Presentation on theme: "Genetics: an Introduction. Genetics: the study of Heredity Heredity: transmission of traits from one generation to the next Gregor Mendel: father of modern."— Presentation transcript:
Genetics: an Introduction
Genetics: the study of Heredity Heredity: transmission of traits from one generation to the next Gregor Mendel: father of modern genetics – worked in 1860’s – bred garden peas – used applied mathematics: statistics!!!!
Genetics character: observable physical feature – flower color, eye color, etc trait: form of character – purple or white flowers; brown or blue eyes heritable trait: passed from parent to offspring
Garden Peas: who knew? Stamen Carpel Petal
Figure 9.2C_s3 Removal of stamens Carpel White Stamens Transfer of pollen Purple Carpel matures into pea pod Seeds from pod planted Offspring (F 1 ) Parents (P)
Genetics True-breeding varieties result when self-fertilization produces offspring all identical to the parent. The offspring of two different varieties are hybrids. The cross-fertilization is a hybridization, or genetic cross. True-breeding parental plants are the P generation. Hybrid offspring are the F 1 generation. A cross of F 1 plants produces an F 2 generation.
Genetics: Mendel (monohybrid cross) The all-purple F 1 generation did not produce light purple flowers, as predicted by the blending hypothesis. Mendel needed to explain why white color seemed to disappear in the F 1 generation and white color reappeared in one quarter of the F 2 offspring. Mendel observed the same patterns of inheritance for six other pea plant characters.
Mendel developed four hypotheses, described below using modern terminology. 1. Alleles are alternative versions of genes that account for variations in inherited characters. 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. Genetics: Mendel (monohybrid cross)
3.If the alleles of an inherited pair differ, then one determines the organism’s appearance and is called the dominant allele. The other has no noticeable effect on the organism’s appearance and is called the recessive allele. – The phenotype is the appearance or expression of a trait. – The genotype is the genetic makeup of a trait. – The same phenotype may be determined by more than one genotype. Genetics: Mendel (monohybrid cross)
4.A sperm or egg carries only one allele for each inherited character because allele pairs separate (segregate) from each other during the production of gametes. This statement is called the law of segregation. Mendel’s hypotheses also explain the 3:1 ratio in the F 2 generation. –The F 1 hybrids all have a Pp genotype. –A Punnett square shows the four possible combinations of alleles that could occur when these gametes combine Genetics: Mendel (monohybrid cross)
Figure 9.3B_4 F 2 generation P P p p PP Pp pp Eggs from F 1 plant Phenotypic ratio 3 purple : 1 white Genotypic ratio 1 PP : 2 Pp : 1 pp Sperm from F 1 plant
Genetics: Test Cross if you have an individual with the dominant phenotype: you CANNOT know its genotype ( Black = B, b = brown) – Black phenotype could be BB or Bb genotype How do you find out (without DNA testing)? do a TEST CROSS – an y individual with the recessive phenotype MUST have the homozygous recessive genotype
Figure 9.6 What is the genotype of the black dog? Two possibilities for the black dog: Testcross Genotypes Gametes Offspring All black 1 black : 1 chocolate or B_? bb Bb BB B B b b b Bb bb
Genetics: Test Cross if the Black dog is BB BB b b Bb phenotype of ALL offspring = Black genotype of ALL offspring = heterozygous
Genetics: Test Cross if Black dog is Bb B b b b Bb bb phenotype of HALF offspring = Black phenotype of HALF offspring = brown
Genetics: Dihybrid Cross looking at two traits at the same time start with ‘true breeding’ plants for BOTH traits – R = round; r = wrinkled – Y = yellow; y = green RRYY ( homozygous Dominant for both traits ) crossed with rryy ( homozygous recessive for both traits ) = R generation F 1 generation ALL = RrYy (round and yellow) cross F 1 with F 1 to get F 2 generation
Genetics: Dihybrid Cross F2 generation: NEW COMBINATIONS!! –9/16 had round yellow seeds –3/16 had wrinkled yellow seeds –3/16 had round green seeds –1/16 had wrinkled green seeds Independent assortment occurred (genes assort independently into gametes)