1. Patterns of Genetic Inheritance
Chapter 9
Patterns of Genetic Inheritance
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2. Gregor Mendel Deduced Laws of Inheritance

3. 9.1 A blending model of inheritance existed prior to Mendel
Austrian monk Gregor Mendel developed the fundamental laws of heredity after performing a series of experiments with pea plants

4. Figure 9.1 Gregor Mendel examining a pea plant

5. 9.2 Mendel designed his experiments well
Figure 9.2A Garden pea anatomy and the cross-pollination procedure Mendel used

6. Figure 9.2B Garden pea traits and crosses studied by Mendel

7. Single-Trait Crosses Reveal Units of Inheritance and the Law of Segregation

8. 9.3 Mendel’s law of segregation describes how gametes pass on traits
The law of segregation states:
Each individual has two factors for each trait
The factors segregate (separate) during the formation 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. Figure 9.3 Monohybrid cross done by Mendel

10. 9.4 The units of inheritance are alleles of genes
Traits are controlled by alleles - alternate forms of a gene
Found on homologous chromosomes at a particular gene locus
The dominant allele masks the expression of the other allele - the recessive allele
Genotype refers 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.

12. Figure 9.4 Occurrence of alleles on homologous chromosomes

13. Two-Trait Crosses Support the Law of Independent Assortment

14. 9.5 Mendel’s law of independent assortment describes inheritance of 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

15. Figure 9.5 Dihybrid cross done by Mendel

16. 9.6 Mendel’s results are consistent with the laws of probability
Figure 9.6 Use of Punnett square to calculate probable events

17. 9.7 Testcrosses support Mendel’s 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

18. Figure 9.7A One-trait testcross, when the individual with the dominant phenotype is heterozygous

19. Figure 9.7B One-trait testcross when the individual with the dominant phenotype is homozygous

20. Mendel’s Laws Apply to Humans

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 on autosomal chromosomes - all the chromosomes except the sex chromosomes
Carriers - those individuals that carry the abnormal allele but do not express it

22. Figure 9.8A Autosomal recessive pedigree

23. Figure 9.8B Autosomal dominant pedigree

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 - genotype HbS HbS has many symptoms from anemia to heart failure
Individuals who are HbA HbS have sickle-cell trait

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

26. Testing Fetal Cells Testing the Embryo Testing the Egg
APPLYING THE CONCEPTS—HOW 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 to preimplantation 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

27. Figure 9.11A Prepregnancy testing of an embryo

28. Figure 9.11B Prepregnancy testing of an egg

29. Complex Inheritance Patterns Extend the Range of Mendelian Analysis

30. 9.12 Incomplete dominance still follows the law of segregation
Incomplete dominance - heterozygote has an intermediate phenotype between that of either homozygote

31. Figure 9.12 Incomplete dominance

32. 9.13 A gene may have more than two alleles
Multiple alleles - gene has several allelic forms
Example: blood type is determined by multiple alleles
IA = A antigen on red blood cells
IB = 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 of codominance because both IA and IB are fully expressed

33. 9.14 Several genes and the environment can influence a single multifactorial characteristic
Polygenic inheritance occurs when a trait is governed by two or more genes
Multifactorial traits - controlled by polygenes subject to environmental influences

34. Figure 9.14 Polygenic inheritance: Dark dots stand for dominant alleles; the shading stands for environmental influences

35. 9.15 One gene can influence several characteristics
Pleiotropy - when a single gene has more than one effect

36. Figure 9.15A Marfan syndrome illustrates the multiple effects a single human gene can have

37. Chromosomes Are the Carriers of Genes

38. 9.16 Traits transmitted via the X chromosome have a unique pattern 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

39. Figure 9.16 X-linked inheritance

40. 9.17 Humans have X-linked disorders
Color Blindness - the alleles for the red- and green-sensitive proteins are on the X chromosome
Muscular Dystrophy - occurs in males but 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

41. Figure 9.17 X-linked recessive pedigree

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 a linkage group because they tend to be inherited together

43. Figure 9.18 A simplified map of the genes on chromosome 2 of Drosophila

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

45. Figure 9.19 Example of incomplete linkage

46. APPLYING THE CONCEPTS—HOW SCIENCE PROGRESSES 9
APPLYING THE CONCEPTS—HOW SCIENCE PROGRESSES 9.20 Thomas Hunt Morgan is commonly called “the fruit fly guy”
In 1908, Morgan began experimenting with the fruit fly (Drosophila melanogaster)

47. Connecting the Concepts: Chapter 9
The first of Mendel’s laws tells us that an individual has two alleles, but the gametes have only one allele for every trait
The second law tells us that the gametes have all possible combinations of alleles
Polygenic inheritance and X-linked inheritance extend the range of Mendelian analysis
Males are more apt than females to display an X-linked disorder
Genes do have loci on the chromosomes, but today we know that genes are composed of DNA