Name:___________________________
SBI3U
Date:_________________________
Understanding Inheritance
- many of the organisms we see today are the result of careful _______________, like dog breeds
this breeding was based on _________________ ______________ observations and the offspring
closely resembled their parents
- certain traits were identified as highly ___________________, and breeders used careful decision
making and planning so that offspring could have as many of these traits as possible
Early Ideas About Inheritance
Aristotle (384 - 322 B.C.E.)
- proposed the first widely accepted theory of inheritance, called pangenesis
- in this theory the egg and sperm consist of ________________, called pangenes, from all parts of
the _______________, upon fertilization the pangenes develop into the parts of the _________ from
which they were formed
- this theory was accepted for hundreds of years, although no experiments were done to test it
Antony van Leeuwenhoek (1632 - 1723)
- discovered living sperm in semen, using a single-lensed _____________________ he designed
- he believed that he saw a complete, ______________________ ______________ in the head of
the sperm which came from the father, but developed in the mother
Blending
- during the 1800s, the breeding of ____________________ plants became popular and it was
observed that offspring had characteristics of both parents
- the idea of blending became the working theory of inheritance - the characteristics of the parents
blended in the offspring
- they thought that the blending was __________________ so that the original characteristics of the
parents would _______ _______________ in future generations
Gregor Mendel (1822 - 1884)
- Mendel was a monk in an Austrian monastery who sorted out some of the
mysteries of inheritance through his detailed studies of the pea plant
Why the Pea Plant?
- pea plants were available in many _________________ and they show
many ______________
- in genetics, a trait is a specific __________________________ or
feature of an organism, such as the flower colour of a plant
- the laws that Mendel developed through his work on pea plants formed
the foundations of our modern theory of inheritance
Mendel's Pea Plants
- pea plants reproduce through ______________ reproduction, but they
usually __________________________, so the same plant provides both
the male and female gametes
- the plants that Mendel selected for his study self-fertilized to produce offspring with
__________________ traits generation after generation and are therefore called true breeding
- he obtained his true breeding plants through selective breeding techniques until he had plants that
______________________ to produce plants
with predictable traits
- Mendel also needed to be able to control his
experiments, he did this by selectively
__________________ a female gamete with a
specific male gamete, in a process called a
cross (also known as cross pollination)
The Results of Mendel's True-Breeding
Cross
- for his experiments Mendel chose _________
traits that were expressed in two distinguishable
forms (see table)
- Mendel began each experiment with truebreeding plants, which he called the parental or
P generation
- True-breeding plants with one form of a trait
were crossed with true-breeding plants with the
other form of the _________ trait (ex. yellow
seeds with green seeds)
- Offspring of crosses between plants of the P
generation were called the first filial or F1
generation
Monohybrid Cross
- it is called a monohybrid cross because only _____
_________ is monitored in the cross and hybrid plants
(those made from parents of differing forms of traits)
are produced
- as shown in the diagram the monohybrid cross
produced all yellow seeds in the F1 generation, the
green form seemed to have disappeared
- for all seven of the traits studied, Mendel noticed that
when true-breeding organisms with contrasting forms
of a trait were crossed, the offspring
expressed _______ _____ _________ of
that trait
Results of Mendel's F1 Crosses
- Mendel also studied pea plants that result from
a cross between plants of the F1 generation,
these offspring were called the second filial or F2
generation
- Mendel allowed the plants from the F1
generation to ______________________ and
then grew the seeds for the F2 generation (see
results in diagram)
- based on these observations Mendel realized
that the green form had not in fact
________________, instead it was unexpressed
- the ratio of plants with yellow seeds to plants
with green seeds in the F2 generation 6022:2001
or 3.01:1, this is very close to a ratio of 3:1, this
is called the Mendelian ratio
The Law of Segregation
- Mendel concluded that there must be _____
hereditary factors for each trait he studied; today
we refer to those factors as alleles
- so each of Mendel's pea plants had two alleles
for seed colour, for the F1 generation even though all of the seeds were yellow, they all had a copy of
each form of the gene for seed colour - one from each parent
- Yellow is the dominant seed colour an green is the recessive seed color
- This work led to the law of segregation:
- inherited traits are determined by pairs of "factors" or two alleles of a gene, these two
alleles segregate into each of the gametes during meiosis, so that each gamete
contains one of the alleles, upon fertilization each offspring contains one allele from
each of its parents
- the form of the trait that is expressed in an individual _________________ on whether they inherit
the dominant or recessive alleles for the trait, if a dominant allele is present, only the dominant form
will be expressed, therefore expression of the __________________ form requires that an individual
had two recessive alleles for the trait
Genotype & Phenotype
- alleles are often represented using upper case and
lower case letters, a _________________ allele is
represented by the upper-case form of the first letter
of the allele's description, the same letter in lower
case is used to represent the ______________ allele
- The allele for purple blossom colour is represented
by B, and the allele for white blossom colour is
represented by b
Possible combinations: BB, Bb and bb
- the combination of alleles an organism has is called
the genotype, the expression of that genotype is
called the phenotype
- A pea plant with a purple blossom could have a
genotype of Bb or BB, but it would have a phenotype
of purple
- an individual with two identical alleles (BB or bb) is
said to be homozygous for that trait, while an
individual with two different alleles (Bb) is said to be
heterozygous for the trait
Analyzing Genetic Crosses: Punnett Square
- a British geneticist named Reginald Punnett devised a simple, visual technique to help analyze the
results of crosses, which is now called a Punnett square
1) When working with one gene, make a box and divide it into 4 squares
2) above the squares write the genotype of the gametes of one parent
3) beside the squares write the gentoypes of the gametes from the other parent
4) inside each square, write the results of the cross
`
The Inheritance of Two Traits: Dihybrid Crosses
- Mendel designed a second set of experiments that involved following ______ traits instead of just
one - his goal was to find out if the pattern of inheritance for one trait, such as plant height had any
___________ on the pattern of inheritance for another trait, such as flower colour
- this time Mendel started out with plants that were true bred for two traits and each parent differed in
both traits, since two traits are involved, this type of cross is called a dihybrid cross
Mendel's Experiment
- Mendel crossed ______________________ plants
that produced yellow, round seeds (YYRR) with truebreeding plants that produced green, wrinkled seeds
(yyrr)
- Mendel discovered that all the F1 generation plants
displayed the _____________ forms of the trait - they
all produced round, yellow seeds, the same result he
obtained when following only one trait in
_________________ crosses
- when Mendel let the plants self-fertilize, as he had in
the monohybrid crosses, the ________________
forms of each trait reappeared in the F2 generation
(see Punnett square)
Results of Mendel's Experiment:
- in the F2 generation, the ratio of plants with yellow,
round seeds to plants with yellow, wrinkled seeds, to
plants with green, round seeds to plants with green,
wrinkled seeds was 315:101:108:32, which is close to
9:3:3:1
Developing the Law of Independent Assortment
- Mendel performed and repeated numerous _______________ ____________ and he discovered
that no matter which two traits he followed, he always obtained the same ration of 9:3:3:1
- in any dihybrid cross each of the parents could produce _______ possible gametes, therefore there
are 16 possible genotypes produced
- the results that Mendel obtained are expected only if one trait has ___ __________________ on
the inheritance of the other trait, this is the law of independent assortment: " the alleles for one
gene segregate or assort independently of the alleles for other genes during gamete formation"
The Chromosome Theory of Inheritance
- when Mendel performed his
experiments he had no idea how the
traits were passed from generation to
generation, the process of ________
and ___________________ had not
yet been discovered
- in the early 1900's scientists began
to take note of similarities between
how Mendel's traits segregated and
assorted and how chromosomes
behave during meiosis
Sutton Links Mendel's Work to
Chromosome Segregation
- in 1902 Walter Sutton studied the
process of synapsis (segregation of
homologous chromosomes) and
migration of sister chromatids during
meiosis I and meiosis II
- Sutton realized that the behaviour of
chromosomes during Meiosis was
__________ to the behaviour of the
factors in Mendel's experiments with
pea plants: the distribution of
_________________ into developing
gametes follows the pattern for two
alleles of a gene, according to
Mendel's law of ________________
- During gamete formation, alleles
segregate just as homologous
chromosomes do, Sutton proposed
that _________ are carried on
chromosomes, this work formed the
basis for the chromosome theory of
inheritance: "genes are located on
chromosomes, and chromosomes
provide the basis for the segregation
and independent assortment of
alleles."