Extending Mendelian Genetics

• Mendel's pea plant studies revealed that genotype-phenotype relationships are rarely simple
• Most heritable traits are not determined by a single gene with two alleles
• Segregation and independent assortment principles still apply to complex inheritance patterns

Extending Mendelian Genetics for a Single Gene

• Inheritance of single-gene traits can deviate from Mendelian patterns in certain situations
• These deviations include alleles not being completely dominant or recessive, a gene having more than two alleles, and one gene producing multiple phenotypes

Degrees of Dominance

• Complete dominance occurs when a heterozygote and a dominant homozygote have identical phenotypes
• Incomplete dominance is when the F1 hybrid phenotype is somewhere between the two parental phenotypes
• For example, a red flower crossed with a white flower produces pink flowers
• Codominance happens if two dominant alleles are expressed separately and distinctly
• The ABO blood group in humans exhibits codominance

The Relation Between Dominance and Phenotype

• Dominant alleles don't subdue recessive alleles; alleles don't interact this way
• Alleles are variations in a gene's nucleotide sequence
• Dominance/recessiveness relationships depend on the level at which a phenotype is examined

Tay-Sachs Disease

• Tay-Sachs disease leads to lipid accumulation in the brain leading to fatality
• The allele is recessive at the organismal level
• At the biochemical level, the phenotype is incompletely dominant
• At the molecular level, the alleles are codominant

Frequency of Dominant Alleles

• Dominant alleles aren't necessarily more common
• Polydactyly, having extra fingers or toes, affects 1 in 400 US babies
• The recessive allele to have five digits is actually more prevalent

Multiple Alleles

• Most genes have more than two forms
• The ABO blood group in humans has four phenotypes, determined by three alleles(I)
• The I alleles attach A or B carbohydrates to red blood cells, IA, IB, and i.
• The IA allele encodes an enzyme that adds the A carbohydrate
• The IB allele encodes an enzyme that adds the B carbohydrate
• The i allele encodes an enzyme that adds neither

Pleiotropy

• Most genes have multiple phenotypic effects known as pleiotropy
• Certain hereditary diseases, like cystic fibrosis and sickle cell disease, is a result of pleiotropic alleles
• Symptoms of cystic fibrosis includes thick mucus, wheezing, sinus infections like bronchitis and pneumonia, nose polyps, constipation, stomach pains, weight loss or gain, low bone density, and clubbing

Extending Mendelian Genetics for Two or More Genes

• Some traits may be determined by two or more genes

Epistasis

• Epistasis is when a gene at one locus affects the phenotypic expression of a gene at a second locus
• In Labrador retrievers, coat color depends on two genes in many mammals
• One gene determines pigment color B for black and b for brown
• The other gene, with alleles E for color and e for no color, determines if pigment is deposited in the hair

Polygenic Inheritance

• Quantitative characters vary in the population
• Quantitative variation indicates polygenic inheritance
• The additive effect of two or more genes on a single phenotype, such as human skin color

Nature and Nurture: The Environmental Impact on Phenotype

• A departure from Mendelian genetics happens when a character's phenotype depends on environment and genotype
• Polygenic characters' phenotypic range is broadest
• Traits that depend on multiple genes and environmental influences are multifactorial

A Mendelian View of Heredity and Variation

• An organism's phenotype includes physical appearance, internal anatomy, physiology, and behavior
• An organism's phenotype also comes from overall genotype and unique environmental history

Concept 14.4: Many human traits follow Mendelian patterns of inheritance

• Humans aren't great for genetic research because of its long generation time, few offspring, and breeding experiments are considered unacceptable.
• Basic Mendelian genetics is the foundation of human genetics

New Combinations of Alleles: Variation for Normal Selection

• Recombinant chromosomes create new allele combinations in gametes
• Random fertilization increases the number of variant combinations
• Abundant genetic variation is the raw material for natural selection

Alterations of chromosome number or structure cause some genetic disorders

• Large-scale chromosomal alterations in humans and other mammals often lead to spontaneous abortions or developmental disorders
• Plants tolerate such genetic changes better than animals

Abnormal Chromosome Number

• Nondisjunction is when homologous chromosomes don't separate correctly during meiosis
• A gamete gets two of the same chromosome, while one gets no copy

Aneuploidy

• Aneuploidy results from fertilization of gametes with nondisjunction
• Offspring have an abnormal number of a particular chromosome
• A monosomic zygote has one copy of a chromosome
• A trisomic zygote has three copies

Polyploidy

• Polyploidy: An organism has more than two sets of chromosomes
• Triploidy (3n): Three sets of chromosomes
• Tetraploidy (4n): Four sets of chromosomes
• Polyploidy is common in plants, but not animals
• Polyploids appear more normal than aneuploids

Alterations of Chromosome Structure

• Chromosome breakage leads to changes in structure
• Deletion: Removes a segment
• Duplication: Repeats a segment
• Inversion: Reverses a segment's orientation
• Translocation: Moves a segment to another chromosome

Study Guide for Chapter 15

• Thomas Hunt Morgan's experiments used D. melanogaster, studying the wild types and mutants
• Morgan's experiments demonstrated the chromosomal basis of inheritance plus supported segregation and independent assortment

Human Disorders Due to Chromosomal Alterations

• Alterations of chromosome number/structure are linked to severe disorders
• Some aneuploidy types cause less upset so individuals survive to birth
• Survivors have a set of symptoms, or syndrome, based on the aneuploidy type
• Males and females inherit differing X chromosome numbers, thus genes located on autosomes produce patterns of inheritance different than that produced on the X chromosome
• The X chromosome contains numerous genes for characters unrelated to sex.
• Fathers give X-linked alleles to their daughters but none to their sons
• Mothers give X-linked alleles to both sons and daughters
• Red-green color blindness is a mild X-linked disorder

Down Syndrome (Trisomy 21)

• Down syndrome is an aneuploid syndrome with three copies of chromosome 21
• Down syndrome affects 1 in 700 US children
• The frequency of Down syndrome is related to mother's age

Disorders Caused by Structurally Altered Chromosomes

• Cri du chat syndrome results from a chromosome 5 deletion
• Chronic myelogenous leukemia is caused by translocations of chromosomes

Pedigree Analysis

• Pedigree: A family tree showing parent-child relationships through generations
• Pedigrees trace inheritance patterns of traits

Recessively Inherited Disorders

• Genetic disorders are inherited recessively
• Can range from mild to life-threatening

The Behavior of Recessive Alleles

• Recessive disorders appear in those homozygous for the allele
• Carriers: Heterozygous individuals carry a recessive allele, but are phenotypically normal
• Most individuals with recessive disorders are born to carrier parents
• Albinism: A recessive condition defined by lack of skin/hair pigmentation
• The chance of two carriers of a rare disease allele mating is low if the allele is rare
• Consanguineous matings, matings between close relatives, increases the chance of mating between same rare alleles
• Most societies and cultures prevent marriage between close relatives

Hemophilia

• Hemophilia is an X-linked recessive disorder caused by absence of required protein(s) for blood clotting
• A hemophiliac bleeds longer because firm clots are slow to form
• Hemophilia was widespread among European royal families in the 1800s
• Queen Victoria passed the allele to many of her descendants

Cystic Fibrosis

• Cystic fibrosis is the most common lethal genetic disease in the US, striking 1 in 2,500 of European descent
• The cystic fibrosis allele results in defective or absent chloride transport channels which causes a buildup of chloride ions
• Symptoms include: mucus buildup in internal organs and abnormal nutrient absorption

Sickle-Cell Disease: A Genetic Disorder with Evolutionary Implications

• Sickle-cell disease affects 1 in 400 African-Americans
• Caused by a single amino acid substitution in hemoglobin
• In homozygous individuals, all hemoglobin is abnormal (sickle-cell)
• Symptoms: physical weakness, pain, organ damage, paralysis
• Heterozygotes (sickle-cell trait) are usually healthy but may suffer some symptoms
• About one out of ten African Americans has sickle-cell trait
• Heterozygotes are less susceptible to the malaria parasite; advantageous in regions where malaria is common

Dominantly Inherited Disorders

• Some human disorders are caused by dominant alleles
• Lethal dominant alleles are rare and result from mutation
• Achondroplasia is a form of dwarfism caused by a rare dominant allele

Huntington's Disease: A Late-Onset Lethal Disease

• Huntington's disease is a degenerative nervous system disease
• No phenotypic effects appear until the individual is 35-40 years of age
• Deterioration is irreversible and fatal

Marfan Syndrome

• Marfan Syndrome is a dominantly-inherited disease
• Affects connective tissue formation
• Affected individuals are typically tall and lanky

Multifactorial Disorders

• Heart disease, diabetes, alcoholism, mental illnesses, and some cancers have genetic and environmental components
• No matter what our genotype, our lifestyle hugely impacts phenotype

Study Guide for Chapter 14

• Gregor Mendel and garden peas
• Segregation and independent assortment laws
• Key question: "How did Mendel deduce these laws?"
• What is a Punnett square?
• How Punnett squares used to predict progeny from a cross?

Deviations from Mendelian Patterns

• Non-Mendelian inheritance patterns due to:
• Codominance
• Incomplete dominance
• Multiple alleles
• Pleiotropy
• Epistasis
• Polygenic inheritance
• Multifactorial conditions