The Basis of Heredity Inheritance and Meiosis

Definitions Genetics = study of genes, the units on chromosomes that code for traits Heredity = study of how traits are passed on

The Inheritance of Traits: Mendel’s Discoveries Czechoslovakian monk First to discover rules of inheritance Published work in 1865

Mendel’s Hybrid Peas Pure lines: tall plants made tall plants Studied 7 contrasting characteristics

Mendel’s Crosses Mendel looked at one trait at a time by crossing inbred lines. Hybrid = offspring produced by breeding two pure lines. P 1 = purebred parents. F 1 = first generation of offspring. F 2 = second generation of offspring. P = parental F = filial or offspring.

Cross of green and yellow pods

Mendel Crossed Short X Tall P 1 tall x short F 1 all tall (expected medium) F 2 3/4 tall 1/4 short. hypothesis: 2 factors controlling height. Factor for shortness was hidden in F 1.

Short X Tall Continued Tall factor = dominant (masked short) Short factor = recessive (was hidden) Mendel tested his hypothesis - crossed plants with other 6 traits All other traits acted in same way

The Law of Segregation Mendel noticed 2 things factors controlling traits were distinct units traits appeared in constant proportions in F 2 Explained results with Law of Segregation individual carries 2 factors for each trait each parent contributes 1 of the 2 factors TT and Tt = tall and tt = short genes = units of heredity Alleles = 2 separate forms of a gene

Genotype Vs. Phenotype Genotype = the alleles of an organism Phenotype = the appearance of an organism Homozygous = having 2 identical alleles TT or tt Heterozygous = having unlike alleles - Tt

Using Punnett Squares

Test Cross Used to determine if an individual is homozygous or heterozygous

Incomplete Dominance heterozygote is a blend of the two homozygotes

Law of Independent Assortment monohybrid cross: involves one pair of alleles or one gene dihybrid cross: crosses involving two genes Mendel did thousands of dihybrid crosses which led to law of independent assortment the inheritance of one gene does not affect the inheritance of another gene a tall plant can have either smooth or wrinkled seeds

Meiosis The Cellular Basis of Heredity

Chromosome Number somatic cells = body cells most organisms contain two complete sets of chromosomes diploid = 2 sets of chromosomes - 2n n = # of different kinds of chromosomes haploid = 1 set of chromosomes humans n = 23(haploid) and 2n = 46(diploid) homologous chromosomes = matching pairs

Asexual and Sexual Reproduction Asexual reproduction offspring genetically identical to parent offspring formed by mitosis potato production Sexual reproduction 2 haploid cells unite to form a diploid cell haploid cells = gametes (sperm and eggs) fertilization = union of two gametes to form zygote zygote grows by mitosis to form new organism