-generation (F2) progeny with dominant or recessive traits and found a
3:1 ratio of dominant to recessive traits
Mendel
didn’t know about genes or discover genes, but he did speculate that there were
2 factors for each basic trait and that 1 factor
was inherited from each parent.
We now know that Mendel’s inheritance factors are genes, or more
specifically alleles – different variants of the same gene.
2.
Principle of segregation
During reproduction, the
inherited factors (now called alleles) that determine traits are separated into
reproductive cells by a process called meiosis and
randomly reunite during fertilisation.
Separation occurs
during meiosis when the alleles of each gene segregate into individual
reproductive cells (eggs and sperm in animals, or pollen and ova in plants).
3.
Principle of independent assortment
Genes
located on different chromosomes will be inherited independently of each other.
Mendel observed that, when peas with more than
one trait were crossed, the progeny did not always match the parents. This is
because different traits are inherited independently – this is the principle of
independent assortment. For example, he cross-bred pea plants with round,
yellow seeds and plants with wrinkled, green seeds. Only the dominant traits
(yellow and round) appeared in the F1 progeny, but all combinations of trait
were seen in the self-pollinated F2 progeny. The traits were present in a
9:3:3:1 ratio (round, yellow: round, green: wrinkled, yellow: wrinkled, green).
Exceptions
to Mendel’s rules
There are some exceptions to Mendel’s principles, which have
been discovered as our knowledge of genes and inheritance has increased. The
principle of independent assortment doesn’t apply if the genes are close
together (or linked) on a chromosome. Also,
alleles do not always interact in a standard dominant/recessive way,
particularly if they are codominantor have
differences in expressivity or penetrance.
No comments:
Post a Comment