INTRODUCTION TO GENETICS
From Proteins to Mendel
Supplementary Materials
Teacher’s Reference
2.1.2. About blending theory of heredity
Blending theory was the commonly held belief that characteristics were mixed in
each generation. For example, breeding two horses, one with a light-coloured
coat, the other dark, would result in offspring that were all intermediate in coat
colour. If this held true, then eventually all organisms would become more alike
in each generation. Although this theory persisted for many years, it was
eventually supplanted by the work of Mendel and the modern geneticists.1
Pre-Mendelian theory of heredity proposing that hereditary material from
each parent mixes in the offspring; once blended like two liquids in
solution, the hereditary material is inseparable. And the offspring’s traits
are the result of this blend.
Individuals of a population should reach a uniform appearance after many
generations
Once traits blended, they cannot be separated out to appear again in later
generations
QUESTIONS TO THINK ABOUT AND DISCUSS AT THE END
Why do you think Mendel is known as the Father or genetics?” (Because
he discovered the basic underlying principles of heredity)
“Do you think that using mathematics to work out the data was usual in
Mendel’s time? (No, he was the first one to made use ot this scientific
language)
1 Blending inheritance is similar to the modern legitimate idea of incomplete dominance and the
terms are rarely, but incorrectly, used interchangeably by some. However, incomplete dominance
results in blending only of the phenotype, keeping the alleles within the heterozygote
distinct (and, thus still inheritable in successive generations), whereas the theory of
blending inheritance referred to an actual blending of the genetic material (i.e. in modern
terms, alleles would blend together to form a completely new allele).****
To be used when explaining incomplete dominance..
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INTRODUCTION TO GENETICS
From Proteins to Mendel
Supplementary Materials
Teacher’s Reference
What do you think Mendel carried out his breeding experiments with pea
plants? (Because he could observe inheritance patterns in up two
generations a year)
Think and compare how scientific findings are communicate and published
today and in Mendel’s time. (Here can be told that today the language of
science is English and that should help the communication. Remember
Mendel published his work in German. Relate this fact with other cases,
like Wegener…)
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INTRODUCTION TO GENETICS
From Proteins to Mendel
Supplementary Materials
Teacher’s Reference
2.1.3.Words Cards for Definition Team Game
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
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Hybrid
Genetics
Gregor Mendel
Blending theory
Purebred
Genes
Alleles
Genotype
Homozygous genotype
Heterozygous genotype
Phenotype
Recessive allele
Dominant allele
Principle of segregation
Principle of independent assortment
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INTRODUCTION TO GENETICS
From Proteins to Mendel
Supplementary Materials
Teacher’s Reference
2.1.4. Glossary of Mendelian Genetic Terms
1.
Alleles
alternate forms or varieties of a gene.
The alleles for a trait occupy the
same locus or position on homologous chromosomes and thus govern the
same trait. However, because they are different, their action may result in
different expressions of that trait.
2.
Blending theory
an
incorrect
19th
century
theory
about
the
inheritance
of
characteristics. It proposed that inherited traits blend from generation to
generation. Through his plant cross-breeding experiments, Gregor Mendel
proved that this was wrong
3.
Chromosomes
tread-like, gene-carrying bodies in the nucleus of a cell. Chromosomes are
composed primarily of DNA and protein. They are visible only under
magnification during certain stages of cell division.
Humans have 46
chromosomes in each somatic cell and 23 in each sex cell.
4.
Codominance
the situation in which two different alleles for a trait are expressed
unblended in the phenotype of heterozygous individuals. Neither allele is
dominant or recessive, so that both appear in the phenotype or influence
it. Type AB blood is an example. Such traits are said to be codominant.
5.
Cross-pollination
the
mating
of
two
genetically
different
plants
of
the
same
species. Usually, the term is used in reference to the crossing of two pure
breeding (homozygous) plants.
6.
Dominant allele
an allele that masks the presence of a recessive allele in the
phenotype.
Dominant alleles for a trait are usually expressed if an
individual is homozygous dominant or heterozygous.
7.
f1 generation
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INTRODUCTION TO GENETICS
From Proteins to Mendel
the first offspring (or filial) generation.
Supplementary Materials
Teacher’s Reference
The next and subsequent
generations are referred to as f2, f3, etc.
8.
Genes
units of inheritance usually occurring at specific locations, or loci, on a
chromosome. Physically, a gene is a sequence of DNA bases that specify
the order of amino acids in an entire protein or, in some cases, a portion of
a protein. A gene may be made up of hundreds of thousands of DNA
bases. Genes are responsible for the hereditary traits in plants and
animals.
9.
Genetics
the study of gene structure and action and the patterns of inheritance of
traits from parent to offspring. Genetic mechanisms are the underlying
foundation for evolutionary change. Genetics is the branch of science that
deals with the inheritance of biological characteristics.
10. Genotype
the genetic makeup of an individual. Genotype can refer to an organism's
entire genetic makeup or the alleles at a particular locus. .
11. Heterozygous
a genotype consisting of two different alleles of a gene for a particular
trait (Aa). Individuals who are heterozygous for a trait are referred to as
heterozygote.
12. Homologous chromosomes
chromosomes that are paired during the production of sex cells in
meiosis. Such chromosomes are alike with regard to size and also position
of the centromere. They also have the same genes, but not necessarily the
same alleles, at the same locus or location.
13. Homozygous
having the same allele at the same locus on both members of a pair of
homologous chromosomes. Homozygous also refers to a genotype consisting
of two identical alleles of a gene for a particular trait. An individual may be
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INTRODUCTION TO GENETICS
From Proteins to Mendel
Supplementary Materials
Teacher’s Reference
homozygous dominant (AA) or homozygous recessive (aa). Individuals who
are homozygous for a trait are referred to as homozygote.
14. Hybrids
offspring that are the result of mating between two genetically different
kinds of parents--the opposite of purebred.
15. Intermediate expression
the situation in which a heterozygous genotype results in a phenotype that
is
intermediate
between
those
resulting
from
the
homozygous
genotypes. The mid-range baritone male voice is an example.
16. Meiosis
cell division in specialized tissues of ovaries and testes which results in the
production of sperm or ova. Meiosis involves two divisions and results in
four daughter cells, each containing only half the original number of
chromosomes--23 in the case of humans.
17. Mendelian genetics
inheritance patterns which can be explained by simple rules of dominance
and recessiviness of genes.
18. Phenotype
the observable or detectable characteristics of an individual organism--the
detectable expression of a genotype.
19. Principle of independent assortment
Gregor Mendel's second principle of genetic inheritance.
It states that
different pairs of genes are passed to offspring independently so that new
combinations of genes, present in neither parent, are possible. In other
words, the distribution of one pair of alleles does not influence the
distribution of another pair. The genes controlling different traits are
inherited independently of one another.
20. Principle of segregation
21. Principle of segregation
Gregor Mendel's first principle of genetic inheritance. It states that, for
any particular trait, the pair of genes of each parent separate (during the
formation of sex cells) and only one gene from each parent passes on to an
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INTRODUCTION TO GENETICS
From Proteins to Mendel
Supplementary Materials
Teacher’s Reference
offspring. In other words, genes occur in pairs (because chromosomes
occur in pairs). During gamete production, the members of each gene pair
separate, so that each gamete contains one member of each pair. During
fertilization, the full number of chromosomes is restored, and members of
gene pairs are reunited.
22. Probability
the likelihood that a specific event will occur.
Probability is usually
expressed as the ratio of the number of actual occurrences to the number
of possible occurrences.
23. Punnett square
a simple graphical method of showing all of the potential combinations of
offspring genotypes that can occur and their probability given the parent
genotypes. See example below. Punnett squares are commonly used by
genetics counsellors to predict the odds of a couple passing on particular
inherited traits.
24. Purebred
offspring that are the result of mating between genetically similar kinds of
parents--the opposite of hybrid. Purebred is the same as true breeding.
25. Recessive allele
an allele that is masked in the phenotype by the presence of a dominant
allele. Recessive alleles are expressed in the phenotype when the genotype
is homozygous recessive (aa).
26. Sex cell
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INTRODUCTION TO GENETICS
From Proteins to Mendel
Supplementary Materials
Teacher’s Reference
a gamete, either a sperm or an ovum. Sex cells are produced by the meiosis
process.
27. Somatic cell
any cell in the body except those directly involved with reproduction. Most
cells in multicellular plants and animals are somatic cells. They reproduce
by mitosis.
28. Unit inheritance
Gregor Mendel's idea that the characteristics of parents are passed on to
descendants unchanged as units. In other words, the hereditary material
of any organism is made up of discrete units (now called genes).
29. Zygote
a "fertilized" ovum. More precisely, this is a cell that is formed when a
sperm and an ovum combine their chromosomes at conception. A zygote
contains the full complement of chromosomes (in humans 46) and has the
potential of developing into an entire organism.
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INTRODUCTION TO GENETICS
From Proteins to Mendel
Supplementary Materials
Teacher’s Reference
2.2.1. How to solve genetics problems
These steps can help organize student’s attack on a genetic problem.
and of course, unless student understand the terms, such as
homozygous, heterozygous, dominant, recessive, allele, and so on,
he/she cannot begin to think of working problems.
1. Read the problem.
2. Determine what is the relationship between the traits, dominant and which
are recessive, codominant...
3. Are any letters assigned to the genes? If not, you have to. We usually take
the dominant characteristic and use the first letter of that word. For
example, if brown hair is dominant over blond hair, we would pick B for the
dominant gene, and small b for the recessive normal allele. If the alleles are
codominant you do not need to use capital/low case letters.
4. Determine, if possible, the genotypes of the parents. In some problems this
information is given, or at least implied. Sometimes you have to deduce it
from other information given. Write it down so that you can remember what
it is, e.g. Bb.
5. Determine all the possible kinds of gametes that can be made by each parent.
Be careful; remember that a gamete can ordinarily receive only one gene of a
pair of alleles.
6. Make a Punnett square; using each of the gametes for one parent across the
top of each column, those of the other parent go vertically. If you have done
step 5 properly you shouldn’t have any trouble with this step.
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INTRODUCTION TO GENETICS
From Proteins to Mendel
Supplementary Materials
Teacher’s Reference
7. Work the cross carefully.
8. Now read the problem again. Find out exactly what it is asking for.
9. In most problems, these steps should get you through adequately. Some are
slightly altered – for example, if the genotype of one of the parents is
unknown, and that is what the problem wants you to discover. You may assign
that parent something like A? or ?? genotype and see if that helps. Put the
offspring genotypes in the square and work backward.
10. Finally, learn to translate a sentence as, “Mary is normally pigmented but had
an albino father”, into its logical consequence: “Mary is heterozygous for
albinism” and then into “Mary is Cc”.
How to write the genetic report
The report is divided up into three parts.
Part I: content of Drinkwater Family’s pedigree. (Students have to review
lesson 1.1. and probably re-read the story about the Drinkwater
family. They can look at the pedigree done in lesson 1.2. and from that
and with the help of the sheet “how to create a pedigree chart” they
can create the new one. This pedigree has to show:
a) All family members mentioned
b) All affected individuals
c) All known carriers and suspected carriers.
Part II: Paragraph about the Drinkwater Family's Haemophilia
Explain who has the disorder and who does not
Explain who in the family is a carrier and who could be a carrier
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INTRODUCTION TO GENETICS
From Proteins to Mendel
Supplementary Materials
Teacher’s Reference
Explain the risk of the next child having the disorder or being a
carrier of the disorder (a Punnett square may be used to help explain
this section)
What chromosome is the mutation is located in?
Part III: Information about Haemophilia. In order to complete this section
the student can revise the poster they did in lesson 1.2.
Explain how Haemophilia is inherited and what a sex-linked trait is
Explain the difference between haemophilia A and haemophilia B*
Explain the signs and symptoms of haemophilia
Explain the treatment options for haemophilia, including future
treatments being researched (hypothetical) or genetic screening…
How to create a Punnet square
Genetics should help to understand how can be predicted the likelihood of
inheriting traits. One of the easiest ways to calculate the probability of
inheriting a trait is the called Punnett square. Reginald Punnett was a 20th
century British geneticist. This square is a simple graphical way of discovering
all of the potential combinations (probability) of genotypes that can occur in
children, given the progenitor’s genotypes.
1. Draw a grid of perpendicular lines:
2. Write one parent’s genotype across the top and the other’s parent down
the left side. Example:
Father’s genotypes RR
Mother’s genotypes rr. Write only one letter in each box.
It does not matter which parent is on the side
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INTRODUCTION TO GENETICS
From Proteins to Mendel
R
Supplementary Materials
Teacher’s Reference
R
r
r
3. Fill in the empty boxes by copying the row- and columm-head letters
across or down.
r
r
R
Rr
Rr
R
Rr
Rr
4. Now read the “table”. In the example we crossed RR x rr (homozygous x
homozygous) and the 100% of the offspring will be heterozygous. Each
box will expressed 25% or ¼ of the offspring’s genotype. So if the
parents were heterozygous, the results will be 50% Rr, 25% RR and 25%
rr.
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