A Brief HISTORY OF HEREDITY

• The idea that traits are passed down is an old one.

• Characteristics that seem to run in families were noticed ever since humans were able to notice them.

• The earliest recordings of this are from Hippocrates, who lived from 460 – 370 BCE.

• He wrote many aspects on medicine (Hippocratic oath is named after him).

• He mentioned the inheritance of baldness, crossed eyes, epilepsy, and an eye disease that develops in middle age (glaucoma).

Pierre Maupertuis (1698-1759)

• recorded instances of unusual inheritance such as polydactyly—the presence of extra fingers, toes, and nipples.

• He also reported instances of albinism, a lack of pigment which causes the skin to be very fair and the hair white.

• Sometimes the eyes appear violet because the blood vessels in the irises are more easily seen, or light blue.

• Albino animals can be albino too.

• Because they are relatively rare, seeing one was often considered important.

Black bear with a partially albino cub  

Mother with albino daughter.

Jean Baptiste Lamarck(1744-1829)

• suggested that acquired traits could be passed down.

• That is, if you father was muscular, you would be muscular.

• If your mother was educated, you would be smart. (There is more to what he suggested, but we will revisit Lamarck in a later unit).

• Enter Gregor Mendel, a humble monk born in 1822.

• By 1856 he had founded the scientific study of heredity, but never received the credit for it within his life-time.

Mendel’s Laws of Heredity(Ch 10 – P 253-279)

A Brief Bio

• Mendel was born in 1822, attended university of Vienna where he studied chemistry, biology and physics.

• He left before graduating, probably for health reasons.

• Entered the Augustinian monastery in Brno, Austria, and with the support of the abbot, began his investigation of the inheritance of certain traits in pea plants (Pisum sativum).

• Modern genetics began in the 1860’s with Mendel’s discovery of the principles of genetics in breeding garden peas.

• Mendel chose the garden pea for the following reasons: – They were easy to grow because they

reproduce and grow quickly – They were readily available in many

distinguishable varieties with contrasting traits– He was able to exercise strict control over

which plants were bred with another.

Anatomy of a pea flower (p 254 & 642)

• Garden peas reproduce sexually. • This means they produce male and female

sex cells called GAMETES. • The male gamete forms in the pollen

grain. • The female gamete forms in the ovule. • The male gamete unites with the female

gamete in the process of FERTILIZATION.

Flower Anatomy

Anther and Stigma

• The transfer of pollen from the male part of the plant to the female part of the pea plant is called pollination. (Fig 24.12 p 647)

• Petals of pea flowers almost completely enclose the stamen (male) and carpel (female) so in nature, plants self-fertilize (self pollination).

• Mendel ensured self-pollination by covering the flower with a bag so that no pollen from another plant could reach the carpel.

Flower – Life Cycle

The Genetics of Garden Peas

Mendel chose to follow 7 traits in pea plants (pg 256)• Flower colour: purple or white• Seed colour: yellow or green• Seed shape: round or wrinkled• Pod shape: inflated or constricted• Pod colour: green or yellow• Stem length: tall or short• Flower position: axial (side) or terminal (tips)

See Fig 10.3 on p 256

• He picked these traits because they were distinct and easily followed.

• The traits in bold were later found to be significantly more common.

• Over the next 8 years, Mendel conducted experiments and maintained detailed records of his results.

• He designed simple experiments that allowed him to observe the inheritance of one trait at a time.

• He used his mathematics to formulate conclusions based on his results.

How did Mendel Conduct his Experiments?

• He wanted to study what would happen when bred (crossed) different varieties of pea plants.

• Mendel always started with true-breeding varieties (pure breeding)—plants that showed the same trait over several generations. For example, if he worked with tall plants, he used plants from populations of plants that had been tall for many generations and had always produced tall offspring. – Recall that he covered the plants with a bag to

prevent cross pollination

• To cross-fertilize (cross pollinate), Mendel would do the following: (see Fig. 10.1 on p 254) – He would cut off immature stamen to prevent

self-fertilization. This plant was now a female plant.

– He dusted the carpel with pollen from another plant.

– The carpel would develop into a pod, containing the seeds he would later plant.

– When these seeds grew into offspring plants, he observed them for certain traits.

• In this way, Mendel was certain of the parentage of the new plants.

• In this way, Mendel also developed some vocabulary. (See Fig. 10.2 p 255)

The Generations: • Analyzing a single trait in this manor is

called a MONOHYBRID CROSS• The parental plants are called the “P

Generation” • The offspring from the cross are called the

“F1 Generation” (F for filial from the Latin word for son)

• When two F1 generation plants are allowed to pollinate, this new offspring set are called the F2 generation.

• What would happen if a purple flowered plant were crossed with a white flowered plant?

Explaining the Monohybrid Cross

• In pea plants, there are two different forms of the traits that Mendel studied.

• Later, these forms of the same trait were called ALLELES—there is an allele for purple on the genes of one plant, an allele for white on the genes of the other plants.

• In reproduction through meiosis, genes are passed onto offspring through each parent’s gametes.

• Each offspring receives one copy of each gene from each parent– I.E. an offspring receives an allele for a trait

from each parent. (HOMOLOGOUS PAIR)– Problem Solving Lab 10.2 p 264– See Figure 10.10 p 265

• Mendel did the same type of experiment with all 7 traits and discovered the same patterns:– In the F1 generation, one trait was always

hidden, or masked. That hidden trait would show up in the F2 generation in about ¼ of the plants.

– He called the hidden traits in the F1 generation RECESSIVE and the trait that exerted itself was called DOMINANT. In pea plants, purple flowers are dominant to white flowers.

• To show this, the capital letter for the allele trait represents the dominant allele.

• The lowercase letter represents the recessive allele:

P – purple allele p – white allele – A true breeding purple flower: PP– A true breeding white flower: pp

Homozygous

• These represent the P generation in Mendel’s experiment.

• True breeding plants have both alleles the same and are called HOMOZYGOUS. We say “homozygous dominant” (PP) or “homozygous recessive” (pp)

Heterozygous

• HETEROZYGOUS plants show the dominant trait but carry one dominant allele and one recessive allele (often called CARRIERS of the recessive trait).

• A heterozygous purple flower: Pp• These represent the F1 generation of

Mendel’s experiment.• Mendel also used the term HYBRID for

plants that were heterozygous for a trait.

See Fig. 10.5 p 258

Mendel also studied TWO traits at the same time. This type of cross is more complex. It is called a DIHYBRID CROSS. More on that later.

The CONCLUSION:

• Mendel’s use of mathematics allowed him to formulate conclusions based on his results.

• These conclusions are known as Mendel’s Laws or Principles:

1. The Rule of Unit Factors

Traits are controlled by unit factors (genes) that exist in pairs in individuals. One factor (gene) is passed on to the offspring from each parent so that each offspring has a pair of unit factors (genes).

2. The Rule of Dominance:

When two unlike factors for a trait are present in an individual, one masks the expression of another. That is, one factor is dominant to the other, which is recessive. (p 256)

3. The Law of Segregation

During gamete formation in meiosis, factors separate (segregate) randomly so that each gamete receives one form of the trait or the other—each gamete can have only one allele for a trait. The passing down of alleles occurs randomly

4. The Law of Independent Assortment

During gamete formation, segregating pairs of factors (genes) assort independently of each other.

E.g. Colour factors are not linked to height factors—these sort independently of each other.

• In 1865 Mendel published his findings. • The scientific community did not seem to grasp

the significance of his findings, as a result, it was largely ignored.

• Mendel later become abbot (1868) and died in 1884.

• In 1900, three scientists working independently rediscovered and confirmed Mendel’s laws of heredity—Hugo de Vries, Carl Correns and Erich von Tschermak-Seysenegg all gave credit to Mendel.

• Mendel never knew the world would come to embrace him as the father of modern genetics.

Assignment

• Worksheet packet– Pages 9, 10, 18-22, 25 & 26