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Biology 2014-2015

Meiosis and Human Genetics

SWS Biology
SWS Life Science

Sec. 11-4, 14-1, 14-2


Sec. 11-4 Meiosis

  • Genes are located on chromosomes in the cell nucleus.
  • Each organism MUST inherit a single copy of every gene from both its "parents".
  • Each gamete (egg/sperm) contains just ONE set of genes.


Chromosome Number

  • 1/2 of the total # of chromosomes comes from the male and 1/2 of the total # of chromosomes comes from the female.
  • A cell, which contains both sets of homologous chromosomes, is diploid.

                    Diploid means "two sets" or 2N

      In humans, the diploid number is 46 or 2N = 46

  • The gametes of sexually reproducing organisms contain a single set of chromosomes is haploid.

      Haploid means "one set" or N.

      In humans, the haploid number is 23 or N = 23



  • the reduction by division of the number of chromosomes by half in the sex cells (egg/sperm)

Meiosis I : The Reduction Division

  • Before Meiosis I, each chromosome is replicated.

Prophase I

Early prophase

  • Homologs pair
  • Crossing over occurs

Late prophase

  • Chromosomes condense
  • Spindle forms
  • Nuclear envelope fragments
  • Crossing-over is an exchange of alleles between homologous chromosomes and produces a new combination of alleles.

Metaphase I

  • Homolog pairs align along the equator of the cell.

Anaphase I

  • Homologs separate and move to opposite poles.
  • Sister chromatids remain attached at their centromeres.

Telophase I

  • Nuclear envelopes reassemble
  • Spindle disappears
  • Cytokinesis divides cell into two


Meiosis II : The Equational Division


Prophase II

  • Nuclear envelope fragments
  • Spindle forms.

Metaphase II

  • Chromosomes align along equator of cell

Anaphase II

  • Sister chromatids separate and move to opposite poles.

Telophase II

  • Nuclear envelope assembles
  • Chromosomes decondense
  • Spindle disappears

Cytokinesis divides cell into two.


Results of Meiosis

1)      Gametes

2)    Four haploid cells

3)    One copy of each chromosome

4)    One allele of each gene

5)    Different combinations of alleles for different genes along the chromosome


Spermatogenesis:  Sperm Formation

      Stem cells in testes divide mitotically to create a pool of spermatocytes.

      Meiosis produces four spermatids.


Oogenesis: Ovum (Egg) Formation


      One of four meiotic products becomes an ovum.

      The three remaining meiotic products are polar bodies.

      Occurs in the ovaries

      The mature egg has a rich storehouse of nutrients to nourish the young organism that develops after it is fertilized.



         Fertilization is the joining of sperm and ovum.

         Meiosis II in the ovum is completed at the time of fertilization forming one ovum and one polar body.

         Following fertilization, chemical reactions occur preventing additional sperm from entering the ovum.


Human Heredity

Section 14-1 and 14-2


Human Chromosomes

         Each chromosome is composed of a single, tightly coiled DNA molecule.

         The two DNA strands are homologus (duplicates) and are held together by the centromere.

         While they are still attached, the duplicated chromosomes are called sister chromatids.

         Chromosomes can be categorized as two types:

v     Autosomes which are non-sex chromosomes that are the same number and kind between sexes.

v     Sex chromosomes which determine if the individual is male or female.

         Sex chromosomes in the human female are XX and those of the male are XY.

         Males produce X-containing and Y-containing gametes; therefore males determine the sex of the offspring.

         A karyotype is a picture of chromosomes which are grouped together in pairs.

         All animals have a characteristic number of chromosomes in the somatic or body cells called the diploid (or 2n) number.

         The gametes or sex cells (egg & sperm) contain half the number of chromosomes as a body cell; known as the haploid number (n) of chromosomes.

         A pedigree is a chart of the genetic history of family over several generations.

         Scientists or a genetic counselor would find out about your family history and make this chart to analyze.


Blood Type Genes

         Human blood comes in a variety of genetically determined blood groups.

         The best known genes for determining blood types are:

      ABO group

      Rh blood group (two alleles - positive and negative)

         Rh+ is dominant/Rh- is recessive


Sex-linked Traits

         Traits (genes) located on the sex chromosomes

         Sex chromosomes are X and Y

         XX genotype for females

         XY genotype for males

         Most sex-linked traits carried on X chromosome