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Meiosis and Variation

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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Meiosis and Variation

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  1. Meiosisand Variation Chapter 9 • Key Knowledge: • meiosis, gamete production, • variation: mutations, genotype, phenotype; continuous, discontinuous variation • patterns of inheritance in sexually reproducing organisms:

  2. Male Gamete Production • Spermatogenesis • Produced and stored in testes

  3. Human Sperm Human Sperm 3 μm 5μm 50 μm Average ejaculate contains ~180 x 106 sperm (~60 x 106 sperm per mL)

  4. Female Gamete Production • Oogenesis – produced in ovaries

  5. Human Ova 100 μm

  6. Human Gametes The human ovum has approximately 100,000 times the volume of a single human sperm. Ovum Sperm

  7. 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.

  8. 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).

  9. 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.

  10. Independent Assortment of Genes • Random orientations of non-matching chromosomes lead to independent assortment of genes into different gametes.

  11. Crossing Over (Recombination) Crossing-over involves exchange of DNA between chromatids of sister chromosomes, allowing genes to recombine.

  12. 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

  13. 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.

  14. After sequencing of genomes began in 2001, it became apparent that a more complex organism does not necessarily have more genes.

  15. The number of genes vary between species.

  16. Does size matter? The genome of the onion is six times larger that the human genome!

  17. 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.

  18. Gene locus The chromosomal locus of a gene might be written "6p21.3"

  19. 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.

  20. 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).

  21. 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

  22. 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.

  23. 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)

  24. 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.

  25. 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.

  26. 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.

  27. Genetic Variation • Traits can be expressed with continuous variation (e.g. height) or discontinuous (e.g. ABO blood groups)

  28. Discontinuous Variation • Discontinuous variation (e.g. Rhesus blood type) due to single gene affect.

  29. Continuous Variation • Continuous variation usually due to polygene traits (i.e. more than one gene involved)

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