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Genetics and Human Inheritance: Principles, Chromosome Breaks, Genetic Disorders

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  1. Chapter 20 Genetics and Human Inheritance Lecture Presentation Betty McGuireCornell University

  2. Genetics and Human Inheritance • Principles of inheritance • Breaks in chromosomes • Detecting genetic disorders

  3. Principles of Inheritance • Genetic information • Carried on chromosomes that are carried in the egg and sperm in equal numbers

  4. Principles of Inheritance • Homologous pairs of chromosomes • 23 chromosomes received from one parent pair with 23 chromosomes from the other parent • Each member of a homologous pair carries genes for the same traits

  5. Principles of Inheritance • Genes • Segments of DNA • Code for a specific protein that will play a structural or functional role in the cell

  6. Principles of Inheritance • Trait • Characteristic • Produced by the actions of one or more gene-directed proteins

  7. Principles of Inheritance • Alleles • Different forms of a gene • Produce different versions of the trait they determine • Example: gene for freckles • One allele causes freckles to form • Other allele does not

  8. Principles of Inheritance • Homozygous • Individuals with two copies of the same allele • Heterozygous • Individuals with different alleles of a given gene

  9. Principles of Inheritance • Dominant • When the effects of an allele can be detected regardless of the alternative allele • Recessive • When the effects of an allele are masked in the heterozygous condition

  10. Principles of Inheritance • Genotype • Alleles that are present • Genetic composition of an individual • Phenotype • Observable physical traits of an individual

  11. Principles of Inheritance • Law of Segregation • During gamete formation, the two alleles for each gene separate as the homologous chromosomes move toward opposite ends of the cell during meiosis • Each chromosome is inherited independent of the other chromosomes, following the Law of Independent Assortment

  12. Principles of Inheritance • Gregor Mendel • Studied how single genes are inherited from parent to offspring • First used one-trait crosses • Then used two-trait (dihybrid) crosses

  13. Principles of Inheritance • Punnett square • Matrix used to predict genetic makeup of offspring of individuals of particular genotypes • Rows represent possible gametes of one parent • Columns represent possible gametes of the other parent • Boxes represent possible combinations of gametes

  14. Principles of Inheritance • Monohybrid cross • Cross in which both parents are heterozygous for one trait of interest • Genotypic ratio of offspring • 1 FF : 2 Ff : 1 ff • Phenotypic ratio of offspring • 3 with freckles (FF and Ff) : 1 without (ff)

  15. Principles of Inheritance • Dihybrid cross • Cross in which both parents are heterozygous for two traits of interest • Phenotypic ratio of offspring • 9 : 3 : 3 : 1

  16. Principles of Inheritance Web Activity: One- and Two-Trait Crosses

  17. Principles of Inheritance • Pedigree • Chart showing the genetic connections among individuals in a family • Especially useful in following recessive alleles that are not visible in the heterozygote

  18. Principles of Inheritance • Genetic disorders • Often caused by recessive alleles • Carrier • Someone who displays the dominant phenotype but is heterozygous for a trait • Carries the recessive allele and can pass it to descendants

  19. Principles of Inheritance • Dominant allele • Often produces a functional protein that the recessive allele does not

  20. Principles of Inheritance • Example: albinism • Ability to produce brown pigment melanin is lacking • Ability to produce melanin depends on the enzyme tyrosinase • Dominant allele that results in pigmentation produces functional tyrosinase • Recessive allele that results in albinism produces nonfunctional tyrosinase

  21. Principles of Inheritance • Complete dominance • Heterozygote exhibits the trait associated with the dominant allele but not that of the recessive allele

  22. Principles of Inheritance • Codominance • Effects of both alleles are apparent in a heterozygote • Example: blood type AB • The protein products of both the A and B alleles are expressed on the surface of the red blood cell

  23. Principles of Inheritance • Incomplete dominance • Expression of the trait in a heterozygous individual is in between the way the trait is expressed in a homozygous dominant or homozygous recessive person • Example: sickle-cell allele • Heterozygote has sickle-cell trait (HbAHbS)

  24. Principles of Inheritance Web Activity: Codominance and Incomplete Dominance

  25. Principles of Inheritance • Pleiotropy • One gene having many effects • Sickling of red blood cells caused by abnormal hemoglobin affects many areas of the body

  26. Principles of Inheritance • Multiple alleles • When three or more forms of a given gene exist across many people in the population • Example: ABO blood types • Gene has three alleles: IA, IB,i

  27. Principles of Inheritance • Polygenic inheritance • Variation in a trait, such as height, independent of environmental influences • Involves two or more genes, often on different chromosomes

  28. Principles of Inheritance • Genes on the same chromosome • Usually inherited together • Described as being linked

  29. Principles of Inheritance • Sex-linked genes • Y chromosome is much smaller than X chromosome • Y carries fewer genes • Most genes on the X chromosome have no corresponding alleles on the Y chromosome • Known as X-linked genes • Different pattern of inheritance • Recessive phenotype of X-linked genes more common in males • Son can inherit X-linked recessive only from mother

  30. Principles of Inheritance • Examples of disorders caused by X-linked recessive alleles • Red-green color blindness • Two forms of hemophilia • Duchenne muscular dystrophy

  31. Principles of Inheritance Web Activity: Sex-Linked Traits

  32. Principles of Inheritance • Sex-influenced genes • Autosomal genes whose expression is influenced by sex hormones • Example: male pattern baldness • More common in men than in women because its expression depends on both the presence of the allele for baldness and the presence of testosterone

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