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Transcript
Fundamentals of Genetics Chapter 9 Genetics The field of biology dedicated to understanding how characteristics are transmitted from parent to offspring. Studied by Gregor Mendel Heredity- the transmission of characteristics from parents to offspring. Gregor Mendel Austrian monk & science teacher in the mid 1800’s Worked with pea plants in his garden Studied 7 characteristics of peas Each characteristics had 2 different traits EX) tall/short, purple/white Page 176 Mendel’s Experiments: 1. Started off with 2 pure plant groups for a particular characteristic P1 generation 2. Cross pollinated these plant groups (sexual reproduction) to produce the next generation of plants F1 generation 3. When the seeds matured from the F1 generation, he counted & recorded the traits of the offspring. 4. Then he allowed the F1 generation plants self pollinate (sexual reproduction with self) to produce the next generation of plants F2 generation (notice that the predicted ratios & the actual ratios are not exact!) Mendel’s Results: The F1 generation always showed only 1 of the 2 traits for the characteristic Mendel named this the dominant factor The F2 generation always showed a 3:1 (or a 75% to a 25%) ratio between the 2 traits Mendel named the second the recessive factor Example: Purple is dominant White is recessive F1 generation: all purple F2 generation: 75% purple 25% white The Law of Segregation Mendel stated that… a pair of factors is segregated, or separated, during the formation of gametes. So, What does this mean? Each egg and sperm receives only one factor from each parent. The Law of Independent Assortment Mendel also stated that … factors for different characteristics are distributed to gametes independently. So, What does this mean? The factors for different characteristics are not connected. Human traits link Molecular Genetics The study of the structure & function of chromosomes & genes Allele: alternate form of a gene Mendel called them “factors” Abbreviations: Dominant allele = capital letter Recessive allele = lower case letter (B) (b) 9.2 Vocabulary: Genotype: the genetic makeup of the organism (BB, Bb, bb) Phenotype: the physical appearance of the organism (Black, white) Genotype Vocabulary: Homozygous: “same genes” When the organism has the same alleles for the characteristic BB = homozygous dominant bb = homozygous recessive Heterozygous: “different genes” When the organism has different alleles for the characteristic Bb Genetic Probability The likelihood (probability) of offspring of known parents can be determined by Punnett squares Punnett Squares: diagrams biologists use to predict the probability that certain traits will be inherited by offspring. 3 Types: Monohybrid Cross – One characteristic Dihybrid Cross – Two characteristics Trihybrid Cross – Three characteristics Monohybrid Cross A cross between individuals that involves one pair of contrasting traits. 1 characteristic 2 contrasting traits Question: In rabbits, the allele for black coat color (B) is dominant over the allele for brown coat color (b). Predict the results of a cross between a rabbit homozygous for black coat color and a rabbit homozygous for brown coat color. What is the genotypic ratio? What is the phenotypic ratio? Brown hair color is dominant over blond hair, and brown eyes are dominant over blue eyes. Q- If 2 heterozygous parents for both traits mate, what is the phenotypic ratio of their offspring? __________ Brown hair, Brown eyes __________ Brown hair, Blue eyes __________ Blond hair, Brown eyes __________ Blond hair, Blue eyes Dihybrid Cross A cross between individuals that involves 2 pairs of contrasting traits. • 2 characteristics • 4 contrasting traits •Assumes the genes aren’t linked How do you find an unknown genotype? Testcross Example) Is the red flower a pure (RR) or hybrid (Rr) ? •Cross the unknown flower with a pure recessive flower (rr). •This will allow the recessives to show up in the next generation. The results: If the flower is hybrid, the offspring will be 50% red and 50% white. Result if flower is hybrid If the flower is pure, the offspring will be all red. Result if flower is pure Complications! 1) Incomplete Dominance: occurs when the F1 offspring has a phenotype between that of the parents. neither allele is completely dominant Example) Incomplete Dominance Four o’clock flowers Q- What would happen if you crossed a red four o’clock flower with a white four o’clock flower? A- a pink flower! 2) Codominance Occurs when both alleles are expressed in a heterozygous offspring. Neither allele is dominant or recessive Q- What would happen if you crossed a horse with a white coat with a horse with a red coat? A- A horse with a roan coat (both red and white hairs) ! Codominance Ex) ABO blood groups Dominant Dominant Codominant Recessive 3) Epistasis occurs when the alleles of one gene cover up or alter the expression of alleles of another gene Ex) Mouse coat color: B - black coat b - brown coat C - pigment c - no pigment 4) Polygenic Inheritance One trait determined by multiple genes Ex) skin color: at least 3 genes