1

Mendelian genetics:

Gene: a specific sequence of nucleotides (or heriditaryunit) that codes for a particular polypeptide

Allele: an alternate form of a gene

Homozygous: having two copies of the same allele of a gene.

Heterozygous: having two different alleles of the gene in question.

Phenotype: the physical appearance or manifestation coded for by a genotype.

Genotype: the genetic make-up of an organism.

Mutations:

1) Somatic mutations: occur in somatic (body) cells and can be passed on to daughter cells (those that result from mitotic divisions), but are not passed on to sexually-reproduced offspring.

2) Germ line mutations: mutations that occur in the cells that give rise to gametes (usually during the process of meiosis). These are often passed on to sexually-reproduced offspring.

2

Mutations (cont.):

1) Aneuploidy: failure of chromosomes to separate during anaphase I or II of meiosis leading to gametes with one extra or one fewer of a particular chromosome than a normal gamete.

2) Polyploidy: individuals with cells having one or more complete extra sets of chromosomes (triploids, tetraploids, etc.). This occurs either via autopolyploidy (malfunction during meiosis) or allopolyploidy (hybridization involving diploid gametes from different species).

Central Dogma, revisited…

DNA↓

RNA↓

Protein

3

The Universal Genetic Code

codons Amino acids coded for by specific codons

Point mutations: change in a single nucleotide.

Silent mutations: nucleotide change does not affect amino acid sequence.

4

Redundancy in Genetic Code: different codons code for same amino acid.

Missence mutations: point mutations that change codons and amino acid sequence and, in turn, the protein produced.

5

Nonsense mutations: point mutations that create a Stop codon in place of an “active” codon and halts translation.

Frame-shift mutation: the insertion of a nucleotide into a normal sequence; this alters all the following codons beyond the insertion.

6

Types of chromosomal mutations:

1) Deletions

2) Duplications

3) Inversions

4) Translocations

Deletion: removal of chromosomal segment

Duplication and deletion occur when homologs break at different places and unequally swap parts during crossing-over.

7

Inversions: the flipping of chromosomal segments.

Reciprocal translocations: the exchange of segments between non-homologous chromosomes

Gregor Mendel’s experiments with peas

Cross two parental plants with different characteristics.

Gather mature seeds and grow them.

8

Cross-breed the offspring from those seeds.

Analyze the seeds from the above crosses for seed characteristics.

Both parents are homozygous; one dominant (SS, smooth seeds) and the other recessive (ss; wrinkled seeds).

F1 generation are all heterozygous (Ss), but appear smooth seeded.

F2 generation are ¼ homozygous dominant,½ heterozygous & ¼ homozygous recessive.

9

S S

s

s

Homozygous dominant = SS; heterozygous = Ss; homozygous recessive = ss; Phenotypes: both homozygous dominant and heterozygous individuals have smooth seeds; homozygous recessives have wrinkled seeds.

Ss Ss

Ss Ss

Punnett Square: parental cross

Punnett Square: F1 generation

S

s

S s

SS Ss

Ss ss

Genotypes: ¼ homozygous dominant, ½ heterozygous, ¼ homozygous recessives. Phenotypes: ¾ smooth, ¼ wrinkled.

10

Phenotypic ratio:9:3:3:1

Dihybrid cross: a cross involving two loci (2 genes) each with two alleles.

Incomplete dominance: where one allele is not expressed preferentially to another when they are combined in the heterozygote state.

11

Incomplete Dominance: when heterozygotes have a phenotype that is intermediate between the two homozygous conditions.

Codominance: when both alleles are expressed in the heterozygous individuals.

Example: blood types.

12

Pleiotrophy: when a single allele causes more than one distinguishable phenotypic effect.

Epistasis: when the phenotypic effects of one gene are influenced by another gene.

Many characteristics (height, skin color, etc) are the product of the interaction of many genes. These interactions usually produce a wide variation in trait expression (quantitative traits).

Many genes have multiple alleles (not just one or two).

13

Gregor Mendel (July 20, 1822 - January 6, 1884). Moravian monk (ordained as priest, August 6, 1847). Discoverer of the fundamental principles of "mendelian" genetics.

14

Exercise: Work in partners or small groups (tables, perhaps).

Beads represent alleles (i.e., black = dominant allele, white = recessive). Two blacks will equal the homozygous dominants, a black and white the heterozygotes, two white beads the homozygous recessive condition).

With beads combined in equal numbers in a jar, choose beads and assign characters to a hypothetical plant.

Example:Black beads White beads

purple flwrs white flwrs

Axial flowers terminal flwrs

yellow seeds green seeds

smooth seed wrinkled seeds

Tall stems short stems

pod inflated pod constricted

Construct ten plants based on all the above characteristics.

Calculate and record the ratio of each set of characteristics (i.e., purple flwrs vs white flwrs; yellow seeds vs green seeds, etc).

Now do the same with 50 plants.