Download Early Beliefs and Mendel

4.1
hybrids: offspring that differ from their
parents in one or more traits. Interspecific
hybrids result from the union of two different
species.
Early Beliefs and Mendel
The idea that biological traits are inherited existed long before the mechanisms
of inheritance and gene interaction were understood. Stone tablets crafted by the
Babylonians 6000 years ago show the pedigrees of successive generations of
champion horses. Other carvings from the same period show the artificial crosspollination of date palms. Early records kept by Chinese farmers provide evidence
of methods used for improving different varieties of rice. The selection of desired
traits was based on keen observation and, to a great extent, trial and error.
Early naturalists made assumptions about some incredible cross-species
hybrids. The giraffe, for example, was thought to have been a cross between a
leopard and a camel. (The fact that camels and leopards are not compatible did
not seem to deter the theory.) The banana was thought to have been a hybrid of
the acacia and the palm.
Humans also mated selectively to produce desirable traits. The ancient
Egyptians encouraged the intermarriage of royalty to preserve bloodlines. For
example, Cleopatra married her younger brother. Plato, a Greek philosopher of
the early 4th century B.C., called for the segregated mating of the elite. To maintain a line of strong warriors, the ancient Spartans practised infanticide, killing
babies with undesirable characteristics.
Pioneer of Genetics: Gregor Mendel
Figure 1
Gregor Mendel was an Austrian monk whose
experiments with garden peas laid the foundation for the science of genetics.
One of the classic scientific experiments on inheritance was performed by an
Austrian monk named Gregor Mendel (1822–1884) during the mid-19th century
(Figure 1). Mendel’s work with garden peas not only explained the mechanism of
gene inheritance in plants, but provided a basis for understanding heredity in general. When Mendel’s work was rediscovered many years later, it provided the
missing piece in the theory of how organisms survive and reproduce.
Why did Mendel choose the garden pea on which to perform his work? First,
he observed that garden peas have a number of characteristics that are expressed
in one of two ways (see Figure 2). For example, some garden peas produce green
characteristics
dominant
trait
recessive
trait
round
wrinkled
yellow
green
inflated
constricted
green
yellow
purple
white
seed shape
dominant
trait
recessive
trait
side of stem
end of stem
tall
short
flower
position
seed colour
pod shape
pod colour
Figure 2
The seven characteristics Mendel studied
in his experiments with garden peas. Flower
colour and seed colour are correlated. Plants
with white flowers produce seeds that are
yellow, and plants with violet-purple flowers
produce seeds that are green.
130 Chapter 4
stem
length
flower colour
4.1
seeds (peas), while others produce yellow seeds. Some plants are tall, while others
are short. Mendel also noticed different flower positions on the stem and different flower colours. The fact that there were only two ways for each trait to be
expressed would make it easy to see which traits had been inherited from generation to generation.
A second reason for using garden peas is the way the plant reproduces.
Garden peas are both self-fertilizing and cross-fertilizing. Fertilization occurs
when pollen produced by the stamen, the male part of the plant, attaches to the
pistil, the female part (Figure 3). The pistil consists of the stigma, style, and
ovary. The pollen grains fertilize the egg cells in the ovary. This process is called
pollination. In self-fertilization, pollination occurs within one flower and the
traits of the offspring are easily predicted. Mendel cross-pollinated the pea
plants rather than allowing them to self-pollinate. He made sure to use purebreeding plants, that is, plants that always produce identical offspring. For
example, tall plants produce only tall plants. If any offspring in any generation
was not tall, then Mendel did not consider the parent plant to be pure and did
not use it in his experiment. He transferred the pollen from one plant to the
pistil of another plant, thus combining the male and female sex cells of different
plants. To ensure that the recipient plant didn’t pollinate itself, he first removed
its anthers. The pollen present then had to originate from the donor plant,
resulting in seeds produced from cross-pollinated plants (Figure 4).
stamen
filament
style
anther
ovary
pollen
stigma
pistil
Figure 3
The structure of a flower
Mendel’s Experiments
Mendel’s predecessors had hypothesized that the crossing of different traits
would create a blend. According to this theory, crossing a plant that produced
round seeds with one that produced wrinkled seeds would result in slightly wrinkled seeds. However, Mendel proved that this was not the case. When he crossed
the pollen from a plant that produced round seeds with the eggs of one that produced wrinkled seeds, the offspring were always round. Did this mean that the
pollen determines the seed coat? To test this idea, Mendel crossed the pollen from
a wrinkled seed plant with the eggs from a round seed plant. Once again, all the
offspring were round. In fact, the round trait dominated, regardless of whether
the trait came from the male (pollen) parent or the female (seed) parent.
Mendel repeated the procedure for other characteristics. He discovered that
one trait always dominated another, whether the sex cell came from the male or
female part of the plant. Tall plants produced tall offspring when cross-pollinated
with short plants; likewise, plants that had yellow seeds produced offspring with
yellow seeds when cross-pollinated with plants that had green seeds. Mendel reasoned that things called factors control the traits of a plant. The factors were later
transfer pollen from pollen
parent to seed parent
remove anthers
from seed parent
Figure 4
The donor plant is also known as the pollen
parent and the recipient is also known as the
seed parent.
Genes and Heredity
131
alleles: two or more alternate forms of a
gene. The alleles are located at the same
position on one of the pairs of homologous
chromosomes.
dominant: alleles of this type determine
the expression of the genetic trait in offspring
recessive: alleles of this type are overruled
by dominant alleles, which determine the
genetic trait
called genes. He assumed that the genes control the inheritance of particular
traits, such as seed colour and plant stem height. He also realized that there are
alternate forms of a gene. Today, the alternate forms of a gene are called alleles.
Green and yellow are expressions of the different alleles for seed colour. Tall
stems and short stems are expressions of the different alleles for stem height. In
garden peas, the traits that were expressed most often were considered to be
dominant and those expressed less frequently were recessive. The allele for a
yellow seed is dominant over the allele for a green seed; the allele for tall stems
is dominant over the allele for short stems.
Mendel cross-pollinated many plants and kept track of all the results. For
each type of cross, he recorded the number of offspring that exhibited the dominant trait versus the recessive trait. He created a system of symbols to show what
traits were passed to offspring. In this system, letters are used to represent traits.
Uppercase letters stand for dominant traits, and lowercase letters stand for recessive traits. For the dominant trait of yellow seeds, Y represents the allele for yellow
seeds; y represents the allele for green seeds, the recessive trait. Today, Mendel’s
system is still in use.
Mendel continued his experimentation by crossing two hybrid plants with
round seeds from the first generation. He referred to the first generation as filial
generation one, or F1 generation. The word filial comes from the Latin for son.
Both of these F1 plants contain R and r alleles, one from each of their parents.
This makes them hybrids. Remember, the R represents the round allele, while the
r represents the wrinkled allele.
You might predict an equal number of round and wrinkled offspring in the
second, or F2, generation. However, this is not the ratio Mendel discovered when
the two hybrids were crossed. He was astonished to find that 75% of the offspring
expressed the dominant round trait, while only 25% expressed the wrinkled trait.
How can these results be explained?
Figure 5 shows what happens when the sex cells, or gametes, from the F1
generation recombine to form an F2 generation. All members of the F1 generation are round, but wrinkled offspring appear during the F2 generation. Any
members of the F2 generation with an R allele will be round because the round
allele is dominant over the wrinkled allele. To be wrinkled, the offspring must
Rr
Meiosis occurs.
Each gamete has one
of the homologous
chromosomes.
Figure 5
The result of crossing two hybrid pea plants
with round seeds from the first generation
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R
Rr
r
R
r
RR
Rr
Rr
rr
round
round
round
wrinkled
F2 generation inherits
alleles from the gametes
of the F1 generation.
4.1
Try This
Activity
Creating a Personal Profile
Table 1 lists human traits controlled by dominant and recessive alleles.
Table 1
Trait
eye colour
hair colour
hairline
freckles
earlobe
hair texture
eyesight
eyelashes
nose line
fingers
Rh blood factor
ear rim
thumb joint
finger hair
folded hands
tongue rolling
clenched fist
chin dimple
blood type
eyes
Dominant
brown or black or green
brown or black
pointed on forehead
present
suspended
curly
near or farsighted
long
convex tip
6 fingers
positive Rh factor
curled rim
last joint bends out
present
left thumb over right
can be rolled into U shape
two wrist cords
dimple in middle
type A, B, AB
astigmatism
Recessive
blue or grey
blonde or red
straight across forehead
absent
attached to head
straight
normal vision
short
concave or straight
5 fingers
negative Rh factor
not curled rim
last joint is straight
absent
right thumb over left
cannot be rolled
three wrist cords
no dimple
type O
no astigmatism
Copy Table 2. Use information in Table 1 to complete your personal
profile. Which additional traits can you include?
Table 2
Trait (use the
letter indicated)
eye colour
E/e
hairline
L/l
earlobe
T/t
ear rim
R/r
freckles
F/f
thumb joint
J/j
finger hair
P/p
tongue rolling Y/y
folded hands D/d
nose line
N/n
hair colour
H/h
chin dimple
G/g
clenched fist K/k
Appearance or
Dominant or
physical condition recessive
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Possible
genetic makeup
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Genes and Heredity
133