Mcdonald Bio11 Chapter5 August24c 2011

7/23/2019 Mcdonald Bio11 Chapter5 August24c 2011

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UNIT 2: Genetic Processes

Chapter 4: Cell Division and Reproduction

Chapter 5: Patterns of Inheritance How are traits inherited, and how can

inheritance be predicted?

Chapter 6: Complex Patterns o Inheritance


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5: Patterns of Inheritance

Canola (Brassica napus) is a

Canadian success story. It was

developed in a traditional selective

breeding program in the 1!"s. It is

now a valuable Canadian crop that

benefits from continued modern#

molecular genetics research.

Chapter !: Patterns o InheritanceUNIT 2


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5.1 $nderstanding Inheritance%hile people bred animals and plants for thousands of years without understanding th e mechanisms of inheritance# eventually theories and e&planations of how breeding wor'ed were proposed.

he first widely accepted theory was pangenesis# proposed by ristotle. It suggested that sperm an d egg contained tiny par ticles from all body parts.

*thers thought that only the sperm had such an essence. In fact# it was proposed that an entire miniature huma n being was inside the sperm+

,y the 1-""s# people settled on the idea that traits from the parents were irreversibly blended in the offspring.

one of these theories was based on scientific evidence.

UNIT 2 Chapter !: Patterns o Inheritance "ection !#$


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/regor 0endels 2&periments/regor 0endel (1-334-)# an ugustinian mon'# used scientific methods to solve the mystery of how traits were inherited. ,efore his t ime at the monastery# he studied botany and mathematics# which proved invaluable to his observations.

*ne of the 'eys to his discovery was the plant type he chose to wor' with: pea plants. Pea plants come in many varieties and show different traits (characteristics e&hibited by an organism). In addition# they usually self6fertili7e# which allowed 0endel to start

with plants that were true breeding (same outcome traits every generation). 8e carefully cross-pollinatedtrue6breeding pea plants.


%et&een $'!6 and $'6()

*endel +red) tended) andanal,-ed more than 2' ...

pea plants in his monaster,

/arden#


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0endels 0onohybrid 2&periments0endel started every e&periment with plants that were true breeding for a trait but that e&hibited a different

form of the trait. 8e called this the parental# or P generation. *ffspring were called the first filial (91)

generation. hese e&periments were called monohybrid crossesbecause only one (mono) trait was monitoredat a time. 8owever# 0endel studied seven different traits in his e&periments.



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0endels esults

Example 1:P generation of male yellow6pea6producing plant and female green6pea6producing plant

P generation cross results: ll offspring (91generation) were the same seed colour: yellow# i.e.# one parents seed colour

trait seemed to disappear. his result was the same for each of the seven traits he studied.


Continued


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0endels esultsExample 2:91generation of yellow6pea producing plants

91generation cross results: In the 93generation# some peas were yellow and some green. 0athematically# the ratio was ;:1yellow:green. his ratio was the same for all seven traits that 0endel studied.



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he factors? for each trait. oday we call those factors >alleles.? ecall that diploid organisms have two alleles for each gene.

8e also concluded that one factor@allele is always dominant# and one is recessive. In the e&ample# yellow colour is dominant over green when it comes to the colour of seeds in the pea plant.

0endel proposed the >law of segregation? to e&plain this: raits are determined by pairs of alleles that segregate during meiosis so that each gamete receives one allele (updated terminology).



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/enotype and Phenotypeo e&press alleles easily in written form# upper and lower case letters are used. dominant a llele is represented by the first letter of the alleles description. he recessive allele then receives

the lower case of the same letter.

Aellow pea allele: Y /reen pea allele:y

In each plant# two alleles are present so the possible combinations are:YY# Yy# oryy. his is the plants genotype.

he actual colour of the peas is the plants phenotype.


C


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=ection 5.1 eview


UNIT 2 Ch t ! P tt I h it " ti ! 2


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5.3 =tudying /enetic Crosses

he possibility of a certain allele pac'aged in a gamete is B since there aretwo alleles in a diploid cell and only one is pac'aged in a haploid gamete.

UNIT 2 Chapter !: Patterns o Inheritance "ection !#2

hus# when determining

the possible outcomes of a

monohybrid cross# there is

B B D E# or a 35F

chance of eachcombination of alleles in

the offspring. %e use a grid

called a Punnett sGuare to

show the law of

segregation and possiblecross outcomes.



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$sing Punnett =Guares Punnett sGuare demonstrates the possible 91outcomes from a cross between two

hetero7ygous parents. In this case# the parents are hetero7ygous for flower colour.

he phenotype ratio is ;:1 for flower colour (purple to white).




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est Crosses%hen geneticists want to 'now if an individual is hetero7ygous or homo7ygous for a dominant phenotype#

they do a test cross. test cross is a cross between an individual of un'nown genotype for a trait and anindividual that is homo7ygous recessive for that trait. naly7ing the phenotype should provide insight into

the un'nown genotype.


In a test cross) i an,

o the osprin/ sho&

the recessive

phenot,pe) the

un0no&n /enot,pe o

the parent must +e

hetero-,/ous#



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%hat do you thin' the 93generation loo'ed li'eH

2&plain your answer.

0endels ihybrid Crosses0endel also designed e&periments to follow the inheritance pattern of two traits to determine if the inheritance of one trait affected another.

8e crossed true6breeding plants that produced yellow# round seeds (YYRR) with true6breeding plants that produced green# wrin'led seeds (yyrr). he peas in the 91

generation all displayed the dominant trait for both traits (yellow and round).


Continued



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0endels ihybrid Crosseshe 91generation self6fertili7ed to create the 93generation. It had a mi& of four phenotypes but came

close to the ratio :;:;:1 (yellow# round to yellow# wrin'led to green# round to green# wrin'led).


UNIT 2 Chapter !: Patterns o Inheritance "ection ! 2


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0endels esults Punnett sGuare can show the

segregation of the gametes for two traits.

2ach parent can pac'age the alleles in the

gametes in four different ways.




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he


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he Chromosome heory of Inheritance%hen 0endel performed his e&periments and formulated his laws of inheritance# the process of meiosis and the e&istence of chromosomes had not been discovered. ,y the early 1""s# scientists began to see the lin' between both.


Continued



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he Chromosome heory of Inheritance

In 1"3# %alter =utton showed that the behaviour of chromosomes during meiosis was related to the behaviour of 0endels factors. 8e reali7ed

that during gamete formation# alleles segregate Just as homologous chromosomes do# and proposed that genes are carried on chromosomes. his

formed the basis of the chromosome theory of inheritance: /enes are located on chromosomes# and chromosomes provide the basis for the

segregation and independent assortment of alleles.




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=ection 5.3 eview


UNIT 2 Chapter !: Patterns o Inheritance "ection ! (


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5.; 9ollowing Patterns of Inheritance

in 8umans/eneticists who study human inheritance collect as much information as they can and use it to create adiagram called a pedigree. pedigreeis a type of flow chart that uses symbols to show the inheritance

patterns of traits in a family over many generations. hey help uncover the genotype of a particular member

of a family# and they can be used to predict phenotypes and genotypes of future offspring.

UNIT 2 Chapter !: Patterns o Inheritance "ection !#(

8ow is human genetic research different from genetic

research on plants and animalsH

Continued



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9ollowing Patterns of Inheritance in 8umans




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utosomal InheritanceAutosomal inheritancerefers to the inheritance of traits whose genes are found on the autosomes (chromosomes 1 22). These chromosomes hold

normal, functioning genes (hair colour, freckles) as well as disorder genes (cystic fibrosis, Huntington disease).

An autosomal dominantdisorder occurs when the disease-causing allele is dominant and an individual has one or both copies of the allele. An

autosomal recessivedisorder occurs when the disease-causing allele is recessive and an individual has both copies of the allele.




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utosomal Inheritance%hen using a pedigree to study a disorder# you can determine

if the pattern is autosomal dominant or autosomal recessive.


8untington isease: utosomal ominant

1n unaected child +orn o t&o

aected parents indicates autosomal

dominant

inheritance#

This pedi/ree sho&s the inheritance pattern

or an autosomal dominant disorder# Notice

that an aected child must have at least one

aected parent to +e aected#



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utosomal Inheritance


Cystic 9ibrosis: utosomal ecessive

In autosomal recessive inheritance) i+oth parents are hetero-,/ous or the

disorder) the, &ill have an aected child# This pedi/ree sho&s the inheritance pattern

or an autosomal recessive disorder#

Notice that the appearance o the recessive

phenot,pe can s0ip /enerations) and that t&o

unaected parents can have an aected

child#



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ests for /enetic iseases




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/enetic Counselling


genetic counsellorhas special training in human genetics

and in counselling. family may see' a counsellor whenthere is a history of a genetic disorder in the family.

Counsellors often use pedigrees to determine offspring ris'.



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=ection 5.; eview


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Mcdonald Bio11 Chapter5 August24c 2011