Mendelian Inheritance ... the outward physical manifestation of internally coded, inheritable, information.
Gregor Mendel • Father of Genetics • 1823-1884
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
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
Why Peas? • Either or traits • Easy to grow • Many offspring • Easy to regulate pollination
Tall Tall Short
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
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
T t Tt TT tt
T T TT TT TT
t t tt tt tt
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
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
Discovery of Chromosomes in 1900 Confirms Law of Segregation • Chromosomes are in pairs • Each chromosome has one of the allele pair
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
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
Chromosomes separate Each each daughter cell gets doubled chromosomes t t T T
Allele • Member of a paired gene • One allele comes from each parent • Represented by a single letter
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
Homozygous • Both alleles alike • AA or aa or A A a a
Heterozygous • Alleles are different • Aa A a
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.
Phenotype • A trait • Genotype determines the phenotype • Tall is a phenotype • “Think adjective!” • Descriptive This is the "outward, physical manifestation" of the organism.
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.
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
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
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
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
Albinism • Lack of pigment • Skin • Hair • Eyes
a A AA = Normal pigmentation Aa = Normal pigmentation aa = Albino Enzyme Melanin Pigment Amino Acids
A man & woman are both carriers (heterozygous) for albinism. What is the chance their children will inherit albinism?
AA = Normal pigmentation Aa = Normal pigmentation (carrier) aa = Albino A A a a Man = Aa Woman = Aa
A A a a Aa AA Aa aa
Aa AA Aa aa Genotypes 1 AA, 2Aa, 1aa Phenotypes 3 Normal 1 Abino Probability 25% for albinism
Dwarfism Dwarfism = D Normal height = d DD = Dwarfism Dd = Dwarfism dd = Normal height Dwarf Band
A man with heterozygous dwarfism marries a woman who has normal height. What is the chance their children will inherit dwarfism? Dwarfism is dominant.
DD = Dwarf Dd = Dwarf dd = Normal Dd Dd dd dd d d D d
Dd Dd dd dd Genotypes 2 Dd, 2dd Phenotypes 2 Normal 2 Dwarfs Probability 50% for Dwarfism