Post on 25-Dec-2015
Genetic PrinciplesDiscovered by Mendel
• Gregor Mendel (1822-1884) laid down the basic principles of heredity.
• Plant hybrids
Dominance and Recessiveness
• Homozygous vs. heterozygous• Recessive traits are not expressed in
heterozygotes.• Dominant traits are governed by an allele that
can be expressed in the presence of another, allele.
Punnett square
• 1⁄4 of the plants can be expected to be homozygous tall (TT).
• 1/2 can be expected to be tall but will be heterozygous (Tt).
• The remaining 1⁄4 are homozygous for the recessive “short” allele (tt).
Principle of Independent Assortment
• The distribution of one pair of alleles into gametes does not influence the distribution of another pair.
• The genes controlling different traits are inherited independently of one another.
Phenotype
• The observable or detectable physical characteristics of an organism.
• The detectable expressions of genotypes, frequently influenced by environmental factors.
• Phenotypic ratio - The proportion of one phenotype to other phenotypes in a group of organisms.
Mendelian Traits
• Characteristics that are influenced by alleles at only one genetic locus.
• Examples include many blood types, such as ABO.
• Many genetic disorders such as sickle-cell anemia and Tay-Sachs disease are also Mendelian traits.
Mendelian Inheritance in Humans
• Over 4,500 human trains are known to be inherited according to Mendelian principles.
Some Mendelian Traits in Humans: Dominant
Condition Manifestations
Achondroplasia Dwarfism due to growth defects involving the long bones of the arms and legs; trunk
and head size usually normal.
Brachydactyly Shortened fingers and toes.
Familial hyper-cholesterolemia
Elevated cholesterol levels and cholesterol plaque deposition;
a leading cause of heart disease, with death frequently occurring
by middle age.
Some Mendelian Traits in Humans: Recessive
Condition Manifestations
Cystic fibrosis Among the most common genetic disorders among European Americans; abnormal secretions of the exocrine glands, with
pronounced involvement of the pancreas; most patients develop obstructive lung
disease.
Tay-Sachs disease
Most common among Ashkenazi Jews; degeneration of the nervous system
beginning at about 6 months of age; lethal by age 2 or 3 years.
ABO Genotypes and Associated Phenotypes
Genotype Antigens onRed Blood Cells
ABO Blood Type(Phenotype)
AA, AO A A
BB, BO B B
AB A and B AB
OO None O
Codominance
• The expression of two alleles in heterozygotes.
• In this situation, neither allele is dominant or recessive so they both influence the phenotype.
Autosomal Dominant Trait
• Inheritance of an autosomal dominant trait: a human pedigree for brachydactyly. How can individuals 5, 11, 14, 15, and 17 be unaffected?
Pattern of Inheritance of Autosomal Dominant Traits
• (a) Diagram of possible gametes produced by one parent with brachydactyly and another with normal hands and fingers.
• (b) Punnett square depicting possible genotypes in the offspring of one parent with brachydactyly (Bb) and one with normal hands and fingers (bb).
Partial Pedigree for Albinism
• Individuals 6 and 7, children of unaffected parents, are affected. Four individuals are definitely unaffected carriers. Which ones are they?
An African Albino
• An African albino. This young man has a greatly increased likelihood of developing skin cancer.
Phenotypically Normal Parents, Both Carriers of the Albinism Allele
• Offspring: Homozygous dominants, Heterozygotes, (carriers), or Homozygous recessives
Mendelian Disorders Inherited as X-Linked Recessive Traits
Condition Manifestations
G-6-PD deficiency
Lack of an enzyme in red blood cells; produces severe anemia in the presence of
certain foods and/or drugs.
Muscular dystrophy
One form; other forms can be inherited as autosomal recessives; progressive weakness and atrophy of muscles beginning in early
childhood; continues to progress
Mendelian Disorders Inherited as X-Linked Recessive Traits
Condition Manifestations
Red-green color blindness
Two separate forms, one involving the perception of red and the other affecting the
perception of green.
Lesch-Nyhan Impaired motor development noticeable by 5 months; progressive motor impairment,
disease diminished kidney function, self-mutilation, and early death.
Mendelian Disorders Inherited as X-Linked Recessive Traits
Condition Manifestations
Hemophilia In hemophilia A, a clotting factor is missing; hemophilia B is caused by a defective clotting
factor. Both produce abnormal internal and external bleeding from minor injuries; severe
pain is a frequent accompaniment; without treatment, death usually occurs before
adulthood.
Mendelian Disorders Inherited as X-Linked Recessive Traits
Condition Manifestations
Ichthyosis A skin condition due to lack of an enzyme; characterized by scaly, brown lesions on the
extremities and trunk. In the past, people with this condition were sometimes exhibited in
circuses and sideshows as “the alligator man.”
Polygenic Inheritance
• Polygenic traits are continuous traits governed by alleles at more than one genetic locus.
Discontinuous Distribution of Mendelian Traits
• Shows the discontinuous distribution of ABO blood type in a hypothetical population.
• The expression of the trait is described in terms of frequencies.
Continuous Expressionof a Polygenic Trait
• Represents the continuous expression of height in a large group of people.
Pleiotropy
• Pleiotropy is a situation where a single gene influences more than one phenotypic expression. – Example: The autosomal recessive disorder
phenylketonuria (PKU). • Individuals who are homozygous for the PKU
allele don’t produce phenylketonurase, the enzyme involved in the conversion of the amino acid phenylalanine to the amino acid, tyrosine.
Mitochondrial Inheritance
• Animals of both sexes inherit their mtDNA, and all mitochondrial traits, from their mothers.
• All the variation in mtDNA is caused by mutation.
The Modern Synthesis
Evolution is a two-stage process:1. The production and redistribution of
variation.2. Natural selection acting on this variation.– A current definition of evolution - a change in
allele frequency from one generation to the next.
Genetic Drift
• Genetic drift occurs when some individuals contribute a disproportionate share of genes to succeeding generations.
Founder Effect
• Genetic drift in which allele frequencies are altered in small populations that are taken from, or are remnants of, larger populations.
Recombination
• In sexually reproducing species both parents contribute genes to offspring and the genetic information is reshuffled every generation.
Natural Selection
• Natural selection provides directional change in allele frequency relative to specific environmental factors.
• If the environment changes, selection pressures also change.
• If there are long-term environmental changes in a consistent direction, then allele frequencies should also shift gradually each generation.
Genetic Variation
• Cheetahs, like many other species, have passed through a genetic bottleneck.
• As a species, they have little genetic variation.
Levels of Organization in the Evolutionary Process
EvolutionaryFactor
Level Evolutionary Process
Mutation DNA Storage of genetic information; ability to replicate; influences
phenotype by production of proteins
Mutation Chromosomes A vehicle for packaging and transmitting DNA
Levels of Organization in the Evolutionary Process
EvolutionaryFactor
Level Evolutionary Process
Recombination(sex cells only)
Cell Basic unit of life, contains chromosomes, divides for
growth and production of sex cells
Natural selection Organism The unit that reproduces and which we observe for
phenotypic traits
Drift, gene flow Population Changes in allele frequencies between generations