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Meiosis and Variation. Chapter 9. Key Knowledge: m eiosis, gamete production, variation : mutations, genotype, phenotype; continuous, discontinuous variation patterns of inheritance in sexually reproducing organisms: . Male Gamete Production. Spermatogenesis
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Meiosisand Variation Chapter 9 • Key Knowledge: • meiosis, gamete production, • variation: mutations, genotype, phenotype; continuous, discontinuous variation • patterns of inheritance in sexually reproducing organisms:
Male Gamete Production • Spermatogenesis • Produced and stored in testes
Human Sperm Human Sperm 3 μm 5μm 50 μm Average ejaculate contains ~180 x 106 sperm (~60 x 106 sperm per mL)
Female Gamete Production • Oogenesis – produced in ovaries
Human Ova 100 μm
Human Gametes The human ovum has approximately 100,000 times the volume of a single human sperm. Ovum Sperm
Inherited Variation • Inherited 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.
Variation • Individuals 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).
Variation • Also 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 Genes • Random 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 Meiosis • Sometimes during meiosis,sisterchromosomes 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. • Allelesare 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.
Does 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 locus The chromosomal locus of a gene might be written "6p21.3"
Non-Nuclear Genes • There 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 Genes • In plants, there are also a very small number of genes located on the circular pieces of DNA in each chloroplasts (ctDNA).
Genotype and Phenotype • Genotype - the genes of a trait • Homozygous - both alleles of a gene are the same • QQ stands for a homozygous dominantgenotype (uppercase letters) • qqstands 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 = Phenotype
Location of Genes • Apart 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 SRYgene which determines ‘maleness’ of an embryo. • Without the SRYgene, individuals develop as phenotypically female. • XY phenotypic ‘females’ are possible if the SRY gene or the receptors for the SRY gene product are faulty.
X-Chromosome Inactivation • In 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-chromosomegenes as the male, who only possess a single copy of the X-chromosome. • X inactivation in marsupials is always paternal (i.e. the father’s 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 Alleles • Complete 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 Environment • Whilst we have found that an individual’s 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 Variation • Traits can be expressed with continuous variation (e.g. height) or discontinuous (e.g. ABO blood groups)
Discontinuous Variation • Discontinuous variation (e.g. Rhesus blood type) due to single gene affect.
Continuous Variation • Continuous variation usually due to polygene traits (i.e. more than one gene involved)