Genetic Information

Duncanrig Secondary School S3 Biology Course

Genetic Information

Pupils’ Activity Booklet

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What you should know by the end of this unit:

A species is a group of organisms that can interbreed to produce fertile offspring.

Variations are differences that exist between members of a species.

Many characteristics showing variation are inherited and are determined by genetic information received from both parents.

Genes are segments of DNA which control an inherited characteristic

by providing a set of instructions to make proteins. Each body cell has two matching sets of chromosomes. Sex cells are called gametes.

Gametes have one set of chromosomes. After fertilisation the new cell formed has two matching sets of

chromosomes. Sexual reproduction can lead to variation found within a species. Phenotype is the physical appearance of an organism. Each characteristic is controlled by two forms of a gene. Each parent contributes one of the two forms for each characteristic. Each gamete carries one of the forms of the gene. Genotype is the genes an organism has for a specific characteristic. The sex of an organism is determined by the sex chromosomes

called X and Y. In humans females have two X chromosomes (XX) and males have

an X and a Y chromosome (XY).

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Skills that you should be able to carry out by the end of this unit are:

Identify examples of different phenotypes of a characteristic.

Identify examples of dominant and recessive forms of a gene from the

phenotypes and numbers given in a genetic cross.

Determine the sex of an individual with reference to inheritance of the

sex chromosomes.

Work as part of a group and take responsibility to support the work of the

group.

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Inheritance

A species is a group of organisms that can interbreed to produce fertile offspring.

All humans belong to the same species. Although all members of the same species are very similar they are not identical. Differences between organisms are called variations.

Brown wavy hair

Green eyes

boy

Many of the characteristics showing variation may be passed on from generation to generation when organisms reproduce. These features are inherited characteristics.

Write the heading Inherited Characteristics in your jotter. Make a list of six examples of inherited characteristics in humans.

Blonde straight hair

Blue eyes

girl

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Inheritance

Inherited variations for example eye colour and blood group are determined by genetic information.

Inside every cell of each living thing (plants and animals) are sets of instructions called genes. Each gene controls an inherited characteristic by providing a set of instructions to make proteins. These determine what is the organism looks like, how it is to survive, and how it will interact with its surrounding environment.

The genes are strung together in long strands of material called deoxyribonucleic acid (DNA). Each of these long strands is called a chromosome and they are found in the nucleus of every cell.

Gene : A segment of DNA

The number of chromosomes varies from species to species. Humans have 46 chromosomes.

Each normal body cell has two matching sets of chromosomes in the nucleus. The 46 chromosomes found in human cells can be arranged into 23 pairs, giving two sets of 23 chromosomes.

A chromosome

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Chromosomes

The diagram below shows human chromosomes arranged into 23 pairs. 22 of these pairs match each other but the remaining pair, pair 23, can differ from each other. These are the sex chromosomes. A human female has two X chromosomes, whereas a human male has an X chromosome and a Y chromosome.

Sex Cell Production

Sex cells are called gametes. The cells that produce them are called gamete mother cells. When gametes are formed the gamete mother cell divides to produce 4 sex cells each containing half the genetic material of a normal cell.

Gametes have one set of chromosomes in the nucleus of the cell.

In animals the male gametes are called sperm and the female gametes are called eggs. In plants the male gametes are found in pollen grains and the female gametes are found in ovules.

AMAZING FACT -

A human cell contains about 25,000-30,000 genes packed into the nucleus.

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How is genetic information passed on to the next generation?

In sexual reproduction two parents are usually involved. Each egg cell and each sperm cell contains half of the genetic information needed for an individual.

This means that each human egg cell and each human sperm cell carries 23 chromosomes.

At fertilisation the nuclei of the sperm and the egg fuse to create a new cell called a zygote. The zygote has a two sets of chromosomes containing all the genetic information needed for an individual. Since both parents contribute half the genetic material that their offspring inherit, the offspring show a mixture of genetic traits from both parents.

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Key Questions (Answer in sentences)

1. What is meant by a species?

2. What are chromosomes and where are they found?

3. What is a gene?

4. From whom do humans receive their genetic information?

5. Compare the number of sets of chromosomes present in a normal

cell to a gamete (sex cell).

Egg cell Sperm cell

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Phenotype

Phenotype is the organism’s physical appearance which has resulted from the genetic information inherited from both parents.

Characteristic Possible phenotypes

Eye Colour brown/blue/green

Freckles

A, B, AB, O

Gender

Earlobes

Write down the definition of the word phenotype in your jotter Copy and complete the table below to show human characteristics and possible phenotypes.

Genotype

Genotype is the genes an organism has for a specific characteristic. As organisms inherit genetic material from both parents they have two copies of every gene (one from mum and one from dad). Genes exist in a variety of forms, but only two forms are ever present in a cell.

Different forms of a gene are called alleles.

For example there are two forms of the gene for the presence or absence of freckles in humans; the form for freckles and the form for no freckles.

The two genes in each pair can be the same form or they can be different forms.

Write down the definition of the word genotype in your jotter.

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Dominant and Recessive

The different forms of a gene can be dominant or recessive. The form of a gene which always shows its effect in the phenotype of the offspring, even when another form is present is said to be the dominant form. Since the other form is masked by the dominant form this is said to be recessive.

Symbols are used to represent the dominant and recessive forms of a gene.

Since every body cell have two forms of each gene, the organisms genotype is represented by two symbols.

For example in humans, freckles is dominant to no freckles. The dominant form is freckles. This may be given the symbol F. The recessive form, no freckles, may be given the symbol f. (Note-a capital letter is used to denote the dominant form and the small version of the same letter is used to denote the recessive form).

The possible combinations of genes which may be inherited are FF, Ff or ff.

A person with freckles has two possible genotypes, FF or Ff.

Even although the person may have the ‘no freckles’ form of the gene, this is masked by the freckles form and they will have freckles.

A person who has no freckles must have the two recessive forms of the gene and would have the genotype ff.

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Dominant Recessive Characteristic

Widow’s peak Straight hairline Hair

Dimples No dimples Facial features

Broad lips Thin lips Facial features

Double-jointedness Normal joints Facial features

The table below shows some examples of dominant and recessive characteristics in humans.

True-breeding

An organism is said to be true breeding if on being crossed with a member of the same strain it always produces more organisms with exactly the same phenotype. This is because the organisms genotype is made up of two identical forms of the gene for that characteristic. For example a true breeding black rabbit when crossed with another true breeding black rabbit will always produce more black rabbits.

Parents

black rabbit black rabbit

Offspring

all black rabbits

x

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Key Questions (Answer in sentences)

The diagram below shows 3 individuals with different forms (alleles) of

genes for eye colour. B is the symbol for brown eyes and b the symbol

for blue eyes.

1. Write down the genotypes and phenotypes of individuals A, B

and C.

2. In humans earlobes can either be attached to the head or free. Free

earlobes are dominant to attached ear lobes. Using E as the symbol

for free earlobes and e as the symbol for attached earlobes, write

down the possible genotypes for each of the individuals in the

photographs.

B b B B b b

Person 1 Person 2

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Child A Child B Child C Child D

Collect a coloured copy of this picture from your teacher. Consider which features the children might have inherited from mum and dad. Make 2 lists in your jotter to show what was inherited from each parent.

1. Working with a partner, collect a Reebops Activity Instruction Card and a set of Mum and Dad Chromosomes from your teacher. 2. Read the card carefully and follow the instructions.

Activity : Reebops

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Key Questions (Answer in sentences)

1. What do you notice about the features of the babies?

2. Are any of the babies identical?

3. How many babies are the same as their parents?

4. How much genetic material does each parent provide?

5. Write a couple of sentences explaining how sexual reproduction

introduces variation in a species.

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Why do we all look so different?

From the Reebops activity you will have learned that each parent contributes the same amount of genetic information to their offspring. The different combinations of genes that we inherit from our parents determines our characteristics. The mixing of one parent’s genetic material with that of another produces an individual that is different from both parents and from all other members of the species. Sexual reproduction contributes greatly to the tremendous variation we see within a species.

Family Trees

Family trees can be used to show the inheritance of genetic information from parents to offspring.

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. Tasting is dominant to non-tasting.

Study the family tree and answer the questions which follow.

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Key Questions (Answer in sentences)

1. (a) What are the two phenotypes, concerning PTU, that are shown

in the family tree?

(b) Which of the following couples are probably true-breeding?

Peter and Alison

Vivien and Eddie

Marion and John

2. Use the letters T for tasting and t for non-tasting to show the forms

of the gene present in the sex cells of Rab and Audrey.

3. Cystic fibrosis is an inherited disorder of the human body. Sufferers

produce abnormally thick mucus that blocks the air passages in the

lungs. The condition is caused by a recessive gene. Copy and

complete the family tree below.

grandfather normal

grandmother normal

father __________

Nn

NN nn

mother normal

son __________

daughter __________

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Patterns of Inheritance

The way in which inherited characteristics are passed on from parents to offspring follows a pattern. Patterns of inheritance have been worked out using breeding experiments.

The first generation of offspring produced are called the first filial generation (F1). When they reach adulthood they may be cross bred to produce a second generation called the second filial generation (F2).

A monohybrid cross is the study of the inheritance of one characteristic.

A punnet square can be used to show the possible phenotypes of the offspring.

The example below shows how to write a monohybrid cross using hair colour in humans, where the form of the gene (allele) for non-red hair (H) is dominant to the allele for red hair (h).

It shows the offspring produced by a true-breeding non-red haired parent and a true-breeding red haired parent.

A Punnet Square

fertilisation

The predicted F2 genotypes are 1/4 HH, 1/2 Hh and 1/4 hh.

The predicted F2 phenotypes are 3/4 non-red hair and 1/4 red hair.

This can also be described as 3 : 1 non-red to red hair phenotypic ratio in the F2 generation.

Parents (P)

Phenotype

True-breeding non-red hair

True–breeding red hair

Genotype HH hh

Gametes H h

F1 Genotype Hh

F1 Phenotype All non-red hair types

To work out the F2

F1 crossed Hh Hh

gametes H h

H HH Hh

h Hh hh

x

x

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Sex Determination

Whether a fertilised egg (zygote) develops into a boy or a girl depends upon what sex chromosomes it contains. In humans females have two X chromosomes (XX) and males have an X and a Y chromosome (XY).

Sperm and egg cells contain half the genetic information of normal cells. This means that egg cells produced by females will all contain one X chromosome, whereas 50% of sperm cells produced by a male will contain an X chromosome 50% will contain a Y chromosome.

Egg producing cell Sperm producing cell

Gametes

When fertilisation takes place, if the egg cell is fertilised by a sperm cell containing an X chromosome then the offspring will be a girl (XX). If the egg cell is fertilised by a sperm cell containing a Y chromosome then the offspring will be a boy (XY).

Fertilisation

Egg cells Sperm cells

female male

XX XY

X X X Y

X X Y

XX XY

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Key Questions (Answer in sentences)

The diagram below shows the inheritance of the sex chromosomes in

humans.

Copy the diagram below into your jotter and complete it to show:-

1. The genotypes of the gametes (sex cells) produced by each parent.

2. Possible genotypes of the offspring produced from this cross.

3. What is the ratio of males to females in the offspring?

XY

XX

Sperm cells

Egg cells

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Glossary of Terms

Alleles Different forms of a gene

Chromosome Thread-like structure found inside the

nucleus of a cell

Dominant

Form of a gene which always shows its

effect in the phenotype of the offspring,

even when another form is present

First filial generation (F1 ) The first generation of offspring produced

from a cross

Gamete Sex cell, containing one set of chromosomes

in its nucleus

Gene

A segment of DNA which controls an

inherited characteristic by providing a set of

instructions to make proteins

Genotype Genes an organism has for a specific

characteristic

Monohybrid cross Breeding experiment involving the

inheritance of one characteristic

Phenotype Physical appearance of an organism

Recessive The form of a gene that is always masked

by the dominant form

Second filial generation (F2) The second generation of offspring resulting

from a cross between members of the F1

True-breeding

An individual whose genotype is made up of two identical forms of a gene (ie both alleles are the same)

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Glossary of Terms

X chromosome Type of sex chromosome present in

human males and females

Y chromosome Type of sex chromosome present in

human males

Zygote Single cell formed by the nuclear

fusion of two gametes