Thomas Hunt Morgan –
Lexington, KY native and graduate of U.K in 1886.
Revolutionized the study of genetics

• Every living thing has a set of characteristics inherited from its parents – heredity.
• Genetics – the scientific study of heredity.

• Austrian monk named
Gregor Mendel is considered the grandfather of genetics.
• Worked in monastery and taught high school
• Also tended monastery garden and his worked changed biology forever.

• Gregor Mendel worked with garden peas
• Each flower produces pollen which contains male’s reproductive cells – sperm
• The female portion of the flower produces egg cells
• Male and female sex cells = gametes.
• During sexual reproduction, male and female reproductive cells join, a process called fertilization.

• Fertilization produces a new cell = embryo
• Pea flowers self-pollinate, meaning that the sperm cells fertilize egg cells in the same flower.
• These seeds inherit all of the characteristics of their parents.
• In effect they have a single parent

• True-breeding organisms are those that produce offspring identical to themselves.
• A stock of tall plants would produce tall plants
• A stock of short plants would produce short plants
• Some plants bred only green seeds, others only yellow seeds

• Mendel wanted to produce seeds by joining male and female cells from different plants
• He had to prevent selfpollination so he cut off the male parts (ouch) and then dusted pollen from another plant onto the flower

• Mendel studied 7 different pea plant traits
• A trait is a specific characteristic, such as seed color, that varies between individuals.
• Mendel crossed plants with each of the seven varying characteristics with contrasting plants.

• The original pair of plants is called the P (parental) generation
• Their offspring are called the F1 (first filial) generation.
– Filius and filia are Latin for “son” and “daughter”
• The offspring of crosses between parents of different traits are called hybrids.
P

• Did the traits blend in the offspring?
• No, not at all. All of the offspring had the trait of only one of the parents. The other trait seemed to have disappeared completely.
• Mendel made two conclusions:
– 1. Inheritance passed from one generation to the next. Now we call these factors passed genes.
– 2. Each trait had two forms. These forms of each gene are called alleles.
• For example: the gene for plant height has two forms: one that produces tall plants and another that produces short forms. Short and tall are alleles of the plant height gene.

• The principle of dominance states that some alleles are dominant and others are recessive.
• An organism with a dominant allele for a trait will always exhibit that form; it is dominant.
• An organism with a recessive allele for a trait will only exhibit that form when the dominant form is absent.
• In Mendel’s pea, the allele for tall plants was dominant over the allele for short plants.

• Just because an allele of a gene is dominant, doesn’t mean it is necessarily superior or even more common, it is simply just dominant.
• Achondroplasia (dwarfism) is a dominant form
• Huntington’s disease, a neurological disorder, is dominant

1
• Mendel asked another question: had the recessive alleles disappeared or were they still present in the F1 plants?
• So he allowed all seven kinds of F1 hybrids to produce an F2 generation by self-pollination.
• He found that in the F2 generation, the recessive traits reappeared, in about ¼ of plants

2

• After viewing his results, Mendel suggested that the alleles for height, color, etc. segregated from each other during the formation of gametes.
• When each F1 plant flowers and produces gametes, the two alleles segregate from each so that each gamete carries only a single copy of each gene.
• Therefore, each F1 plant produces two types of gametes – those with the allele for tallness and those with the allele for shortness.

• Capital T’s represent the allele for Tallness
– dominant allele = capital letter
• Lowercase t’s represent the allele for shortness – recessive allele = lowercase letter.

• Song About Gregor Mendel