1. Chapter 19.3: Genetic Inheritance

2. Clarification…..  Codominance: a cross between organisms with two different phenotypes produces offspring with a third phenotype in which both of the parental traits appear together.  Ex: red x white = red and white spots  Incomplete dominance : a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits.  Ex: red x white = pink

3. Multiple Alleles  Most genes have more than two alleles, known as multiple alleles  Ex: blood type is determined by single gene with three alleles: I A, I B, I O GENOTYPEBLOOD TYPE

4. Sex inheritance  In humans, sex is determined by the 23 rd chromosomes pair, called the sex chromosomes  The other 22 pairs are called autosomes

5. Sex inheritance  Sex chromosomes differ from the autosomes in that the two sex chromosomes in a cell are not always alike  They do not always have the same genes at the same loci, so they are not always homologous

6. Sex inheritance  There are two types of sex chromosome: X and Y  X chromosomes is much larger and carries more genes  XX= female, XY=male

7. Sex linkage  X chromosomes contains many different genes, one of them codes for the production of a protein needed for blood clotting, called factor VIII  Two alleles for this gene: dominant H and recessive h, those who are hh will not form this protein and will suffer from haemophilia, in which blood fails to clot

8. Sex linkage  Because this gene is located on the X chromosome, it is inherited differently than autosomal genes  Since females have two X chromosomes and males have one, the possible genotypes for males and females are different.

9. Sex linkage  The factor VIII gene is said to be a sex- linked gene, which is a gene that is found on a part of X chromosome not matched by the Y, and therefore not found on the Y chromosome  Genotypes representing sex-linked genes are always represented by symbols that show that they are on the X chromosome  Ex: X H X h

10. Sex inheritance

11. Dihybrid crosses  Genetic diagrams that cross only one gene are called monohybrid crosses  Dihybrid crosses look at the inheritance of two genes at once

12. Dihybrid crosses  In tomato plants, there is a gene that codes for stem color. This gene has two alleles:  A= purple, a=green stem  A different gene at a different locus on a different chromosomes code for leaf shape. This gene has two alleles:  D= jagged cut leaves, d= smooth edges

13. Dihybrid crosses  At metaphase of meiosis I, the homologous chromosome pairs line up on the equator independently of each other  At the end of meiosis II, there are four types of gametes produced (compared to two with a single gene)

14. Dihybrid crosses  Ex: if we cross two plants that are heterozygous for both traits, the gametes formed will be:

15. Dihybrid crosses  From this heterozygote cross, the possible phenotypes of the offspring will be produced in a 9:3:3:1 ratio  9 double dominant, 3 mix dominant/recessive of each, 1 double recessive