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Understanding Mendelian Inheritance and its Impact on Genetics

Mendelian inheritance provides the foundation for genetics, detailing how traits are passed from parents to offspring through alleles. Gregor Mendel, the Father of Genetics, conducted pioneering experiments with pea plants to uncover the laws of heredity in the 1860s. His work revealed concepts such as the Law of Segregation and the distinction between dominant and recessive alleles. By understanding genotype and phenotype, along with tools like the Punnett square, we gain insights into genetic variation, inheritance patterns, and the basis of many human genetic disorders.

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Understanding Mendelian Inheritance and its Impact on Genetics

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  1. Mendelian Inheritance ... the outward physical manifestation of internally coded, inheritable, information.

  2. Genetics: expression of genes

  3. Gregor Mendel • Father of Genetics • 1823-1884

  4. Gregor Mendel • Austrian monk • Studied science and mathematics at University of Vienna • Conducted breeding experiments with the garden pea Pisum sativum • Carefully gathered and documented mathematical data from his experiments • Formulated fundamental laws of heredity in early 1860s • Had no knowledge of cells or chromosomes • Did not have a microscope

  5. One-Trait Inheritance • Mendel performed cross-breeding experiments with pea plants • Used “true-breeding” (homozygous) plants • Chose varieties that differed in only one trait (monohybrid cross) • Performed reciprocal crosses • Parental generation = P • First filial generation offspring = F1 • Second filial generation offspring = F2 • Formulated the Law of Segregation

  6. Why Peas? • Either or traits • Easy to grow • Many offspring • Easy to regulate pollination

  7. Mendel’s method

  8. Tall Tall Short

  9. Mendel’s Hypotheses • Each parent has two factors (alleles) • Each parent gives one of those factors to the offspring • Tall has TT • Short has tt • Tall is dominant • Short is recessive

  10. Law of Segregation • Each individual has a pair of factors (alleles) for each trait • The factors (alleles) segregate (separate) during gamete (sperm & egg) formation • Each gamete contains only one factor (allele) from each pair • Fertilization gives the offspring two factors for each trait

  11. T t Tt TT tt

  12. T T TT TT TT

  13. t t tt tt tt

  14. Modern Genetics View • Each trait in a pea plant is controlled by two alleles (alternate forms of a gene) • Dominant allele (capital letter) masks the expression of the recessive allele (lower-case) • Alleles occur on a homologous pair of chromosomes at a particular gene locus • Homozygous = identical alleles • Heterozygous = different alleles

  15. Law of Segregation and Random Fertilization… genetic variation • Alleles separate during gamete production • Gametes have one allele for each trait • During fertilization gametes combine at random to form individuals of the next generation

  16. Discovery of Chromosomes in 1900 Confirms Law of Segregation • Chromosomes are in pairs • Each chromosome has one of the allele pair

  17. Homologous Chromosomes • Chromosomes of the same pair • Each homologue will have one allele for a paired gene • Homologous chromosomes pair up during meiosis • Only one of each homologue will be in each gamete

  18. Meiosis I Metaphase Chromosomes line up in a double row. T T t t Assume a T allele on each red chromatid and a t allele on each green chromatid

  19. Chromosomes separate Each each daughter cell gets doubled chromosomes t t T T

  20. Doubled Chromosomes Separate in Second Meiotic Division T T t t

  21. Each gamete will have a T allele or a t allele T T t t

  22. Allele • Member of a paired gene • One allele comes from each parent • Represented by a single letter

  23. Dominant & Recessive Alleles • Dominant alleles are expressed • Recessive alleles are not expressed in the presence of a dominant allele • Recessive alleles are only expressed if both alleles are present

  24. Homozygous • Both alleles alike • AA or aa or A A a a

  25. Heterozygous • Alleles are different • Aa A a

  26. Genotype • Genetic make up • Represented by alleles • TT & Tt are genotypes for tall pea plants This is the "internally coded, inheritable information" carried by all living organisms.

  27. Phenotype • A trait • Genotype determines the phenotype • Tall is a phenotype • “Think adjective!” • Descriptive This is the "outward, physical manifestation" of the organism.

  28. Phenotype: red flowers • Cells contain red granules • Enzymes help convert colorless pigment into red pigment • Most enzymes are proteins • Most traits are produced by the action of proteins.

  29. Summary: Genotype Versus Phenotype • Genotype • Refers to the two alleles an individual has for a specific trait • If identical, genotype is homozygous • If different, genotype is heterozygous • Phenotype • Refers to the physical appearance of the individual

  30. Punnett Square • Table listing all possible genotypes resulting from a cross • All possible sperm genotypes are lined up on one side • All possible egg genotypes are lined up on the other side • Every possible zygote genotypes are placed within the squares

  31. Monohybrid Testcross • Individuals with recessive phenotype always have the homozygous recessive genotype • However, Individuals with dominant phenotype have indeterminate genotype • May be homozygous dominant, or • Heterozygous • Test cross determines genotype of individual having dominant phenotype

  32. One-Trait Test CrossUnknown is Heterozygous

  33. One-Trait Test CrossUnknown is Homozygous Dominant

  34. Human Genetic Disorders • Autosome - Any chromosome other than a sex chromosome • Genetic disorders caused by genes on autosomes are called autosomal disorders • Some genetic disorders are autosomal dominant • An individual with AA has the disorder • An individual with Aa has the disorder • An individual with aa does NOT have disorder • Other genetic disorders are autosomal recessive • An individual with AA does NOT have disorder • An individual with Aa does NOT have disorder, but is a carrier • An individual with aa DOES have the disorder

  35. Albinism • Lack of pigment • Skin • Hair • Eyes

  36. a A AA = Normal pigmentation Aa = Normal pigmentation aa = Albino Enzyme Melanin Pigment Amino Acids

  37. A man & woman are both carriers (heterozygous) for albinism. What is the chance their children will inherit albinism?

  38. AA = Normal pigmentation Aa = Normal pigmentation (carrier) aa = Albino A A a a Man = Aa Woman = Aa

  39. A A a a Aa AA Aa aa

  40. Aa AA Aa aa Genotypes 1 AA, 2Aa, 1aa Phenotypes 3 Normal 1 Abino Probability 25% for albinism

  41. Dwarfism Dwarfism = D Normal height = d DD = Dwarfism Dd = Dwarfism dd = Normal height Dwarf Band

  42. A man with heterozygous dwarfism marries a woman who has normal height. What is the chance their children will inherit dwarfism? Dwarfism is dominant.

  43. DD = Dwarf Dd = Dwarf dd = Normal Dd Dd dd dd d d D d

  44. Dd Dd dd dd Genotypes 2 Dd, 2dd Phenotypes 2 Normal 2 Dwarfs Probability 50% for Dwarfism

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