Presentation on theme: "Chapter 13 MENDEL AND THE GENE Why do we look like family members or not?"— Presentation transcript:
Chapter 13 MENDEL AND THE GENE Why do we look like family members or not?
History It started with farmers and botanists Knight used pure breeding peas, one variety with purple flowers, one variety with white flowers. ▫Crossing the two varieties he found that the offspring all had purple flowers. ▫When he crossed the offspring, some had purple flowers, some had white flowers. ▫Conclusion: some traits have a stronger tendency to appear than others. No Numbers
History Mendel, an Austrian monk, repeated Knight’s experiments: ▫Also used true-breeding peas and studied 7 different traits, fig 13.2. ▫Cross-fertilized peas showing two variations of the same trait, ex. round peas vs. wrinkled peas.
Figure 13-2 Trait Phenotypes Seed shape Seed color Pod shape Pod color Flower color Flower and pod position Stem length TallDwarf Terminal (at tip) Axial (on stem) Purple White YellowGreen Inflated Constricted GreenYellow RoundWrinkled
Figure 13-1 Self-pollination Female organ (receives pollen) Eggs Male organs (produce pollen grains, which produce male gametes) Cross-pollination CROSS- POLLINATION 3. Transfer pollen to the female organs of the individual whose male organs have been removed. 2. Collect pollen from a different individual. 1. Remove male organs from one individual. SELF- POLLINATION
Mendel Studied a Single Trait Mendel cross-fertilized two plants, one with white flowers with one with purple flowers. The hybrids, the F1 generation, all had purple flowers. Studying one trait through cross-fertilization is termed a monohybrid cross.
Mendel Studied a Single Trait Mendel’s experiments cont’d ▫Mendel allowed F1 generation plants to self- fertilize. ▫Their offspring, the F2 generation, expressed (demonstrated) both purple and white flowers. The ratio of plants with purple to white flowers was always 3:1. ▫Where did these white flowered plants come from?
Mendel Studied a Single Trait Mendel cont’d ▫The F1 generation plants all resembled only parent plant; i.e. one variation of the trait is dominant. ▫The F2 generation showed plants with both variations of the character, purple and white. The variation of the trait that was only seen in the F2 generation (white flowers) is recessive.
Mendel Studied a Single Trait Mendle cont’d ▫The F2 generations were allowed to self-fertilize. Looking at the F3 generation, Mendel discovered that the F2 generation actually consisted of 3 different types of plants: Pure breeding purple Not pure breeding purple (produced both purple and white flowered plants. Pure breeding white. The ratio was actually 1:2:1.
Mendel Studied a Single Trait Conclusions (cross involving 1 trait)
Genes and Mendel’s Findings Traits are carried by genes. An individual has 2 genes or alleles for each trait, 1 on each homologous chromosome. Meiosis results in separation of the homologous chromosomes and the alleles so that each is carried by a different gamete.
Genes and Mendel’s Findings An individual with 2 identical alleles is said to be homozygous, while an individual with 2 different alleles is said to be heterozygous. The genetic make-up of an individual is its genotype. The appearance or expression of the genotype is called its phenotype.
Genes and Mendel’s Findings Mendel’s results can be predicted using Punnett squares. ▫Dominant genes are represented by uppercase letters, ex. round peas (R). Expressed when there is 1 or 2 dominant alleles present. ▫Recessive genes are represented by lowercase letters, ex. wrinkled peas (r). Only expressed when there are 2 recessive alleles present.
Figure 13-4 Homozygous mother Meiosis Offspring genotypes: All Rr (heterozygous) Homozygous father Male gametes Female gametes Meiosis Offspring phenotypes: All round seeds A cross between two homozygotes A cross between two heterozygotes Female gametes Heterozygous mother Heterozygous father Offspring genotypes: 1/4 RR : 1/2 Rr : 1/4 rr Male gametes Offspring phenotypes: 3/4 round : 1/4 wrinkled
Genes and Mendel’s Findings Mendels’ Principle of Segregation, fig 13.7:
Figure 13-7 Meiosis I Alleles segregate Recessive allele for seed shape Gametes Chromosomes replicate Rr parent Dominant allele for seed shape Meiosis II Principle of segregation: Each gamete carries only one allele for seed shape, because the alleles have segregated during meiosis.
Mendel Studied 2 Traits Mendel then looked at two traits simultaneously – dihybrid cross. Ex. plants that produced round (R), yellow (Y) peas and plants that produced wrinkled (r), green (y) peas. The pure breeding parents’ genotypes were RRYY and rryy, fig 13.5. What is the genotype and phenotype of the F1 generation? The F2 generation?
Figure 13-5a Hypothesis of independent assortment: Alleles of different genes don’t stay together when gametes form. Male gametes Male parent F 1 PUNNET SQUARE F 1 offspring all RrYy F 2 female parent Female parent Female gametes Male gametes F 2 offspring genotypes: 9/16 R–Y– : 3/16 R–yy : 3/16 rrY– : 1/16 rryy F 2 offspring phenotypes: 9/16 : 3/16 : 3/16 : 1/16 F 2 male parent Female gametes F 2 PUNNET SQUARE Alleles at R gene and Y gene go to gametes independently of each other
Genes and Mendel’s Findings Mendel’s Principle of Independent Assortment:, fig 13.8.
Figure 13-8 Meiosis I Replicated chromosomes prior to meiosis Gametes Alleles for seed shape Meiosis II Principle of independent assortment: The genes for seed shape and seed color assort independently, because they are located on different chromosomes. Meiosis II Meiosis I Alleles for seed color 1/4 RY 1/4 ry 1/4 Ry 1/4 rY Chromosomes can line up in two ways during meiosis I R R y y r Y Y R R RR R R rr rr r r r Y Y Y Y Y Y y y y y y y y y yy y y RR R R R R rr rr r r YY Y Y Y Y
Peas are Easy Most phenotypes (expression of genes) are the result of the action of more than one gene. Continuous variation:
Peas are Easy Pleiotropic effects: An individual gene may have effects on many traits. Example: A dominant gene causes yellow hair in mice. If the mouse is homozygous for the gene it dies = a lethal defect. (In this case the gene was acting as if it was a recessive gene). Other examples: Marfan’s syndrome.
Figure 13-17 Flower color is variable in four-o’clocks. Incomplete dominance in flower color Parental generation F 2 generation F 1 generation Self-fertilization Purple Lavender White
Peas Are Easy Co dominance: Some phenotypes represent both alleles, ex. blood types. ABO Blood groups - CoDominance ▫2 dominant alleles, A and B, one recessive allele, i. ▫Alleles code for different RBC membrane proteins. These protein act as antigens (can cause an immune response). ▫Immune response = antibodies.
Peas Are Easy ABO Blood Groups, cont’d ▫Type A blood type has I A,I A or I A,i alleles ▫Type B blood type has I B, I B or I B,i alleles ▫Type AB blood type has I A, I B alleles ▫Type O blood type has i, i alleles (recessive form, no antigens on their RBCs). Rh blood group: the Rh factor consists of 8 different antigens. A person that has even one of these antigens is Rh+ while those having none of the antigens is Rh-.
Human Genetics Random mutation of genes occurs constantly, but most do not produce changes in phenotype or disease symptoms. Most gene disorders are rare, i.e. the frequency of occurrence of a defective allele is low. Exceptions: “closed societies”, ex. Tay Sachs, Sickle Cell
Human Genetics Most gene disorders are recessive and only expressed when both alleles are recessive forms of the gene. Exceptions: Huntington disease is caused by a dominant gene.
Figure 13-21 Carrier maleCarrier female Affected male Affected female I II III IV Each row represents a generation Carriers (heterozygotes) are indicated with half-filled symbols Pedigree of a family with an autosomal recessive disease
Figure 13-22 Affected female I II III IV Unaffected male If a child shows the trait, then one of the parents shows the trait as well Pedigree of a family with Huntington’s Disease
Patterns of Inheritance in Humans Controlled mating is not practical. Solution: Pedigrees – constructed from the progeny of matings over many generations. Ex. – hemophilia in family of Queen Victoria. The defective gene is recessive and occurs on the X chromosome. Heterozygous females are carriers. Because the male Y chromosome does not express many of its genes, the defective gene is expressed in males, i.e. it is sex-linked.
A Pedigree of an X-Linked Recessive Disease Queen Victoria Prince Albert Female carrier of hemophilia allele I II III IV Affected male
Patterns of Inheritance in Humans Some genetic disorders arise from the mutation of a single base on the DNA. This can alter one amino acid of a single protein and have lethal effects. Ex. Sickle cell anemia.
Gene Therapy Replacing a defective gene with a functional gene. In the past, this type of therapy has worked in some isolated instances. Problems: The functional gene is carried as part of the DNA of an adenovirus (cold virus), the vector. The virus can cause a strong immune response causing 1) the destruction of the virus and the gene destroyed or 2) death of the patient. The gene may also be incorporated into the patient’s DNA at random and cause lethal mutations.
Gene Therapy Gene therapy was banned for several years but a new vector AAV (paravirus with only 2 genes of its own) is showing promise. Animal trials have shown positive results with few problems. Clinical trials are underway for cystic fibrosis, etc.