BSC1005 UNIT 3 Genetics and Human Heredity Gregor Mendel 1866 He discovered the existence of genes which he called particulate factors He also discovered rules governing transmissions from generation to generation These rules hold for all types of life He laid the foundation of the use of precise statistical relationships in sciences At the time of Mendel the Blending Theory of inheritance held that the appearance of the offspring would be an intermediate of the parents Artificial Selection Around 10 000 BC people started selecting plants for their desirable traits and breeding those thus inheriting those traits The variation was already present but people elected for certain traits People have domesticated animals and plants The domestication of plants came earlier and this selective breeding for better crops let them sustain civilizations Blending Theory of Inheritance blending phenotypes of the parents With this theory everyone is going towards average height ability etc In an example of crossing flowers P1 is the parental level This consisted of red and white flowers F1 is the offspring whereas F2 is the next generation of off springs and so on F1 hybrid was 100 red This showed that only one characteristic red was expressed meaning it s dominant F2 705 flowers were red and 225 were white This is a ratio of about 3 1 saying about of the flowers were red and about of the flowers were white This showed that the characteristic of being white was hidden in F1 This also dispelled the notion of blending as for it that was true they should all be pink Another prevailing theory was the Fixity of Species that animal and plant groups remained unchanged in form from the moment of their appearance on earth in ancient Jewish history it was discovered how hemophilia was inherited At the time they had set rules that if two boys in a family die from bleeding following the circumcision then later born sons from the same mother or her sisters don t have to be circumcised They didn t know however that hemophilia gets inherited through an X linked gene transmitted from an unaffected mother to her sons What Mendel did that made his approach different from his predecessors was that he quantified his notes meaning that he recorded the numbers of progeny generated in each cross Mendel used garden peas Pisum sativum for his genetic experiments as they reproduced quickly and he could control the breeding Mendel used pure breeding varieties so a certain type of seed was guaranteed to create a certain color flower Using peas he could do the fertilization by hand creating hybrids In this plant the petals enclose the stamen which houses the pollen The embryo develops in the pistil This made it possible for him to physically put the pollen from another flower into one of his choosing Peas are self pollinating or self fertilizing Mendel had 30 traits to look at in peas He studied the inheritance which is the same for all traits Monohybrid Cross genetic cross between parental varieties differing in a single trait Dominant trait the phenotype that is observed Recessive trait the phenotype that is masked Mendel s Interpretations from the Pea Plant Experiments 1 Genetic characteristics are controlled by factors which come in pairs Now we call these factors genes Each individual has a pair of genes for each trait 2 Genes can be present in alternate forms called alleles True breeding populations have identical alleles making them homozygous In a hybrid where two pure lines are crossed each parent contributes one allele one of which is for the dominant phenotype and one of which is for the recessive Since the alleles are different this is reffered to as heterozygous In the formation of gametes eggs and sperm each member allele of a gene pair segregates into a gamete so each gamete only carries one allele either dominant or recessive Half carry one and half carry the other 3 4 During fertilization the gametes fuse at random and irrespective of the other gene pairs involved Fertilization brings together the gene pairs The principle of segregation Mendel s first law states that during the formation of the gametes the paired hereditary determinants separate segregate in such a way that each gamete is equally likely to contain either paired member Segregation during meiosis the separation of homologous chromosomes into different gametes Independent assortment the random distribution of genes on different chromosomes into gametes Heterozygous having two different alleles for a trait Homozygous having two identical alleles for a given trait considered true breeding Punnett Square this allows you to diagram the cross and trace the segregation of alleles The purpose of a test cross is to determine the genotype homozygous or heterozygous of an individual with the dominant phenotype In a test cross plants of an unknown genotype are mated with homozygous recessive plants The genotype of the parental generation can then be determined from the phenotype observed in the progeny Dihybrid cross used to determine if any relationship existed between different allelic pairs Crosses were made between true breeding plants with different traits Remember in this example Yellow is dominant and represented with G Green is recessive and is represented with g Round is dominant and represented with W Wrinkled is recessive and is represented with w Parental cross P1 yellow round seed GGWW and green wrinkled seed ggww F1 generation all yellow round GgWw Each parent the parent is from the F1 generation so is GgWw can form 4 genetically distinct gametes GW Gw gW and gw When two members of the F1 generation are bred we can track the dihybrid cross and observe all possibilities for the inheritance of both traits Male Gametes GW Gw gW gw Female Gametes GW GGWW Yellow round GGWw Yellow round GgWW Yellow round GgWw Gw GGWw Yellow round GGww Yellow wrinkled GgWw Yellow round Ggww gW GgWW Yellow round GgWw Yellow round ggWw green round ggWw gw GgWw Yellow round Ggww Yellow wrinkled ggWw green round Ggww Yellow round Yellow wrinkled green round Green wrinkled So to recap what s in this table 9 out of the 16 had the phenotype of yellow round Of these 9 the genotypes were 1 GGWW 2 GGWw 2 GgWW and 4 GgWw 3 out of the 16 had the phenotype green and round Of these 3 the genotypes were 1 ggWW and 2 ggWw 3 out of the 16 had the phenotype yellow and wrinkled Of these 3 the genotypes were 1 GGww and 2 Ggww 1 out of the 16 had the phenotype green and wrinkled The
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