HEREDITY (OR HOW TRAITS ARE PASSED ON FROM ONE GENERATION TO THE NEXT)

INTRODUCTION

• There was very little knowledge of inheritance prior to the 1860s (with the work of Gregor Mendel).
• During the late1600s-early 1700s, the spermists and ovists presented ideas about how inheritance may occur.  The improvement of lense grinding by Leeuwenhoek (1632-1723) allowed scientists using microscopes to observe the sperm and eggs of animals.  The spermists thought they could see a small human in human sperm.  Thus they believed that when the sperm fertilized the egg and the embryo developed in the womb, the small human form (called a homunculus) was simply nurtured and grew to term.  Therefore the baby would inherit the characteristics of the father.  I rival school of thought soon developed by scientists who thought they could see a small human form in the egg rather than the sperm, thus they were call ovists. The ovists believed that during fertilization of the egg, the sperm only stimulated the growth of the egg and that the developing embryo inherited its characteristics from the mother.  The spermists and ovists thought the process would be similar for other animals.Of course, the work of plant breeders (particularly of ornamental flowers) quickly showed these ideas of how traits are passed on were not accurate.  Gardeners found that traits were passed on regardless of whether they were associated with pollen (which carries male sex cells) or the egg (which carries female sex cells).
• Blending Inheritance.  While writing The Origin of Species, Darwin realized that one of the major problems with his theory of natural selection was explaining how traits are passed on from one generation to the next. The following direct quote from Darwin (1859) illustrates the problem: "The laws governing inheritance are for the most part unknown.  No one can say why the same peculiarity in different individuals of the same species, or in different species, is sometimes inherited and sometimes not so."
• The critics of Darwin's theory of natural selection quickly pointed this out and stated that even if beneficial heritable traits were to arise in a population (by mutation) they would be quickly blended out in the reproductive process from one generation to the next (and next, etc).  However, Darwin devoted three chapters of The Origin of Species to discussing this topic and presenting evidence that characters do not blend out from one generation to the next.  For example, a certain trait may disappear in one generation, only to reappear in a later generation.  How could this be if traits were blended out.  However, Darwin's critics were not convinced and still preached this concept of "blending out" of heritable traits from one generation to the next.  The science of genetics would eventually come to the rescue of Darwin's theory.

GREGOR MENDEL AND GENETICSGregor Mendel (1823-1884)

• Critics of natural selection said that Darwin and Wallace could not account for the origin of variation and how variation is maintained in populations.
• Gregor Mendel (1823-1884), an Austrian Monk (and called the Father of Genetics), solved this problem and founded the science of genetics. {Go to these websites and read about Mendel: 1)  Gregor Mendel (1823-1884) , 2) Gregor Mendel's Legacy }
• Mendel did experiments with true breeding, self-fertilizing pea plants to establish that variable characters could be passed on to offspring and would not be blended out by combination with other characters.  Mendel did his work in the 1860s and his results could have helped Darwin and Wallace in their arguments for natural selection.  However, Mendel published his results in an obscure journal that did not draw much attention.  He did send Darwin a copy of his paper, but evidently Darwin did not read it.  In fact, Mendel's results went almost unnoticed by the scientific community and was independently rediscovered about 1900 by several researchers.
• Mendel's Pea Experiments: Mendel looked at characters like height, flower color, seed coat color, seed shape, etc. in his pea experiments.  Pea plants normally self-fertilize (self-pollinate). Mendel mechanically (surgically) removed the anther (male flower part that contains the male sex gamate - pollen) so he could cross-fertilize peas with different characteristics.  Typically, in self-fertilization pollen from the anther falls on the stigma (female flower part that produces the eggs).  The pollen contains sperm nuclei which fertilizes the egg, forming a zygote.  The zygote (having recieved genetic material from the male part and female part) develops into an embryo.  The embryo is coated with an envelope of nutrients and a coat to form a seed.  The seed can then develp into a new individual.
•      When Mendel crossed purple flowered pea plants with white flowered pea plants all the offspring were purple in the first generation cross (F-1 cross).  Mendel then let the F-1 generation self-fertilize to give an F-2 generation.  In the second generation (F-2), white flowers would again reappear, with a ratio of about 3 purple to 1 white (Mendel did a large number of trials).
•      Mendel treated the results mathematically and based on the outcomes of his experiments he concluded that traits must appear in pairs.  These pairs must combine and recombine according to laws of probability. GO TO THIS WEBSITE AND READ ABOUT MENDEL'S EXPERIMENTS, MONOHYBRID CROSSES, PUNNETT SQUARES, PHENOTYPES, AND GENOTYPES: Mendel's Genetics.
•      Mendel's traits (he called them Elemente) we now call genes.  Different expressions of genes (such as purple flowers verus white flowers) we now refer to as alleles.  Mendel stated four laws of inheritance, restated as follows:
• MENDEL'S LAWS OF INHERITANCE:
• 1) Mendel concluded that genes occur in pairs, each member of a pair is called an allele (however, there may be several alleles for a particular character trait; example for eye color: brown, blue, green, etc.).
• 2) Mendel concluded tha a pea seed got one allele from the pollen and one allele from the ovule (egg in the stigma).  In general, when sex cells are formed, the two alleles of each pair separate from one another, and each sex cell recieves only one allele of each pair.  This is called the Law of Segregation.
• 3) When two alternative forms of the same gene (i.e., two different alleles) are present in an individual, often only one of the alleles is expressed.  This is called the Law of Dominance.
• (Things are not always this simple, there are many cases of incomplete dominance and the crosses will have intermediate charateristics)
• 4) Mendel concluded that, if two or more separate characteristics are considered in a cross, flower color, tallness, etc., each trait is inherited without relation to other traits.  Thus all possible combinations of independently inherited characteristics will occur in the sex cells (gamates).  This is called the Law of Independent Assortment (however, as we now know, this is only true for genes located on different chromosomes).

	P	p
P	PP	Pp
p	pP	pp

• Example of a Punnett sqaure of a F-2 cross (letting F-1 peas self-fertilize).  This results in phenotypes of 3 purple pea flowers and 1 white pea flower (ratio of 3:1 for phenotypes; so 75% purple phenotypes and 25% white phenotypes).  However, the genotypes will be 1 PP, 2 Pp, 1 pp (ratio of 1:2:1 for genotypes, so 25% or 1/4 are PP; 50% or 1/2 are Pp; 25% or 1/4 are pp).  This is the result of a simple monohybrid cross in the F-2 generation of one gene with two possible alleles.  PP is said to be homozygous dominant; Pp is heterozygous; and pp is homozygous recessive.
•      The importance of Mendel's work is that factors (genes) controlling characteristics (traits) are transmitted as discrete entities, and even though not always expressed, are not lost.
•      Thus some variation is accounted for by alternate expression of genes and variation can be maintained.

GENES AND CHROMOSOMES

• Only sex cells pass on genetic information, thus variations and mutations of genes can only be transmitted by the sex cells.
• Genes are located on chromosomes (composed of DNA) - thread-like structures in the cell nucleus.
• The number of chromosomes is specific for a particular species, thus varies among different species.
• Humans have 46 chromosomes.  Horses have 64.
• Chromosomes always occur in pairs, so humans have 23 pairs whereas horses have 32 pairs.
• Sex cells have only one-half of the number of chromosomes as do body cells.  Sex cells undergo a type of division called meiosis (see page 43 in text and diagram below).  This reduces the number of chromosomes to one-half (called the haploid number).  Sex cells end up with one each of the paired chromosomes.  Body cells undergo a different type of cell division called Mitosis (see page 43 in text and diagram below).  In mitosis each daughter cell has the same number of chromosomes as the parent cell (called the diploid number).
• COMPARISON OF MEIOSIS AND MITOSIS:
• 
• From:  Meiosis at http://www.accessexcellence.org/AB/GG/comparison.html
• Each chromosome is made up of thousands of genes.
• Paired chromosomes are said to be Homologous Chromosomes. Study the diagram below to see homologous chromosome pairs, allelic genes vs. non-allelic genes, etc.
• 
• Thus for every characteristic in a simple monohybrid cross, one allelic gene is supplied from each parent (or from each sex gamate)  (in many cases it is much more complex than this and several genes may control a specific trait).  However, in a simple monohybrid cross and based on the laws of probablity, we can predict the possible combinations.
• 

MENDELIAN GENETICS AND TEST CROSSES

MENDELIAN GENETICS AND POPULATIONS - The Hardy - Weinberg Equilibrium Principle

• The Hardy-Weinberg Equilibrium Principle assumes random combination of alleles (i.e. random mating), no selection, and no additions or changes to gene pool frequencies of alleles by migration or mutation.  The principle also assumes that no genetic drift is occurring (thus we have a large population).  In other words, the population is not evolving.  If the Hardy-Weinberg Equilibrium is violated in a population, then the population is evolving.

MENDEL AMENDED

• Gene Linkage (Many Genes, Few Chromosomes)
• Multiple Alleles (One Gene, Many Alleles)
• Multiple Genes - Polygenes (Many Genes, one trait)