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Chapter 5 Heredity
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Section 5.1 Mendel & Peas
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Heredity Passing of genetic traits from parent to offspring How Traits are Inherited Genes made up of DNA Genes found on chromosomes Genes control all traits Pairs of genes separate when chromosomes separate
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Known as Father of Genetics Discovered the principles of heredity by studying pea plants Noticed that traits can skip a generations
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Chose peas because they grow quickly, many different kinds, & are self-pollinating (has both male & female reproductive parts – pollen from one flower can fertilize same flower) Mendel able to grow true breeding plants (all offspring have same trait as parents) Peas can also cross-pollinate (pollen from one plant fertilizes the egg of another plant) Pollen carried by insects, wind, & other animals
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Mendel studied one characteristic at a time (seed shape, plant height, flower color) Another example: hair color in humans is a characteristic & different forms, like red, brown, or blonde, are traits
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Dominant vs. Recessive Dominant traits are always expressed (observed) in first generations when parents w/different traits are bred Recessive traits are masked or hidden in first generations but reappear in the second generation when parents w/different traits are bred
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Mendel decided to figure out ratio of dominant to recessive traits Ratio is relationship between two different numbers (often shown as a fraction) Example: 3:1
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Section 5.2 - Traits & Inheritance
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Genes: Set of instructions for an inherited trait Alleles: Different forms of a gene (ex. all have a gene for eye color but each have different alleles such as blue, brown, green)
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Dominant trait that is always expressed shown with a capital letter (B) can be homozygous or heterozygous
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Recessive trait that is not expressed when dominant allele is present; dominant allele mask or covers it up. shown with a lowercase letter (b) must be homozygous for recessive trait to be expressed
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Purebred: Both alleles are the same (ex. BB or bb) Can be homozygous dominant (BB) or homozygous recessive (bb) Means the same as Homozygous Hybrid: Both alleles are different (ex. Bb) Means the same as Heterozygous
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Phenotype: Physical appearance or what you see Example: such as brown eyes vs. blue eyes. Genotype: Genetic makeup of an organism Set of alleles can be - Bb, BB, or bb
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Punnett Square diagram that shows the expected offspring of 2 parents (shows all the possibilities) Capital letter = dominant allele Small letter = recessive allele Letters of one parent written along top of square and letters of other parent written along the left side of the square Cannot always figure out genotype by looking at the phenotype
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Example: First Generation Alleles from homozygous tall parent Alleles from homozygous short parent Genotypes of offspring: Phenotype of offspring:
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Second Generation Alleles from heterozygous tall parents Genotypes of offspring: Phenotype of offspring:
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Probability Predicts the chance that something will happen (ex. coin toss 1 out of 2 or a 50% chance) Think lottery, weather forecasting.
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Incomplete Dominance Phenotype is intermediate (in between) to the 2 homozygous parents Neither allele for color was dominant – colors blended to make new color Example: four o’clock flowers have alleles for red and white flowers Red x White = Pink
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One gene-Many Traits Sometimes one gene can influence more than 1 trait Single trait that is produced by a combination of many genes Example: In tigers the gene for fur color also carries the gene for eye color white tiger - -> white fur caused by single gene but this gene also influences other traits like eye color (tiger has blue eyes)
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Section 5.3 Meiosis
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Asexual Reproduction Only one parent cell is needed (these cells have 46 chromosomes) Structures are copied and then parent cell divides making two identical cells (this is Mitosis & occurs in body cells) Daughter cells are identical (46 chromosomes) Bacteria, single celled organisms
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Sexual Reproduction Two parent cells join together to form an offspring that are different from both parents Two sex cells join – one from each parent (each sex cell has 23 chromosomes)
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Sexual Reproduction Human sex cells join (1 egg, 1 sperm) Human sex cells have 23 chromosomes Process is known as Meiosis
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Meiosis Process in cell division in which number of chromosomes are reduced to half the original number Cells go through cell division 2 times
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Meiosis Sex cell in the female is the egg (23 chromosomes) Sex cell in the male is the sperm (23 chromosomes) Sperm and egg join to make offspring with 46 chromosomes (23 + 23 = 46)
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Homologous Chromosomes Chromosomes that carry the same set of genes (like a pair of shoes)
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Steps of Meiosis 1.Starts with parent cell (46 chromosomes) 2.Interphase-chromosomes copy (sister chromatids) 3.Prophase I -chromosomes become visible; nuclear membrane disappears 4.Metaphase I -homologous chromosomes pair up, pairs of chromosomes line up at the center of the cell
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Steps of Meiosis 5. Anaphase I - homologous pairs start to separate 6. Telophase I -homologous pairs move to opposite ends of cell; nuclear membrane reforms 7. Now you have 2 cells
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Steps of Meiosis 8.Prophase II - chromosomes become visible; nuclear membrane disappears 9.Metaphase II - chromosomes line up at equator of cell 10.Anaphase II - sister chromatids of chromosomes separate
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Steps of Meiosis 11.Telophase II - chromatids move to opposite ends of the cell; nuclear envelope reforms 12.Cytokinesis - cytoplasm divides 13.END with 4 sex cells- each has ½ number of chromosomes (23)
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Sex Chromosomes X and Y chromosomes Male has XY Female has XX All eggs have an X chromosome Sperm will EITHER have an X OR a Y chromosome When they join determines if offspring is male or female
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Sex Linked Disorders Some inherited conditions are carried on a sex chromosome Females get 2 X chromosomes-if one is unhealthy they have a back up Males only get one X chromosome-if it is unhealthy they will have a disorder
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Color-Blindness Sex linked disorder Trouble distinguishing red from green Males are color-blind more often than females If mom has an allele for colorblindness and passes it on to a son he will be color blind Females must have a dad who is color-blind and a mom who carries the color-blind allele to be color-blind
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Hemophilia Sex linked disorder Blood does not clot properly People with this disorder bleed a lot from small cuts Can be fatal
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Sickle-cell anemia (recessive disorder) Homozygous recessive disorder Blood cells are sickle shaped (like a C) instead of normal disc shaped (like a C) instead of normal disc shaped Cannot deliver enough oxygen Blood cells get stuck together Most people with disorder die as children
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Cystic fibrosis (recessive disorder) Homozygous recessive Normally thin fluid lubricates lungs but people with CF have Thick mucus clogs lungs Makes it hard to breathe causes lung damage 1 in 20 people carries a recessive allele for this disorder
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Pedigree Charts Family tree that traces a trait in a family Purebred dogs might have a pedigree chart Used in tracking disease & genetic counseling
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Pedigree Charts Square = male Shaded in = has trait Circle = female Half shaded = carrier Not shaded = normal Not shaded = normal
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Selective Breeding Organisms with desirable characteristics are mated Humans have been selectively breeding for thousands of years (10,000) Started after the last ice age Examples: chickens that produce larger eggs, dog breeding, thorn-less roses
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