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