Mendelian Genetics - Mendel used the scientific approach to identify two laws of inheritance o He discovered the basic principles of heredity breeding garden peas- Advantages of pea plants for genetic studyo There are many varieties with distinct heritable features, or characters; and character variants called traits (purple or white flowers) o Mating can be controlledo Each flower has sperm-producing organs and an egg-producing organ - He also used varieties that were true-breeding (plants that produce offspring of the same variety when they self-pollinate) o Almost like pure-bred animal o The purple flower only has purple genes - Mendel mated 2 contrasting, true-breeding varieties, a process called hybridization - The true-breeding parents are the P generationo The hybrid offspring are the F1 generation - The Law of Segregationo When he crossed the purple and white flowers, the F1 hybrids were purpleo When he crossed the F1 generation, many of the F2 plants were purple but some were white - The purple flower is exerting its effects, not the white flowero Called the purple flower color dominant trait whereas the white flower color was the recessive trait o The white gene did not disappear - He observed the same pattern of inheritance in 6 other pea plant characters, each represented by 2 traits- What Mendel called a heritable factor is what we call a gene- Mendel’s Modelo He developed a hypothesis to explain the 3:1 inheritance patter he observed in F2 plants- First concept: alternative versions of genes account for variations in inherited characterso These alternative versions of a gene are now called alleleso Each gene resides at a specific locus on a specific chromosome - Second concept: for each character, an organism inherits two alleles, one from ech parent o Mendel made this deduction without knowing about the role of chromosomeso The two alleles at a particular locus may be identical, as in the true-breeding plants of Mendel’s P generationo Alternatively, the two alleles at a locus may differ- Third Concept: if the two alleles at a locus differ, then the one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance o For the flowers, the purple was the dominanto - Fourth concept (The Law of Segregation): the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes o Thus, an egg or a sperm gets only one of the two alleles that are present in an organismo You will never give 2 of your alleles to a gamete (you only pass on one of your alleles) since gametes are haploid - Homozygous is an organism with two identical alleles for a character- Heterozygous an organism that has two different alleles for a gene o They are not true-breeding, they have two different alleles - A phenotype is the physical appearance - Genotype is the genetic makeup - Testcross: breeding the mystery individual with a homozygous recessive individualo If any offspring display the recessive phenotype, the mystery plant must be heterozygous - The Law of Independent Assortmento Mendel derived the law of segregation by following a single character o The F1 offspring produced in the cross were monohybrids, individuals that are heterozygous for one character- Crossing two true-breeding parents differing in two characters produced dihybrids in the F1 generation, heterozygous for both characters o A dyhybrid cross is how he found out the law of independent assortment - Alleles from a certain gene separate into gametes independently of how alleles from another gene separate into gametes - The law of independent assortment: each pair of alleles segregates independently ofeach other pair of alleles during gamete formationo Only for genes on two different chromosomes o Genes located on the same chromosome tend to be inherited together - Inheritance of characters by a single gene may deviate from a simple Medenlian patterns in the following situation:o When alleles are not completely dominant or recessiveo When a gene has more than 2 alleles o When a gene produces multiple phenotypes - Degrees of dominanceo Complete dominance: when phenotypes of the heterozygote and dominant homozygote are identical o In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varitieso In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways 2 alleles for skin color, part of your skin will be the one color and part of your skin will be the other color - A dominant allele does not reduce a recessive allele; alleles don’t interact in that way - Alleles are simply variations in a gene’s nucleotide sequence- Dominant alleles are not necessarily more common in populations than recessive alleles - Most genes exist in populations in more than two allelic formso The four phenotypes of the ABO blood groups in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i- Most genes have multiple phenotypic effects, a property called pleiotropy - Some traits may be determined by two or more geneso Height, weight, blood pressure- One gene might also effect more than one trait- Polygenic Inheritanceo Quantitative characters are those that vary in the population along a continuum - In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locuso In Labs coat color depends on two geneso One gene determines the pigment coloro The other gene determines whether the pigment will be deposited in the hair- The norm of reaction is the phenotypic range of a genotype influenced by the environment o Ex. Hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity - Norms of reaction are generally broadest for polygenic/quantitative characters- Such characters are called multifactorial because genetic and environmental factors collectively influence phenotype - An organism’s phenotype includes its physical appearance, internal anatomy, physiology, and behavior o An organism’s phenotype reflects its overall
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