Transcript 3.4 Mendel
Introduction to Mendelian Genetics Gregor Mendel (1822-1884) • Used pea plants to study genetics, heredity and variation. • Looked at 7 different traits that only had two variations (two different alleles) • Discovered Mendel’s Laws of genetics 19th Century Britain • Before Mendel, from breeding ornamental plants, scientists believed in a blending theory of inheritance. • Charles Darwin theorized that offspring have variations of their parents' characteristics, but he was unable to explain why (1870s). Gregor Mendel (1822-1884) • Austrian monk • Considered the father of genetics (now aka Mendelian Genetics) • Why the pea plant? – grows easily and quickly – has seven traits that are easily identified – it is easy to manipulate 7 traits Mendel looked at Remember: • We have 2 copies of every gene (2 alleles) – We get 1 allele from father, 1 allele from mother – Both of these alleles combined give us our traits • A trait is true bred if both of the alleles are the same – For example, a true bred “tall” plant would have 2 “tall” alleles Mendel's Pea Plant Experiment • Mendel selected true bred plants for different traits and cross-fertilized them to see what would happen • For example: – He bred true bred TALL with true bred SHORT • What do you think the offspring looked like? X • Normally, you would assume that they would be medium height (blended between tall and short) X • However, Mendel found that 100% of the offspring plants were tall! • This shows that some alleles (like tallness) are dominant over other alleles • Mendel then let the F1 generation self-pollinate (fertilize each other) • F1 generation – (filial generation) offspring from the cross (reproduction) of the Parent or P generation. x • this time most were tall, but some were short and they were ALWAYS 3:1 ratio • This shows that even though “tall” dominated “short”, the short allele is still hidden in the organism Why? • Each plant in the F1 generation carried an allele from the P generation; a tall allele and a short allele. – This is a hybrid. – AKA heterozygous • Traits are dominant or recessive. If a dominant trait is inherited, it will be expressed. Mendel’s “Key Terms” • Phenotype: the appearance of traits in an organism (eg. short or tall) • Genotype: the specific alleles that an organism has (ex. Tt or TT or tt) • Every individual has a phenotype and genotype for every trait. Genotype vs. Phenotype Which Allele is expressed? Can you tell the genotype of an individual by their phenotype? A flower species is dominant for the purple trait and recessive for the white trait. What would happen if we crossed two hybrid (heterozygous) parents? We use a Punnett square to find out. Punnett Square Genotypic proportions The genotypic proportions of the offspring from the cross of two heterozygous parents (Bb) are: • BB __/4 • Bb __/4 • bb __/4 Phenotypic proportions The phenotypic proportions of the offspring are: • Purple __/4 or __% • White __/4 or __% Monohybrid cross problem Curly hair is dominant over straight hair. If a mother is homozygous dominant for curly hair and the father has straight hair, what type of hair will the children have? Test Crosses • When geneticists want to know if an individual is heterozygous or homozygous – Why? For breeding purposes • Cross between unknown phenotype and homozygous recessive individual – (Pp or PP) x pp • Pp x pp = 50% purple, 50% white • PP x pp = 100% purple Mendel's Laws and Principles • Law of Segregation: when any individual produces gametes, the copies of a gene separate so that each gamete receives only one copy of a gene and therefore only one allele for all possible traits. Principle of Segregation Demystified Law of Segregation Segregation The principle of segregation is explained by the behavior of homologous chromosomes at meiosis. • Law of Independent Assortment: alleles of different genes assort independently of one another during gamete formation, so different traits are inherited independently of each other