Unit Overview – pages 250-251

2. Unit Overview – pages 250-251 Genetics Patterns of Heredity and Human Genetics When Heredity Follows Different Rules

3. Section 12.2 Summary – pages 315 - 322 Complex Patterns of Inheritance • Patterns of inheritance that are explained by Mendel’s experiments are often referred to as simple. • However, many inheritance patterns are more complex than those studied by Mendel.

4. Section 12.2 Summary – pages 315 - 322 Incomplete dominance: Appearance of a third phenotype • When inheritance follows a pattern of dominance, heterozygous and homozygous dominant individuals both have the same phenotype. • Incomplete dominance: the phenotype of heterozygous individuals is intermediate between those of the two homozygotes.

5. Section 12.2 Summary – pages 315 - 322 Incomplete dominance: Appearance of a third phenotype • For example, if a homozygous red-flowered snapdragon plant (RR) is crossed with a homozygous white-flowered snapdragon plant (R′ R′), all of the F1 offspring will have pink flowers. • A prime symbol is used to show incomplete dominance, a lower case letter is not used.

6. Section 12.2 Summary – pages 315 - 322 Incomplete dominance: Appearance of a third phenotype White Red All pink Red (RR) Pink (RR’) White (R’R’) Pink (RR’) All pink flowers 1 red: 2 pink: 1 white

7. Section 12.2 Summary – pages 315 - 322 Codominance: Expression of both alleles • Codominant alleles: cause the phenotypes of both homozygotes to be produced in heterozygous individuals; both alleles are expressed equally.

8. Example of Codominance • Ex: Feather colors in chickens • Black (BB) x White (WW) = Black and White checkered Chicken B B BW BW W BW BW W

9. Section 12.2 Summary – pages 315 - 322 Multiple phenotypes from multiple alleles • Although each trait has only two alleles in the patterns of heredity you have studied thus far, it is common for more than two alleles to control a trait in a population. • Multiple alleles: traits controlled by more than two alleles

10. Multiple phenotypes from multiple alleles • In pigeons a single gene controls feather color. There are 3 alleles for feather color. • BA = ash red feathers • b = chocolate feathers • B = Blue feathers

11. Multiple phenotypes from multiple alleles • b is recessive • B is dominant to b but recessive to BA • BA is dominant over both B and b.

12. BABA, BAB, BAb

13. BB,Bb

14. bb

15. Section 12.2 Summary – pages 315 - 322 Sex determination • In humans the diploid number of chromosomes is 46, or 23 pairs. • Autosomes: chromosomes that come in homologous chromosomes (22 pairs in humans). Homologous autosomes look alike. • The 23rd pair of chromosomes differs in males and females.

16. Section 12.2 Summary – pages 315 - 322 Sex determination • Sex chromosomes: determine the sex of an individual, are called and are indicated by the letters X and Y.

17. Section 12.2 Summary – pages 315 - 322 Sex determination • If you are female, your 23rd pair of chromosomes are homologous, XX. X X Female • If you are male, your 23rd pair of chromosomes XY, look different. X Y Male

18. Section 12.2 Summary – pages 315 - 322 Sex determination • Males usually have one X and one Y chromosome and produce two kinds of gametes, X and Y. • Females usually have two X chromosomes and produce only X gametes. • It is the male gamete that determines the sex of the offspring.

19. Section 12.2 Summary – pages 315 - 322 XY Male Sex determination X Y X XX Female XY Male XX Female X XY Male XX Female

20. Section 12.2 Summary – pages 315 - 322 Sex-linked inheritance • Sex-linked traits: traits controlled by genes located on sex chromosomes • The alleles for sex-linked traits are written as superscripts of the X or Y chromosomes. • Because the X and Y chromosomes are not homologous, the Y chromosome has no corresponding allele to one on the X chromosome and no superscript is used.

21. Section 12.2 Summary – pages 315 - 322 Sex-linked inheritance • Also remember that any recessive allele on the X chromosome of a male will not be masked by a corresponding dominant allele on the Y chromosome.

22. Section 12.2 Summary – pages 315 - 322 Sex-linked inheritance White-eyed male (XrY) F2 Females: Red-eyed female (XRXR) all red eyed Males: 1/2red eyed 1/2white eyed F1 All red eyed

23. Section 12.2 Summary – pages 315 - 322 Polygenic inheritance • Polygenic inheritance: the inheritance pattern of a trait that is controlled by two or more genes. • The genes may be on the same chromosome or on different chromosomes, and each gene may have two or more alleles. • Uppercase and lowercase letters are used to represent the alleles.

24. Section 12.2 Summary – pages 315 - 322 Polygenic inheritance • However, the allele represented by an uppercase letter is not dominant. All heterozygotes are intermediate in phenotype. • In polygenic inheritance, each allele represented by an uppercase letter contributes a small, but equal, portion to the trait being expressed.

25. Section 12.2 Summary – pages 315 - 322 Polygenic inheritance • The result is that the phenotypes usually show a continuous range of variability from the minimum value of the trait to the maximum value. • AABBCC is a 16 cm tall plant, aabbcc is a 4 cm tall plant. • The difference in height is 12 cm or 2 cm/allele.

26. Section 12.2 Summary – pages 315 - 322 Polygenic inheritance • If a plant has genotype AaBbCc, how tall would it be? • The base height is 4 cm and you add 2cm for each dominant allele, so 4 cm + 6 cm = 10 cm tall.

27. Section 12.2 Summary – pages 315 - 322 Environmental Influences • The genetic makeup of an organism at fertilization determines only the organism’s potential to develop and function. • As the organism develops, many factors can influence how the gene is expressed, or even whether the gene is expressed at all. • Two such influences are the organism’s external and internal environments.

28. Section 12.2 Summary – pages 315 - 322 Influence of external environment • Temperature, nutrition, light, chemicals, and infectious agents all can influence gene expression.

29. Section 12.2 Summary – pages 315 - 322 Influence of external environment • In arctic foxes temperature has an effect on the expression of coat color.

30. Section 12.2 Summary – pages 315 - 322 Influence of external environment • External influences can also be seen in leaves. Leaves can have different sizes, thicknesses, and shapes depending on the amount of light they receive.

31. Section 12.2 Summary – pages 315 - 322 Influence of internal environment • The internal environments of males and females are different because of hormones and structural differences. • An organism’s age can also affect gene function.