Aim: How do different combinations of genes control traits in an organism? Do Now: Where are genes found?

How many copies of each gene do we have in a diploid cell?  Two copies  One from mom and one from dad.  Copies of the same gene are called alleles.  Example: for flower color there is a different allele for each color.

 Homologous chromosomes: Homologous chromosomes are similiar but not identical. Each carries the same genes in the same order, but the alleles for each trait may not be the same.

 Heterozygous – the two alleles of a gene are different.  Example: Brown and Blue  Homozygous – the two alleles of a gene are the same.  Example: Blue and Blue  Brown and Brown

Why are alleles important?  Determine what physical trait will be seen in the offspring.  If you have a brown eye allele and a blue eye allele, what color eyes will you have?  Which is dominant, which is recessive?  Dominant – the trait that is seen with a heterozygous pair of alleles.  Recessive – the trait that is only seen with a homozygous pair of alleles.

Representing alleles  Each gene is given a letter.  The dominant gene is capitalized.  The recessive gene is in lower case.  Ex:  Eye color: the letter “b”  Brown eyes (dominant): ‘B’  Blue eyes (recessive): ‘b’

 Genotype: the combination of alleles for a gene.  Example: bb  Phenotype: the physical characteristic that is seen based on the genotype.  Example:

Punnett Square  The Punnett square is a diagram that is used to predict the outcome of a cross between alleles for a gene.

How to do a Punnett Square:  Determine parent genotypes from given phenotypes.  “split” the letters of the genotype for each parent  Put the letters on the outside of the punnett square.  Finally carry parent alleles down and over to fill in the empty spaces.  Summarize results (genotypes & phenotypes of offspring) Genotype Ratios: Phenotype Ratios:

 Genotype Ratios:  Phenotype Ratios:

 If mom is homozygous for curly hair and dad is homozygous for straight hair, what kind of hair will the offspring have? (curly hair is dominant, straight hair is recessive)  If mom cannot curl her tongue and her son can curl his tongue, where did he get the ability from?

 In pea plants (which Gregor Mendel studied), tall pea plants are dominant over short pea plants. Using Punnett Squares, you can predict the genotypes and phenotypes of the offspring of a cross between a homozygous (purebred) tall pea plant and a homozygous (purebred) short pea plant.