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Patterns of Inheritance
Chapter 9 Patterns of Inheritance
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Purebreds and Mutts–A Difference of Heredity
Purebred dogs - very similar on a genetic level due to selective breeding (true-breed) Mutts, or mixed breed dogs - show considerably more genetic variation (hybrid)
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The historical roots of genetics:
Early 19th century: traits from mom and dad blend like paints to form kid’s traits Gregor Mendel (1840’s) : “Father of modern genetics” Mendel crossed pea plants that differed in certain characteristics (traits) and traced from generation to generation; used a mathematical approach Why did he choose pea plants? Crossing of traits: Self fertilize (True breed) – cross pollen and egg from same parent plant to get identical offspring Cross Fertilize (hybrid) – cross pollen from one parent plant with the egg of a different parent plant Gregor mendel
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And so on… P generation is true-breeding – Parent generation
F1 generation = Hybrid offspring of P (parents) F2 generation = offspring of F1 plants crossed F3 generation = offspring of F2 plants crossed And so on… Parents (P) White Purple Offspring (F1)
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Mendel hypothesized that there are alternative forms of factors (genes) = units that determine heritable traits Flower color Flower position Seed color Seed shape Pod color Pod shape Stem length Purple White Axial Terminal Round Wrinkled Inflated Constricted Tall Dwarf Green Yellow
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From his experimental data, Mendel deduced that an organism has two genes (alleles) for each inherited characteristic For each characteristic (trait), an organism inherits 2 alleles, one from each parent. Think of TRAITS as CATEGORIES and ALLELES as OPTIONS within each category! P generation (true-breeding parents) F1 generation F2 generation Purple flowers White flowers All plants have purple flowers Fertilization among F1 plants (F1 F1) of plants have purple flowers 3 4 of plants have white flowers 1 Examples: Flower Color (trait) Purple or White (alleles)
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Phenotype = physical appearance of the allele for a specific trait (purple/white flower for flower color trait) Genotype = genetic makeup the alleles that represent the phenotype (one dominant, one recessive; or 2 dominant alleles and 2 recessives) DNA from the Beginning Animations
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Dominant and Recessive Alleles:
If the 2 alleles of an inherited pair are different, then one determines the organism’s appearance and is called the dominant allele. (Dominant will usually show up more often!) The other allele has no noticeable effect on the organism’s appearance and is called the recessive allele. (Is present but does not show up in the appearance) * If dominant allele is present, it takes over and outweighs the recessive!
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Dominant and Recessive alleles:
In a genetic cross, CAPITAL letters are used to represent DOMINANT alleles and lower case letters represent the recessive alleles. MUST USE SAME LETTER FOR EACH TRAIT! (Doesn’t matter the letter you choose!) Example: Flower Color (trait) T = purple, t = white Pea Color R=yellow, r =green
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HOMOZYGOUS and HETEROZYGOUS
When 2 of the SAME ALLELES are present, it is HOMOZYGOUS for that trait. HH = homozygous dominant hh = homozygous recessive When 2 DIFFERENT alleles are present, it is termed HETEROZYGOUS for the trait. Hh= heterozygous With homozygous, you must clarify which allele- either Dominant or recessive!
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We can look at the alleles from each parent to determine the probability of those alleles being passed on to offspring. PUNNETT SQUARE: Shows a genetic mixing (cross) of alleles from both parents for specific traits. Punnett Square are use to PREDICT PROBABILITIES and see inheritance patterns for specific traits!
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Trait= Flower Color H h Parent #2 H =purple h = white Hh Hh Hh Hh
#1 Hh Hh
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Probability of one offspring from parent cross!
Trait= Flower Color * If Dominant allele is present, it takes over and outweighs the recessive! H =purple h = white H h Genotype =genetic makeup (represented by letters!) Parent 1 = hh homozygous recessive Parent 2 = HH homozygous dominant Offspring= 100% Hh Heterozygous Hh Hh Probability of one offspring from parent cross! Phenotype =physical appearance (what the letters represent!) Hh Parent 1 = white Parent 2 = purple Offspring= 100% purple Hh
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LET’S PRACTICE!!!
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Homologous chromosomes bear the two alleles for each characteristic
Reside at the same locus (point) on homologous chromosomes Genotype: PP aa Bb Heterozygous P a b B Gene loci Recessive allele Dominant allele Homozygous for the dominant allele Homozygous for the recessive allele
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Mendel’s law of segregation
Predicts that allele pairs from each parent separate (segregate) from each other during the production of gametes (sperm/eggs) P plants Gametes Genetic makeup (alleles) F1 plants (hybrids) F2 plants PP pp All P All p All Pp Sperm 1 2 P p Pp Eggs Genotypic ratio 1 PP : 2 Pp: 1 pp Phenotypic ratio 3 purple : 1 white
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Actual results support hypothesis
Mendel’s law of independent assortment States that alleles of a pair segregate independently of other allele pairs during gamete formation Hypothesis: Dependent assortment Hypothesis: Independent assortment RRYY rryy Gametes RrYy RY ry Sperm Ry RrYY RRYy rrYY rrYy RRyy Rryy Actual results contradict hypothesis Actual results support hypothesis Yellow round Green round Yellow wrinkled Green wrinkled Eggs P generation F1 generation F2 generation 1 2 4 9 16 3 Figure 9.5 A
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9.8 Genetic traits in humans can be tracked through family pedigrees.
Pedigree Key Dd Joshua Lambert Abigail Linnell D ? John Eddy Hepzibah Daggett dd Jonathan Elizabeth Dd Dd dd Dd Dd Dd dd Female Male Deaf Hearing Female Male Mating Offspring Figure 9.8 B
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Table 9.9
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Recessive Disorders- Most human genetic disorders are recessive
Parents Offspring Sperm Normal Dd D d Eggs D d DD (carrier) dd Deaf Figure 9.9 A
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Dominant Disorders- Some human genetic disorders are dominant
Parents Offspring Sperm Dwarf Dd Normal dd D d Eggs d Achondroplasia – cause of dwarfism Figure 9.9 B
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9.10 New technologies can provide insight into genetic legacy
Identifying Carriers For an increasing number of genetic disorders, tests are available that can distinguish carriers of genetic disorders and can provide insight for reproductive decisions Fetal Testing: Amniocentesis and chorionic villus sampling (CVS) allow doctors to remove fetal cells that can be tested for genetic abnormalities Fetal Imaging- Ultrasound imaging uses sound waves to produce a picture of the fetus
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Chorionic villus sampling (CVS)
Figure 9.10 A Amniocentesis Chorionic villus sampling (CVS) Ultrasound monitor Fetus Uterus Amniotic fluid Fetal cells Several weeks Biochemical tests hours Cervix Suction tube inserted through cervix to extract tissue from chorionic villi Needle inserted through abdomen to extract amniotic fluid Centrifugation Placenta Chorionic villi Karyotyping
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Figure 9.10 B
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Ethical Considerations
Newborn Screening Some genetic disorders can be detected at birth, by simple tests that are now routinely performed in most hospitals in the United States Ethical Considerations New technologies such as fetal imaging and testing raise new ethical questions (Think about “Designer Babies”!)
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NON-MENDELIAN GENETICS
ALL OF THE FOLLOWING ARE EXCEPTIONS TO MENDEL’S RULES!!! Mendel’s principles are valid for all sexually reproducing species, HOWEVER, genotype often does NOT dictate phenotype in the simple way his laws described.
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When an offspring’s phenotype is in between the phenotypes of its parent, it exhibits incomplete dominance. P generation F1 generation F2 generation Red RR Gametes White rr Sperm Eggs Pink Rr R r rR 1 2 Figure 9.12 A
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Involves 3 alleles of a single gene
In a population, multiple alleles (2 or more options) often exist for a single trait. Example: The ABO blood type in humans Involves 3 alleles of a single gene The alleles for A and B blood types are codominant and both are expressed in the phenotype
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Figure 9.13
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Pleiotropy - a single gene can affect a phenotype in many ways
Individual homozygous for sickle-cell allele Abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped Sickle-cell (abnormal) hemoglobin Sickle cells Breakdown of red blood cells Clumping of cells and clogging of small blood vessels Accumulation of sickled cells in spleen Physical weakness Anemia Heart failure Pain and fever Brain damage Damage to other organs Spleen Impaired mental function Paralysis Pneumonia and other infections Rheumatism Kidney 5,555 Figure 9.14
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Polygenic inheritance- creates a multiple variations of phenotypes
P generation F1 generation F2 generation Sperm Eggs aabbcc (very light) AABBCC (very dark) AaBbCc 1 8 64 6 15 20 Skin color Fraction of population Figure 9.15
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THE CHROMOSOMAL BASIS OF MENDEL’S LAWS OF INHERITANCE
All round yellow seeds (RrYy) Metaphase I of meiosis (alternative arrangements) Anaphase I of meiosis Metaphase II of meiosis Gametes F1 generation F2 generation Fertilization among the F1 plants (See Figure 9.5A) 1 4 RY ry R y Y r rY Ry 9 : 3 : 1 Figure 9.18
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Not accounted for: purple round and red long
9.19 Genes on the same chromosome tend to be inherited together Experiment Explanation: linked genes PpLI PpLI Long pollen Observed Prediction Phenotypes offspring (9:3:3:1) Purple long Purple round Red long Red round Parental diploid cell PpLI Most gametes offspring Eggs 3 purple long : 1 red round Not accounted for: purple round and red long Meiosis Fertilization Sperm 284 21 55 215 71 24 P I P L Purple flower Certain genes are called LINKED GENES- tend to be inherited together because they reside close together on the same chromosome. Figure 9.19
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9.20 Crossing over produces new combinations of alleles
Crossing over can separate linked alleleS producing gametes with recombinant chromosomes A B a b Tetrad Crossing over Gametes Figure 9.20 A
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Experiment Gray body, long wings (wild type) GgLI Female Black body, vestigial wings ggll Male Offspring Gray long 965 944 206 185 Black vestigial Gray vestigial Black long Parental phenotypes Recombinant Recombination frequency = = 0.17 or 17% 391 recombinants 2,300 total offspring Explanation (female) (male) G L g l Eggs Sperm Thomas Hunt Morgan: Performed some of the early studies of crossing over using the fruit fly Drosophila melanogaster. Figure 9.20 C
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SEX CHROMOSOMES AND SEX-LINKED GENES
In mammals, a male has one X chromosome and one Y chromosome and a female has two X chromosomes. The Y chromosome has genes for the development of testes. The absence of a Y chromosome allows ovaries to develop. (male) (female) Parents’ diploid cells Sperm Egg Offspring (diploid) 44 + XY XX 22 X Y Figure 9.22 A
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A female has to receive the allele from both parents to be affected
The inheritance pattern of sex-linked genes is reflected in females and males. A male receiving a single X-linked allele from his mother will have the disorder A female has to receive the allele from both parents to be affected Female Male Sperm Xr Y XR Xr Y XR XR XR Xr XR Y Eggs R = red-eye allele r = white-eye allele Xr XR Xr Xr Figure 9.23 B Figure 9.23 C Figure 9.23 D
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Examples: Hemophilia, Color Blindness, and Duchenne Muscular Dystrophy
Sex Linked Most sex-linked human disorders are due to recessive alleles and are mostly seen in males Examples: Hemophilia, Color Blindness, and Duchenne Muscular Dystrophy Queen victoria Albert Alice Louis Alexandra Czar Nicholas II of Russia Alexis Figure 9.24 A Figure 9.24 B
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