BIOLOGY CHAPTER 6 &7 Meiosis and Mendelian Genetics
6.1 & 6.2 CHROMOSOMES AND MEIOSIS 6.6 MEIOSIS AND GENETIC VARIATIONMitosisSomatic cells undergo mitosisOne diploid cell produces 2 genetically identical, diploid cellsThere are 4 stages (prophase, metaphase, anaphase, telophase)DNA is copied once and divided onceMitosis is used for development, growth, and repair
MeiosisGerm cells produce gametes (sex cells) during meiosisOne diploid cell produces 4 genetically different haploid cellsThere are 8 stages (prophase I, metaphase I, anaphase I, telophase I, prophase II, metaphase II, anaphase II, telophase II)DNA is copied once and divided twice
Homologous ChromosomesHalf of an organism’s chromosomes come from the mother, and half from the father. The chromosomes pair up to form homologous chromosomesHomologous chromosomes are two chromosomes with the same length, general appearance, and the same type of genesThe pairing of the genes on the homologous chromosomes are what code for all of an organism’s traits (see examples in section 6.4)
All humans have 23 total pairs of chromosomes, for a total of 46 chromosomes.22 of the pairs are autosomes (body chromosomes)1 pair is the sex chromosomes (XX= female, XY=male)
Meiosis ISame steps as mitosis, except during Prophase I crossing over can occur(see below),during Metaphase I homologous chromosomes pair up in the middle andduring Anaphase I, the sister chromatids DO NOT separateBy the end of Meiosis I, there are 2 haploid cells
Crossing OverHomologous chromosomes exchange genesThis results in a new combination of genes (called recombination)Genes that are located near each other on a chromosome are more likely to be moved (or crossed over) together. This is called genetic linkage (refer to page 191)Meiosis IIThis is exactly the same as mitosis
6.3 MENDEL AND HEREDITYGregor MendelAn Austrian monk who is known as the “father of genetics”He discovered that traits (genes) are inherited from parents by working with pea plantsHe made both purebred plants and crossed plants to see what the results would be.
WHY PEA PLANTS?They reproduce quicklyThey can both self-pollinate(creating purebreds) and cross-pollinate(creating a mix of traits)They have “either-or” traits (the traits do not mix to make a 3rd trait, ex. The purple flowers and white flowers don’t mix to make a pink flower)(refer to page 179 to see the traits and results of Mendel’s experiment)
MENDEL’S CONCLUSIONSMendel’s Law of Segregation: Organisms inherit two copies of each gene, one from each parentOrganisms donate only one copy of each gene in their gametes. Thus, the two copies of each gene segregate, or separate, during gamete formation (Law of Independent Assortment)In other words, the Law of Ind. Assortment is saying that traits are inherited separately.
6.4 TRAITS, GENES, AND ALLELES6.5 TRAITS AND PROBABILITIESGenesEach gene has a specific locus, or place on the chromosomeA gene is a piece of DNA that provides a set of instructions to make a certain protein. The proteins then code for traits.Each trait is represented by alleles.
Letters are used to represent different alleles. There are two alleles per trait (one from mother, one from father)Dominant alleles are represented by capital letters and Recessive alleles are represented by lowercase lettersDominant alleles will always overpower the recessive alleles.
EXAMPLE: P= purple flowers p= white flowersPP=purple flowers (homozygous)pp= white flowers (homozygous)Pp= purple flowers (heterozygous)Genotype-the alleles/genetic code (PP or Pp)Phenotype-the physical trait (purple or white)
PUNNET SQUARESPunnet squares are used to predict the outcome of genotypes when crossing two organismsMonohybrid cross- crosses one trait (always yields a 3:1 phenotypic ratio)Dihybrid cross- crosses two traits (always yield 9:3:3:1 phenotypic ratio)
CHAPTER 7EXTENDING MENDELIAN GENETICSComplex Patterns of InheritanceIncomplete Dominance—the trait is in betweenthe dominant and recessive Ex. RR=red rr=white Rr=pinkCodominance—both traits are expressed at thesame time Ex. RR=red rr=white Rr= both red and white
Polygenic Traits—when two or moregenes influence a traitEx. Human eye color(at least 3 different genes), human skin color (4 different genes)Environmental factors—when theenvironment plays a role in geneexpressionEx. Sea turtle eggs become female in warmer temps and male in colder temps
SEX-LINKED TRAITS—TRAITS CAUSED BY GENES LOCATED ON THE SEX CHROMOSOMES The X contains many genes The Y only determines sex (male) A male is XY, so all of the alleles on the X are expressed in a male, even the recessive ones A female is XX, so dominant alleles can mask the recessive onesCarriers—those who carry a recessive genetic disorder, but do not express the phenotype. They can pass on the disorder to their offspring