2. principles of inheritance

7/29/2019 2. Principles of Inheritance

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Most slide were adapted directly from various sources in the internet


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Native Hybrid

S

EL

E

C

T

I

V

E

breeding


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8000-3000 BC. Egyptian domestication of animals


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How are traits passed on from

parents to children?

The study of how traits are inherited or

how genetic information is passed from

one generation to the next is called

GENETICS

Genetics also explains biological variation


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1. Pre-Mendelian blending theory

2. Gene theory (which traces back to

Mendel)

How are traits passed on from

parents to children?


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1. 19th Century Hypothesis(before Mendel)

inheritance was the blending ofcharacteristics

We look somewhat like our parents characteristics were merely added together:

the next generation is a blend

How would the population look like

after many generations?


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Blending in Parakeet:

Yellow x Green = Blue?

Several shades!!!


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Fact: There is tremendous diversity of life forms

About 8.8 million species; onlya quarter has been discovered!!!


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inheritance as the blending ofcharacteristics was incorrect

No one studied theproblem systematicallyuntil Gregor Mendel

(1822-1884)

1. 19th Century Hypothesis(before Mendel)


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Gregor Mendel (1822-1884)

A little known Austrian monk (flunked out ofcollege twice but became a mathematician)

Spent adult life in relative obscurity doing

basic genetics research and teaching highschool mathematics, physics, and Greek inBrno, Czech Republic

Published his ideas in 1866; work largely wentunrecognized until 1900, which was long afterhis death.


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Why study Mendel

He was the first to demonstrate the principles

of inheritance

He used a systematic scheme based upon an

understanding of mathematics and statistics

before formal statistics was developed

He applied his knowledge of mathematics

with an excellent scientific method


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2. Today: The gene theory

An alternative idea to blending is the

gene idea.

Parents pass on discrete individualheritable units called genes

Genes are segments of DNA in a

chromosome A gene has different forms/variations

called alleles


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What is a gene?

A gene is a stretch of DNA whose sequencedetermines the structure and function of a

specific functional molecule (usually a protein)

DNA

Protein

GAATTCTAATCTCCCTCTC

AACCCTACAGTCACCCATTT

GGTATATTAAAGATGTGTTG

TCTACTGTCTAGTATCC

Computer program

Specific function

functionsf(){document.

f.q.focus()}

Working copymRNA


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Homologous chromosomes bear the two

alleles for each characteristic

Alternative forms of a gene reside at the samelocus on homologous chromosomes


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DNA

(deoxyribonucleic acid)

mRNA

Protein


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Karyotyping

is matching

the paired

chromosomes


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Down syndrome karyotype

(trisomy 21)


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Mendels experiments


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Mendels

data


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Mendel deduced the underlying principles of

genetics from these patterns

1. Segregation

2. Dominance

3. Independent

assortment


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1. Mendels Law of Segregation

A normal (somatic) cell has two variants (alleles) for aMendelian trait.

A gamete (sperm, egg, pollen, ovule) contains one allele,

randomly chosen from the two somatic alleles.

E.g. if you have one allele forbrown eyes (B) and one for

blue eyes (b), somatic cells

have Bb and each gamete

will carry one of B or b

chosen randomly.

B b

B BB Bb

b Bb bb

Eggs

Sperm


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Two types of cell division

Mitosis Meiosis

All body cells Sex cells:

egg/sperm

2 daughter cells 4 d-cells

Equal # of

chromosome/s

Half of the number


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1. Mitosis: by all body cells

Cell cycle:

G Growth

S - Synthesis

M - Mitosis

Homologoue = pair of the same type of chromatids

Diploid = paired state

Haploid = unpaired


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2. Meiosis: in gametes (sperm or egg) only

Gametes are either egg or spermChiasma point of crossing over (recombination)


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Meiosis in a Nutshell

Meiosis is a special type of cell division that

occurs in sexually reproducing organisms.

It reduces the chromosome number by half,enabling sexual recombination to occur.

Itensures that an organism has the samenumberof chromosomes in each generation.


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Terminology

Haploid:

containing one

copy of each

chromosome

(n=23)

B b

B BB Bb

b Bb bbEggs

Sperm

Diploid:

containing two

copies of eachchromosome

(2n=46)

Punnett Square of a Monohybrid Cross


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Terminology

Genotype: the states of the two alleles at one

or more locus associated with a trait

Phenotype: the state of the observable trait

Genotype Phenotype

BB (homozygous) Brown eyes

Bb (heterozygous) Brown eyes

bb (homozygous) Blue eyes


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2. Mendels Law of Dominance

If your two alleles are different (heterozygous,e.g. Bb), the trait associated with only one of

these will be visible (dominant) while the

other will be hidden (recessive).E.g. B is dominant

b is recessive

B b

B BB Bb

b Bb bb

Eggs

Sperm


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2. Law of Dominance...

If your two alleles are different (heterozygous, e.g. Bb),the trait associated with only one of these will be visible

(dominant) while the other will be hidden (recessive).

E.g. B is dominant, b is recessive.

B b

B BB Bb

b Bb bb

Eggs

Sperm


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3. Mendels Law ofIndependent

Assortment

Knowledge of which allele has been inherited at one

locus gives no information on the allele that has been

inherited at the other locus

S/s Y/y

SY Sy sY sy

25% 25% 25% 25%


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S Y

s y

Gametophytes

(gamete-

producing

cells)

S Y

s yGametes

S y

s YRecombinants

Recombination

Segregation

3. Law ofIndependent Assortment.

Red father

Blue - mother


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Simplified view of eye colour inheritance: bi-

allelic Mendelian trait

- Brown dominant: BB, Bb

Blue recessive: bb

Human eye colour

B b

B BB Bb

b Bb bbEggs

Sperm


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Human eye colour

?

B?

B?B?B? bb

bb B?

BB, Bb (Brown, dominant) Bb (blue, recessive)

What are the possible eye color/s of this child?


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Non-Mendelian Inheritance:

Incomplete Dominance

Some alleles for a

gene are not

completely

dominant over the

others. Thisresults in partially

masked

phenotypes which

are intermediate tothe two extremes.


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Non-Mendelian : Sex Linkage

All chromosomes are homologous except on

sex chromosomes.

Sex chromosomes are eitherX orY.

If an organism is XX, it is a female, if XY it ismale.

If a recessive allele exists on the X

chromosome. It will not have a corresponding

allele on the Y chromosome, and will therefore

always be expressed


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The 23rd pair of chromosome during

fertilization

Possible

combinations

Sex: F F M M 1 female: 1 male


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Human Sex Linkage

Hemophilia (X-linked): Disorder of the blood

where clotting does notoccur properly due to afaulty protein.

Occurs on the Xchromosome, and isrecessive.

Thus a vast majority ofthose affected are males.

First known person knownto carry the disorder was

Queen Victoria of England.Thus all those affected arerelated to Europeanroyalty.

P f di i h i k f 1000 f


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Patterns of disease inheritance known for 1000s of

years, e.g. haemophilia


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Colour blindness is usually passed down from

mother to son on the 23rd chromosome (X-linked)


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Y-linked inheritance

baldness hairy ear lobes


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Phenotypes are controlled by more than 1 allele. Eg. Blood types

are regulated by 3 separate genes.

ABO Blood typing Humans have multiple types of surface antigens on RBC's

The nature of these surface proteins determines a person'sBlood Type.

There are 3 alleles which determine blood type IA, IB, or IO. Thisis referred to as having multiple alleles

Human blood types are designated as A, B or O. Type A denotes having the A surface antigen, and is denoted by IA

Type B denotes having the B surface antigen, and is denoted by IB

Type O denotes having neither A or B surface antigen, and isdenoted by IO

There are several blood type combinations possibleA , B, AB (Universal recipient) , O (Universal donor)

Non-Mendelian: Multiple Alleles


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Non-Mendellian: Continuous Variation

Many traits

may have a

wide range of

continuous

values. Eg.Human height

can vary

considerably.

There are notjust "tall" or

"short" humans


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Non-Mendelian: mtDNA

Mitochondrial DNA point mutations are maternally inherited and can

occur in rRNA or tRNA genes, or in genes coding for proteins of

respiratory chain complexes.


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Phenotypes associated with mtDNA

mutations

Longevity

Stroke

Seizures Reduced sperm motility

Common migraine

Bipolar disorder Exercise intolerance,

fatigability


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Non-Mendellian:

Gene interaction: Many biological pathways are governed by multiple

enzymes, involving multiple steps. If any one of thesesteps are altered. The end product of the pathway

may be disrupted.

Environmental effects: Sometimes genes will not be fully expressed owing to

external factors. Example: Human height may not befully expressed if individuals experience poornutrition.


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Summary

Mendel deduced three simple laws ofinheritance:

Segregation

Dominance Random assortment

The majority of traits dont follow these

rules but Mendels laws are neverthelesscrucial to understanding almost all genetic

inheritance


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Levels of biological diversity:

Species diversity

Ecosystem diversity

Genetic diversity Molecular diversity


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Watch

http://highered.mcgraw-

hill.com/sites/0072495855/student_view0/ch

apter2/animation__mitosis_and_cytokinesis.h

tml
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__mitosis_and_cytokinesis.htmlhttp://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__mitosis_and_cytokinesis.htmlhttp://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__mitosis_and_cytokinesis.htmlhttp://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__mitosis_and_cytokinesis.htmlhttp://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__mitosis_and_cytokinesis.htmlhttp://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__mitosis_and_cytokinesis.htmlhttp://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__mitosis_and_cytokinesis.htmlhttp://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__mitosis_and_cytokinesis.htmlhttp://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__mitosis_and_cytokinesis.html


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Web sites to check

http://gslc.genetics.utah.edu/units/basics/tour/inheritance.swf

http://science.nhmccd.edu/biol/genetics.html

http://library.thinkquest.org/20465/games.html
http://gslc.genetics.utah.edu/units/basics/tour/inheritance.swfhttp://gslc.genetics.utah.edu/units/basics/tour/inheritance.swfhttp://science.nhmccd.edu/biol/genetics.htmlhttp://library.thinkquest.org/20465/games.htmlhttp://library.thinkquest.org/20465/games.htmlhttp://library.thinkquest.org/20465/games.htmlhttp://library.thinkquest.org/20465/games.htmlhttp://science.nhmccd.edu/biol/genetics.htmlhttp://gslc.genetics.utah.edu/units/basics/tour/inheritance.swfhttp://gslc.genetics.utah.edu/units/basics/tour/inheritance.swf

2. principles of inheritance

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