Transcript Slide 1
9.1 The science of genetics has ancient roots Pangenesis was an early explanation for inheritance • It was proposed by Hippocrates • Particles called pangenes came from all parts of the organism to be incorporated into eggs or sperm • Characteristics acquired during the parents’ lifetime could be transferred to the offspring • Aristotle rejected pangenesis and argued that instead of particles, the potential to produce the traits was inherited Blending was another idea, based on plant breeding • Hereditary material from parents mixes together to form an intermediate trait, like mixing paint Copyright © 2009 Pearson Education, Inc. 1860’s Mendel’s Methods: 1. Experimentally rigorous 2. Mathematically rigorous Proposed Particulate Hypothesis: Parents pass on separate and distinct factors (genes) responsible for traits to their offspring. Factors retain their trait generation after generation. P generation (true-breeding parents) Purple flowers White flowers F1 generation All plants have purple flowers Fertilization among F1 plants (F1 ´ F1) F2 generation 3 – 4 of plants have purple flowers 1 – 4 of plants have white flowers Mendel observed that • Parental generation: purple flowers white flowers • F1 generation: all plants with purple flowers • F2 generation: 3/4 of plants with purple flowers 1/4 of plants with white flowers Mendel needed to explain • Why one trait seemed to disappear in the F1 generation • Why that trait reappeared in one quarter of the F2 offspring Copyright © 2009 Pearson Education, Inc. Mendel’s Law of Inheritance Four Hypotheses 1. Genes are found in alternative versions called alleles; a genotype is the listing of alleles an individual carries for a specific gene 2. For each characteristic, an organism inherits two alleles, one from each parent; the alleles can be the same or different – A homozygous genotype has identical alleles – A heterozygous genotype has two different alleles Copyright © 2009 Pearson Education, Inc. Mendel’s Laws of Inheritance Four Hypotheses 3. If the alleles differ, the dominant allele determines the organism’s appearance, and the recessive allele has no noticeable effect – The phenotype is the appearance or expression of a trait – The same phenotype may be determined by more than one genotype 4. Law of segregation: Allele pairs separate (segregate) from each other during the production of gametes so that a sperm or egg carries only one allele for each gene Copyright © 2009 Pearson Education, Inc. Genetic makeup (alleles) pp PP P plants Gametes All p All P F1 plants (hybrids) All Pp 1– 2 Gametes 1– 2 P Sperm F2 plants Phenotypic ratio 3 purple : 1 white Genotypic ratio 1 PP : 2 Pp : 1 pp P p P PP Pp p Pp pp Eggs p 9.4 Homologous chromosomes bear the alleles for each character For a pair of homologous chromosomes, alleles of a gene reside at the same locus • Homozygous individuals have the same allele on both homologues • Heterozygous individuals have a different allele on each homologue Copyright © 2009 Pearson Education, Inc. Gene loci Genotype: Dominant allele P a B P a b PP Homozygous for the dominant allele aa Homozygous for the recessive allele Recessive allele Bb Heterozygous Student Practice Should this dog be called Spot? Background: In Labrador retrievers a black coat(B) is dominant to a brown coat(b). • What is the genotype(s) of a dog with a brown coat? • What is the genotype(s) of a dog with a black coat? • If you wanted to determine the exact genotype of a dog with a black coat, what genetic cross (mating) would you perform? A testcross determines unknown genotypes Testcross • Mating between an individual of unknown genotype and a homozygous recessive individual • Will show whether the unknown genotype includes a recessive allele Copyright © 2009 Pearson Education, Inc. Testcross: B_ Genotypes bb Two possibilities for the black dog: BB B Gametes b Offspring Bb or Bb All black b B b Bb bb 1 black : 1 chocolate Uncovering the law of independent assortment by tracking two characters at once Consider the following: • Parental generation: round yellow seeds wrinkled green seeds • F1 generation: all plants with round yellow seeds • F2 generation: 9/16 of plants with round yellow seeds 3/16 of plants with round green seeds 3/16 of plants with wrinkled yellow seeds 1/16 of plants with wrinkled green seeds Mendel needed to explain • Why nonparental combinations were observed • Why a 9:3:3:1 ratio was observed among the F2 offspring Copyright © 2009 Pearson Education, Inc. Law of independent assortment • Each pair of alleles segregates independently of the other pairs of alleles during gamete formation • For genotype RrYy, four gamete types are possible: RY, Ry, rY, and ry Copyright © 2009 Pearson Education, Inc. Hypothesis: Independent assortment Hypothesis: Dependent assortment P generation rryy RRYY ry Gametes RY F1 generation rryy RRYY ry Gametes RY RrYy RrYy Sperm Sperm 1 – 2 F2 generation 1 – 2 RY 1 – 2 1 – 4 ry RY Eggs 1 – 2 ry 1 – 4 RY 1 – 4 rY 1 – 4 Ry Eggs Hypothesized (not actually seen) 1 – 4 RY 1 – 4 rY 1 – 4 Ry 1 – 4 ry RRYY RrYY RRYy RrYy RrYY rrYY RrYy rrYy 9 –– 16 RRYy RrYy RRyy Rryy RrYy rrYy Rryy rryy ry Actual results (support hypothesis) 3 –– 16 3 –– 16 1 –– 16 Yellow round Green round Yellow wrinkled Green wrinkled