Review of Gregor Mendel’s life and work:

Topic 4.3 Theoretical Genetics Some review definitions: Genotype : A pair of alleles expressed by two letters (ie. Bb) Phenotype: A characteristic produced by a genotype Dominant allele: The allele that is expressed in the Bb phenotype Recessive allele: An allele expressed only when homozygous Codominant alleles: Both alleles are expressed

More definitions: Locus: The place, on a chromosome, a gene is located Homozygous: alleles for a gene are identical Heterozygous alleles for a gene are different BB or bb Bb

Carrier: More definitions: A heterozygote - the recessive allele does not have an affect on her phenotype, but she can pass the gene to her offspring where it may be expressed. Test Cross: Crossing a recessive homozygote with an individual with the dominant phenotype can determine the genotype of the individual.

Most genetic research before 1990 was done through pedigree charts. Genetic counselors and Doctors still use them in family histories. Pedigrees trace traits through generations. P1 generation F F F2

F2 5-1

Punnett Squares help us determine the probabilities of the genotypes and phenotypes of offspring of a cross between two individuals. For example, the Sickle cell characteristic T-Boz Tionne Watkins

Steps in making a Punnet Square: 1. Choose letters for alleles: capital for dominant lower case for recessive S s

2. Determine parents genotype: Possibilities: Homozygous dominant = Homozygous recessive = Heterozygous = 3. Put the possible gametes of the parents on the top and along the side of the square: SS ss Ss

4. Fill in the grid: SS X ss Ss X ssSs X Ss 5. Determine the probabilities for offspring genotype and phenotype S s ssss ss SS S s ssss SsSs S

Types of inheritance: 1. Simple Dominant/Recessive Widow’s peak Tongue rolling Hitch hiker’s thumb Attached earlobe Bent pinky

2. Multiple alleles Three or more alleles are possible (though each individual can only have two at a time)

5. Determine the probabilities for offspring genotype and phenotype

3. Sex-linked traits Traits carried on the X (or Y) chromosomes: Color blindness Haemophilia Found most often in males

5. Determine the probabilities for offspring genotype and phenotype

4. polygenic - characteristic determined by more than one gene skin color eye color height

Homework : Punnett Square Work Sheet