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6.1 Chromosomes and Meiosis KEY CONCEPT Gametes have half the number of chromosomes that body cells have. 6.1 Chromosomes and Meiosis You have body cells and gametes. • Body cells are also called somatic cells. • Germ cells develop into gametes. – Germ cells are located in the ovaries and testes. – Gametes are sex cells: egg and sperm. – Gametes have DNA that can be passed to offspring. body cells sex cells (sperm) sex cells (egg) 6.1 Chromosomes and Meiosis Your cells have autosomes and sex chromosomes. • Your body cells have 23 pairs of chromosomes. – Homologous pairs of chromosomes have the same structure. – For each homologous pair, one chromosome comes from each parent. • Chromosome pairs 1-22 are autosomes. • Sex chromosomes, X and Y, determine gender in mammals. 6.1 Chromosomes and Meiosis Body cells are diploid; gametes are haploid. • Fertilization between egg and sperm occurs in sexual reproduction. • Diploid (2n) cells have two copies of every chromosome. – Body cells are diploid. – Half the chromosomes come from each parent. 6.1 Chromosomes and Meiosis • Haploid (n) cells have one copy of every chromosome. – Gametes are haploid. – Gametes have 22 autosomes and 1 sex chromosome. 6.1 Chromosomes and Meiosis • Chromosome number must be maintained in animals. • Many plants have more than two copies of each chromosome. • Mitosis and meiosis are types of nuclear division that make different types of cells. • Mitosis makes more diploid cells. 6.1 Chromosomes and Meiosis • Meiosis makes haploid cells from diploid cells. – Meiosis occurs in sex cells. – Meiosis produces gametes. 6.1 Chromosomes and Meiosis KEY CONCEPT During meiosis, diploid cells undergo two cell divisions that result in haploid cells. 6.1 Chromosomes and Meiosis Cells go through two rounds of division in meiosis. • Meiosis reduces chromosome number and creates genetic diversity. 6.1 Chromosomes and Meiosis • Meiosis I and meiosis II each have four phases, similar to those in mitosis. – Pairs of homologous chromosomes separate in meiosis I. – Homologous chromosomes are similar but not identical. – Sister chromatids divide in meiosis II. – Sister chromatids are copies of the same chromosome. homologous chromosomes sister chromatids sister chromatids 6.1 Chromosomes and Meiosis • Meiosis I occurs after DNA has been replicated. • Meiosis I divides homologous chromosomes in four phases. 6.1 Chromosomes and Meiosis • Meiosis II divides sister chromatids in four phases. • DNA is not replicated between meiosis I and meiosis II. 6.1 Chromosomes and Meiosis • Meiosis differs from mitosis in significant ways. – Meiosis has two cell divisions while mitosis has one. – In mitosis, homologous chromosomes never pair up. – Meiosis results in haploid cells; mitosis results in diploid cells. 6.1 Chromosomes and Meiosis Haploid cells develop into mature gametes. • Gametogenesis is the production of gametes. • Gametogenesis differs between females and males. – Sperm become streamlined and motile. – Sperm primarily contribute DNA to an embryo. – Eggs contribute DNA, cytoplasm, and organelles to an embryo. – During meiosis, the egg gets most of the contents; the other cells form polar bodies. 6.1 Chromosomes and Meiosis KEY CONCEPT Mendel’s research showed that traits are inherited as discrete units. 6.1 Chromosomes and Meiosis Mendel laid the groundwork for genetics. • Traits are distinguishing characteristics that are inherited. • Genetics is the study of biological inheritance patterns and variation. • Gregor Mendel showed that traits are inherited as discrete units. • Many in Mendel’s day thought traits were blended. 6.1 Chromosomes and Meiosis Mendel’s data revealed patterns of inheritance. • Mendel made three key decisions in his experiments. – use of purebred plants – control over breeding – observation of seven “either-or” traits 6.1 Chromosomes and Meiosis • Mendel used pollen to fertilize selected pea plants. – P generation crossed to produce F1 generation – interrupted the self-pollination process by removing male flower parts Mendel controlled the fertilization of his pea plants by removing the male parts, or stamens. He then fertilized the female part, or pistil, with pollen from a different pea plant. 6.1 Chromosomes and Meiosis • Mendel allowed the resulting plants to self-pollinate. – Among the F1 generation, all plants had purple flowers – F1 plants are all heterozygous – Among the F2 generation, some plants had purple flowers and some had white 6.1 Chromosomes and Meiosis • Mendel observed patterns in the first and second generations of his crosses. 6.1 Chromosomes and Meiosis • Mendel drew three important conclusions. – Traits are inherited as discrete units. – Organisms inherit two copies of each gene, one from each parent. – The two copies segregate during gamete formation. – The last two conclusions are called the law of segregation. purple white 6.1 Chromosomes and Meiosis KEY CONCEPT Genes encode proteins that produce a diverse range of traits. 6.1 Chromosomes and Meiosis The same gene can have many versions. • A gene is a piece of DNA that directs a cell to make a certain protein. • Each gene has a locus, a specific position on a pair of homologous chromosomes. 6.1 Chromosomes and Meiosis • An allele is any alternative form of a gene occurring at a specific locus on a chromosome. – Each parent donates one allele for every gene. – Homozygous describes two alleles that are the same at a specific locus. – Heterozygous describes two alleles that are different at a specific locus. 6.1 Chromosomes and Meiosis Genes influence the development of traits. • All of an organism’s genetic material is called the genome. • A genotype refers to the makeup of a specific set of genes. • A phenotype is the physical expression of a trait. 6.1 Chromosomes and Meiosis • Alleles can be represented using letters. – A dominant allele is expressed as a phenotype when at least one allele is dominant. – A recessive allele is expressed as a phenotype only when two copies are present. – Dominant alleles are represented by uppercase letters; recessive alleles by lowercase letters. 6.1 Chromosomes and Meiosis • Both homozygous dominant and heterozygous genotypes yield a dominant phenotype. • Most traits occur in a range and do not follow simple dominant-recessive patterns. 6.1 Chromosomes and Meiosis KEY CONCEPT The inheritance of traits follows the rules of probability. 6.1 Chromosomes and Meiosis Punnett squares illustrate genetic crosses. • The Punnett square is a grid system for predicting all possible genotypes resulting from a cross. – The axes represent the possible gametes of each parent. – The boxes show the possible genotypes of the offspring. • The Punnett square yields the ratio of possible genotypes and phenotypes. 6.1 Chromosomes and Meiosis A monohybrid cross involves one trait. • Monohybrid crosses examine the inheritance of only one specific trait. – homozygous dominant-homozygous recessive: all heterozygous, all dominant 6.1 Chromosomes and Meiosis – heterozygous-heterozygous—1:2:1 homozygous dominant: heterozygous:homozygous recessive; 3:1 dominant:recessive 6.1 Chromosomes and Meiosis • heterozygous-homozygous recessive—1:1 heterozygous:homozygous recessive; 1:1 dominant:recessive • A testcross is a cross between an organism with an unknown genotype and an organism with the recessive phenotype. 6.1 Chromosomes and Meiosis A dihybrid cross involves two traits. • Mendel’s dihybrid crosses with heterozygous plants yielded a 9:3:3:1 phenotypic ratio. • Mendel’s dihybrid crosses led to his second law, the law of independent assortment. • The law of independent assortment states that allele pairs separate independently of each other during meiosis. 6.1 Chromosomes and Meiosis Heredity patterns can be calculated with probability. • Probability is the likelihood that something will happen. • Probability predicts an average number of occurrences, not an exact number of occurrences. number of ways a specific event can occur • Probability = number of total possible outcomes • Probability applies to random events such as meiosis and fertilization. 6.1 Chromosomes and Meiosis KEY CONCEPT Independent assortment and crossing over during meiosis result in genetic diversity. 6.1 Chromosomes and Meiosis Sexual reproduction creates unique combinations of genes. • Sexual reproduction creates unique combination of genes. – independent assortment of chromosomes in meiosis – random fertilization of gametes • Unique phenotypes may give a reproductive advantage to some organisms. 6.1 Chromosomes and Meiosis Crossing over during meiosis increases genetic diversity. • Crossing over is the exchange of chromosome segments between homologous chromosomes. – occurs during prophase I of meiosis I – results in new combinations of genes 6.1 Chromosomes and Meiosis • Chromosomes contain many genes. – The farther apart two genes are located on a chromosome, the more likely they are to be separated by crossing over. – Genes located close together on a chromosome tend to be inherited together, which is called genetic linkage. • Genetic linkage allows the distance between two genes to be calculated.