Section 5 - Inheritance

SECTION 5 - INHERITANCENational 4 & 5 – Multicellular Organisms

Why are we so similar yet different?• We all belong to the same species• We are similar in many ways• But, we show a great deal of variation • - continuous (e.g. height, weight)• - discrete (e.g. eye colour, blood type)• These similarities and differences are mainly determined

by our genes

Learning Outcomes• By the end of this section I will be able to • - identify how genes determine characteristics• - map out patterns of inheritance, including family trees• - identify phenotypes and genotypes using punnet

squares• - define dominant and recessive characteristics, and

identify homozygous and heterozygous individuals

Inherited Characteristics• Our characteristics are determined

by genetic information• E.g. hair colour, eye colour, tongue-

rolling• Each parent passes on 1 piece of

information for a certain characteristic

• The pieces of information from each parent may be the same or different

• A family tree can show how characteristics pass on through several generations

Phenotype• An organisms appearance resulting from genetic

information received from parents• E.g• - wing shape – wild-type, weak, strong • - flower colour – red, white, purple• - eye colour – green, blue, brown

Phenotype and genotypeThe overall appearance of an organism depends on two things:

The full set of genes of an organism is called its genotype.

All the observable characteristics of an organism are called its phenotype.

1. its genes (inherited characteristics)

2. the effects of the environment in which it lives.

An organism’s phenotype therefore depends on its genotype plus environmental effects.

phenotype = genotype + environmental effects

Genes & Alleles• Each cell has two sets of chromosomes• One set from each parent• Each chromosome is made up of units called

genes• Each gene contains information for a particular

characteristic• Each gene normally has at least 2 different

forms• e.g. flower colour could be purple or white• These different forms are called alleles

Inheritance studies• Show the inheritance patterns of certain

characteristics• Usually done with an easily-bred

species • e.g pea plants• Specific characteristics chosen for study

• e.g. flower colour• When only 1 characteristic is examined,

it is said to be a monohybrid cross• Parents (P) are bred to produce the first

filial generation of offspring (F1)• The F1 generation are then interbred to

produce the second filial generation (F2)• Phenotypes are observed to show how

characteristics are passed on

Dominant/Recessive• F1 generation often shows only

one characteristic coming through• This characteristic is said to be

dominant• E.g. purple flower• The other characteristic is often

hidden• It is said to be recessive• e.g white flower• BUT, in the F2 both

characteristics often appear• Why?

Homozygous alleles If the alleles for a characteristic are the same, the organism is said to be homozygous for that characteristic.

What colour eyes will these homozygous pairs of alleles produce?

allele forbrown eyes


allele forblue eyes

allele forblue eyes

Heterozygous alleles

The characteristic expressed by heterozygous alleles will depend on which allele is dominant and which allele is recessive.

If the alleles for a characteristic are different, the organism is said to be heterozygous for that characteristic.

What colour eyes will this heterozygous pair of alleles produce?


allele forblue eyes

?

What eye colour?The allele for brown eyes is dominant over the allele for blue eyes.

The individual will have brown eyes, because the allele for brown eyes masks the allele for blue eyes.


allele forblue eyes

So, what colour will the eyes be of an individual who has both alleles for eye colour?

Inheritance terms

B is the gene for brown eyes b is the gene for blue eyes


ParentsBB bbBody cell in father

with a pair of genes for brown eyes

Body cell in mother with a pair of genes for blue eyes


ParentsBB bbBody cell in father


Body cell in mother with a pair of genes for blue eyes

Gametes


ParentsBB bbbody cell in father


body cell in mother with a pair of genes for blue eyes

Gametes

B Beach sperm has a gene for brown eyes





Gametes


b b each egg has a gene for blue eyes





Gametes



At fertilizationThere are 4 possible combinations of sperm and egg





Gametes




B

B





Gametes




Bb BbBb Bb

B

B

b b

All the children of this F1 generation have genotype Bb and phenotype brown eyes

Parents (F1)father with brown eyes

mother with brown eyes

Gametes

At fertilization

Parents (F1)Bb Bbfather with brown

eyesmother with brown eyes

Gametes

At fertilization



Gametes

B b B b

At fertilization



Gametes

B b B b

At fertilization

B

b

B b



Gametes

B b B b

At fertilization

BB BbBb bb

B

b

B b



Gametes

B b B b

At fertilization

BB BbBb bb

B

b

B b

A child who inherits the genes BB will have brown eyes

A child who inherits the genes Bb will have brown eyes

A child who inherits the genes bb will have blue eyes

Monohybrid Cross• In this example purple is dominant to white• P = purple• p = white• Let’s assume that both parents are

homozygous (true breeding)• One is PP (purple), the other pp (white)• The F1 offspring would gain a purple allele (P)

and a white allele (p)• - therefore only purple flowers in the F1

generation (Pp)• When the F1 interbreed they could put forward

either a purple (P) allele • - or a white (p) allele • A punnet square allows us to map out the

possible combinations in the F2

• We discover that the numbers produced are 3 purple:1white

• This is called the phenotypic ratio

• Green flower (G) is dominant

• Yellow (g) is recessive• All the F1 generation have

the Gg genotype• They are all therefore green• Then the F1 plants are

crossed• The results of the F2 are:• 1GG:2Gg:1gg• - this is called the

genotypic ratio• - 3 green:1 yellow• - this is the phenotypic

ratio

1 GG = green2 Gg = green1 gg = yellow

Expected vs Observed ratio• With a monohybrid cross, a 3:1 ratio would always be expected in the F2

generation• However, there is often a difference between the expected and the

observed results

This is because fertilisation is a random process, involving an element of chanceA punnet square only shows the likely outcomes, not what will actually occurIn real life, if you toss a coin 20 times, you would expect 10 tails:10 heads – rarely occurs

Generation Green YellowParents (P) 6 (GG) 6 (gg)

F1 198 (Gg)

F1 cross Gg x Gg

F2 147 53

Using the previous example the 200 offspring from the F2

generation doesn’t exactly match a 3:1 ratio

Using a test-cross

To help identify it’s genotype, it is crossed with a white flower(a white flower can only have a ppgenotype)

In this example purple (P) is dominant to white (p)

We have a flower that is purple, but don’t know if it is homozygous (PP) or heterozygous (Pp)

The offspring produced should prove what the unknown genotype is

On occasion, an organisms genotype may be uncertain, but needs to be identified.

A test-cross is used to prove an unknown genotype

http://www.google.co.uk/url?sa=i&rct=j&q=testcross+mice&source=images&cd=&cad=rja&docid=vJ19vQyK_w4zpM&tbnid=tspG7FMYfb3fgM:&ved=0CAUQjRw&url=http://bioserv.fiu.edu/~walterm/GenBio2004/new_chap13_inheritance/pics.htm&ei=Ly6WUcz0MKyg0wWTsIGICg&bvm=bv.46471029,d.d2k&psig=AFQjCNHN9lhfFcEtaPdfo8g-RMa5FQxHPw&ust=1368882927633055

http://www.google.co.uk/url?sa=i&rct=j&q=testcross+mice&source=images&cd=&cad=rja&docid=vJ19vQyK_w4zpM&tbnid=tspG7FMYfb3fgM:&ved=0CAUQjRw&url=http://bioserv.fiu.edu/~walterm/GenBio2004/new_chap13_inheritance/pics.htm&ei=Ly6WUcz0MKyg0wWTsIGICg&bvm=bv.46471029,d.d2k&psig=AFQjCNHN9lhfFcEtaPdfo8g-RMa5FQxHPw&ust=1368882927633055

For example, when a red snapdragon plant is crossed with a white snapdragon plant, all the offspring flowers are pink.

What is incomplete dominance?Sometimes two different alleles are neither fully dominant or recessive to each other.

In heterozygous individuals, this creates a phenotype that is a mix of the other two. This is called incomplete dominance.

- because both the red and white alleles are expressed.

What is co-dominance?The human blood group system is controlled by three alleles: A, B and o.

In heterozygous individuals who have A and B alleles, both are fully expressed

This is called co-dominance.

A and B are dominant while o is recessive.

- creating an extra phenotype (AB)

http://www.google.co.uk/url?sa=i&rct=j&q=abo+blood+group+inheritance&source=images&cd=&cad=rja&docid=ODp_yuIXxTph3M&tbnid=E7Xe0mSs-k1ShM:&ved=0CAUQjRw&url=http://ghr.nlm.nih.gov/handbook/illustrations/codominant&ei=eSaCUayuM4eP0AXavYGoCA&bvm=bv.45921128,d.d2k&psig=AFQjCNH6kOhXSwkyJouc-xTAIZldm_GZUw&ust=1367570401356610

Co-dominance in humans

The life and work of Gregor Mendel

Mendel’s experimentsOver seven years, Mendel experimented on more than 28,000 pea plants! Why were his experiments so successful?

Pea plants grow quickly.

Pea plants are available in pure-breeding (homozygous) strains.

Many pea plant characteristics show discrete variation; they are either one form or another.

This means that their phenotypes are easily distinguishable.

What are sex chromosomes?Humans cells contain one pair of sex chromosomes, which control gender.

Males have one X andone Y chromosome (XY).

Y chromosomes are small and contain 78 genesX chromosomes are larger and contain 900–1,200 genes.

Because females can only produce X gametes, it is the sperm that determine the sex of the offspring.

X chromosome

Y chromosome

Females have two X chromosomes (XX).

Boy or girl?

Multiple-choice quiz

Anagrams

Glossary (1/4)acquired – A characteristic of an organism that depends on environmental

factors.

allele – One version of a gene, found at a specific location along a chromosome.

carrier – An individual with a recessive allele, whose effect is masked by a dominant allele.

characteristic – A specific feature of an organism, such as eye colour.

co-dominance – A situation where two alleles are equally dominant.

continuous – Variation represented by a continuous range of values and which can be measured.

Glossary (2/4)discontinuous – Variation represented by discrete categories.

dominant – An allele that is always expressed, even if the cell only contains one copy.

gene – The unit of inheritance.

genotype – The full set of genes of an organism.

heterozygous – Having two different alleles of a specific gene.

homologous chromosomes – A matched pair of chromosomes that carry genes for the same characteristics.

homozygous – Having two identical alleles of a specific gene.

Glossary (3/4)incomplete dominance – A situation where two alleles are both

partially expressed, producing an intermediate phenotype.

inherited – A characteristic of an organism that depends on its genes.

monohybrid cross – A cross in which one pair of characteristics is studied.

phenotype – All the observable characteristics of an organism.

recessive – An allele that is only expressed if two versions of it are present in a cell.

Glossary (4/4)

test cross – A situation where an individual with an unknown genotype is bred with a homozygous recessive individual to reveal the unknown genotype.

variation – The difference between individuals within a population.

Section 5 - Inheritance

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