Genetics and Heredity
 More than 100 trillion cells make up an adult human. And it is an
intricate process to make all of these cells.
 Heredity is the passing of information from one generation to the
next. The orderly expression of heredity information is required for
development.
 Karyotype – is all of the chromosomes within an organism (e.g. in
humans our karyotype consists of 23 pairs of chromosomes or 46
chromatids)
Mendelian Genetics and Beyond
 The term Mendelian genetics refers to certain basic inheritance
patterns. The term is to honour the Austrian monk Gregor Johann
Mendel (1822-1884).
 Mendel’s Discovery
o Mendel chose the garden pea of his studies because of its
easy cultivation, its ability to control it pollination, and the availability of variety with differing traits.
o He controlled pollination by moving pollen from one plant to another; thus he knew the parentage of
the offspring in his experiments.
o A suitable trait would be one of ‘true-breeding’, heritable (passable from parent to offspring), and well
defined (easily seen)
o The plants providing and receiving pollen were the parental generation, designated (P)
o In due course, seeds formed and were planted. The resulting new plants constituted the first filial
generation (F1)
o In some experiments the F1 were allowed to self pollinated and thusly they were called the F2
generation
 Experiment 1
o Included a monohybrid cross – one in which the parents where both hybrid for a single trait (the
offspring of genetically different parents)
o He took pollen from plants of a true-breeding strain for dented seeds and placed it on the stigmas of
flowers of true-breeding spherical seeds. He also formed the reciprocal.
o In both cases the F1 plants from the cross were spherical.
o In the self-pollinating F2 plants they were ¾ spherical and ¼ dented.
o Mendel observed that the spherical trait was dominant because it was expressed over the dented trait,
which he called recessive.
 Terminology for Mendelian Genetics
o Because there was not a blending of traits which one would have predicted. Mendel purposed a
particulate theory; in which heredity carriers are present as discrete unites that retain their integrity in
the presence of other units. His units are now called genes.
o The physical appearance of a character is its phenotype. And the genetic constitution is called the
genotype.
o In the example above there are 2 phenotypes (dented and spherical) and 3 genotypes (Ss, ss, SS). The
different forms of a gene (S and s) are called alleles.
o If they have 2 copies of the same allele it is said to be homozygous (ss and SS), and if they have one of
each they are called heterozygous (Ss).
 Segregation of Alleles
o
o
o
According to Mendel’s model each F1 individual produced gametes, the alleles segregate (separate) so
that each gamete receives only one member of is pair of genes (Meiosis). Thus these gametes combine
to form the F2 generation.
Mendel’s First law (Law of Segregation)
 Alleles segregate from one another during the formation of gametes
Doing this can be simplified using a Punnett Square.
The Test Cross
 Because phenotypes and genotypes do not always match, (SS, Ss have the same phenotype) we can not
determine between 2 genotypes.
 So we use a test cross to test whether an individual is a homozygous or heterozygous genotype.
 In a test cross we cross the individual in question with a individual known to be homozygous for the recessive
trait.
 If the individual is heterozygous the results will be ½ and ½ for each trait.
 If the individual is homozygous the results will be 100% domination trait.
Independent Assortment of Alleles
 What about those crosses between 2 parents that differ at 2 or more loci (places on the chromosome). When a
double heterozygote (AaBb) makes gametes. To answer these question Mendel preformed a series of dihybrid
crosses (crosses made between parents identically heterozygous at 2 loci)
 Mendel’s Second Law (Law of independent assortment)
o Alleles of different genes assort independently of one another during gamete formation