1. Introduction to Punnett Squares
I LOVE punnett squares!!

2. Let’s review first…
DNA – is made up of nucleotides and is the ‘blueprint’ for an organism, it contains all the information that determines inherited characteristics
Chromosomes – the structures of DNA made up of genes
Homologous Chromosomes – chromosomes that have the same sequence of genes, ‘pairs’

3. http://www.youtube.com/watch?v=lJzZ7p- 47P8&feature=related
Let’s watch!!

4. Ok… I know what homologous chromosomes are… but can I draw it?
Draw out a pair of homologous chromosomes (label where a gene falls at a particular location)

5. Genes – are a series of nucleotides found within the DNA that encodes for a given trait
Alleles – different forms of the same genes (example eye color)

6. Two types of alleles:
Dominant – describes an allele that is fully expressed whenever the allele is present (usually represented by an uppercase letter)
Recessive – describes an allele that is expressed only when there is no dominant allele present in an individual (usually represented by a lowercase letter)

7. Example: In pea plants…
R=round and r=wrinkled
Something with the genes Will look
RR round Rr round rr wrinkled

8. RR Two Types of Genetic Expression:
Genotype – represents the alleles that are inherited, you have two alleles per trait (letter combinations)
Phenotype – actual traits that we physically see (what the trait looks like) RR

9. R=round and r=wrinkled
Genotype Phenotype
RR round Rr round rr wrinkled

10. Example: If an organism inherited an R from the female and r from the male, what would be its genotype? Phenotype?
Genotype – ?? Phenotype – ??

11. Homozygous – when the alleles are the same, the individual is said to be homozygous, or ‘true breeding’, so letters would be either both uppercase or lowercase (BB, dd, RR, rr)
Heterozygous – when the alleles are different, in this case the dominant allele is expressed (Pp, Aa)

12. http://www.youtube.com/watch?v=d4izVAkh MPQ&feature=related
Wow I’m so excited to check this one out!!

13. Solving Punnett Squares
Read the problem…
Underline what the dominant allele and recessive allele represent.
Put a circle around the genotypes of the parents (when writing out the genotypes, any dominant alleles always go first)
Write out the genotypes of the Punnett square, separating the alleles (letters)
Fill in the squares down and across using the alleles in each column and row. Remember dominant alleles always go first!

14. Example: In garden peas, round seed coats (R) is dominant over wrinkled seed coats (r).  What will the results be of a cross between a homozygous dominant male and a recessive female? 

15. Example 2: In humans, straight toes (S) is dominant over curled toes (s).  What would be the result of a cross between a recessive male and a heterozygous female?

16. More Practice
In purple people eaters, one-horn is dominant and no horns is recessive.
Draw a Punnet Square showing the cross of a purple people eater that is hybrid for horns with a purple people eater that does not have horns.
Summarize the genotypes & phenotypes of the possible offspring.

17. More Practice
Ms. Pilarz and Gregory Mendel (famous geneticist we will learn about soon) fell in love and married. Both of them have had family members die from a recessive disease called Biologist Syndrome, which is brought on by loving Biology too much. If we assume they are both heterozygous for this disease, what are the chances of Ms. Pilarz and Mr. Mendel having a child with Biologist Syndrome?

18. Let's say that in seals, the gene for the length of the whiskers has two alleles.  The dominant allele (L) codes long whiskers & the recessive allele (l) codes for short whiskers.
a)  What percentage of offspring would be expected to have short whiskers from the cross of two long- whiskered seals, one that is homozygous dominant and one that is heterozygous?
b) If one parent seal is pure long- whiskered and the other is short- whiskered, what percent of offspring would have short whiskers?