Alleles: Their Expression and Inheritance · Normal female Normal male ... Widow’s peak Long Eyelashes Hair on the back of your hand! Thursday, ... who shows the dominant phenotype

Alleles: Their Expression and Inheritance

For Pedigree Analysis: Visit Physical AnthropologyThursday, October 28, 2010

http://highered.mcgraw-hill.com/sites/0072485949/student_view0/chapter3/interactive_activity.html

http://highered.mcgraw-hill.com/sites/0072485949/student_view0/chapter3/interactive_activity.html

What is a genome?

A genome is all of the chromosomes and DNA that an organism can possess.

The study of genomes is called genomics.

Genome size does not reflect complexity.

The number of genes varies between species.

table hereThursday, October 28, 2010

Principle of Segregation

The principle of segregation is founded on the generation ofgametes by meiosis. It assumes that

* Alleles occur in pairs because chromosomes occur in pairs

* During gamete production, members of each gene pair separate so each gamete contains one member of each pair

* During fertilization, the full number of chromosomes is restored and members of a gene are reunited

Thursday, October 28, 2010

Genotypes and Phenotypes

A genotype is the genetic composition of an individual, especially in terms of

alleles for particular genes.

A gene that controls one function can exist in different

forms called alleles.

The expression of the genotype is called the phenotype. This expression might be

physical, chemical or physiological.

Each characteristic that makes up a phenotype is called a trait.

We might say that the flower shown here is

displaying incomplete, or partial, dominance- it’s

phenotype is a mixture of the traits coded by its two

different colour alleles.


Alleles Control Traits- ABO Blood Groups

AoAA

or

or

BoBB

AB

oo

Genotype Trait RelationshipCombination

Homozygous AA orHeterozygous Ao

Homozygous BB orHeterozygous Bo

Heterozygous AB

Homozygous oo

Phenotype

A

B

AB

O

Phenotype for allele A displays dominance over the phenotype for allele o

Phenotype for allele B displays dominance over the phenotype for allele o

Phenotype for allele B displays codominance with the

phenotype for allele B

Phenotype for allele o is only observed in the

phenotype of individuals homozygous for the allele.

It is a recessive phenotype.


ABO Blood Groups

3. Mary (AB) marries Barry. Barry’s mother was type O and his father type A. Mary and Barry have a son with type B blood. Can Barry’s blood type and genotype be determined?

The correct notation for blood group alleles is IA, IB, and i. Answer the following questions using this system.

1. If two people who have blood type A (both with the IAi genotype) have children, what proportion of their children would be expected to have the O blood type? Why?

2. Can these parents have a child with AB blood? Why or why not?


Genetics Terminology Checkpoint

Genotype: The genetic makeup of an individual; the alleles at a particular locusPhenotype: The observable or detectable physical characteristics of an organism

Heterozygous: Having different alleles at the same locus on members of a chromosome pairHomozygous: Having the same allele at the same locus on both members of chromosome pair

Recessive traits will only be expressed where there are two copies of the alleleDominant traits prevent the expression of recessive traits in heterozygotesCodominance occurs where two traits are expressed together in a heterozygous genotype. Incomplete dominance occurs where the phenotype is intermediate of two traits.


Pedigree Analysis

Pedigree analysis is a way of determining the pattern of inheritance of particular traits. You just applied pedigree analysis of an autosomal trait to the previous questions on ABO blood groups.

Pedigree analysis is often used in genetic counseling to determine the probability that offspring might carry particular traits or diseases.

Normal female Normal male

Affected female Affected male

A Simple PedigreeKey

1. How many children and of what sex?

2. Which child is affected?

3. Is the trait dominant or recessive?

A pedigree chart shows us phenotypes.

Often we can also use a pedigree to determine genotypes.


Pedigree Charts


Pedigree Charts

Carrier female (sex linked recessive)

Carrier (autosomal recessive)If you see symbols like these around the place this is what

they mean


* Both males and females can beaffected

* All affected individuals will have atleast one affected parent

* A heterozygote will show the trait

* In large samples equal numbers ofmales and females will be affected

* Once the trait disappears from abranch of the pedigree it does notreappear

If individual II-9 married an unaffected woman what would be the chances of their first child being affected?

ExamplesHuntington’s DiseaseWidow’s peakLong EyelashesHair on the back of your hand!


* Both males and females may be affected

* Two unaffected parents can have an affected child

* The trait may skip generations

* The trait is only expressed in a homozygous state

If individual II-12 married an affected male what is the likelihood their first child will be affected?

ExamplesCystic FibrosisPhenylketonuriaSickle-Cell disease


* The male will pass on the trait to all his daughter but not his sons.

* A female will pass the trait to both her daughter and her sons.

* Every affected person has at least one parent with the trait.

If individuals III-8 and III-9 had a son (ew!) what is the probability he would show the dominant trait?

ExamplesVery uncommonRett Syndrome


If individual III-9 had a daughter with II-6 (ew!) what is the probability she would be affected?

* All sons of a female with the trait will be affected

* All the daughters of an affected male will be carriers of the trait

* In a large sample more males than females show the trait.

ExamplesDuchenne Muscular DystrophyHaemophilia A & BRed-Green Colour Blindness


Pedigree Analysis 1: Video here


http://www.youtube.com/watch?v=HbIHjsn5cHo

http://www.youtube.com/watch?v=HbIHjsn5cHo

Pedigree Analysis 2: Video


http://www.youtube.com/watch?v=ej2hFc8u_zQ&feature=related

http://www.youtube.com/watch?v=ej2hFc8u_zQ&feature=related

Pedigree Analysis 3: Video


http://www.youtube.com/watch?v=UU3Ou0c9u0U&feature=related

http://www.youtube.com/watch?v=UU3Ou0c9u0U&feature=related

Mendelian Inheritance: Independent Assortment

For Mono & Dihybrid Crosses: Visit Mendelian Genetics


http://www.biology.arizona.edu/mendelian_genetics/mendelian_genetics.html

http://www.biology.arizona.edu/mendelian_genetics/mendelian_genetics.html

Mendelian Inheritance

Gregor Mendel (1822-1884)

Mendel established an early model of inheritance through a study of pea plants.

Mendel had no knowledge of meiosis but observed that the inheritance of many traits followed the same patterns and ratios throughout subsequent generations.

His work, published in 1866, was ignored by biologists for thirty years. At this time much of the scientific community was focused on Darwin’s Origin of Species and the differences between species- rather than those differences that might exist within a species.

In 1900 Mendel’s model of inheritance was rediscovered and accepted.

You have already covered the basic foundations of Mendelian genetics by conducting pedigree analyses. The emphasis now is on ratio and experimental design.


Mendelian Inheritance

Mendel studied the inheritance of a number of traits in pea plants.

Why pea plants?

Mendel took purebred plants bearing different traits and bred them. This type of experiment is called a cross and Mendel conducted two major versions.

* Monohybrid cross: A study of the inheritance of one trait in each generation.

* Dihybrid cross: Two traits are studied simultaneously in each generation.

When we study test crosses we use the following terms to refer to the different generations.

P Parental generationF1 First filial generationF2 Second filial generation


Monohybrid Cross

Mendel fertilised over a thousand pea plants in one of his most famous monohybrid crosses and these were his results.

Can you

1. Determine which alleles are dominant and recessive?

2. Explain why all the F1 generation are tall?

3. Explain why a 3:1 ratio was observed for the population of tall:short plants in the F2 generation?


Monohybrid Cross- The Punnet Square

The punnet square is a useful tool for predicting the approximate phenotypic ratio of our test crosses.

P generation gametes T T

t Tt Tt

t Tt Tt

All the F1 generation will be heterozygous and tall.

F1 generation gametes

T t

T TT Tt

t Tt tt

We can expect 3/4 of the F2 generation to be tall and 1/4 to be short.

When answering questions about crosses always state your answers as a probability


Dihybrid cross

Heredity

The dihybrid cross studies the inheritance of two traits in an experiment.

So what do we observe if we watch for the inheritance of both these traits?

First, lets cross two purebred parents and look at the F1 generation.

P generation gametes

RR ; yy

YY ; rr

Try this yourself: What would our F1 generation look like if we crossed purebreds YYRR and yyrr?


Dihybrid cross

Heredity

What will be the pattern among our F2 generation?

F1 generation gametes

Rr ; Yy

Rr ; Yy

How many of each pea do we end up with?

Express this as a ratio.


Dihybrid cross

The dihybrid cross adds another trait into the experiment.


Mendelian Inheritance: Summary

Heredity

What similarities did we observe between the monohybrid and dihybrid crosses of purebred pea plants?

F1 generation displayed the same genotypesF1 generation displayed only the dominant phenotypes

F2 generation showed variation in genotypes* 3:1 for monohybrid crosses* 9:3:3:1 for dihybrid crosses


Test Crosses & Linkage


Test crosses

Heredity

A test cross involves one parent who is homozygous for the recessive trait crossed with another parent who shows the dominant phenotype.

The purpose of a test cross is to

* Identify the genotype of the parent expressing the dominant trait as either homozygous or heterozygous

* Establish if the genes are linked in the case of a dihybrid cross

For example, if I want to discover the exact genotype of this round yellow pea I might cross it with a wrinkled green one.

Why would it be important to generate a large number of offspring for this experiment?

RR ; YYor

Rr ; Yy

rr ; yy

All offspring show dominant phenotype: The round yellow pea is likely to have been homozygous.

Some offspring show recessive phenotype: The round yellow peas is likely to have been heterozygous.


Linkage

In the previous examples of dihybrid crosses we assumed considered that the alleles for each trait will sort themselves independently of each other when forming gametes. This is called Independent Assortment.

But what if we were to observe that one allele is usually accompanied by another? We could then assume that the genes are linked- because they are located on the same chromosome the two alleles will often be inherited together. This is called Linkage.

Independent Assortment

An individual with genotype

Aa ; Bb

is capable of these gametes

Linkage

If the alleles are on the same chromosome however

AB / ab

the individual is likely to produce gametes of the type

A a B b

A B A b a B a b

AB / ab

AB ab


Demonstrating No Linkage

Apply a test cross using aa bb (homozygous recessive for both traits)

P generation

gametesAB Ab aB ab

ab Aa Bb Aa bb bb Bb aa bb

Phenotypic ratio 1 1 1 1

Aa Bb x aa bb

If the two gene loci are NOT linked the outcome of the test cross will show four classes of phenotypes in EQUAL proportions.


Demonstrating Linkage

Linkage can be suggested by two kinds of results.

Aa Bb x aa bb

P generation

gametesAB ab

ab AB / ab ab / ab

Phenotypic ratio 1 1

A phenotypic ratio of 1 : 1 will indicate that traits are linked in a dihybrid cross.

Linkage can be suggested by two kinds of results.Thursday, October 28, 2010

Demonstrating Linkage

Linkage can be suggested by two kinds of results.Aa Bb x aa bb

P generation

gametesAB ab

ab AB / ab ab / ab

Phenotypic ratio 1 1

A phenotypic ratio of 1 : 1 will indicate that traits are linked in a dihybrid cross.

Linkage can be suggested by two kinds of results.

P generation

gametesAB Ab aB ab

ab Aa / Bb Aa / bb bb / Bb aa / bb

Phenotypic ratio 1 few few 1

A low incidence of two phenotypes might be accounted for by crossing over during meiosis.

Such a ratio is another indicator of linkage.


Allele Symbols: Video here


http://www.youtube.com/watch?v=mT3Uz9mQsd0

http://www.youtube.com/watch?v=mT3Uz9mQsd0

Trait Variability


Trait Variability- Monogenic and Polygenic

Monogenic traits are due to the action of a single gene with two or more allelic forms.

These traits show discrete (or discontinuous) variation.

Polygenic traits are due to the action of many genes and their allelic forms.

These traits show continuous variation.

Attached earlobes32.9%

Unattached earlobes67.1%

Japanese Earlobes!

Not every gene is responsible for a specific trait. Genes can interact in various ways to produce more varied phenotypes.


Trait Variability- Environmental Factors

Sometimes environmental factors influence gene expression to create a different phenotype.

In acidic soils (low pH) hydrangeas are blue

In alkaline soils (high pH) hydrangeas are pink


Trait Variability- X Inactivation in Female Mammals

Females have two X chromosomes- so how do males get by with only one?

Females don’t need two X chromosomes to survive.

In fact, on one X chromosome

The X chromosome that is inactivated in somatic cells is called a Barr Body.

Female phenotypes can therefore present geneticists with a confusing variety of traits.

75% of alleles on one X will be switched off during embryonic development15% remain activated with another10% may vary their activation state


Trait Variability

Monogenic traits Controlled by a single gene Discrete variation displayed between traits

Polygenic traits Controlled by multiple genes Continuous variation

Environmental Factors

When the phenotype is altered by a change in gene expression in response to the environment

Vary with environmental conditions

X-inactivation75% of alleles on one X are switched off

15% remain activated with another10% may vary their activation state

Increased variety in female phenotypes.

What sort of trait does blood typing belong to?


Epigenetics

Epigenetics refers to changes in phenotype due to modification of the activation of genes such as methyl groups or by chromatin remodeling.

Two different conditions produced by the same mutation on chromosome 15 is explained by epigenetic effects. In this case it is called genomic imprinting.Angelman

Syndrome

Severe learning difficulties, jerky

movements.

Prader-Willi Syndrome

Learning difficulties, insatiable hunger,

behavioural problems

Paternal deletion

Maternal deletion


Documents

Alleles: Their Expression and Inheritance · Normal female Normal male ... Widow’s peak Long Eyelashes Hair on the back of your hand! Thursday, ... who shows the dominant phenotype