Jan 13, 2016
Genetics often comes down to probability Mendel stated that traits in an organism are
controlled by different factors◦ We know that each allele is controlled by a unique
gene (sequence of nucleotides) Principle of segregation showed individual
genes separate during gamete formation Each allele produced in equal numbers, so
equal probability that a gamete with a given allele will make it to fertilization
The possible offspring of sexual reproduction can be shown in a Punnett square
We see some offspring with two of the same allele (homozygous – BB or bb), or one of each (heterozygous – Bb)
The description of their genes is their genotype, whereas the physical characteristic is the phenotype
(think: GEnotype – GEnes, PHenotype – PHysical characteristic)
BB -> genotype Purple flower -> phenotype
The seed shapes for pea plants have two alleles, round (R – dominant) and wrinkled (r – recessive). If a homozygous dominant plant is crossed with a heterozygous plant, what is the expected genotypes and phenotypes of the offspring?
round
wrinkled
First, we indentify the homozygous dominant as RR and the heterozygous as Rr
Next, we fill in the Punnett square
R RR
r
Although the genotypes work out to be 1:1 homozygous dominant to heterozygous, the phenotypes are 100% round
What if the example was two heterozygotes crossing?
R rR
r
Now the genotype ratio is 1:2:1 (homozygous dominant : heterozygous : homozygous recessive)
And the phenotype ratio is 3:1 (round to wrinkled)
A Punnett square is useful at making predictions as well
However, to be reasonably accurate, the sample size must be quite large
Recall the separation of homologs during meiosis◦ Characteristics will segregate their traits
separately during gamete formation (gene for plant height not linked to gene for seed colour)
This is the principle (or Law) of independent assortment
Punnett squares can show more than one characteristic
A Punnett square of two characterisitcs is called a two-factor cross
Mendel initially attempted this with plants that were true breeding for two different characteristics
Round yellow (RRYY) plants were crossed with wrinkled green (rryy)
However, this merely yielded plants that were round yellow
Then, he tried crossing the F1 generation◦ These were hybrids for two traits, hence the term
“dihybrid” cross The results this time were much different
The results were only described in phenotype
A 9:3:3:1 (round yellow : round green : wrinkled yellow : wrinkled green) was predicted, and Mendel’s experimental results closely matched this
It should be noted that genes for different traits can segregate independently, but isn’t guaranteed to (some are linked)
Genetics isn’t always as simple as Mendel’s experiment illustrated
Some traits have more than two alleles (or even multiple genes) and complete dominance often doesn’t exist
In some cases, two alleles express themselves equally, but separately
Called codominance For example, alleles for white and red
colours in a flower express equally to give a speckled red and white flower
Neither allele masks the other This is similar to incomplete dominance
where both alleles blend to give a combined look
The red and white alleles would express equally to give a pink colour
codominance vs.
Incomplete
Some traits are dictated by more than two alleles (said to have multiple alleles)
A common example of this is blood type
Lastly, some traits are controlled by more than one gene
These are said to be polygenic Human skin colour is polygenic