Meiosis and Mendel Chapter 6
6.1 CHROMOSOMES AND MEIOSIS
Key Concept Gametes have half the number of chromosomes that body cells have.
Body Cells vs. Gametes You have body cells and gametes body cells = somatic cells gametes = sex cells ex: sperm and egg Body Cells Sex Cells (sperm) Sex Cells (egg)
Autosomes vs. Sex Chromosomes Humans have 23 pairs of chromosomes called homologous pairs one from mom, one from dad Pairs 1-22 are autosomes Sex chromosomes, X and Y, determine gender
Diploid vs. Haploid Diploid = 2n 2 copies of every chromosome found in body (somatic) cells Haploid = n 1 copy of every chromosome found in sex cells (sperm & egg) fertilization restores diploid #
Chromosome # Must be maintained in animals plants can have crazy chromosome # s!! Mitosis recall: more diploid cells are made
Chromosome # Meiosis makes haploid cells from diploid cells occurs in sex cells produces gametes
Fertilization 46 meiosis 23 egg 23 23 zygote 46 46 23 fertilization sperm gametes
6.2 PROCESS OF MEIOSIS
Key Concept During meiosis, diploid cells undergo two cell divisions that result in haploid cells.
Process of Meiosis Cells go through 2 rounds of division reduces chromosome # creates genetic diversity
Process of Meiosis 2 rounds: Meiosis I 4 phases homologous chromosomes are separated Meiosis II 4 phases sister chromatids are separated almost identical to mitosis
Overview of Meiosis I.P.M.A.T.P.M.A.T 2n = 4 interphase 1 prophase 1 metaphase 1 anaphase 1 n = 2 n = 2 prophase 2 metaphase 2 anaphase 2 telophase 2 n = 2 telophase 1
Double Division of Meiosis DNA replication 1st division of meiosis separates homologous pairs 2nd division of meiosis separates sister chromatids Meiosis 1 Meiosis 2
Preparing for Meiosis Interphase chromosomes are duplicated during S phase 2n = 6 single stranded 2n = 6 double stranded
Prophase I Double-stranded chromosomes appear Spindle fibers appear Nuclear membrane fades
Metaphase I Chromosome pairs line up at center
Anaphase I Homologous chromosomes separate
Telophase I Cytoplasm divides and 2 cells form
And then NO INTERPHASE!! NO DNA REPLICATION!!
Prophase II Chromatids and spindle fibers reappear
Metaphase II Chromatids line up at center of cell
Anaphase II Chromatids split and move apart
Telophase II 4 sex cells (daughter cells) are produced
Mitosis vs. Meiosis Mitosis 1 division daughter cells genetically identical to parent cell produces 2 cells 2n 2n produces cells for growth & repair no crossing over Meiosis 2 divisions daughter cells genetically different from parent produces 4 cells 2n 1n produces gametes crossing over
6.3 MENDEL AND HEREDITY
Key Concept Mendel s research showed that traits are inherited as discrete units.
Gregor Mendel Laid the groundwork for genetics traits are distinguishing characteristics that are inherited genetics is the study of biological inheritance patterns and variation
Mendel s Data Revealed the patterns of inheritance 3 key things he did right: used purebred plants controlled the breeding observed either-or traits
Mendel s Work Bred pea plants cross-pollinate true breeding parents (P) P = parental raised seed & then observed traits (F1) F = filial allowed offspring to self-pollinate & observed next generation (F2) Pollen transferred from white flower to stigma of purple flower P all purple flowers result F 1 F 2 self-pollinate anthers removed
Results P true-breeding purple-flower peas X true-breeding white-flower peas F 1 generation (hybrids) 100% purple-flower peas Where did the white flowers go? 100% F 2 generation 75% purple-flower peas self-pollinate 25% white-flower peas White flowers came back! 3:1
More Data Mendel gathered data for 7 different traits over 2 generations
3 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 purple white
6.4 TRAITS, GENES, AND ALLELES
Key Concept Genes encode proteins that produce a diverse range of traits.
Genes The same gene can have many versions gene 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
Traits Are influenced by genes All of an organism s genetic material is called the genome
Genotype vs. Phenotype Difference between how an organism looks & its genetics phenotype description of an organism s trait the physical genotype description of an organism s genetic makeup Explain Mendel s results using dominant & recessive phenotype & genotype
Alleles Any alternative form of a gene occurring at a specific locus on a chromosome Each parent donates one allele for every gene
Genotypes Homozygous = same alleles = PP, pp Heterozygous = different alleles = Pp homozygous dominant heterozygous homozygous recessive
Alleles Can be expressed using letters Dominant allele uppercase if present, what you see Recessive allele lowercase masked by dominant
Summary Both homozygous dominant and heterozygous genotypes yield a dominant phenotype Most traits occur in a range and do not follow simple dominantrecessive patterns
6.5 TRAITS AND PROBABILITY
Key Concept The inheritance of traits follows the rules of probability
Punnett Squares Yields the possible ratios of genotypes and phenotypes
Punnett Squares
Monohybrid Cross Examines the inheritance of only one specific trait
Practice 1. Homozygous dominant X Homozygous recessive AA x aa 100% dominant (A)
Practice 2. Heterozygous x Heterozygous Aa x Aa 1:2:1 genotype ratio (AA:Aa:aa) 3:1 phenotype ratios (dominant:recessive)
Practice 3. Heterozygous x homozygous recessive Aa x aa 1:1 genotype ratio (Aa:aa) 1:1 phenotype ratio (dominant:recessive)
Test Cross A cross between an organism with an unknown genotype and an organism with the recessive phenotype x is it PP or Pp? pp
Dihybrid Cross Involves 2 traits 2 heterozygotes Outcome: 9:3:3:1 phenotype ratio
Mendel s Laws Law of Independent Assortment allele pairs separate independently of each other during meiosis
Probability Can calculate heredity patterns Predicts an EXPECTED outcome Applies to random events
6.6 MEIOSIS AND GENETIC VARIATION
Key Concept Independent assortment and crossing over during meiosis result in genetic diversity
Sexual Reproduction Creates unique combinations of gametes independent assortment of chromosomes in meiosis random fertilization of gametes An advantage?
Crossing Over Occurs during meiosis chromosome segments are exchanged Prophase I results in new combinations of genes
Gene Linkage Chromosomes contain many genes if genes are far apart: likely to be separated during crossing over if genes are close together: likely to be inherited together