Genetics

Thomas Hunt Morgan – Lexington, KY native andgraduate of U.K in 1886.

Revolutionized the study of genetics

Inheritance

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

• Genetics – the scientific study of heredity.

Gregor Mendel

• 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.

Fertilization

• 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

• 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

• 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

Cross-Pollination

• Mendel wanted to produce seeds by joining male and female cells from different plants

• He had to prevent self-pollination so he cut off the male parts (ouch) and then dusted pollen from another plant onto the flower

Traits• 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.

Father, Son, Hybrid• 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

Genes and Alleles

• 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.

Genes and Dominance• 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.

Dominance - Clarification• 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

F1 Cross• 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

F2 Generation

Principle of Segregation• 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.

Segregation of Alleles

• Capital T’s represent the allele for Tallness – dominant allele = capital letter

• Lowercase t’s represent the allele for shortness – recessive allele = lowercase letter.

Gregor Mendel

• Song About Gregor Mendel