1.CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 9 Patterns of Genetic Inheritance

2. Gregor Mendel DeducedLaws of Inheritance 9- 3. 9.1 A blending modelof inheritance existedprior to Mendel

Austrian monk Gregor Mendel developed the fundamental laws of heredity after performing a series of experiments with pea plants

9- 4. Figure 9.1 Gregor Mendel examining a pea plant 9- 5. 9.2 Mendel designed his experiments well

Figure 9.2A Garden pea anatomy and the

cross-pollination procedure Mendel used

9- 6. Figure 9.2B Garden pea traits and crosses studied by Mendel 9- 7. Single-Trait Crosses Reveal Units of Inheritance and the Law of Segregation 9- 8. 9.3 Mendels law of segregation describes how gametespass on traits

The law of segregation states:

• Each individual has two factors for each trait

• The factors segregate (separate) during theformation of the gametes

• Each gamete contains only one factor from each pair of factors

• Fertilization gives each new individual two factors for each trait

9- 9. Figure 9.3 Monohybrid cross done by Mendel 9- 10. 9.4 The units of inheritance are alleles of genes

Traits are controlled byalleles- alternate forms of a gene

• Found on homologous chromosomes at a particulargene locus

Thedominant allelemasks the expression of the other allele - therecessive allele

Genotyperefers to the alleles an individual receives at fertilization

• Homozygous -an organism has two identical alleles at a gene locus

• Heterozygous -an organism has two different alleles at a gene locus

Phenotype -the physical appearance of the individual.

9- 11. 9- 12. Figure 9.4 Occurrence of alleles on homologous chromosomes 9- 13. Two-Trait Crosses Support the Law of Independent Assortment 9- 14. 9.5 Mendels law of independent assortment describes inheritanceof multiple traits

The law of independent assortment states the following:

• Each pair of factors separates (assorts) independently (without regard to how the others separate)

• All possible combinations of factors can occur in the gametes

9- 15. Figure 9.5 Dihybrid cross done by Mendel 9- 16. 9.6 Mendels results are consistent with the laws of probability

Figure 9.6 Use of Punnett square to calculate probable events

9- 17. 9.7 Testcrosses support Mendels laws and indicate the genotype

Testcross- intentional breeding in order to determine underlying genotypes

• One-trait Testcross -When a heterozygous individual is crossed with one that is homozygous recessive, the results are always a 1:1 phenotypic ratio

• Two-trait Testcross - when an individual is heterozygous for two traits is crossed with one that is recessive for the traits, the offspring have a 1:1:1:1 phenotypic ratio

9- 18. Figure 9.7A One-trait testcross, when the individual with the dominant phenotype is heterozygous 9- 19. Figure 9.7B One-trait testcross when the individual with the dominant phenotype is homozygous 9- 20. Mendels Laws Apply to Humans 9- 21. 9.8 Pedigrees can reveal the patterns of inheritance

Some genetic disorders are medical conditions inherited from parents

Some may be due to the inheritance of abnormal alleles onautosomal chromosomes -all the chromosomes except the sex chromosomes

• Carriers -those individuals that carry the abnormal allele but do not express it

9- 22. Figure 9.8A Autosomal recessive pedigree 9- 23. Figure 9.8B Autosomal dominant pedigree 9- 24. 9.9 Some human genetic disorders are autosomal recessive

Tay-Sachs Disease -uncontrollable seizures, and paralysis prior to dying

• Results from a lack of the enzyme Hex A

Cystic Fibrosis -most common lethal genetic disease of Caucasians in the U.S.

• Genetic testing for the recessive allele is possible

Phenylketonuria -most commonly inherited metabolic disorder affecting nervous system

• Many diet products have warnings that they contain phenylalanine

Sickle-cell Disease -genotypeHb SHb S has many symptoms from anemia to heart failure

• Individuals who areHb AHb S have sickle-cell trait

9- 25. 9.10 Some human genetic disorders are autosomal dominant

Neurofibromatosis- many children with neurofibromatosis have learning disabilities and are hyperactive

Huntington disease- a neurological disorder that leads to progressive degeneration of brain cells

Achondroplasia- a common form of dwarfism associated with a defect in the growth of long bones

9- 26. APPLYING THE CONCEPTSHOW BIOLOGY IMPACTS OUR LIVES 9.11 Genetic disorders may now be detected early on

Testing Fetal Cells

• Amniocentesis -long needle withdraws a small amount of the fluid that surrounds the fetus and contains a few fetal cells

• Chorionic Villi Sampling (CVS) -tube is inserted through the vagina into the uterus and fetal cells are obtained by suction

Testing the Embryo

• A single cell can be removed from the 8-celled embryo and subjected topreimplantation genetic diagnosis (PGD)

Testing the Egg

• Polar bodies(nonfunctional cells produced during egg formation) receive a haploid number of chromosomes

• When a woman is heterozygous for a recessive genetic disorder, about half the polar bodies have received the mutated allele, while the egg has received the normal allele

9- 27. Figure 9.11A Prepregnancy testing of an embryo 9- 28. Figure 9.11B Prepregnancy testing of an egg 9- 29. Complex Inheritance Patterns Extend the Range ofMendelian Analysis 9- 30. 9.12 Incomplete dominance still follows the law of segregation

Incomplete dominance -heterozygote has an intermediate phenotype between that of either homozygote

9- 31. Figure 9.12 Incomplete dominance 9- 32. 9.13 A gene may havemore than two alleles

Multiple alleles -gene has several allelic forms

• Example: blood type is determined by multiple alleles

• • I A = A antigen on red blood cells

• • I B = B antigen on red blood cells

• • i= Neither A nor B antigen on red blood cells

• Possible phenotypes and genotypes for blood type:

This is an example ofcodominancebecause bothI A andI B are fully expressed

9- 33. 9.14 Several genes and theenvironment can influence a singlemultifactorial characteristic

Polygenic inheritanceoccurs when a trait is governed by two or more genes

• Multifactorial traits -controlled by polygenes subject to environmental influences

9- 34. Figure 9.14 Polygenic inheritance: Dark dots stand for dominant alleles; the shading stands for environmental influences 9- 35. 9.15 One gene can influence several characteristics

Pleiotropy -when a single gene has more than one effect

9- 36. Figure 9.15A Marfan syndrome illustrates the multiple effects a single human gene can have 9- 37. Chromosomes Are theCarriers of Genes 9- 38. 9.16 Traits transmitted via the X chromosome have a uniquepattern of inheritance

X-linked alleles have a different pattern of inheritance than autosomal alleles

• The Y chromosome cannot offset the inheritance of an X-linked recessive allele

Affected males always receive their X-linked recessive mutant allele from the female parent

9- 39. Figure 9.16X-linked inheritance 9- 40. 9.17 Humans haveX-linked disorders

Color Blindness -the alleles for the red- and green-sensitive proteins are on the X chromosome

Muscular Dystrophy -occurs in malesbut the recessive allele remains in the population through passage from mother to daughter

Hemophilia -1 in 10,000 males is affected by both external and internal bleeding

9- 41. Figure 9.17 X-linked recessive pedigree 9- 42. 9.18 The genes on one chromosome form a linkage group

Gene linkage -the existence of several genes on the same chromosome

• Genes on a single chromosome form alinkage groupbecause they tend to be inherited together

9- 43. Figure 9.18 A simplified map of the genes on chromosome 2 ofDrosophila 9- 44. 9.19 Frequency of recombinant gametes maps the chromosomes

A linkage map can also be called a chromosome map because it tells the order of gene loci on chromosomes

9- 45. Figure 9.19 Example of incomplete linkage 9- 46. APPLYING THE CONCEPTSHOW SCIENCE PROGRESSES 9.20 Thomas Hunt Morgan is commonly called the fruit fly guy

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