1. Genetic Crosses

2. Homozygous vs. Heterozygous  Recall:  Alleles received by offspring may either be heterozygous or homozygous  Homozyous = two alleles are the same  Heterozygous = two alleles are different Phenotype & Genotype  Genotype: the genetic makeup of an organism  Phenotype: the physical characteristics of an organism (what the organism looks like)

3. Probability  Probability: the likelihood that a specific event will occur  Can be expressed as a decimal, percentage, or a fraction  Probability = number of times an event is expected to happen number of times an event could happen

4. Predicting Results of Monohybrid Crosses  Monohybrid cross: a cross in which only one characteristic is tracked  Biologists use a diagram called a Punnett square to help predict the probability of inherited traits of offspring

5. How To: Monohybrid Cross 1.The alleles of the first parent will be placed on the top of the square 2.The alleles of the second parent will be placed along the left side of the square 3.The possible gene combinations of the offspring will be placed inside the squares  Letters will represent the alleles  A capital letter represents a dominant allele  A lower case letter represents a recessive allele

6.  Example 1: homozygous x homozygous  What happens when you cross a homozygous purple (PP) flower with a homozygous white (pp) flower?  There is a 100% probability that the offspring will have the genotype Pp = purple flower color GenotypesPhenotypes 4/4 Pp4/4 purple

7.  Example 2: homozygous x heterozygous  What happens when you cross a guinea pig that is homozygous dominant trait of black coat color (BB) and a guinea pig that is heterozygous for this trait (Bb)?  There is a 2/4 (50%) chance for genotype BB and 2/4 (50%) change for Bb GenotypesPhenotypes 2/4 BB 2/4 Bb 4/4 black  You Try! What if we crossed one that was homozygous for a brown coat (bb) and heterozygous for black (Bb)?  2/4 (50%) black & 2/4 (50%) brown

8.  Example 3: heterozygous x heterozygous  What happens when you cross two rabbits heterozygous for black coats (Bb)?  There is a 1/4 (25%) chance for genotype BB, 2/4 (50%) chance for Bb, and 1/4 (25%) for bb  = 3/4 (75%) will have black coat &1/4 (25%) will have brown coat GenotypesPhenotypes 1/4 BB 2/4 Bb 1/4 bb 3/4 black 1/4 brown  Genotypic ratio: the ratio of genotypes that appear in offspring  Ex: 1 BB : 2 Bb : 1bb  Phenotypic ratio: the ratio of the offspring’s phenotypes  Ex: 3 black : 1 brown

9. You Try!  In dogs, the allele for short hair (B) is dominant over the allele for long hair (b). Two short haired dogs have a litter of puppies. Some of the puppies have short hair and some of the puppies have long hair.  What are the genotypes of the parents?  Bb and Bb  How would the Punnett square look like?  What is the probability of offspring with short hair?  3/4 short hair  Long hair?  1/4 long hair  If the litter of puppies contained 12 pups, how many would you expect to have short hair?  3/4 of 12 = 9 pups  How many would you expect to have long hair?  1/4 of 12 = 3 pups

10. Predicting Results of Dihybrid Crosses  Mendel designed an experiment that helped to follow two different genes as they passed from parent to offspring  Used a dihybrid cross  Dihybrid cross: a cross in which two characteristics are tracked  Offspring of a dihybrid cross are called dihybrids

11. Steps in Setting Up a Dihybrid Cross 1.Determine the parental genotypes 2.Determine the gametes using “FOIL” method 3.Set up a large 4x4 Punnet square & place one set of gametes from one parent on top & the other set on the side 4.Write the genotype of the offspring in each box

12. Sample Problem Homozygous x Homozygous  A pea plant that is homozygous for round, yellow seeds (RRYY) is crossed with a one that is homozygous for wrinkled, green seeds (rryy). Determine the offspring.

13. Heterozygous x Heterozygous  Determine the results of crossing two pea plants heterozygous for round, yellow seeds. Sample Problem #2 Shortcut! In any case where the parents are heterozygous for both traits (AaBb x AaBb) you will always get a 9:3:3:1 ratio 9 is the number for the two dominant traits 3 is the number for the dominant/recessive combo 1 individual will display both recessive traits

14.  There are some exceptions to Mendel’s principles  Not all genes show a pattern of dominance and recessiveness  For some genes, there are more than two alleles  Many times, traits are controlled by more than one gene Exceptions to Mendel’s Principles

15. Genes & the Environment  Gene expression is always the result of the interaction of genetic potential with the environment  The presence of the gene is not all that is required for the expression of a trait  There must be proper environmental conditions present  The phenotype of any organism is the result of interaction between genes and the environment

16. Incomplete Dominance  All traits are not so clear-cut as dominant & recessive traits – some genes seem to blend together  Sometimes you have offspring that show incomplete dominance  Incomplete dominance: offspring have a phenotype that is a blend of its parents  Ex: Some flowers will be pink if you cross a red flower with a white flower

17. Codominance  Codominance: occurs when both alleles for a gene are expressed in a heterozygous offspring  = both alleles of a trait show  Neither allele is dominant or recessive, nor do the alleles blend in the phenotype How does this differ from incomplete dominance?