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8/3/2019 Bio Genetic Ex.

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F 6 Biology - Ch 9: Heredity and Genetics

Name:____________________________( )

1

INTRODUCTION

Heredity - the tendency of an individual to resemble his parents

Variation - the differences between individuals of the same species,

e.g. height, intelligence, eye colour, hair colour, etc

Genetics - the study of the method of inheritance of 'characteristics' of living organisms

In sexual reproduction a new offspring is derived only from the gametes of its parents. Gametes

contain all the necessary

hereditary information.

9.1.1

THE WORK OF MENDEL

MENDEL'S BLEEDING EXPERIMENT

Mendel carried out cross-

pollination of two varieties of peaflowers. He collected the seeds

and sowed them the following

year. The characteristics of the

daughter plants were then

carefully examined.

He carried out a mono-hybrid cross of flower colour:

pure red and pure white

The F2 generation

gave rise to a ratio of approximately

3 reds: 1 white - monohybrid ratio

Mendel's 1st law:

The characteristics of an organism

are determined by internal factors

which occur in pairs. Only one of a

pair of such factors can be represented in a single gamete.

9.1.2

Interpretation of the results of

Mendel's breeding experiment

Alleles control the expression of flower colour: Red - R; White - r

R gene suppresses the effect of r gene: R gene is dominant;

r gene is recessive




Name:____________________________( )

2

9.1.3 Genetic Representation of the Monohybrid Cross

(see p 152 - 153 of text-book)

9.1.4 Genes and Alleles

Phenotype - external appearance of the organism,

e.g. red flower, white flower

Genotype - genetic composition of the organism,

e.g. RR, Rr or rr

A gene is the basic unit of inheritance.

It is a region of the chromosome or a length of the

DNA molecule which has a particular

function. Each gene may have two, or

occasionally more, alternative forms.

Each form of the gene is called anallele

.

96-I-7(a): Distinguish between gene and allele

(2 marks)

Homozygous (pure-breeding) -

individual with its genotype of a particular characteristic consists of two identical alleles,

e.g. RR or rr

It could be homozygous dominant (RR) or homozygous ___________ ( )

Heterozygous (hybrid) - genotype consists of two different alleles, e.g. Rr

A dominant gene can suppress the expression of its recessive allele,

e.g. Rr is red because the red allele R is ___________ over the white allele r

SOME OTHER DOMINANT & RECESSIVE CHARACTERISTICS IN PLANTS AND

ANIMALS

1. In man: tongue rolling - RR, Rr

eye colour - BB, Bb

skin pigment (albinism)- AA, Aa

2. In maize: colour of seeds in a cob - CC, Cc

9.1.5 Dihybrid inheritance (Mendel's Law of Independent Assortment)

Dihybrid inheritance refers to the simultaneous inheritance of two characters:

Each of a pair of contrasted characters may be combined with either of another pair. OR

Each member of an allelic pair may combine randomly with either of another pair.

TABLE 9.2 RESULTS OF MENDEL'S DIHYBRID CROSS

Seed shape

Round Wrinkled Total Approx. ratio

Seed Yellow 315 101 416 3 yellow

colour Green 108 32 140 1 green

Total 423 133

App. ratio 3 round 1 wrinkled

Approximate ratio - round yellow : round green : wrinkled yellow : wrinkled green

___ : ___ : ___ : ___




Name:____________________________( )

3

Genetic representation of the dihybrid cross

In the following list, “-“ represents either the dominant or the recessive allele.

R-G- = round, yellow seed 9 (315)

R-gg = round, green seed 3 (108)

rrG- = wrinkled, yellow seed 3 (101)

rrgg = wrinkled, green seed 1 (32)

Allowing for statistical error, Mendel's results were a reasonable approximation to the

expected 9 : 3 : 3 : 1 ratio.

9.2. The Test Cross

FINDING OUT THE GENOTYPE OF AN ORGANISM

- The genotype of an organism with recessive characteristic must be __________

- An organism having dominant characteristic can either be ________________ or

________________

For example: white flower -

red flower -

- If the organism is a self-pollinating plant, we can find out its genotype by self-pollination:

Case 1 - if all of the offspring are red flowered, the parent plant must be __

Case 2 - if a mixture of red-flowered & white flowered offspring, the parent plant must be ___

TEST CROSS for animals which are unisexual & cannot carry out self-fertilization.




Name:____________________________( )

4

Test Cross: For animals which are unisexual and cannot carry out self-fertilization.

A homozygous recessive brown mouse is used.

It is possible to perform a dihybrid test cross:

A round and yellow seed has 4 possible genotypes -

To determine its genotype, cross it with one with wrinkled green seed -

Possible genotypes of

round yellow seeds

Possible

gametes

Genotypes of offspring

crossed with wrinkled green

seeds(gamete=rg)

Phenotype

(type of seeds

produced)

½

9.3 Sex Determination

autosomes:

sex chromosomes (heterosomes):

homogametic sex:

heterogametic sex:

* Sex determination differ in other

organisms:

Male being XX, female being XY,

e.g.

Male being XO, female being XX,

e.g.

In fruit flies, the female is XX, the

male is XY. But the Y chromosome has a completely different shape:




Name:____________________________( )

5

9.4 Linkage

Linked genes are

All the genes on a single chromosome form a linkage group.

Under normal circumstances, all the linked genes remain together during cell division and so pass

into the gamete, and hence the offspring, together. They do not therefore segregate in accordance

with Mendel's Second Law of Independent Assortment.

9.4.1 Crossing Over and Recombination

It is known that genes for flower colour and fruit colour in tomatoes are on the same

chromosomes. Plants with yellow flowers bear red fruit while white flowers bear yellow fruit.Let R = allele for red fruit (dominant) and r = allele for yellow fruit (recessive)

W = allele for yellow flowers (dominant) and w = allele for white flowers (recessive)

Yellow flowers, red fruit x White flowers, yellow fruit

F1 generation: All yellow flowers and red fruit (WwRr)

If the F1 generation is self-pollinated, the following results should be expected:

Genotypes: WWRR : WwRr : WwRr : wwrr

Phenotypes: 3 yellow flowers, red fruit : 1 white flowers, yellow fruit




Name:____________________________( )

6

However, the following results were obtained in F2 if the F1 generation is self-pollinated:

Yellow flowers and red fruit - 68

Yellow flowers and yellow fruit - 7

White flowers and red fruit - 7

White flowers and yellow fruit - 18

** The new recombinants are the result of crossing over in prophase I of meiosis.

9.4.2 Sex Linkage

Sex linkage refers to the carrying of genes on the sex chromosomes. These genes determine body

characters and have nothing to do with sex. The X chromosome carries many such genes. The Y

chromosome has very few.

Crosses in fruitflies:

*Wild type - a term to describe and organism as it normally occurs in nature

*Reciprocal cross - a cross of the male and female with the same genetic features

but sexes are reversed

R = allele for the red eye (dominant gene)

r = allele for the white eye (recessive gene)

Red-eye female x white-eye male




Name:____________________________( )

7

Red-eyed female x red-eyed male:

Reciprocal cross: white-eye female x red-eye male

Reciprocal cross: red-eye female x white-eyed male:

Inheritance of red green colour-blindness:

* Carriers: The heterozygous females are not affected by the defect but are capable of passing the

recessive gene to their offspring.

Let X

B

represent the allele fornormal sight and

Xb represent the allele for

colour-blindness




Name:____________________________( )

8

The inheritance of haemophilia:

The inability of the blood to clot leading to slow and persistent bleeding, especially in the joints ---

a potentially fatal disease.

Haemophiliac females are highly improbable, and are unlikely to have children

because

Haemophilia is the result of an individual being unable to produce one of the many clotting factors,

namely factor 8 or antihaemophiliac globulin (AHG).

Any mutant recessive gene, such as that causing haemophilia, is normally rapidly diluted among

the many normal genes in a population. Its expression is rare but it occurrence is high in many

European royal families, e.g. England’s Queen Victoria. Why ?




Name:____________________________( )

9

9.5 Allelic Interaction

Codominance: when neither allele completely dominates the other

Multiple al leles : when more than two alleles exist for a given gene

9.5.1 Incomplete dominance/Co-dominance

In snapdragon: red flower x white flower → F1 with pink flowers

9.5.2 Partial Dominance

Sometimes both alleles express themselves in the phenotype, but one more so than another. This

an intermediate stage between complete dominance and codominance. There are many blends of

partial dominance which lead to a wide range of intermediate varieties between two extremes, e.g.

a cross between a black dog and a white dog give patches of predominantly black fur in their

offspring.

9.5.3 Multiple Alleles

In humans the inheritance of the ABO blood group is determined by a gene I which

has 3 different alleles. Any 2 of these can occur at a single locus at any one time - codominance

Allele IA

causes production of antigen A on RBCs

Allele IB causes production of antigen B on RBCs

Allele IO

causes no production of antigens on RBCs

Alleles IA

and IB

are codominant and allele IO

is recessive to both




Name:____________________________( )

10

Table 9.4 Possible genotypes of blood groups in the ABO system

The transmission of these alleles occurs in normal Mendelian fashion.

A cross between individuals of blood group A and certain individuals of blood group B may

produce offspring with any one of the 4 blood groups:

Paternity suits

- to show an individual could not possibly be the father

Dominance series

Coat colour in rabbits is determined by a gene C which has 4 possible alleles:

Allele CF determines full coat colour and is dominant to

Allele CCH which determines chinchilla coat and is in turn dominant to

Allele CH which determines Himalayan coat and is in turn dominant to

Allele CA which determines albino coat colour

There is a dominance series, and each type has a range of possible genotypes.

Inheritance follows the Mendelian fashion.

Table 9.5 Possible genotypes of rabbits with different coat colour




Name:____________________________( )

11

9.5.4 Lethal Genes

A lethal gene will cause death of the offspring when present in homozygous condition,

e.g. in mice: homozygous yellow embryo always dies

Let Y represent the allele for yellow fur (dominant)

and y represent the allele for grey/agouti fur (recessive)

9.6 Gene Interaction - the alleles of more than one gene (at more than one locus) interact

9.6.1 Simple Interaction

This occurs when a group of genes or a gene complex act together to determine a single character.

Inheritance with an intermediate form arises.

Example in humans - skin pigmentation controlled by two genes A and B with

AABB being darkly pigmented,

AaBB or AABb being dark brown,

AAbb or aaBB or AaBb being half-coloured,

Aabb or aaBb being light brown,

aabb being white.

Example in poultry (epistasis)- blending leads to new features:

comb shape in chicken with entirely new features

rose comb x pea comb → F1 all walnut type → interbreed to give 4 types of combs in F2




Name:____________________________( )

12

9.6.2 Polygenes

Many genes acting together are referred to as polygenes. For example, a character determined by

5 genes, each gene having a dominant or recessive allele. An

organism inheriting 5 dominant alleles will lie at one end of

the spectrum and one with 5 recessive alleles will lie at the

other. Between these extremes will a continuum of types

depending on the relative proportions of dominant and

recessive alleles. Polygenes give rise to continuous

variation.

A normal distribution of continuous variation characteristics

Choice of species for genetic crosses

Organisms favouring for genetic research are fruit fly ( Drosophila melanogaster ), mice and maize

plants combine most, if not all, the following factors:1. Easy to breed

2. Readily grown

3. Cheap and easy to feed

4. Small size

5. Short life cycle

6. Production of many offspring

7. Early sexual maturity

8. Obviously recognizable features

9. Sexual dimorphism




Name:____________________________( )

13

Exercise:

1. 2000-I-10

2. Distinguish between gene and allele. 96-I-7 (a))

3.Briefly explain the following terms: gene, allele, chromosome, locus. (8 marks) (80-II-5)

4. 81-I-4




Name:____________________________( )

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5. 82-I-1

6.

7. 84-I-1

D istinguish betw een a) test cross and back cross b) gene pool and genotype (4 m arks)

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