 Mendel noticed similar results every time he performed a certain cross.  Example: Whenever Mendel crossed two plants that were hybrid for stem length, he always obtained…  ¾ of the plants were tall  ¼ of the plants were short  Probability: the likelihood that an event will occur

 Coin toss example: toss a coin three times.  What is the likelihood that it will land heads up 3x in a row? ◦ 2 sides  2 possible outcomes  each time there is a 50% chance of it landing on heads (1/2) ◦ Each toss is independent of the results of the others ◦ ½ x ½ x ½ = 1/8 ◦ So, there is a 1/8 chance of flipping heads up 3 times in a row. ◦ Principles of probability can be used to predict outcomes of genetic crosses.

 Diagram used to determine possible outcomes of genetic crosses  The possible gametes are placed on the top and left of the square.  The F 2 combinations appear in the boxes of the square  Example: Tt x Tt ◦ T = dominant for tallness ◦ t = recessive for shortness

 Homozygous – two identical alleles (TT,tt)  Heterozygous – two different alleles (Tt)  Phenotype – physical characteristics  Genotype – genetic makeup/combination of alleles

 The ratios of offspring supported the idea of segregation (separation of possible alleles into gametes).  For a cross of two hybrids, the outcome was always ¾ dominant phenotype (TT, Tt, Tt) and ¼ recessive (tt).  Ratio was 3:1  In order to have expected ratios, more individuals are better, since probabilities predict a large number of events.

 Genes for different traits can segregate independently during the formation of gametes.  Example: Round Yellow x Wrinkled Green  RRYY x rryy  F 1 generation was all RrYy (What did they look like?)

 Genes for different traits can segregate independently during the formation of gametes.  Example: Round Yellow x Wrinkled Green  RRYY x rryy  F 1 generation was all RrYy (What did they look like?) – Yes!  Round Yellow

 When the F 1 was crossed, what happened?  Let’s do it together…

 When the F 1 was crossed, what happened?  Let’s do it together…

 1. Organisms inherit genes from their parents which determine biological characteristics.  2. Some forms of a gene may be dominant and some may be recessive.  3. In most sexually reproducing organisms, adults have 2 copies of each gene – one from each parent, and these are segregated during gamete formation.  4. Alleles for different genes usually segregate independently of one another.

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