Genetics: Mendel's Laws - Lecture 4

1 Lecture 4 2 3 Review of what has been studied 4 Course contents At the end of this part, you should be able to: Mendelian inheritance Mendel’s Laws Types of dominance 5 Lecture Objectives At the end of this Lecture, you should be abl...

1 Lecture 4 2 3 Review of what has been studied 4 Course contents At the end of this part, you should be able to: Mendelian inheritance Mendel’s Laws Types of dominance 5 Lecture Objectives At the end of this Lecture, you should be able to: Mendel’s Laws (3. Law of Independent assortment)  6 Homologous chromosomes Homologous chromosomes are pairs of chromosomes in diploid organisms, with one chromosome originally inherited from each parent. Gene loci Homologous chromosomes  Homologous chromosomes bear 7 the alleles for each character These chromosomes are similar in size, shape, and genetic content and bear the alleles for each character. Gene loci P a B Homologous chromosomes P a b  Homologous chromosomes bear 8 the alleles for each character Alleles are alternative forms of a gene that may produce different variants of a particular trait. Gene loci Dominant allele P a B Homologous chromosomes P a b Recessive allele Genotype: PP aa Bb Homozygous Homozygous Heterozygous, with for the for the one dominant and one dominant allele recessive allele recessive allele 9 Mendel’s Laws 3. Law of Independent assortment: “Members of one gene pair segregate independently from other gene pairs during gamete formation”.  Law of Independent assortment 10 The alleles of two more genes get sorted into gametes independent of each other. The allele received for one gene does not influence the allele received for another gene. During gamete formation, one pair of trait segregates from another pair of traits independently. This gives each pair of characters a chance of expression.  11 Segregation of alleles during meiosis: When the F1 plants produce gametes (sex cells) and self- pollinate, the two alleles for the same gene separate from each other so that each gamete carries only one copy of each gene. Remember, gametes are haploid. In the example, we use “T” to represent the dominant, tall allele and “t” to represent the recessive, short allele.  Law of Independent assortment 12 Mendel identified his law of inheritance by following two characters at the same time. Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters  Dihybrid cross 13 A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently  Dihybrid cross 14 Cross involving two traits e.g. Seed shape & Seed color (2 traits, 2 alleles per trait). 15 Dihybrid cross RrYy x RrYy Each parent can produce 4 types of gametes. RY Ry rY ry Cross is a 4 X 4 with 16 possible offspring. 16 Dihybrid Cross Traits: Seed shape & Seed color Alleles: R Round r Wrinkled Y Yellow y Green All possible gamete combinations RY Ry rY ry RY Ry rY ry 17 Dihybrid Cross RrYy x RrYy SPERM RY Ry rY ry RY EGGS Ry rY ry 18 Dihybrid Cross RY Ry rY ry Round/Yellow: 9 RY RRYY RRYy RrYY RrYy Round/green: 3 Ry RRYy RRyy RrYy Rryy wrinkled/Yellow: 3 rY RrYY RrYy rrYY rrYy wrinkled/green: 1 ry RrYy 9:3:3:1 phenotypic Rryy rrYy rryy ratio 19 Law of Independent assortment Mendel needed to explain why the F2 offspring: had new nonparental combinations of traits and had a 9:3:3:1 phenotypic ratio. Mendel suggested that the inheritance of one character has no effect on the inheritance of another, suggested that the dihybrid cross is the equivalent to two monohybrid crosses, and The law of independent assortment states that allele pairs separate independently of each other during meiosis. Called this the law of independent assortment.  Law of Independent assortment 20  21 Test cross When you have an individual with an unknown genotype, you do a Test cross. Test cross is used to determine the genotype of an individual. There are two possible test crosses: Homozygous dominant x Hybrid Homozygous recessive x Hybrid  Homozygous dominant x Hybrid 22 Trait: Seed Shape Alleles: R – Round r – Wrinkled Cross: Round seeds x Round seeds RR x Rr R r Genotype: RR, Rr R RR Rr Phenotype: Round Genotypic Ratio: 1:1 R RR Rr Phenotypic Ratio: All alike 23 Homozygous recessive x Hybrid Trait: Seed Shape Alleles: R – Round r – Wrinkled Cross: Wrinkled seeds x Round seeds rr x Rr R r Genotype: Rr, rr Phenotype: Round & r Rr rr Wrinkled Genotypic Ratio: 1:1 r Rr rr Phenotypic Ratio:1:1 24 Phenotype vs. Genotype PP and Pp – Two different genotype – The same phenotype How do you distinguish these two genotypes experimentally?  25 How can we know if it is homozygous (PP) or heterozygous (Pp)? Test Cross: A mating between an individual of unknown genotype and a homozygous recessive individual. 26 The following figure demonstrates how single gene test cross can be performed to determine the genotype of a purple flowers: 27 Test Cross: Confirmation of Segregation 28 Two gene test cross 29 ?? ?? x rryy 1. RRYY 2. RRYy 3. RrYY ? 4. RrYy