Chapter 11 Introduction to Genetics

Chromosomes and Cells Two general types of cells –Somatic cells-body cells that make up the tissues and organs –Gametes-sex cells (eggs and sperm)

Chromosomes and Cells Characteristic number of chromosomes in cells of each species’ body cells Gametes have ½ of that number of chromosomes Human body cells have 46 chromosomes in every body cell but only 23 chromosomes in each gamete

Chromosomes and Cells Chromosomes are grouped in pairs (homologous pairs) according to size, shape, and genes they carry There are 23 pairs of chromosomes in human cells –Pairs 1-22 are autosomes –Pair 23 is sex chromosomes

Chromosomes and Cells Somatic cells are diploid (having the “normal” number of chromosomes / cell = 46 for human somatic cells) Gametes are haploid (having ½ the normal number of chromosomes / cell = 23 for human gametes) –Gametes are haploid because the diploid number is reinstated at fertilization

Karyotype

Meiosis Germ cells undergo meiosis to produce gametes Meiosis is a form of nuclear division that divides diploid cells into haploid cells –Reduces the number of chromosomes –Essential for sexual reproduction

Meiosis Germ cells undergo meiosis to produce gametes Meiosis is a form of nuclear division that divides diploid cells into haploid cells –Reduces the number of chromosomes –Essential for sexual reproduction

Comparing mitosis and meiosis MITOSIS –1 nuclear division –Begin with 1 diploid cell –End w/ 2 diploid cells –Occurs in somatic cells –Daughter cells are identical MEIOSIS –2 nuclear divisions –Begin w/1 diploid cell –End w/4 haploid cells –Occurs in gametes or sex cells –Daughter cells are NOT identical

11-4 Meiosis Involves 2 nuclear divisions Results in the production of gametes or sex cells (egg and sperm) Reduction division to reduce the number of chromosomes by half in the gametes (chromosomes return to normal number at fertilization)

Meiosis I Begin with a diploid cell Prophase I –Homologous chromosomes pair up to form TETRADS (4 chromatids per tetrad) –Crossing over occurs: chromatids exchange genes to create larger genetic variation –Nucleolus and nucleus dissolve –Spindles begin to form

Meiosis I Metaphase I –Tetrads line up at equator –Each centromere is attached to a spindle

Meiosis I Anaphase I –Homologous chromosomes separate and begin moving to opposite poles Telophase I and cytokinesis –Nuclear membranes form around each set of chromosomes –Cell separates into two new cells –Each new cell is now HAPLOID

Meiosis II Begin with 2 haploid cells Prophase II –Nuclei dissolve –Chromosomes are visible Metaphase II –Chromosomes line up at the equator of each cell –Centromeres are attached to spindles

Meiosis II Anaphase II –Chromatids split in each cell and move to opposite poles Telophase II and cytokinesis –Nuclei form around each set of chromosomes –Each cell divides into two new haploid cells

Results of Meiosis 4 haploid daughter cells that are NOT identical These cells will develop into gametes in a process known as gametogenesis –Males: all 4 cells develop into sperm –Females: only 1 cell receives enough cytoplasm to become an egg; the other 3 become polar bodies and are reabsorbed

11-1 Gregor Mendel Father of genetics Born 1822 Austrian Monk Attended the University of Vienna Did all genetic research in the gardens at the monastary Studied pea plants

Pea plants Naturally true-breeding (can self-fertilize) Can manipulate pollination for cross breeding to produce hybrid offspring Hybrids are offspring from two parents having contrasting characters for a trait

Traits studied on peas Seed shape Seed color Seed coat color Pod shape Pod color Flower position Plant height

Genes Genes are chemical factors that control traits Located on chromosomes Have alternate forms call ALLELES Each individual has 2 alleles for each trait (one allele coming from the mother one coming from the father)

Dominant and recessive Dominant traits –show in all generations –Represented by capital letters Recessive traits –Absent in first generation but reappear in second generation –Represented by lower-case letters

11-2 Probability Probability is the likelihood that an event will occur. Probabilities are used to predict the outcomes of genetic crosses To show probabilities of genetic crosses Punnett Squares are used

Punnett Squares Show all possible gene combinations Can be used to predict and compare genetic variations that result from a cross Show genotypes (genetic makeup) using letters Relate phenotypes (physical characteristics)

Genotypes Homozygous have two of the same alleles –rr –RR Heterozygous have two different alleles –Rr –Tt

11-3 Independent assortment Genes that segregate independently do NOT influence each other’s inheritance Genes are not inherited together Leads to large variations in genetics of offspring

Crosses Monohybrid cross involves one trait Dihybrid cross involves two traits P1 generation- true-breeding parents F1 generation-first generation of hybrid offspring from P1 F2 generation-second generation of hybrid offspring (F1 parents)

Mendel’s principles Inheritance of characteristics is determined by genes Genes may have alternate forms called alleles; some dominant, some recessive In sexually reproducing organisms, adults have 2 copies (alleles) for each gene-one from each parent Alleles segregate independently

Other forms of inheritance Incomplete dominance Codominance Multiple alleles Polygenic traits

Incomplete dominance Neither allele is completely dominant over the other Heterozygotes will exhibit a “mixing” of traits or an intermediate phenotype

Codominance Both alleles are equally dominant and contribute equally to the heterozygous phenotype

Multiple alleles More than 2 alleles involved with the trait of that particular population Rabbit fur color –C – brown; c ch -gray, c h -white w/ brown areas, c-albino

Polygenic traits Traits produced by the interaction of several genes At least 3 genes involved Example: human skin color or human height

Genetic recombination Sexual reproduction gives genetic variation in the offspring –Due to independent assortment of chromosomes in meiosis –Mixing of alleles when gametes fuse in fertilization –Produces unique combinations of alleles

Genetic recombination Crossing over –Increases genetic variation –Occurs only in prophase I of meiosis –Chromatids of homologous chromosomes will exchange (trade) some alleles

Genetic linkage Genes located close together on a chromosome are usually inherited together because they are “linked” Genes located far apart or on different chormososmes are inherited independently

Genetic linkage