National 5 - Unit 2 - Multicellular Organisms...National 5 Biology 2.4 Variation and Inheritance Pupil Course Notes Page 1 of 22 Duncanrig Secondary School 2017 Multicellular Organisms

National 5 Biology 2.4 Variation and Inheritance Pupil Course Notes

Page 1 of 22 Duncanrig Secondary School 2017

Multicellular Organisms

Sub Topic 2.4 Variation and Inheritance

On completion of this sub topic I will be able to state that:

Variations are differences that exist between members of a species or different

species.

Genes are short sections of DNA that code for proteins by specifying the order

in which amino acids must be joined together.

Combining genes from separate parents contributes to variation within a

species.

Alleles are different forms of a gene.

Discrete variation (single gene inheritance) is shown in characteristics which can

be grouped into categories.

Discrete variation (single gene inheritance) is found in characteristics that are

coded for by a single gene with a limited number of alleles.

Continuous variation (polygenic inheritance) is found in characteristics which

show a continuous range of possibilities between a minimum and a maximum

value.

Continuous variation occurs because several different genes influence the same

characteristic. Such characteristics are said to be polygenic.

State examples of characteristics which can be described as discrete or

continuous variation.

Phenotype is the physical appearance of an organism for a particular

characteristic.

Genotype is the term given to describe the alleles that an organism possesses

for a particular characteristic.

When two different alleles for a particular characteristic are present the

dominant allele will show its effect in the phenotype of an organism.

When two different alleles for a particular characteristic are present the

recessive allele will be masked and will not show its effect in the phenotype.



When an organism has two alleles the same for a particular characteristic the

organism is said to be homozygous for that characteristic.

When an organism has two alleles which are different for a particular

characteristic the organism is said to be heterozygous for that characteristic.

Identify the P (parental), F1 (first filial) and F2 (second filial) generations in a

monohybrid cross.

Understand that the phenotypes of the F1 produced from a homozygous cross

are all uniform (the same).

Predict the proportions of the phenotypes of the F2 offspring produced in a

monohybrid cross.

Explain a monohybrid crosses in terms of the genotypes produced.

State that the reason why the predicted ratios in a monohybrid cross are not

always observed is due to the random process of fertilisation and that sample

sizes in the observed crosses may be too small.



Discrete variation

Fingerprint

Ear lobe shape

Tongue rolling

(roller / non roller)

Blood group

Eye colour

Variation

Members of a species can interbreed and produce fertile young. Individuals in a

species are not all identical, but show slight differences. The differences that occur

within a population (a group of organisms of the same species) are called variation.

These genetic variations occur because the fusion of gametes, which takes place

during fertilisation, is a random process.

There are two types of variation: continuous and discrete.

Discrete Variation (single gene inheritance)

Discrete variation describes a characteristic that can be separated into distinct

categories or groups. Examples of discrete variation include eye colour, fingerprints,

blood groups, ability or inability to tongue roll, attached or detached ear-lobes.

Discrete variation is found in characteristics that are coded for by a single gene with

a limited number of alleles.

http://www.google.co.uk/imgres?imgurl=http://robertronsson.co.uk/wp-content/uploads/2012/08/eye-colour-chart.jpg&imgrefurl=http://robertronsson.co.uk/?p=501&h=300&w=400&sz=25&tbnid=0hCS4sik6shjnM:&tbnh=87&tbnw=116&prev=/search?q=eye+colour+images&tbm=isch&tbo=u&zoom=1&q=eye+colour+images&usg=__Pc0PMjwD6g0aqfRAmuQfMYijt3o=&docid=WD1skQNi0w_lwM&sa=X&ei=GEyKUYLJF-et0QXziIG4Dw&ved=0CDUQ9QEwAg&dur=1313



Continuous variation

Height Tail length in mice

Shell diameter

Discrete variation can be shown by a bar graph.

Continuous Variation (polygenic inheritance)

Continuous variation is found in characteristics which show a continuous range of

possibilities between a minimum and a maximum value.

Continuous variation occurs because several different genes influence the same

characteristic. Such characteristics are said to be polygenic.

Examples of continuous variation include height, hand span, tail length in mice,

diameter of a shell, length of a leaf

Nu

mb

er

of

Pe

op

le

Blood Group



Continuous variation can be shown by a bell-shaped curve on a line graph or

histogram.

Key Question:

Complete the table by adding examples of continuous and discrete variation. Try to

add as many as you can including any that are not mentioned in your notes.

Discrete Variation Continuous Variation

Nu

mb

er

of

Pe

op

le

Height (cm)



Pick one of the examples that you have listed above to survey the class.

Class survey title: _____________________________________

Show your results in a table with the appropriate headings.

Once you have completed the survey, use your results to construct a graph.

* Remember: discrete variation should be drawn as a bar graph and

continuous variation should be drawn as a line graph or histogram.



Genetics

When organisms reproduce, genetic information is passed from parents to offspring on

the chromosomes in the gametes (sex cells).

The way in which inherited characteristics are passed on from parents follows a

pattern. The study of this pattern of inheritance is called genetics.

Genes

The units of inheritance are called genes which make up chromosomes. Each

chromosome consists of about ten thousand genes.

An individual has two sets of genes in each cell, one set inherited from the mother

and one set inherited from the father. Therefore, chromosomes can be arranged into

pairs. The two chromosomes of each pair are identical in shape, size and type of

genes.

There can be more than one form (types) of any one gene. Different forms of a gene

are known as alleles. For any given characteristic, an individual can carry the same

two alleles or two different alleles. These alleles are usually abbreviated using letters

for shorthand.

For example: Eye colour is an inherited characteristic in humans. If the gene for

brown eyes is represented using a B and the gene for blue eyes is represented using a

b, then a person can have two brown alleles (BB), two blue alleles (bb), or can have

one brown allele and one blue allele (Bb).

Dominant and Recessive Alleles

Where an individual carries two different forms of a gene for a particular characteristic,

very often one version will be stronger than the other. The stronger version of the

gene is said to be dominant and the weaker one is recessive. The recessive allele

will be masked by the dominant allele and won’t show in the phenotype of the

organism.

Capital letters are used for dominant alleles and lower case for recessive alleles.

For example: the allele for brown eyes (B) is dominant to the allele for blue eyes

(b). Therefore a person with both alleles (Bb) will have brown eyes.



Phenotype and Genotype

The physical appearance of an organism is referred to as its phenotype. For

example eye and hair colour in animals or leaf shape and petal colour in plants. Most

physical characteristics have more than one phenotype.

Complete the table by adding the possible phenotypes for each of the characteristics.

Physical Characteristic Phenotypes

eye colour

blood group

flower colour

The genes an organism has for a particular characteristic is known as its genotype.

In eye colour for example, there are three possible genotypes for brown or blue; BB,

Bb and bb. It is not always possible to identify the genotype of an individual with a

dominant characteristic as they could have the recessive allele which would be

masked.

If both alleles are the same as each other, the individual is said to be homozygous

(BB or bb).

If the alleles are different from each other, the individual is said to be heterozygous

(Bb).

Using the example of eye colour, complete the following table.

Genotype Homozygous or

heterozygous? Phenotype

BB homozygous

Bb brown eyes

bb

It is therefore possible for organisms to have a different genotype but the same

phenotype.

The dominant allele will always show up in the phenotype of an organism, even when

the organism is heterozygous. This is demonstrated in the angelfish example on the

following page.



Inheritance of colour in angelfish:

Homozygous black angelfish X Homozygous gold angelfish

Parents

First generation

(the offspring)

Key Question:

The diagram below shows the same sections of matching chromosomes found in four

fruit flies A, B, C and D. The genes are for body type and antennae type.

The genes shown on the chromosomes can be identified using the following key:

Which fly is homozygous for both characteristics?

Fly D Fly C

Fly B Fly A

Form of gene for abnormal antennae

Form of gene for normal antennae

Form of gene for unstriped body

Form of gene for striped body Key

All the baby angelfish in are black,

therefore black must be dominant.



Monohybrid Cross

Genetic crosses involving only one characteristic are called monohybrid crosses.

The parents in these crosses will usually have different phenotypes and are

homozygous for that characteristic, such as the angelfish described on the

previous page. Inheritance of colour in angelfish is controlled by a gene that exists in

two forms; black which is dominant to gold.

When studying genetics, shorthand symbols are used:-

The original generation (the parents): P

The first generation: F1

The second generation: F2

Using B to represent the black allele and b to represent the gold allele, the

monohybrid cross is shown.

P phenotypes: homozygous black X homozygous gold

genotypes: BB

bb

gametes: all B

all b

F1 phenotypes: all black

genotypes: Bb



Heterozygous Monohybrid Cross

When two heterozygous organisms are crossed, the expected ratio of offspring can be

predicted using a Punnett square. Using inheritance of colour in angelfish, complete

the monohybrid cross below where two of the F1 offspring are self crossed.

Three quarters of the angelfish in the F2 generation have the same phenotype, they

all appear black; BB, Bb and Bb. However, these black angelfish do not have the same

genotype.

F1 phenotypes: black X black

genotypes: _____________

_____________

gametes: _____________

_____________

Punnett square

F2 phenotypes: __________________________________

genotypes: __________________________________



Backcross

When an organism shows a dominant phenotype, it is not obvious whether its

genotype is homozygous or heterozygous for that characteristic. The identity of an

unknown genotype can be found by carrying out a backcross with a homozygous

recessive organism.

The outcome of the cross is used to tell whether the individual is homozygous or

heterozygous. Your teacher will help you complete the cross using inheritance of

colour in angelfish.

Backcross 1: Backcross 2:

phenotype: black X gold black X gold

genotype: BB _______ Bb _______

gametes: _______ _______ _______ _______

Punnett squares:

Results: All the offspring have the

______genotype and are

_______________ in colour.

All black offspring indicates

the genotype of the parent

must be ____ .

Half the offspring have the

genotype _____ and are

____________ in colour. Half

have the ______ genotype and

are _____________ in colour.

A ratio of 1 black : 1 gold

angelfish indicates the

genotype of the parent must

be ____ .



Ratios

A cross between BB (homozygous dominant) and bb (homozygous recessive)

will always give offspring which are all the same phenotype as the dominant parent.

A cross between Bb and Bb (heterozygous) will give the ratio of:

3 dominant : 1 recessive

In reality the actual or observed ratios of offspring in crosses do not match the

predicted ratios.

This can be for two reasons:

1) fertilisation is a random process.

2) the numbers of organisms in the cross are too small.

Complete the monohybrid crosses on the following pages using the worked

example of the angelfish as a guide to help you.



Key Questions:

1) Homozygous pea plants were bred to produce two generations, as shown

below:

(a) Using the letter R for round seeds and r for wrinkled seeds, copy and

complete the following table.

The seeds from the F2 were counted and the results are shown in the bar chart.

(b) What is the simple whole number ratio of round to wrinkled seeds?

(Clue: the ratio should be written as ____ round : ____ wrinkled)

Plant Genotype

Parent with round seeds

All F1

F2 with wrinkled seeds

Wrinkled seeds

Number of seeds

80

70

60

50

40

30

20

10

0

Type of seed

Round seeds



2) Sorghum is an important food crop in some parts of the world. The colour of

the seed husk (coat) is controlled by a single gene. Purple husk colour (H) is

dominant to tan husk colour (h).

(a) A homozygous purple husk plant is crossed with a homozygous tan husk plant.

(i) Copy and complete the genotypes of the parental (P) generation below

into your jotter:

P purple x tan

P genotypes ___________ ___________

(ii) State the phenotype (s) of the F1 plants.

(b) An individual from the F1 generation is crossed with a homozygous tan husk

plant.

(i) Copy and complete the Punnett square to show the expected results of

this cross.

(ii) State the expected ratio of purple to tan husk offspring of this cross.

Ratio = ____ purple : ____ tan

(iii) The actual results of this cross produced an observed ratio of

2 purple : 1 tan, explain one possible reason for the difference between the

expected and the observed ratio.

purple husk tan husk

Genotypes of gametes from F1 plant

Genotypes of gametes from tan husk plant



3) (a) Fruit flies show variation in wing structure which can be inherited. Vestigial

wings are shorter in length than normal.

Flies were crossed as shown below.

(i) Using N for the normal form and n for the vestigial (short) form, give the

genotypes of each of the following:

1 Parent with normal wings

2 A fly from the F1 generation

3 An F2 fly with vestigial wings

(ii) Some of the normal wing flies in the F2 will be NN and some will be Nn,

what method could be used to determine the genotype of these flies?

(b) What term is used to describe the different forms of a gene?

P

F1

F2

X

homozygous normal winged male

homozygous vestigial winged female

all flies have normal wings

F1 flies were self-crossed

some flies have normal wings and

some have vestigial wings



Family Trees

Unlike other organisms that have been described thus far (fruit flies, angelfish and

plants), humans do not breed to suit geneticists (scientists that study genetics). In

addition, humans produce too few offspring to allow reliable conclusions. However,

the same laws of genetics apply to humans and inheritance in humans can be traced

by constructing a family tree.

The family tree below shows the inheritance of PTU tasting.

The ability to taste a chemical called PTU is controlled by a single gene. People can either be tasters or non-tasters.

Family trees always include a key where a square () represents a male and a circle is

used to represent a female (O).

Use the information given to help you answer the questions on the following pages

that include a family tree.



Key Questions:

1) Tay-Sachs disease is an inherited condition which affects the nerves. Different

forms of the same gene determine its effect.

T (dominant) represents the normal form of the gene.

t (recessive) represents the form of the gene which causes the disease.

The family tree diagram shows a pattern of inheritance of the disease.

(a) Copy and complete the table by writing the genotypes of persons A, D and K.

(b) A carrier of the disease is someone who does not show the symptoms of the

disease but can pass it to their offspring.

Give the letter of one person from the F2 generation who must be a carrier of the

disease.

(c) What type of variation is shown by Tay-Sachs disease? Explain your answer.

Person Genotype

A

D

K

= normal male

= normal female = affected female

= affected male

B

C

G J

A

D F

I

E

H K

P generation

F1 generation

F2 generation



2) The family tree diagram shows how hair type is genetically inherited. Use the

diagram to answer the questions below.

In humans, the allele for wavy hair (H) is dominant to the allele for straight hair

(h).

(a) Copy the family tree into your jotter and add the missing genotypes beside the

phenotype for each member of the family. (It is not necessary to copy the

pictures).

(b) From whom did Wendy receive the genetic information that gave her straight

hair? Explain your answer.

grandfather grandmother grandmother grandfather

wavy: HH wavy: HH

wavy: ____ wavy: ____ wavy: ____ wavy: ____

wavy: HH wavy: Hh

straight: hh

straight: ____

straight: ____

aunt father mother uncle

brother sister Wendy



3) The diagram below shows the two ways that hands can be clasped together.

The diagram below shows whether members of a family clasp their hands with the left

or right thumb on top.

T = the left thumb on top t = right thumb on top

(a) What information from parent B proves that left thumb on top is the dominant

form of the gene?

(b) Use the information from the diagram to copy and complete the following table.

(c) If person D has a child with a man with the same genotype, what is the %

chance of their first child clasping their hands with the left thumb on top?

(d) When 1000 people were surveyed, 625 were found to clasp their hands with the

left thumb on top. What is the simple whole number ratio of left to right thumb

people?

_______ left thumb : ______ right thumb

Person Genotype

C

E

F

right thumb on top left thumb on top

B

E C

A

D

F G

tt Tt = right thumb male

= left thumb male

= right thumb female

= left thumb female

Key:



Key Questions:

Complete the following table using your notes to add the definition for each word.

Term Meaning

Discrete variation

Continuous variation

Gene

Phenotype

Genotype

Allele

Dominant

Recessive

Homozygous

Heterozygous



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National 5 - Unit 2 - Multicellular Organisms...National 5 Biology 2.4 Variation and Inheritance Pupil Course Notes Page 1 of 22 Duncanrig Secondary School 2017 Multicellular Organisms