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.