INHERITANCETOPIC 3 - 2015

INHERITANCEThings to coverReview of Topics 1 & 2

Genes & allelesHeterozygous & homozygousGenotypes & phenotypesDominant traits & recessive traitsAutosomal & sex-linked traitsPunnett Squares & pedigrees

REVIEW

REVIEW – TOPIC 1

DNA is composed of _______________. Each nucleotide consists of:

◦ A _______________ sugar ◦ A _______________ group◦ A _______________ base (_____ types)

The subunits form chains. Two chains twist to form the _______________ shape of the DNA molecule.

Chemical bonds hold the DNA molecule together. Strong _______________ bonds hold the nucleotides together, while weak _______________ bonds form between the bases.

cells nucleotides phosphate double helix covalent nitrate 4

pentose hexose phosphorus linear hydrogen nitrogen 2

REVIEW – TOPIC 1 DNA is composed of nucleotides. Each nucleotide consists of:

◦ A pentose sugar

◦ A phosphate group

◦ A nitrogen base (4 types) The subunits form chains. Two chains twist to form the

double helix shape of the DNA molecule. Chemical bonds hold the DNA molecule together. Strong

covalent bonds hold the nucleotides together, while weak

hydrogen bonds form between the bases.

REVIEW – TOPIC 2

DNA is coiled up tightly to form _______________

There are _______________ of chromosomes in every cell.o Most of these are _______________ , controlling general

characteristics.o One pair are the _______________ , controlling both sexual

and general characteristics.

autosomes 2 pairs chromosomes centrosomes

thymine sex chromosomes cytosine 23 pairs

REVIEW – TOPIC 2

DNA is coiled up tightly to form chromosomes There are 23 pairs of chromosomes in every cell.

o Most of these are autosomes, controlling general characteristics.

o One pair are the sex chromosomes, controlling both sexual and general characteristics.

autosomes 2 pairs chromosomes centrosomes

thymine sex chromosomes cytosine 23 pairs

REVIEW – TOPIC 2

Cell division is necessary for _______________ , _______________ and reproduction.

One type of cell division, _______________ , is used in the formation of new body cells. It is also used by unicellular organisms for _______________ reproduction. The ______ daughter cells formed are _______________ to the parent cell and have ______ chromosomes.

The other type of cell division, _______________ is used to form sex cells. Another name for the products of this type of division are _______________. This process takes place in the reproductive organs. The ______ daughter cells formed are _______________ to the parent cell and have ______ chromosomes. They are also _______________ in size.

growth mitosisgamete

sasexua

lidentic

al46 23 differentiation

meiosis

sexual larger repair smaller 4 2 non-identical

REVIEW – TOPIC 2

In order for cell division to occur, the DNA must ____________________ and the chromosomes must divide equally between the cells.

____________________ can occur if this does not happen correctly. For example, disorders involving additional or missing ____________________.

Down Syndrome is one example. This disorder results from the presence of ____________________ Chromosome 21. This is called ____________________ 21.

replicate mutants trisomy monosomy differentiate

mutationschromosome

sgenes an extra

a missing

REVIEW – TOPIC 2 Cell division is necessary for repair, growth and reproduction. One type of cell division, mitosis, is used in the formation of new

body cells. It is also used by unicellular organisms for asexual reproduction. The 2 daughter cells formed are identical to the parent cell and have 46 chromosomes.

The other type of cell division, meiosis, is used to form sex cells. Another name for the products of this type of division are gametes. This process takes place in the reproductive organs. The 4 daughter cells formed are non-identical to the parent cell and have 23 chromosomes. They are also smaller in size.

REVIEW – TOPIC 2 In order for cell division to occur, the DNA must replicate and the

chromosomes must divide equally between the cells.

Mutations can occur if this does not happen correctly. For example,

disorders involving additional or missing chromosomes. Down Syndrome is one example. This disorder results from the

presence of an extra Chromosome 21. This is called Trisomy 21.

VARIATION

VARIATIONThe population of the Earth is more than 6 billion people, and no two individuals (apart from identical twins) are genetically the same. Why?People are different because they inherit different

characteristics (or traits) from their parents.Children carry a unique set of genes; half from

their mother and half from their father.

VARIATION Some characteristics, such as eye colour and earlobe

shape, are only determined by genes. These are called inherited characteristics.

Other types of characteristics, such as scars and hair length, are not inherited but depend on environmental factors. These are called acquired characteristics.

In some cases, it can be difficult to say how much influence the environment has over the expression of a trait.

VARIATIONAll the observable characteristics of an

organism are called its phenotype.The full set of genes of an organism is called its

genotype.

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

VARIATION Sexual reproduction is the most important cause of

genetic variation because it mixes up genetic material.

◦ Meiosis creates a variety of gametes

◦ Any male gamete can combine with any female gamete during fertilisation.

◦ Mate selection is random. All these events occur randomly and

create new combinations of genetic material.

GENES VS

ALLELES

GENES

The basic unit of inheritance

A segment of DNA

Control one specific characteristic.

Contain the code needed to produce a protein.

Located in the same position on thesame chromosome. This position iscalled its locus.

ALLELES

Different versions of a gene

Code may only differ by a few bases:◦ eg. The base eye colour gene has

2 options: blue and brown AGTACGGTACG = blue eyes allele (b) AGTCAGGTACG = brown eyes allele (B)

This diagram shows one chromosome, with ten genes. There are 20 alleles shown(two for each gene) in various combinations.

A

b

C

D

E

f

G

H

i

j

a

B

c

D

e

f

G

h

i

J

Gene

Chromosome

EXPRESSION

All alleles contain a base sequence (code) that is used by the cell to synthesise (build) a protein.◦ eg. eye colour pigment

The allele that will have its code used is determined by its dominance.

DNA protein gene expression

DOMINANCE

Most genes are controlled by 2 alleles.

Some alleles are dominant over others, so some traits can be hidden (= recessive) by others (eg. blue eyes)

DOMINANCE Dominant alleles:

◦ always expressed in a cell’s phenotype

◦ only one copy of the dominant allele needs to be inherited in order for it to be expressed

◦ represented by an upper case letter (eg. B)

Recessive alleles:◦ only expressed in a cell’s phenotype if two copies are present

◦ if only one copy is present, its effect is “masked” by the dominant allele

◦ represented by an lower case letter (eg. b)

HOMOZYGOUS If the alleles for a characteristic are the same, the

organism is said to be homozygous for that trait. If there are two dominant alleles,

the organism is said to be homozygous dominant for that gene.

If there are two recessive alleles, the organism is said to be homozygous recessive for that gene.

HETEROZYGOUS If the alleles for a characteristic are different, the

organism is said to be heterozygous for that trait. The protein made (and trait expressed)

will depend on which allele is dominant and which allele is recessive.

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 masksthe allele for blue eyes.

TYPES OF INHERITANCE

TYPES OF INHERITANCE1) Complete dominance

2) Co-dominance

COMPLETE DOMINANCE

Complete dominance = NORMAL!!!!! If the dominant allele is present, it will be

expressed eg. Brown eyed allele (B) always

expressed when present (BB or Bb)

Notation used:◦ Choose 1 letter ◦ Use lowercase (recessive) and uppercase (dominant)◦ eg. BB, bb or Bb only B trait shown

CODOMINANCEThere is no dominant allele!The phenotype is a blend of both alleles

presenteg. red allele + white allele = pink phenotypeeg. Type AB blood from combination of

A and B alleles

Notation used:◦Choose 2 letters◦Use uppercase for both◦eg. RR, WW or RW mixture of R & W traits is shown

AWESOME EXAMPLE: ABO BLOOD GROUPING To determine your blood type, there are three alleles:

◦ A – IA

◦ B - IB and ◦ O - ί

The alleles IA and IB are codominant However, both of these alleles are completely dominant

over ί

This results in four different phenotypes.

ABO BLOOD GROUPINGPhenotype

or blood group Genotypes

Type A IAIA or IAίType B IBIB or IBί

Type AB IAIB

Type O ίί

TYPES OF INHERITANCE3) Autosomal inheritance

4) Sex-linked inheritance

AUTOSOMAL INHERTIANCE

Autosomal inheritance = NORMAL!!!!! The gene is located on an autosome Traits can be:

◦ autosomal dominant – meaning that a trait/disorder is determined by the presence of a dominant allele

◦ autosomal recessive –determined by the presence of two recessive alleles

Notation used:◦ BB, bb, Bb

SEX-LINKED INHERTIANCE The gene is located on a sex chromosome Can be either the X chromosome or the Y chromosome

(but is usually X) eg. haemophilia, colour blindness Traits can be:

◦ sex-linked dominant – meaning that a trait/disorder is determined by the presence of the dominant allele on the X chromosomes◦ sex-linked recessive – meaning that a trait/disorder is determined by the presence of one or two recessive alleles on the X chromosomes

Notation used:◦ Females:

Homozygous XNXN or XnXn

Heterozygous XNXn (carrier)

◦ Males (only one X chromosome): Unaffected XNY Affected XnY Why are there no male

heterozygotes??

SEX-LINKED INHERTIANCE

CARRIERS Someone who is heterozygous for a genetic disorder They do not have the disorder themselves However, the disorder can be passed on to the next

generation

eg.

◦CC = no Cystic Fibrosis

◦cc = Cystic Fibrosis

◦Cc = carrier of CF

PUNNETT SQUARES

PUNNETT SQUARES Also called monohybrid crosses

Method used to find the expected genotype and phenotype ratios of offspring when the parental genotypes and/or phenotypes are known.

Steps:1. Write down the parental genotypes

You may need to choose letter to represent your trait – choose easy ones like T, G, H not C, S, Y

2. Write down the possible alleles that they are able to pass on in their gametes (sex cells)

3. Construct the Punnett square4. Fill in the potential genotypes of the offspring 5. Determine the genotype ratio of the offspring6. Determine the phenotype ratio of the offspring

PUNNETT SQUARES

PUNNETT SQUARESExample: An alien that is homozygous dominant for green skin

has chosen an alien who has purple skin for his bride.

Determine the likelihood that their children will have purple skin.

Like in Maths – you need to state what symbols you will be using to represent your traits:

eg. Let G = green; g = purple

PUNNETT SQUARESAnswer: Parents: GG and gg Gametes: G and g First generation (F1):

F1 genotype ratio: 100% Gg F1 phenotype ratio: 100% green skin

G G

g Gg Gg

g Gg Gg

PUNNETT SQUARESExample: One of the couples sons (named Neyp) married a

female alien (named Gjup) who was homozygous recessive for the skin colour trait.

Work out the genotype and phenotype percentage of their potential offspring.

PUNNETT SQUARESAnswer: Parents: Gg and gg Gametes: G,g and g Second generation (F2):

F2 genotype ratio: 50% Gg : 50% gg F2 phenotype ratio: 50% green skin : 50% purple skin

G g

g Gg gg

PUNNETT SQUARESExample: One of Neyp’s sons married a female alien who was

heterozygous for haemophilia. Neyp did not have haemophilia.

Calculate the likelihood that they could have a child with haemophilia.

Again – specify your symbols! It is a sex-linked recessive disorder!!! So:

Let H = normal; h = haemophilia

PUNNETT SQUARESAnswer: Parents: XHXh and XHY Gametes: XH, Xh and XH, Y First generation (F1):

F1 genotype ratio: 25% XHXH : 25% XHXh : 25% XHY : 25% XhY

F1 phenotype ratio: 75% normal: 25% haemophilia

XH Xh

XH XHXH XHXh

Y XHY XhY

PEDIGREE CHARTS

PEDIGREE CHARTS Shows the members of a family and how they are

related to each other.

A genetic family tree!

Pedigree charts can also be used to study the inheritance of a characteristic.

PEDIGREE CHARTS Circles = females Squares = males Shading = affected individuals Parents = linked by a horizontal line Children = vertical lines running down from parents Siblings = linked by

a horizontal lineabove them

PEDIGREE CHARTSExample: Draw a pedigree chart for Neyp and his wife Gjup,

including their parents and children. They had 3 children, 2 boys and a girl (in order of age):

◦ Their youngest son (Guol) had green skin and was also married with one purple daughter (Zcug).

◦ Their daughter (Hefg) had purple skin and married an alien who also had purple skin. They had one son (Yerg).

NB. Being purple is the recessive trait!

PEDIGREE CHARTS

Gg gg

gg gg

gg

Gg/gg Gg

I

II

III

IV

GG gg Gg/gg Gg/gg

? ?

Gg/gg

gg

? ?

PEDIGREE CHARTS Pedigree charts can also be used to determine if:

◦ The characteristic is dominant or recessive.◦ The characteristic is sex-linked or autosomal.

Things to look for:◦ Gender bias of males:females

more males indicates sex linkage; balanced ratio indicates autosomal

◦ Affected offspring from unaffected parentsindicates a recessive condition

◦ Affected sons from affected fathersindicates an autosomal condition

http://www.youtube.com/watch?v=CBezq1fFUEA&feature=share&list=EC3EED4C1D684D3ADF