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Chapter 10 Patterns of Inheritance

Chapter 10 Patterns of Inheritance. 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. Alleles.

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Chapter 10 Patterns of Inheritance

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  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

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