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Rollercoaster of Genes by Dr. Annette M. Parrott
Sung to the tune “Rollercoaster of Love”
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It’s genetics, Inheritance. It’s genetics: How traits are passed on
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It’s genetics, Inheritance. It’s genetics: How traits are passed on
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It’s genetics, Inheritance. It’s genetics: How traits are passed on
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It’s genetics, Inheritance. It’s genetics: How traits are passed on
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We each have genes Found on every chromosome and They code for traits
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Your eyes are green But his are grey and mine are brown All caused by genes
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You have two genes, one from mom and one from dad
They’re called alleles
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Two of the same and homozygous is your name
You’re heterozygous if it’s one of each heterozygous
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It’s genetics, Inheritance. It’s genetics: How traits are passed on
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It’s genetics, Inheritance. It’s genetics: How traits are passed on
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Phenotype expression of genes, what you look like
These are your traits
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Genotype the genes that control characteristics
On chromosomes
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Dominance expressing one trait over another
Recessive is masked
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Autosomes they do not determine gender,
Like sex chromosomes
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…this has been a Dr. Production...
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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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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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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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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 law of segregation 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 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 the 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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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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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 Figure 9.8A
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Dirk Brings his family tree to class
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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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Pedigree Time
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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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A few are caused by dominant alleles
Examples: achondroplasia, Huntington’s disease Figure 9.9B
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Table 9.9
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VARIATIONS ON MENDEL’S PRINCIPLES
9.11 The relationship of genotype to phenotype is rarely simple Mendel’s principles are valid for all sexually reproducing species However, often the genotype does not dictate the phenotype in the simple way his principles describe
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9.12 Incomplete dominance results in intermediate phenotypes
When an offspring’s phenotype—such as flower color— is in between the phenotypes of its parents, it exhibits incomplete dominance P GENERATION Red RR White rr Gametes R r Pink Rr F1 GENERATION 1/2 R 1/2 r 1/2 R 1/2 R Eggs Sperm Red RR 1/2 r 1/2 r Pink Rr Pink rR F2 GENERATION White rr Figure 9.12A
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Incomplete dominance in human hypercholesterolemia
GENOTYPES: HH Homozygous for ability to make LDL receptors Hh Heterozygous hh Homozygous for inability to make LDL receptors PHENOTYPES: LDL LDL receptor Cell Normal Mild disease Severe disease Figure 9.12B
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9.13 Many genes have more than two alleles in the population
In a population, multiple alleles often exist for a characteristic The three alleles for ABO blood type in humans is an example
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The alleles for A and B blood types are codominant, and both are expressed in the phenotype
Blood Group (Phenotype) Antibodies Present in Blood Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left Genotypes O A B AB Anti-A Anti-B O ii IA IA or IA i A Anti-B IB IB or IB i B Anti-A AB IA IB Figure 9.13
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Let’s do a mini-lab
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9.14 A single gene may affect many phenotypic characteristics
A single gene may affect phenotype in many ways This is called pleiotropy The allele for sickle-cell disease is an example
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Individual homozygous for sickle-cell allele
Sickle-cell (abnormal) hemoglobin Abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped Sickle cells Clumping of cells and clogging of small blood vessels Breakdown of red blood cells Accumulation of sickled cells in spleen Physical weakness Heart failure Pain and fever Brain damage Damage to other organs Spleen damage Anemia Impaired mental function Pneumonia and other infections Kidney failure Rheumatism Paralysis Figure 9.14
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9.16 A single characteristic may be influenced by many genes
This situation creates a continuum of phenotypes Example: skin color
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Fraction of population
P GENERATION aabbcc (very light) AABBCC (very dark) F1 GENERATION AaBbCc AaBbCc Eggs Sperm Fraction of population Skin pigmentation F2 GENERATION Figure 9.16
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THE CHROMOSOMAL BASIS OF INHERITANCE
9.17 Chromosome behavior accounts for Mendel’s principles Genes are located on chromosomes Their behavior during meiosis accounts for inheritance patterns
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The chromosomal basis of Mendel’s principles
Figure 9.17
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9.18 Genes on the same chromosome tend to be inherited together
Certain genes are linked They tend to be inherited together because they reside close together on the same chromosome
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Figure 9.18
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9.19 Crossing over produces new combinations of alleles
This produces gametes with recombinant chromosomes The fruit fly Drosophila melanogaster was used in the first experiments to demonstrate the effects of crossing over
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A B a b B A a b A b a B Tetrad Crossing over Gametes Figure 9.19A, B
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Figure 9.19C
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9.20 Geneticists use crossover data to map genes
Crossing over is more likely to occur between genes that are farther apart Recombination frequencies can be used to map the relative positions of genes on chromosomes Chromosome g c l 17% 9% 9.5% Figure 9.20B
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Alfred H. Sturtevant, seen here at a party with T. H
Alfred H. Sturtevant, seen here at a party with T. H. Morgan and his students, used recombination data from Morgan’s fruit fly crosses to map genes Figure 9.20A
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A partial genetic map of a fruit fly chromosome
Mutant phenotypes Short aristae Black body (g) Cinnabar eyes (c) Vestigial wings (l) Brown eyes Long aristae (appendages on head) Gray body (G) Red eyes (C) Normal wings (L) Red eyes Wild-type phenotypes Figure 9.20C
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SEX CHROMOSOMES AND SEX-LINKED GENES
9.21 Chromosomes determine sex in many species A human male has one X chromosome and one Y chromosome A human female has two X chromosomes Whether a sperm cell has an X or Y chromosome determines the sex of the offspring
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Parents’ diploid cells
(male) (female) Parents’ diploid cells X Y Male Sperm Egg Offspring (diploid) Figure 9.21A
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9.22 Sex-linked genes exhibit a unique pattern of inheritance
All genes on the sex chromosomes are said to be sex-linked In many organisms, the X chromosome carries many genes unrelated to sex Fruit fly eye color is a sex-linked characteristic Figure 9.22A
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Their inheritance pattern reflects the fact that males have one X chromosome and females have two
These figures illustrate inheritance patterns for white eye color (r) in the fruit fly, an X-linked recessive trait Female Male Female Male Female Male XRXR XrY XRXr XRY XRXr XrY XR XR Xr Xr XR XR XRXr Y XRXR Y XRXr Y Xr Xr XRY XrXR XRY XrXr XRY XrY XrY R = red-eye allele r = white-eye allele Figure 9.22B-D
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9.23 Connection: Sex-linked disorders affect mostly males
Most sex-linked human disorders are due to recessive alleles Examples: hemophilia, red-green color blindness These are mostly seen in males A male receives a single X-linked allele from his mother, and will have the disorder, while a female has to receive the allele from both parents to be affected Figure 9.23A
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Czar Nicholas II of Russia
A high incidence of hemophilia has plagued the royal families of Europe Queen Victoria Albert Alice Louis Alexandra Czar Nicholas II of Russia Alexis Figure 9.23B
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