Transcript egg rooster
Mendel’s Legacy • Genetics is everywhere these days – and it will continue as a dominant force in biology and society for decades to come. • Wouldn’t it be nice if people understood it better? Discussing Some Terms • Heredity – the passing of traits to offspring • Trait – each form of a characteristic • Purebred – an organism that carries only one trait • Allele – the forms of a gene that control the trait • Dominant and Recessive Dangerously Ahead of the Game - A Eugenics Exhibit at the 1920 Kansas State Fair Can history be repeated? Genes, Alleles, and Chromosomes Mendel’s Model Organism – the Garden Pea The Reality of “Round and Wrinkled” – Two Alternative Traits of the Seed Shape Character Note that each of seed is a new individual of a different generation – seeds are not of the same generation as the plant that bears them. Mendel’s Monohybrid Cross – Purebred to F1 A Punnett square, something we’ll cover in a moment. Staying the Course – Mendel Continued Crosses to the F2 (the grandchildren) • What was learned? • The green trait was not lost or altered, even though it disappeared in the F1. • One trait is dominant to the other in its expression • The reappearance of the recessive trait in ¼ of the F2, suggests genes come in pairs that separate in the formation of sex cells. Monohybrid Crosses and the Principle of Segregation • A cross between individuals differing in single character is a monohybrid cross. • The analysis of monohybrid crosses allowed Mendel to deduce the Principle of Segregation .... • Genes come in pairs that separate in the formation of sex cells (and these sex cells unite randomly at fertilization). A Punnett Square is a Handy Way of Analyzing Crosses In a Punnett square for a monohybrid cross, the Principle of Segregation is applied. Different Genotypes Can Produce the Same Phenotype Consistency is Good No matter what the character, Mendel observed a 3:1 ratio of characters in the F2. Characters investigated by Mendel Monohybrid Crosses Yielded Consistent Results Therefore, the Principle of Segregation indeed is a general principle of genetics. What Works for Peas Also Works for Humans In the cross Aa x Aa, where A is a dominant allele for wild type (standard) pigmentation and a is a recessive allele for no pigmentation (albinism), ¾ of offspring will be wild type and ¼ will be albino. An albino woman Are Different Characters Like Color and Shape Inherited Together or Inherited Independently? • Mendel performed dihybrid crosses to find out. • Mendel’s conclusion: Different characters are inherited independently. The Punnett Square for a Dihybrid Cross • Note that we’re simultaneously applying the Principles of Segregations and Independent Assortment. Some More Terms • Genotype – the genetic characteristics of the organism • Phenotype – the physical characteristics of the organism • Codominance – two non-dominant alleles can be present in the same animal, such as a black and white rooster (FB and FW) Predicting Probability • What is probability? • Use the Punnett Square to estimate what percentage the offspring will be. But How Do We Reproduce? • Most plants and animals use a unique process called Meiosis, which creates sex cells. • These sex cells are called gametes, and there is a different one for each sex. • The male sex cell is called the sperm, and the female the egg. Sexual Reproduction • The sperm and egg unite (either inside or outside the body, and not always the female!!) and then begin to grow and develop into the offspring. • But how do we combine two cells to make one cell? Cell Meiosis • The process of creating sex cells is called Meiosis. • It is different from cell Mitosis because it has two stages of cell division, but the process is the same (Pro, Meta, Ana, Telo) Cell Meiosis • In Meiosis 1, the cell goes through the normal Mitosis process. • At the end of Meiosis 1, two daughter cells are formed • These cells then split again, without replicating DNA Cell Meiosis Cell Meiosis • When the daughter cells split again in Meiosis 2, they are left with half the number of chromosomes! • That way, when they join up with the opposite sex cell, the fertilized egg will have the regular number of chromosomes again!! Cell Meiosis Cell Meiosis • A human chromosome has 23 pairs of chromosomes, or a total of 46. • One of those pairs are the sex chromosomes, and the other 22 pairs control other functions of the cell.