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CHAPTER 9 Patterns of Inheritance
Modules 9.1 – 9.10
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Purebreds and Mutts — A Difference of Heredity
Genetics is the science of heredity These black Labrador puppies are purebred—their parents and grandparents were black Labs with very similar genetic makeups Purebreds often suffer from serious genetic defects
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canine hip dysplasia
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The parents of these puppies were a mixture of different breeds
Their behavior and appearance is more varied as a result of their diverse genetic inheritance
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9.1 The science of genetics has ancient roots
MENDEL’S PRINCIPLES 9.1 The science of genetics has ancient roots The science of heredity dates back to ancient attempts at selective breeding Until the 20th century, however, many biologists erroneously believed that characteristics acquired during lifetime could be passed on characteristics of both parents blended irreversibly in their offspring
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The inheritance of acquired characters (or characteristics) is the hereditary mechanism by which changes in physiology acquired over the life of an organism (such as muscle enlarged through use) are purportedly transmitted to offspring. It is also commonly referred to as the theory of adaptation equated with the evolutionary theory of French naturalist Jean-Baptiste Lamarck ( ).
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Genetic Disproof There are many formulations of the genetic disproof, but all have roughly the same structure as the following: Acquired traits do not affect an organism's genome. Only the genome is passed to the offspring. Therefore, acquired traits cannot be passed to the offspring.
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9.2 Experimental genetics began in an abbey garden
Modern genetics began with Gregor Mendel’s quantitative experiments with pea plants Stamen Carpel Figure 9.2A, B
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This illustration shows his technique for cross-fertilization
Mendel crossed pea plants that differed in certain characteristics and traced the traits from generation to generation White 1 Removed stamens from purple flower Stamens Carpel 2 Transferred pollen from stamens of white flower to carpel of purple flower PARENTS (P) Purple 3 Pollinated carpel matured into pod This illustration shows his technique for cross-fertilization 4 Planted seeds from pod OFF-SPRING (F1) Figure 9.2C
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Mendel studied seven pea characteristics
FLOWER COLOR Purple White FLOWER POSITION Axial Terminal He hypothesized that there are alternative forms of genes (although he did not use that term), the units that determine heredity SEED COLOR Yellow Green SEED SHAPE Round Wrinkled POD SHAPE Inflated Constricted POD COLOR Green Yellow STEM LENGTH Figure 9.2D Tall Dwarf
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Punnett Squares Professor Reginald Crundall Punnett, FRS (June 20, 1875 – January 3, 1967) was a British geneticist who co-founded, with William Bateson, the Journal of Genetics in Punnett is probably best remembered today as the creator of the Punnett square, a tool still used by biologists to predict the probability of possible genotypes of offspring. His Mendelism (1905) is sometimes said to have been the first textbook on genetics; it was probably the first popular science book to introduce genetics to the public.
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3/4 of plants have purple flowers 1/4 of plants have white flowers
9.3 Mendel’s principle of segregation describes the inheritance of a single characteristic From his experimental data, Mendel deduced that an organism has two genes (alleles) for each inherited characteristic One characteristic comes from each parent P GENERATION (true-breeding parents) Purple flowers White flowers All plants have purple flowers F1 generation Fertilization among F1 plants (F1 x F1) F2 generation 3/4 of plants have purple flowers 1/4 of plants have white flowers Figure 9.3A
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GENETIC MAKEUP (ALLELES)
A sperm or egg carries only one allele of each pair GENETIC MAKEUP (ALLELES) P PLANTS PP pp Gametes All P All p The pairs of alleles separate when gametes form This process describes Mendel’s 1. Law of segregation 2. Alleles can be dominant or recessive F1 PLANTS (hybrids) All Pp Gametes 1/2 P 1/2 p P P Eggs Sperm PP F2 PLANTS p p Pp Pp Phenotypic ratio 3 purple : 1 white pp Genotypic ratio 1 PP : 2 Pp : 1 pp Figure 9.3B
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9.4 Homologous chromosomes bear the two alleles for each characteristic
Alternative forms of a gene (alleles) reside at the same locus on homologous chromosomes- One allele is dominant over the other with regard to Mendelian Traits. GENE LOCI DOMINANT allele P a B P a b RECESSIVE allele GENOTYPE: PP aa Bb HOMOZYGOUS for the dominant allele HOMOZYGOUS for the recessive allele HETEROZYGOUS Figure 9.4
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9.5 The principle of independent assortment is revealed by tracking two characteristics at once
By looking at two characteristics at once, Mendel found that the alleles of a pair segregate independently of other allele pairs during gamete formation This is known as: 3. Principle of independent assortment
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HYPOTHESIS: DEPENDENT ASSORTMENT HYPOTHESIS: INDEPENDENT ASSORTMENT
RRYY rryy P GENERATION RRYY rryy Gametes RY ry Gametes RY ry F1 GENERATION RrYy RrYy Eggs 1/2 RY 1/2 RY Sperm Eggs 1/4 RY 1/4 RY 1/2 ry 1/2 ry 1/4 rY 1/4 rY RRYY 1/4 Ry 1/4 Ry RrYY RrYY F2 GENERATION 1/4 ry 1/4 ry RRYy rrYY RrYy RrYy RrYy RrYy RrYy Yellow round 9/16 Actual results contradict hypothesis Green round rrYy RRyy rrYy 3/16 ACTUAL RESULTS SUPPORT HYPOTHESIS Rryy Rryy Yellow wrinkled 3/16 Yellow wrinkled rryy 1/16 Figure 9.5A
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Independent assortment of two genes in the Labrador retriever
Blind Blind PHENOTYPES Black coat, normal vision B_N_ Black coat, blind (PRA) B_nn Chocolate coat, normal vision bbN_ Chocolate coat, blind (PRA) bbnn GENOTYPES MATING OF HETEROZYOTES (black, normal vision) BbNn BbNn PHENOTYPIC RATIO OF OFFSPRING 9 black coat, normal vision 3 black coat, blind (PRA) 3 chocolate coat, normal vision 1 chocolate coat, blind (PRA) Figure 9.5B
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A dihybrid cross
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The Female Calico Cat-X-Linked
Calico coloring is a mix of phaeomelanin based colors (red) and eumelanin based color (black, chocolate and cinnamon). Cats of this coloration are believed to bring good luck in the folklore of many cultures.[1] The spotting gene causes white patches to cover the colored fur
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4 Sex-Linked Traits: 1. Normal Color Vision: A: 29, B: 45, C: --, D: 26 2. Red-Green Color-Blind: A: 70, B: --, C: 5, D: -- 3. Red Color-blind: A: 70, B: --, C: 5, D: 6 4. Green Color-Blind: A: 70, B: --, C: 5, D: 2
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9.6 Geneticists use the testcross to determine unknown genotypes
The offspring of a testcross often reveal the genotype of an individual when it is unknown TESTCROSS: GENOTYPES B_ bb Two possibilities for the black dog: BB or Bb B B b GAMETES b Bb b Bb bb Figure 9.6 OFFSPRING All black 1 black : 1 chocolate
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Most such disorders are caused by autosomal recessive alleles
9.9 Connection: Many inherited disorders in humans are controlled by a single gene Most such disorders are caused by autosomal recessive alleles Examples: cystic fibrosis, sickle-cell disease Normal Dd Normal Dd PARENTS D D Eggs Sperm DD Normal d d Dd Normal (carrier) Dd Normal (carrier) OFFSPRING dd Deaf Figure 9.9A
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9.7 Mendel’s principles reflect the rules of probability
Inheritance follows the rules of probability The rule of multiplication and the rule of addition can be used to determine the probability of certain events occurring F1 GENOTYPES Bb female Bb male Formation of eggs Formation of sperm 1/2 B B 1/2 B B 1/2 b b 1/2 1/4 b B B b 1/4 1/4 b b F2 GENOTYPES 1/4 Figure 9.7
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9.8 Connection: Genetic traits in humans can be tracked through family pedigrees
The inheritance of many human traits follows Mendel’s principles and the rules of probability OMIM- Figure 9.8A
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Family pedigrees are used to determine patterns of inheritance and individual genotypes
Dd Joshua Lambert Dd Abigail Linnell D_ John Eddy ? D_ Hepzibah Daggett ? D_ Abigail Lambert ? dd Jonathan Lambert Dd Elizabeth Eddy Dd Dd dd Dd Dd Dd dd Female Male Deaf Figure 9.8B Hearing
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A few are caused by dominant alleles
Examples: achondroplasia, Huntington’s disease Figure 9.9B
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