11-2 Probability and Punnett Squares Genetics and Probability How do geneticists use the principles of probability? Probability – the likelihood that a particular event will occur The principles of probability can be used to predict the outcomes of genetic crosses. Sexual Reproduction and Genetics Chapter 10

Punnett Squares – a tool used to determine the combinations from a genetic cross Shows all possible combinations of gametes and likelihood each will occur Punnett Squares Sexual Reproduction and Genetics Chapter 10

Setting up a Punnett Square Draw the punnett square – size of your square depends on how many traits you are using (Ex. 1 trait = 4 boxes, 2 traits = 16 boxes) Gametes produced by each parent are shown along the top and left side. Possible gene combinations for the offspring appear in the four boxes. Sexual Reproduction and Genetics Chapter 10

Mendel’s Experiment Gametes from the F 1 generation parents are used to get the offspring for the F 2 generation. Mendel only looked at one trait at a time in his first experiments Monohybrid cross – a cross involving one trait Sexual Reproduction and Genetics Chapter 10

Probablility and Segregation One fourth (1/4) of the F 2 plants have two alleles for tallness (TT). 2/4 or 1/2 have one allele for tall (T), and one for short (t). One fourth (1/4) of the F 2 have two alleles for short (tt).

Mendel’s Law of Segregation Sexual Reproduction and Genetics  Two alleles for each trait separate during meiosis.  During fertilization, two alleles for that trait unite.  Heterozygous organisms are called hybrids Mendelian Genetics Chapter 10

Monohybrid Cross Sexual Reproduction and Genetics  A cross that involves hybrids for a single trait is called a monohybrid cross Mendelian Genetics Chapter 10

Sexual Reproduction and Genetics Dihybrid Cross  The simultaneous inheritance of two or more traits in the same plant is a dihybrid cross.  Dihybrids are heterozygous for both traits Mendelian Genetics Chapter 10

Sexual Reproduction and Genetics Law of Independent Assortment  Random distribution of alleles occurs during gamete formation  Genes on separate chromosomes sort independently during meiosis.  Each allele combination is equally likely to occur Mendelian Genetics Chapter 10  Law of independent assortment occurs during metaphase I of meiosis.

Sexual Reproduction and Genetics Punnett Square— Dihybrid Cross  Four types of alleles from the male gametes and four types of alleles from the female gametes can be produced.  The resulting phenotypic ratio is 9:3:3: Mendelian Genetics Chapter 10

Polyploidy Sexual Reproduction and Genetics  Polyploidy is the occurrence of one or more extra sets of all chromosomes in an organism.  A triploid organism, for instance, would be designated 3n, which means that it has three complete sets of chromosomes Gene Linkage and Polyploidy Chapter 10