Meiosis and Variation
Embed Size (px)
Transcript of Meiosis and Variation
Meiosis and VariationChapter 9Key Knowledge:meiosis, gamete production,variation: mutations, genotype, phenotype; continuous, discontinuous variationpatterns of inheritance in sexually reproducing organisms: Male Gamete ProductionSpermatogenesisProduced and stored in testes
Human Sperm5 m50 m3 mAverage ejaculate contains ~180 x 106 sperm (~60 x 106 sperm per mL) Female Gamete ProductionOogenesis produced in ovaries
100 mHuman Gametes
OvumSpermThe human ovum has approximately 100,000 times the volume of a single human sperm.Inherited VariationInherited variations are packaged into sperm and ova (gametes).This occurs in the gonads.The gonads in males are the testes and the gonads in females are the ovaries. The gametes produced by the gonads are always haploid.VariationIndividuals differ in all sorts of ways, even when they are offspring of the same parents. These differences are called variation. Most characteristics, such as height, are determined by several genes working together. They are also influenced by environmental factors such as climate, diet and lifestyle.Variation among individuals is due to meiosis.During meiosis Anaphase I, the chromosomes of each pair migrate to opposite poles randomly (Independent Assortment of Genes).VariationAlso during Metaphase I in meiosis, a recombination of genetic material occurs.During this recombination process, a significant event called crossing-over occurs between sister chromatids.This allows genes to be mixed and hence allow for greater genetic variation of offspring.The last type of variation is mutation. To be inherited the mutation must occur in the gonads. Somatic cell mutations are not inherited.Independent Assortment of GenesRandom orientations of non-matching chromosomes lead to independent assortment of genes into different gametes.
Crossing Over (Recombination)Crossing-over involves exchange of DNA between chromatids of sister chromosomes, allowing genes to recombine.
Errors in MeiosisSometimes during meiosis, sister chromosomes fail to separate and therefore you end up with two copies of a chromosome instead of the usual one.This is called non-disjunction and result in aneuploidy (missing a chromosome) and the reciprocal polyploidy (more than two chromosomes) in gametes.A is the normal process, B and C show non-disjunction
What is a Gene?A unit of heredity made up of a sequence of DNA.Alleles are alternate forms of a gene.A whole set of genes in an organism is a genome.Genes vary in size form about 100 to 2.5 million base pairs.Humans are thought to have 20,000 30,000 genes.After sequencing of genomes began in 2001, it became apparent that a more complex organism does not necessarily have more genes.Common nameApprox. size of genome (mbp)Fruit fly180Human3,100Onion18,000Newt84,000Amoeba670,000The number of genes vary between species.Common NameNumber of genesHuman30,000Mustard plant27,000Fruit fly14,000Bakers yeast6,000Does size matter?The genome of the onion is six times larger that the human genome!
Where are genes located?Each gene has a particular position, called a locus (plural=loci), on a specific chromosome. p or q denotes which arm of the chromosome. p denotes the short arm, and q denotes the long arm.
Gene locusThe chromosomal locus of a gene might be written "6p21.3"ComponentExplanation6The chromosome numberpp=short arm or q=long arm 21.3The numbers that follow the letter represent the position on the arm: band 2, section 1, sub-band 3. The bands are visible under a microscope when the chromosome is suitably stained. Each of the bands is numbered, beginning with 1 for the band nearest the centromere. Non-Nuclear GenesThere are also a very small number of genes located on the circular DNA of each mitochondria (mtDNA).There are many copies of the mtDNA in one mitochondrion.
Non-Nuclear GenesIn plants, there are also a very small number of genes located on the circular pieces of DNA in each chloroplasts (ctDNA).
Genotype and PhenotypeGenotype - the genes of a traitHomozygous - both alleles of a gene are the sameQQ stands for a homozygous dominant genotype (uppercase letters)qq stands for a homozygous recessive genotype (lowercase letters). Heterozygous means that each of the alleles are different (e.g. Qq, IAIB). Phenotype refers to the physical or observable traits of the individual. Genotype + Environment = PhenotypeLocation of GenesApart from the 22 pairs of autosomes, genes are also found on the sex chromosomes (X and Y). Genes are mainly found on the X-chromosome and very rarely on the Y-chromosome.The Y-chromosome contains the SRY gene which determines maleness of an embryo. Without the SRY gene, individuals develop as phenotypically female.XY phenotypic females are possible if the SRY gene or the receptors for the SRY gene product are faulty.
23-22X-Chromosome InactivationIn a normal female, one of the two X-chromosomes are randomly switched off (and turned into a Barr Body) in each of the somatic cells (e.g. tortoiseshell coloured cats)X inactivation occurs so that the female, with two X-chromosomes, does not have twice as many active X-chromosome genes as the male, who only possess a single copy of the X-chromosome.X inactivation in marsupials is always paternal (i.e. the fathers X is always switched off)Tortoiseshell Coloured Cats
Only female cats can be tortoiseshell. The gene for orange coat colour is on the X chromosome. In a heterozygous cat (Oo) one of the two X chromosomes are randomly inactivated giving the blotchy phenotype.Expression of AllelesComplete dominance this occurs when the phenotype of the heterozygous individual displays the dominant allele (e.g. the heterozygous Qq will exhibit a widows peak ).Incomplete dominance occurs when there is a blending of the two alleles in the heterozygous individual (e.g. an allele for red flowers and an allele for white flowers results in pink flowers)Codominance this occurs when the heterozygous individual expresses both alleles equally.
Genotypes and EnvironmentWhilst we have found that an individuals genotype is largely responsible for their phenotype, it is still important to keep in mind that the environment plays a major role too.For example, individuals who have the condition PKU, can live relatively normal lives on a special diet.Some flowers (e.g. hydrangea) can change colour depending on the pH (acidity) of soil.Genetic VariationTraits can be expressed with continuous variation (e.g. height) or discontinuous (e.g. ABO blood groups)
Discontinuous VariationDiscontinuous variation (e.g. Rhesus blood type) due to single gene affect.
PhenotypeRh+Rh-GenotypeDD and DdddContinuous VariationContinuous variation usually due to polygene traits (i.e. more than one gene involved)