Mendels Laws of Heredity Main Concepts a. Sexual reproduction involves the random distribution of genes. b. We have thousands of genes. c. Genes determine your traits. d. Genes line up on your chromosomes. e. Chromosomes are in the nucleus of the cell. f. Chromosomes duplicate and divide in cell division. g. Chromosomes are typically in pairs. h. A diploid cell has complete set of chromosomes. Frog: 13 pairs Pea: 7 pairs Apple: 17 pairs Chimpanzee: 24 pairs Dog: 39 pairs 1.HEREDITY the passing of characteristics from parents to offspring 2.Characteristics that are inherited are called TRAITS 1.Gregor Mendel was an Austrian monk and was the first person to successfully study GENETICS, the branch of biology that studies heredity 1.Mendel chose to study the garden pea plant: a)The male gamete is pollen and fuses with the female gamete in a process called FERTILIZATION i.Fertilized egg = zygote b)The transfer of male pollen to the female reproductive organ is called POLLINATION 1.Pea plants can reproduce by SELF- POLLINATION because each plant has both male and female gametes 2.Mendel performed a process called CROSS- POLLINATION by dusting pollen from one plant to any other plant of his choosing. This allowed him to control which plants mate with which plants 1.Mendel was a careful researcher: a)He studied only one TRAIT at a time (e.g. Height) and analyzed the data mathematically b)Mendel used TRUE BREEDING plants for his crosses. This means the plants have the same traits for many generations (e.g. all purple flowers) i.Pure bred 1.When Mendel crossed two different ALLELES for a trait he called this a HYBRID 2.When Mendel only crossed one trait at a time he called this a MONOHYBRID CROSS 1.All pea plants are either tall (6ft) or short (2ft) 2.The 1 st Generation: a)Mendel cross-pollinated a true breeding tall pea plant with a true breeding short pea plant. The offspring were all tall pea plants b)P 1 Tall x short = All tall plants 1.The 2 nd Generation: a)Mendel allowed the tall offspring to self- pollinate. b)F 1 Tall x Tall = 75% Tall/25% Short c)3:1 ratio of tall Short pea plants X Tall pea plants Q: What did he get? A: All Tall pea plants!!!! Then, Tall X Tall pea plants Q: What did he get? A: 787 Tall pea plants & 277 short pea plants 3:1 Ratio = 75% tall : 25% short 1.The original (true breeding) parents are known as the P 1 generation a)P = parent 2.The offspring of the P 1 generation are known as the F 1 generation a)F = filial (son or daughter) 3.Crossing two F 1 plants creates the F 2 generation 1.Factors = Genes 2. Law of Dominance: in any pair of genes, one may hide the appearance of the other. a)Dominant: expressed b)Recessive: masked, hidden, not expressed 3. Law of Segregation: During Meiosis chromosomes separate, and genes separate also. 4. Law of Independent Assortment: the pairs of genes (and chromosomes) separate randomly! 1.Mendel also concluded that one allele is DOMINANT and one is RECESSIVE 2.The dominant allele is the one that shows up in the F 1 generation (Tall) a)Capital letter T 3.The recessive allele is the trait that is hidden in the F 1 generation (Short) a)Lowercase letter t 4.Capital letter is always written before the lowercase letter Tt tT 1.A dominant trait (tall) is the result of either TT or Tt a)TT = tall b)Tt = tall 2.A recessive trait (short) can ONLY be the result of tt a)tt = short 3.The dominant allele always overrides the recessive allele, therefore, dominant traits are more common than recessive traits Purple is dominantWhite is recessive Purple is present (White is hidden) AAaa Aa AA aa 1.Mendel concluded that each organism must have two factors that control each of its traits 2.We now know that traits are controlled by GENES and are located on chromosomes. 3.Genes exist in alternative forms called ALLELES a)Gene plant height i.Alleles tall or short ii.Alleles dominant or recessive 1.Alleles are located in exactly the same position on homologous chromosomes 2.In a diploid organism, there are two alleles for a given gene a)One from mother; one from father 3.A plant might have: a)2 copies of the tall allele (TT) b)2 copies of the short allele (tt) c)1 tall and 1 short allele (Tt) 1.The Law of Segregation every individual has two alleles for each gene and each gamete receives one of these alleles Law of Segregation Each gamete only gets one allele Each offspring has one allele from each parent 1.The Law of Independent Assortment genes for different traits are inherited independently of each other a)Height does not affect flower color; they are independent of each other b)Independent assortment results in increased genetic diversity because of the shuffling of individual genes Law of Independent Assortment 1.PHENOTYPE: physical appearance; what it looks like. 2.GENOTYPE: genetic make-up; combination of genes. 3.Represented by 2 letters. 4.3 possible genotypes. EX. Green pea pod, pink flower, tall stems EX. GG, Gg, gg 1.Homozygous two alleles are the SAME a)Homozygous dominant TT b)Homozygous recessive tt c)True breeding (pure bred) 2.Heterozygous two alleles are DIFFERENT a)Tt b)Hybrids 1.Dominant 2.Free earlobe 3.Left handedness 4.Tongue rolling 5.Left arm on top 6.Left thumb on top 1. Recessive 2. Cleft chin 3. Dimples 4. Attached earlobe 5. Second toe longer 6. Widows peak 7. Double jointedness Dimples Cleft Chin 1.Reginald Punnett devised an easy system to predict genotypic outcomes of a cross called a PUNNETT SQUARE 2.Parents gametes are combined in every possible combination 3.Monohybrid cross one gene (height) 4.Dihybrid cross two genes (height and color) Fathers Gametes (sperm) Mothers Gametes (egg) Possible Offspring #1 Possible Offspring #2 Possible Offspring #3 Possible Offspring #4 Monohybrid Cross TTxtt Setting up the Punnett square TTtt (Short)(Tall) MomDad T = tall t = short T T x tt Solving the Punnett Square T t T tt T t tt TT T (Tall)(Short) Tall (Heterozygous) T = tall t = short MomDad TTtxTt Setting up the Punnett square Ttt (Tall) TT T t x Tt Solving the Punnett Square TT t t t Tt Tt t (Tall) Tall Short (Homozygous) (Heterozygous) 1.Offspring outcomes can be expressed as: a)Fractions or b)Percent 25% or 50% or 75% c)Ratio 3:1 or 1:2:1 1.A TEST CROSS is a cross of an individual of unknown genotype with an individual with a known genotype a)Test crosses use homozygous recessive individuals because the phenotype always indicates the genotype (short = tt) 2.Example: a)Tall pea plant: genotype unknown (TT or Tt) b)Short pea plant: known genotype (tt) c)Cross the tall plant with the homozygous recessive and look at the offspring i.If the offspring are all tall then the unknown genotype is TT ii.If the offspring are 50% tall and 50% the unknown genotype is Tt Tall Plant (TT or Tt?) Short Plant (tt) P1P1 Test Cross F1F1 TTTt OR