Download Les 2 Hist. of Heredity

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