and his peas are the basis of modern genetic theory. Take time to learn this
building block on which genetic theory is based. It is a practical
demonstration of what happens as the result of the ideas in chapter 1 and
how you can use deduction to guess what might happen. Mendel was a monk that
lived in the 19th Century who gained lasting fame because of the work he
published based on inheritance patterns he observed in peas. He was able to
deduce from his experiments how genetic material behaved even though he did
not know about chromosomes.
careful to use pure breed pea varieties, which when mated to their own type
produced offspring that resembled the parents. He then used varying
characteristics, which sharply differed from each other: green or yellow
round, wrinkled or smooth and tall or short. Not a complete list but what
will be referred to. This enabled him to count the numbers in each group and
work out the ratios between them.
experiment he crossed a true breeding tall plant (TT) with a true breeding
short plant (tt).
of the 2 possible forms a gene may be. In this case a gene that results in a
alternate form of a gene. In this case it represents the gene that results
in a short plant
alternate forms are known as alleles.
the first generation offspring were as tall as the tall parents. The
offspring were then mated to each other and these produced offspring that
had ¾ tall and ¼ short. He got the same result whether he bred a tall female
to a short male or a short female to a tall male. The sex of the parent made
no difference as to what resulted in the offspring. The factor responsible
was not affected by the sex of the parents.
In another similar experiment he
added pollen (male) from a line of yellow peas (YY) to the flower (female)
from a green line of peas (yy). Instead of the offspring being intermediate,
all of the plants in the new generation looked like one of the parents and
not the other. They all had yellow peas not a blend of yellow/green. From
this generation, the eggs were fertilized by pollen from the same
individual. Unexpectedly, both the original colors, yellow and green,
reappeared in the next generation. Whatever had made green peas could still
do so even though it spent a generation within a plant with yellow peas.
This did not fit with the idea that the properties of each parent blended
together. Mendel also added up the numbers of yellow and green peas in each
generation. In the first generations (pure bred) all yellow. In the second
generation, three yellow to one green. The same ratio as the tall and short
plants (3/4 yellow and 1/4 green).
also made crosses using other characteristics and found that the same ratios
applied. The four ways pollen and eggs can be combined to apply (one quarter
yellow to yellow, one quarter green to green and one half yellow to green)
also apply to different characteristics considered together. He found plants
with yellow smoothed peas crossed with green wrinkled peas did not alter his
laws of color inheritance. There was no blending of color inheritance
influenced by shape. He deduced that completely separate genes were involved
for each characteristic. We can distinguish in the cross two sorts of
individuals, true breeding individuals with both alleles the same (called
homozygotes (YY) or (yy), and individuals in which the two alleles continue
to segregate (heterozygotes (Yy) or carriers. The gene that we are
considering is said to be homozygous or heterozygous respectively.
this Mendel deduced the fundamental law of genetics.
Mendel discovered can be put very simply.
two characteristics behaved completely independently of each other. He
called this the rule of
assortment. This could be explained if it
were assumed that each plant had two copies of the factor influencing the
trait. We call the factor responsible a gene. More than one form of the gene
can exist and we call these alternative forms alleles.
explained the results by suggesting that each plant contained two alleles
which did not blend together but which remained unchanged. In the
next generation the plants passed one or other allele at random into a
gamete to be combined with a gamete from the other parent. The non-blending
followed by separation into the next generation is the
Summary of what the results showed:
Although all the first generation offspring looked like the parents and had
the same phenotype (physical appearance) they had a different genotype
(genetic makeup). Unlike the parents they were not true breeding.
Pea color or height, he deduced was controlled by pairs of genes. Each
adult plant had two genes for pea color or height, but the pollen or egg
only one. On pollination a new plant with two genes was reborn since some of
the second-generation plants were green or short. The color or height was
determined by what genes the plant carried. In the original pure lines the
individuals carried either two yellow/tall or two green/short genes
depending on the lines they came from. When breeding to pure lines, the next
generation was identical to the parents.
Although these plants were yellow or tall they each carried the hidden gene.
The yellow gene being expressed in the appearance of the offspring that
still carries the green gene. The gene for yellow being dominant to the
green gene which is recessive. The gene for tallness being dominant to the
recessive short gene.
Both the male and female gametes must make an equal contribution to the
offspring because the reverse crosses gave the same result.
Since the male gamete is in the nucleus this is where the heredity material
must be. In the nucleus. The material is now known as genes.
Since the offspring receives genes from both parents, pairs of genes
controlled color/stem height. Although the genes had come together in the
first generation some of the offspring must have carried only one kind of
gene because they were true breeding. Therefore, when the first generation
plants produced gametes they must have only form of the gene. That is, the
genes for color or height must have segregated.
Mendel concluded that offspring are not the average of their parents and
that inheritance is based on differences rather than similarities.