Mitosis and Meiosis. See Mitosis and Meiosis on the class web page

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1 Mitosis and Meiosis Mitosis and Cellular Reproduction. A cell s hereditary material (DNA) is located on chromosomes in the cell s nucleus. In the process called mitosis, a cell s hereditary material is duplicated and the duplicate copies are distributed equally between two new cells. The stages of mitosis: Interphase Prophase Metaphase Anaphase Telophase Interphase See Mitosis and Meiosis on the class web page Meiosis. The genetic material (the genes [DNA]) is located on the chromosomes. The chromosomes come in pairs. Therefore, the genes come in pairs. Gametes = sex cells. Female cells: Ova (ovum) = eggs Male cells: Sperm. When gametes form, the pairs of chromosomes (genes) 1

2 are separated (segregated) into different gametes. Gametogenesis: The formation of gametes (ova and sperm). Meiosis: The process by which the members of pairs of chromosomes are separated from one another when gametes form. Each gamete has half the number of chromosomes that an ordinary cell has. A full set of chromosomes constitutes the diploid number of chromosomes. Half of a set (one chromosome from each pair) constitutes the haploid number of chromosomes. Example: An ordinary human cell has 46 chromosomes (23 pairs). 46 = diploid number. Each human gamete that forms (egg or sperm) will have 23 chromosomes (one member of each of the 23 pairs). 23 = haploid number. See Mitosis and Meiosis on the class web page 2

3 Classical (Mendelian) Genetics Mendel s Law of Segregation: When gametes form, the pairs of genes (alleles) are segregated from one another so that each gamete has only one member of each pair of genes (alleles). Actually, it s the chromosomes that are segregated from one another (the genes are on the chromosomes). Mendel didn t know about chromosomes or the chemical nature of the genes. Background information and terminology by way of an example: 1. Some people can curl their tongue; others cannot! it s hereditary. 2. Since the alleles (genes) come in pairs. Everyone has a pair of alleles concerned with tongue curling. 3. There actually are two alleles (alternative genes) concerned with this trait. There s an allele that gives the ability to curl the tongue ( it can be represented as T ) and an allele that makes curling the tongue impossible ( this alternative allele can be represented as t ). 4. Each person has a pair of alleles so their genetic 3

4 make up with respect to this trait can be represented as TT, Tt, or tt. 5. The genetic make up of a person is called the genotype so TT, Tt, and tt are genotypes. 6. If a person has the genotype TT or Tt, he/she will be able to curl his/her tongue. T is a dominant allele and t is a recessive allele. 7. The physical expression of the genotype is called the phenotype. There are two phenotypes for the tongue-curling condition (a person can either curl the tongue or not) and three genotypes (TT, Tt, and tt). 8. If both of a person s alleles are the same he/she is homozygous. If the two alleles are different, he/she is heterozygous. The concept of how something like tongue curling can be illustrated with the help of a Punnet Square. Suppose a man and a woman are heterozygous for tongue curling. With respect to this trait, what genotypes and phenotypes could be expected among their children? Tt X Tt 4

5 Tt X Tt T 0.5 t 0.5 T 0.5 TT 0.25 Tt 0.25 t 0.5 Tt 0.25 tt % of the couple s children can be expected to be homozygous dominant (TT). 50% of the couple s children can be expected to be heterozygous (Tt). 25% of the couple s children can be expected to be homozygous recessive (tt). Let s consider another example of an inherited human characteristic. Some people have free earlobes. That s a dominant condition and we can represent the allele for free earlobes as F. The allele for attached earlobes is recessive so it can be represented as f. The genotype of a person with free earlobes can be either FF or Ff while the genotype of a person with attached earlobes only can be ff. Suppose a couple is heterozygous for free earlobes (Ff). With respect to earlobes, what can you expect among their children? Ff X Ff 5

6 Ff X Ff F 0.5 f 0.5 F 0.5 FF 0.25 Ff 0.25 f 0.5 Ff 0.25 ff 0.25 The situation is the same as with tongue curling; only the letters have been changed. Mendel s Law of Independent Assortment: When gametes form, the pairs of alleles segregate independently of one another. To illustrate Mendel s second law, we can combine the examples of tongue curling and earlobes! after all, individuals have both a tongue which they can or cannot curl and earlobes which are either free or attached. Suppose both individuals in a couple are heterozygous for tongue curling and free earlobes. With respect to these two traits, what can you expect among their children? TtFf X TtFf 6

7 Again, a Punnet square can be helpful. The couple s genotypes are as follows: TtFf X TtFf TF Tf tf tf TF TTFF TTFf TtFF TtFf Tf TTFf TTff TtFf Ttff tf TtFF TtFf ttff ttff tf TtFf Ttff ttff ttff It s not likely that the couple will have 16 children but, with respect to these two traits, there are 16 possible combinations so the probability is based on 16. The chances are 9/16 that a child will be a tongue curler with free earlobes. 9/16 T-F-. The dash means that the allele could be either the dominant or the recessive form: TTFF, TtFF, TTFf, or TtFf. The chances are 3/16 that a child will be a tongue curler with attached earlobes. 3/16 T-ff. The chances are 3/16 that a child will be a non tongue curler with free earlobes. 3/16 ttf-. The chances are 1/16 that a child will be a non tongue curler with attached earlobes. Ttff 7

8 Some other patterns of inheritance: 1. Incomplete dominance. 2. Multiple alleles 3. Inheritance of sex and sex linkage. 8