Chapter 10 Patterns of Inheritance

1. Chapter 10Patterns of Inheritance

2. Genetics • Genetics is the branch of science that studies how the characteristics of living organisms are inherited. • In classic or Mendelian genetics, the central question is how are characteristic seen in parent distributed among offspring.

3. Alleles • An allele is a specific version of a gene. • Examples: eye color, hair color, earlobe type • The two different alleles are on the same part of a chromosome

4. Fundamentals of Genetics • The interaction of alleles determines the appearance of the organism. • The genotype of an organism is the combination of alleles that are present in an organism’s cells • Ex. BB, Bb, bb • Homozygous – two identical alleles • Heterozygous – two different alleles • The phenotype of an organism is how it appears outwardly and is a result of an organism’s genotype • Blue eyes, brown eyes

5. Fundamentals of Genetics • A dominant allele masks the recessive allele in the phenotype of an organism • Dominant allele is usually shown by a capital letter • Recessive alleles are usually shown by a lower-case letter. • B – brown-eyes, b – blue-eyes • BB • Bb • bb

6. Fundamentals of Genetics • Genetic cross is a planned mating between two organisms • Punnett square shows the possible offspring of a particular genetic cross • Aa x Aa

7. Punnett Square • Earlobes: E=free, e=attached EE x eeEe x EeEE x Ee EE: EE: EE: Ee: Ee: Ee: ee: ee: ee:

8. The Father of Genetics • Gregor Mendel (1822-1884) • Mendel was an Augustian monk in the Czech Republic. • He studied physics and botany at the University of Vienna. • He worked for 12 years on his genetic experiments and published them in the local natural history journal in 1866. • Mendel’s results were forgotten until the early 20th century.

9. Pea Plants • Mendel performed experiments concerning the inheritance of seven certain characteristics of pea plants.

10. Pea Plants • Mendel had one pure breeding purple and one pure breeding white flower plant and crossed them. This generation, is known as the Parental, or P. • CC x cc

11. Pea Plants • As seen in the previous slide, all the offspring of the P generation resulted in a purple color, with a genotype of Cc. This generation is known as the F1 generation. Mendel allowed the F1 generation to self-fertilize. • This results in the F2 generation.

12. Mendel • Mendel recognized four genetic principles • Organisms have two pieces of genetic information for each trait (later called alleles) • Law of Dominance states that some alleles interact with each other in a dominant and recessive manner, where the dominant allele masks the recessive trait • Gametes fertilize randomly • Law of Segregation says when a diploid organisms forms gametes, the two alleles for a characteristic separate from one another.

13. Single-Factor Cross • Let’s try a single-factor cross of our own now • Two heterozygous for pod color; Green is dominant and yellow is recessive

14. Seed color - green (y) Seed color - green (y) Seed color - yellow (Y) Seed color - yellow (Y) Seed shape - rough (r) Seed shape - rough (r) Seed shape - round (R) Seed shape - round (R) Double-Factor Cross • Genes on non-homologous chromosomes are inherited independently. • Step 1) One parent has yellow, round seeds (YYRR) and the other has green, rough seeds (yyrr).

15. Seed color - yellow Seed color - green Seed shape - rough Seed shape - round Double-Factor Cross • To predict the number of different gametes, use 2n where n = # of heterozygous genes on non-homologous chromosomes. • In this case, 22 = 4 different gametes. • Gamete Possibilities:

16. Double-Factor Cross • Time for the Punnett Square for F1

17. Double-Factor Cross • Now lets do an F2 generation of heterozygous (YyRr) • Possible gametes:

18. Double-Factor Cross • Time for the Punnett Square for F1

19. Double-Factor Cross

20. Human Traits – Dominant or Recessive • Cleft in chin - No cleft dominant, cleft recessive • Widow peak dominant, straight hairline recessive • Free lobe dominant, attached recessive • Freckles dominant, no freckles recessive • Roller dominant, nonroller recessive

21. Autosomal Dominant Traits: Polydactyly • Polydactyly is the appearance of more than the normal number of digits on the hand or the foot. • While the overall frequency is 1 in 500 births, it is more common in some populations (e.g., Amish in U.S.). • Polydacytly is the result of an autosomal dominant genetic trait.

22. Autosomal Dominant Traits: Polydactyly • What will the children look like if one parent is a heterozygous polydactylous parent and other is a “normal” pentadactylous parent? 6 5 5 5 • The chance that any child will be polydactylous (65) is 50:50. • The chance that any child will be pentadactylous (55) is 50:50.

23. Fig. 10.4, pg. 200 Autosomal Recessive Traits: Phenylketonuria • Phenylketonuria (PKU) is caused by a mutation on chromosome 12 which prevents the synthesis of an enzyme which degrades phenyalanine, an amino acid.

24. Autosomal Recessive Traits: Phenylketonuria • Phenylketonuria appears in 1 in every 17,000 births. • Phenylketonuria is an autosomal recessive trait. • Only individuals who are homozygous recessive (2 copies of the phenylketonuria allele) will have this disorder. • Because heterozygotes have one normal copy of the gene which makes the key enzyme, they will not show the disorder • Heterozygous individuals are called carriers.

25. Autosomal Recessive Traits: Phenylketonuria • What will the children look like if two parents are both carriers of the phenyketonuria? • The chance that any child will be totally free of the disorder (NN) is 25%. • The chance that any child will be a carrier (Nn) is 50%. • The chance that any child will have phenyketonuria (nn) is 25%.

26. Complete Dominance • In the genes that Mendel examined, one allele demonstrated complete dominance. • In heterozygotes, the dominant allele was expressed in the phenotype and the alternative allele (recessive) was repressed. • An individual with a dominant phenotype could have either a homozygous dominant genotype (PP) or a heterozygous genotype (Pp).

27. Incomplete Dominance • In other genes, a heterozygous individual has a phenotype that is intermediate. • A heterozygous snapdragon CRCW is pink. • F2 offspring of P1 homozygous cross will show three phenotypes and genotypes in a 1:2:1 ratio.

28. Incomplete Dominance • Let’s do a Punnett Square for Incomplete Dominance. FRFW x FWFW

29. Codominance • In codominance the effects of both alleles are visible as distinct effects on the phenotype. • Like incomplete dominance, the F2 offspring of a monohybrid cross of two codominant alleles will lead to 3 types of offspring with 3 genotypes in a 1:2:1 ratio. • A good example of codominance is expression of the A and B blood type alleles in humans.

30. Codominance • Multiple Alleles refers to situations in which there are more than 2 possible alleles that control a particular trait • For blood type there are three different alleles • IA – blood has type A antigen on rbc surface • IB – Blood has type B antigen on rbc surface • i – Blood type O has neither type A nor type B antigens on rbc surface

31. Interactions Among Alleles • Type A blood has anti-B antibodies. • Type B blood has anti-A antibodies • Type O blood has no antibodies for A or B

32. Codominance • Another Punnett Square for a child who has a parent with type A blood and type O blood.

33. Codominance • Type O individuals (ii) are universal donors and type AB are universal recipients

34. Polygenic Inheritance • The final phenotype may depend on the additive effects of several genes.

35. Pleiotropy • Pleiotropy occurs when the alleles from a single gene have multiple phenotypic effects.

36. Linkage Groups • A linkage group is a set of genes located on the same chromosome. • They will be inherited together • Crossing-over may occur in prophase I of Meiosis I, which may split up these linkage group • A child can have gene combinations not found in either parent alone • The closer together two genes are to each other, the less likely crossing over would occur

37. Autosomal and Sex Linkage • Autosomesare chromosomes not directly involved in sex detrmination • Sex Linkage occurs when genes are located on the chromosomes that determine the sex of an individual • The Y chromosome is shorter than the X chromosome and has few genes for traits found on the X chromosome • So, the X chromosome has many genes for which there is no matching gene on the Y chromosome

38. Sex Linkage • Males have both a Y chromosome with a few genes on it and the X chromosome has many of the recessive characteristics present on the X chromosome appear more frequently in males than females. • X-linked genes are only found on the X chromosome • Y-linked genes are only found on the Y chromosome

39. Sex Linkage • Color-Blindness

40. Sex Linked • Color Blindness

41. Sex Linkage • One last Punnett Square for a mother who is homozygous for normal color vision, and a father who is color-defective vision

42. Sex Linkage