2.  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

4.  The possible offspring of sexual reproduction can be shown in a Punnett square

6.  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

7.  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

8.  First, we indentify the homozygous dominant as RR and the heterozygous as Rr  Next, we fill in the Punnett square

9. RR R r

10.  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?

11. Rr R r

12.  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

13.  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

14.  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

16.  Then, he tried crossing the F 1 generation ◦ These were hybrids for two traits, hence the term “dihybrid” cross  The results this time were much different

18.  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)

19.  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

20.  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

21.  The red and white alleles would express equally to give a pink colour codominancevs.Incomplete

22.  Some traits are dictated by more than two alleles (said to have multiple alleles)  A common example of this is blood type

24.  Lastly, some traits are controlled by more than one gene  These are said to be polygenic  Human skin colour is polygenic