Mendelian Genetics and Inheritance

Mendel Chap 14

Mendel's Laws of Inheritance

  • Mendel used the scientific approach to identify two laws of inheritance.
  • The laws of probability govern Mendelian inheritance.
  • Inheritance patterns are often more complex than predicted by simple Mendelian genetics.
  • Many human traits follow Mendelian patterns of inheritance.

Mendel's Experimental Approach

  • Pea plants were used because they were available in many different varieties.
  • Varieties have distinct heritable features, or characters (e.g., flower color).
  • Each variant for a character, such as purple or white flowers, is called a trait.
Why the Pea Plant?
  • Easily categorizable characters.
  • Easy to control pollination.
  • Short generation time.
  • Large number of offspring.

Crossing Pea Plants: Generations

  • Hybridization: Mating two contrasting, true-breeding varieties.
  • True-breeding parents are the P generation.
  • Hybrid offspring of the P generation are called the F1 generation.
  • When F1 individuals self-pollinate or cross-pollinate with other F1 hybrids, the F2 generation is produced.
Inquiry: F1 Hybrid Crosses
  • When Mendel crossed F1 hybrids, many F2 plants had purple flowers, but some had white.
  • Ratio of about 3 to 1, purple to white flowers, in the F2 generation.

The Law of Segregation

  • Mendel reasoned that only the purple flower factor was affecting flower color in F1 hybrids.
  • Purple flower color was called a dominant trait, and white flower color a recessive trait.
  • The factor for white flowers was not diluted or destroyed because it reappeared in the F2 generation.
  • Mendel observed the same pattern of inheritance in six other pea plant characters [each represented by two traits].
  • What Mendel called a heritable factor is now called a gene.
Alleles
  • Alternative versions of a gene are called alleles.
  • Each gene resides at a specific locus on a specific chromosome.
Mendel’s Law of Segregation
  • Punnett square: a diagram for predicting the results of a genetic cross between individuals of known genetic makeup.
  • Visualization of genotypes and phenotypes for simpler hybrid and dihybrid crosses.
  • Gets too complicated for 3+ traits.
Useful Vocabulary
  • Homozygous: An organism with two identical alleles for a character.
  • Heterozygous: An organism that has two different alleles for a gene.
  • Heterozygotes are not true-breeding.
  • Phenotype: Physical appearance.
  • Genotype: Genetic makeup.
Phenotype vs Genotype
  • PP and Pp plants have the same phenotype (purple), but different genotypes.
Research Method: The Testcross
  • Testcross: Breeding the mystery individual with a homozygous recessive individual.
  • If any offspring display the recessive phenotype, the mystery parent must be heterozygous.

Probability and Mendelian Inheritance

  • Genotype Ratios from the cross [Gg x Gg] are: 14\frac{1}{4} GG : 12\frac{1}{2} Gg : 14\frac{1}{4} gg
  • The F2 homozygous/true-breeding offspring [GG and gg] are half the progeny.

Pedigree Analysis

  • Pedigree: A family tree describing interrelationships of parents and children across generations.
  • Used to trace and describe inheritance patterns of particular traits.
Recessive Traits
  • Albinism as an example of a recessive trait, where individuals must inherit two copies of the recessive allele to express the trait.

Complex Inheritance Patterns

  • The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied.
  • Many heritable characters not determined by only one gene with two alleles.
  • However, basic principles of segregation and independent assortment apply even to more complex patterns of inheritance.
Degrees of Dominance
  • Complete dominance: Phenotypes of the heterozygote and dominant homozygote are identical.
  • Incomplete dominance: Phenotype of F1 hybrids is in between the phenotypes of the two parental varieties.
  • Codominance: Two dominant alleles affect the phenotype in separate, distinguishable ways.
Multiple Alleles
  • Example: ABO blood types.
  • IAI^A allele adds the A carbohydrate.
  • IBI^B allele adds the B carbohydrate.
  • i allele adds neither.
Pleiotropy
  • Pleiotropy = one gene, many effects.
  • Most genes have multiple phenotypic effects.
  • Pleiotropic alleles are responsible for multiple symptoms of some hereditary diseases, such as cystic fibrosis and sickle-cell disease.
Extending Mendelian Genetics for Multiple Genes
  • Some traits may be determined by two or more genes.
  • Epistasis: A gene at one locus alters the phenotypic expression of a gene at a second locus.
  • Example: coat color in Labrador retrievers.
  • Polygenic Inheritance: Additive effect of two or more genes on a single phenotype.
Epistasis Example
  • One gene determines pigment color (alleles B for black and b for brown).
  • The other gene (with alleles E for deposition) determines whether pigment will be deposited in the hair.
  • Gene E/e is epistatic to B/b and therefore the coat color is determined by the E/e locus.
Polygenic Inheritance Examples
  • Quantitative characters are those that vary in the population along a continuum.
  • Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype.
  • Examples: Skin color, hair color, height, intelligence, blood pressure, obesity, autism.
Multifactorial Disorders
  • Many diseases, such as heart disease, diabetes, alcoholism, mental illnesses, and cancer have both genetic and environmental components.
  • Lifestyle has a tremendous effect on phenotype.
  • Little is understood about genetic contribution to most multifactorial diseases.

Genetic Disorders

Cystic Fibrosis
  • Most common lethal genetic disease in Canada affecting 1/2,500 people of European descent.
  • Due to defective or absent chloride transport channels in plasma membranes leading to a buildup of chloride ions outside the cell.
  • Symptoms from mucus buildup and abnormal absorption of nutrients in the small intestine.
Sickle-Cell Disease
  • Affects 1/400 African-Americans.
  • Caused by the substitution of a single amino acid in hemoglobin protein in red blood cells.
  • Symptoms include physical weakness, pain, organ damage, and even paralysis.
  • In homozygous individuals, all hemoglobin is abnormal (sickle-cell).
Evolutionary Implications
  • Heterozygotes are usually healthy but may suffer some symptoms.
  • About 1/10 African Americans carries the sickle cell allele.
  • Heterozygotes are less susceptible to the malaria parasite, so there is an advantage to being heterozygous where malaria is common.
Dominantly Inherited Disorders
  • Some human disorders are caused by dominant alleles.
  • Dominant alleles that cause a lethal disease are rare and arise by mutation.
  • Achondroplasia is a form of dwarfism caused by a rare dominant allele.

Examples of Genetic Diseases

  • Tay Sachs Disease [Autosomal Recessive]
  • Cystic Fibrosis [Autosomal Recessive]
  • Sickle Cell Anemia [Autosomal Recessive]
  • Achondroplasia [Autosomal Dominant]
  • Hemophilia A [X-Linked Recessive]
  • Colour Blindness [X-Linked Recessive]