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Transcript
Introduction to Genetics Chapter 11 What is inheritance? • Genetics – the scientific study of heredity. • Gregor Mendel – an Austrian monk who observed patterns of inheritance in pea plants. • Why peas? Breeding could be easily controlled (pollen and eggs), they grew fast, and had variable genetic traits (characteristics). Peas had varying traits: Round, wrinkled, green, yellow, tall, short, axial flowers, terminal flowers, etc… Mendel’s observations of peas: • Some plants were true-breeding – they always produced the same offspring. Always tall, green… • Pea plants could be selectively bred by brushing on or inhibiting pollen. • Certain traits would “disappear” for a generation, then return again. • Repeating patterns emerged. Mendel’s observations • P generation – parent generation. • F1 generation – first filial, offspring of P generation. • Hybrid – offspring produced by parents of different traits. Patterns emerging… • When Mendel bred the purple and the white, all the F1 generation were purple. The white disappeared. • White flower trait is recessive. • All offspring were purple. Purple flower trait was dominant. • Different versions of a gene are called alleles. Would the recessive trait return? • Mendel bred the F1 generation to produce an F2 generation: flower color, pod color, shape…The recessive returned. What ratio was observed? Mendel’s conclusions: • Each trait has two alleles (versions). Plant height: Tall (T) or Short (t).. • These alleles segregate, or separate when gametes (egg and sperm) are formed. TT makes T gametes, Tt makes T gametes and t gametes.. • The allele from a sperm and the allele from the egg is the organism’s genotype. TT, Tt, or tt.. • What the organism looks like is its phenotype. Tall plant, short plant.. Genotype, phenotype, homozygous (same) and heterozygous (different) traits Probability and Punnett Squares: • When gametes are formed, there is a 50/50 chance that it will get one version of a trait. Like a coin flip! • The different possible offspring, and ratios of genotypes produced in a cross can be shown using a Punnett square. • Did the 3:1 ration for Mendel’s F1 cross make sense? A Monohybrid (one trait) cross: Dihybrid (two traits) cross, Trihybrid… • In reality, all traits are passed on to the gametes. • Most traits go into the gametes independent of each other. • Example: A homozygous tall, yellow plant (TTYY) produces only the following gamete: TY. • Another example: a heterozygous tall, yellow plant TtYy can produce the following gametes: TY, Ty, tY, ty. Try a Dihybrid cross: Other modes of inheritance…. • Incomplete dominance – the dominant gene does not completely show • The heterozygous is in-between • Japanese four-oclocks, snapdragons Other modes… • Codominance – both alleles show up in the phenotype. • Examples: coat color in cows, speckled hens, sickle-cell trait, blood type… • Multiple alleles – more than two alleles for a trait. Meiosis – Gamete production! • In order to make an individual with the full component of chromosomes (diploid), gametes must be created (haploid). • Gametes are sex cells with half the amount of chromosomes. • We have 23 homologous (same) pairs of chromosomes, total = 46. • Gametes (egg or sperm) have 23. Meiosis… • Meiosis consists of 2 divisions of a diploid or 2N cell, to create 4 haploid cells (1N each). • Before the first division, the chromosomes replicate to form the tetrad (X X figure). The homologous pairs and their copies go through crossover. • Crossover produces new combinations of alleles, It makes you unique! Crossover… • Crossover happens in prophase I, and sections of one homologue are traded with sections of another. • Alleles are shuffled from one to another. This is a random event, with an infinite number of combinations! Crossover… • If genes are close together on an arm of a chromosome, they are more likely to transfer together. • Because of this, they are said to exhibit linkage. • This helps scientists map the genes on a chromosome! Gene maps Meiosis continued… • Meiosis I – chromosomes duplicate, crossover occurs, IPMAT, results in 2 diploid cells. • Meiosis II – PMAT, results in haploid cells. • Note that these 4 gametes are each genetically unique! • Non-disjunction - if the chromosomes fail to separate properly, disorders occur. Meiosis