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Lecture notes study book Human Molecular Genetics of Tom Strachan, Andrew Read - ISBN: 9780815345893, Edition: 5th New edition, Year of publication: -
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Transmission geneticsWeeks 1 to 3: Are there units of inheritance?
Introduction
The old idea was a ‘blending’ theory of inheritance
This is whereby inherited factors blend together generation after generation
This is opposite the idea of inherited factors remaining as discrete units generation after generation
Mendel studied seven traits that bred true
The morphological characteristics of an organism are termed characters or traits
A variety that produced the same trait over several generations is termed a true- breeder
The seven traits that Mendel studied are:
1. Flower colour is either purple or white
2. Flower position is axil or terminal
3. Stem length is short or long
4. Seed shape is round or wrinkled
5. Seed colour is yellow or green
6. Pod shape is inflated or constricted
7. Pod colour is yellow or green
Mendel’s experiment
Each of the seven traits Mendel studied showed two variants or forms found in the same species (plant height
variants were tall and dwarf)
His first experiments crossed only two variants of one trait at a time. This is termed monohybrid cross
A pea plant contains two discrete hereditary factors, one from each parent
The two factors may be identical or different
When the two factors of a single trait are different (one is dominant and its effect can be seen, whereas the other is
recessive and isn’t expressed)
During gamete formation, the paired factors segregate randomly so that half of the gametes received one factor and
half of the gametes received the other
Mendel also performed a dihybrid cross, which is a cross between two parents that differ by two pairs of alleles
(AABB x aabb). Therefore these individuals also differ in two traits
The dominance relationship between alleles for each trait was already known to Mendel when he made this cross.
The purpose of the dihybrid cross was to determine if any relationship existed between different allelic pairs
, Yellow = G; Green = g: Round = W; Wrinkled = w
He came to three important conclusions from these experiment results
1. That the inheritance of each trait is determined by ‘units’ or ‘factors’
2. They are passed onto descendants unchanged (these units are now called genes’
3. Than an individual inherits on such unit from each parent for each trait. A trait may not show up in an individual,
but can still be passed to the next generation
He came up with two principles: the principle of segregation and the principle of independent assortment
, According the the principle of segregation, for any particular trait, the pair of alleles of each parent separate and only
one allele passes from each parent on to an offspring. Which allele in a parent's pair of alleles is inherited is a matter
of chance. We now know that this segregation of alleles occurs during the process of sex cell formation
According to the principle of independent assortment, different pairs of alleles are passed to offspring independently
of each other. The result is that new combinations of genes present in neither parent are possible. For example, a
pea plant's inheritance of the ability to produce purple flowers instead of white ones does not make it more likely
that it will also inherit the ability to produce yellow pea seeds in contrast to green ones. Likewise, the principle of
independent assortment explains why the human inheritance of a particular eye color does not increase or decrease
the likelihood of having 6 fingers on each hand
Interpreting the data
The F2 generation contains seeds with novel combinations (i.e. not found in the parentals): round and green, and
wrinkled and yellow
These are called nonparentals
If the genes on the other hand assort independently, then the predicted phenotypic ratio in the F2 generation would
be 9:3:3:1
Mendel’s data was very close to segregation expectations
Thus, he proposed the law of independent assortment
During gamete formation, the segregation of any pair of hereditary determinants is independent of the segregation
of other pairs
Sex determination
Sex is an inherited trait
In many animal species, chromosomes play a role in sex determination
This implicates chromosomes as the physical site of this information
Mendel: the data
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