Genetics from the beginning Basic principles of genetics
Genetics from the beginning! Basic principles of genetics University of Utah Review of Genes etc!
The study of Heredity Inheritance of traits coded for by genes
What is a gene pool? All the genes in a given population
Evidence for Heredity Similar phenotype caused by similarities in genotype
Gregor Mendel Austrian Monk!
Mendel’s Pea Plants Showed Traits that were Markedly Contrasting Could tell tall from short easily
Traits studied by Mendel Markedly contrasting
Ma le m e F e l a t r pa th i w g g e par t wit hp olle n
Cross Pollination/Cross Fertilization Pollen from anther transferred to stigma of female part
Gregor Mendel • Cross pollinated pea plants • Studied traits whose expression were markedly contrasting • eg Height: tall vs. short…. . no in between • Pure Tall X Pure Short……. cross 1 • All offspring Tall
• He then crossed offspring from cross 1 • Offspring: 75% Tall : 25% Short • He reasoned that…… • Gene for tall is dominant • Gene for short is recessive
Summary of Mendel’s Cross • TT x tt ----Tt • Tt x Tt ---- 3 Tall : 1 Short • Parents referred to as P 1 • First generation referred to as F 1 (F=Filial) • Second generation referred to as F 2
Monohybrid Cross: Involving 1 trait T T t Tt Tt Phenotypes All Tall Punnett Square Genotypes All Tt Heterozygous
Monohybrid Cross T t T TT Tt tt Phenotypes 3 Tall: 1 Short Genotypes 1 TT : 2 Tt : 1 tt
Genotypes of parents F 1 F 2 Punnett Square
Crossing Tall and Short Plants Which characteristic appears to be recessive?
Genetics Terminology • • • Physical appearance = phenotype Genetic make up causing pheno = genotype Genotype described using minimum of 2 letters All traits controlled by at least 2 genes These referred to as alleles of a pair For height: alleles are T and t
• If genotype is TT, plant is tall (homozygous dominant) • If genotype is Tt, plant is also tall but also a carrier for short (heterozygous) ? ? y te e H h y w a … s o ant t ed in e m n o No us d o g y z ro Hybrid & Heterozygous often confused
More terminology…Yikes!!. • If genotype is tt, plant is short (homozygous recessive or just recessive) • Phenotypic ratio in cross 1 = all tall • Phenotypic ratio in cross 2 = 3 tall : 1 short • Genotypic ratio in cross 2 = 1 TT: 2 Tt: 1 tt • G. R. & P. R. found easily using punnett square
Identify the recessive allele & phenotype Using the following punnett square
Monohybrid Cross t t T Tt Tt t tt tt Phenotypes 1 Tall: 1 Short Genotypes 1 Tt : 1 tt
Conclusions from Mendel’s work • Inherited characteristics controlled by factors (genes) found in gametes • All traits controlled by 2 alleles (minimum). – Some alleles mask effect of others. • These are called dominant alleles
• Law of Segregation: Pairs of genes segregate when homologous chromosomes move to opposite poles during meiosis • Law of Independent Assortment: Different pairs of alleles segregate independently of each other
Mendel’s law of segregation
Independent Assortment
Test Cross • Used to determine genotype of individual expressing dominant characteristic • eg. If pea plant is tall • Genotype could be TT or Tt • Only one way to find out! • Cross with homozygous recessive
Test Cross t t T Tt Tt Phenotypes All Tall No short offspring Genotypes All Tt
Test Cross t t T Tt Tt t tt tt Phenotypes 2 Tall : 2 short Short offspring Genotypes 2 Tt : 2 tt
Summary of Test Cross Results • If some recessive individuals show up…. . • Genotype of unknown must be heterozygous • If no recessives show up…. • Genotype is probably homozygous dominant
Testcross with monohybrids
Test Crossing Sheep • Sheep ranchers like producing white wool • Easier to dye and less brittle than black • White is dominant, black recessive • If farmer suspects white sheep in herd are heterozygous…what can she/he do? ?
• Farmer may find out for certain by crossing suspected carriers with homozygous recessive (ww) • If some black sheep show up as offspring… • Sheep was a carrier for black wool! (heterozygous) • These sheep make better meat!!!!
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