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Page 1: 2.1.2. About blending theory of heredity - XTEC · 2011-12-02 · 2.1.2. About blending theory of heredity Blending theory was the commonly held belief that characteristics were mixed

INTRODUCTION TO GENETICS

From Proteins to Mendel Supplementary Materials

Teacher’s Reference

Rosa Macaya IES Pobla de Segur 1

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

Rosa Macaya IES Pobla de Segur 2

� 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

Rosa Macaya IES Pobla de Segur 3

2.1.3.Words Cards for Definition Team Game

01 Hybrid

02 Genetics

03 Gregor Mendel

04 Blending theory

05 Purebred

06 Genes

07 Alleles

08 Genotype

09 Homozygous genotype

10 Heterozygous genotype

11 Phenotype

12 Recessive allele

13 Dominant allele

14 Principle of segregation

15 Principle of independent assortment

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INTRODUCTION TO GENETICS

From Proteins to Mendel Supplementary Materials

Teacher’s Reference

Rosa Macaya IES Pobla de Segur 4

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 Supplementary Materials

Teacher’s Reference

Rosa Macaya IES Pobla de Segur 5

the first offspring (or filial) generation. 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

Rosa Macaya IES Pobla de Segur 6

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

Rosa Macaya IES Pobla de Segur 7

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

Rosa Macaya IES Pobla de Segur 8

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

Rosa Macaya IES Pobla de Segur 9

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

Rosa Macaya IES Pobla de Segur 10

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

Rosa Macaya IES Pobla de Segur 11

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 Supplementary Materials

Teacher’s Reference

Rosa Macaya IES Pobla de Segur 12

3. Fill in the empty boxes by copying the row- and columm-head letters

across or down.

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.

R R

r

r

R R

r Rr Rr

r Rr Rr