Patterns of Inheritancefaculty.collin.edu/cdoumen/1408/1408_Ch8_9/1408_Ch9A.pdf · 2015-11-22 ·...

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Patterns of Inheritance

 Dogs are one of man’s longest genetic experiments.

  Over thousands of years, humans have chosen and mated dogs with specific traits.

Introduction

© 2012 Pearson Education, Inc.

The results :

•  an incredibly diversity

•  distinct body types and

•  behavioral traits.

9.1 The science of genetics has ancient roots

  This breeding of animals is actually accelerated evolution, where man instead of nature pre-determined what traits he/she wanted to see in the next offspring.

  This idea that hereditary materials can be passed on and mixed in forming offspring, was termed the blending hypothesis, –  suggested in the 19th century by scientists studying

plants but

–  later rejected because it did not explain how traits that disappear in one generation can reappear in later generations.


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9.2 Experimental genetics began in an abbey garden

Gregor Johann Mendel (1822–1884)

  Was an Austrian friar (monk) when he began the field of genetics in the 1860s

  He deduced the principles of genetics by breeding garden peas

  relied upon a background of mathematics, physics, and chemistry ( and obviously biology as well).



  In 1866, Mendel –  correctly argued that parents pass on to their offspring

discrete “heritable factors” and

–  stressed that the heritable factors (today called genes), retain their individuality generation after generation.

 A heritable feature that varies among individuals, such as eye color or flower color, is called a character.

 Each variant for a character, such as purple or white flowers, is a trait.



Although the significance of Mendel's work was not recognized until the turn of the 20th century, the independent rediscovery of these laws formed the foundation of the modern science of genetics


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 Heredity is the transmission of traits from one generation to the next.

 Genetics is the scientific study of heredity.

In the following, we will discuss the experiments that provided the basis for Mendel’s discoveries.

Thus we need to first look at some simple plant anatomy and sexuality.



The reproductive parts of a flowering plant are located within the flowers

•  Pollen is located on the anther parts of the stamen.

•  Pollen contains the plant sperm.

•  Pollination is the movement of pollen from the anthers to the stigma; fertilization occurs in the ovule

•  The fertilized ovules produce seeds that are the next generation


Many flowers are capable of self-fertilization. Thus, the pollen of a flower pollinates the stigma of the same flower and results in seed formation.


One would expect that the resulting seeds produce offspring all identical to the parent.

This is however not always the case as we will see later.

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–  Science often requires a good dose of scientific design and luck.


Mendle was fortunate that these pea plants he worked with were •  easy to manipulate •  capable of self- fertilization and

cross fertilization. •  Showed many traits that

occurred in an either/or format (blue/white; tall/short). Stamen

Carpel

Petal

Experimental design

Pollination in nature occurs by different mechanisms ( wind, insects, birds). But plant breeders early on knew they could do this manually with simple tools such as fine paint brushes.


Removal of stamens

Carpel

White

Stamens Transfer of pollen Purple Parents

(P)

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To prevent self fertilization, the stamen of the flowers would be removed for example.

Some Important definitions

  True-breeding varieties are defined as plants that, when self-fertilized, produces offspring all identical to the parent.

  True-breeding parental plants are called the P generation.

  If we look for example at the trait of flower color: the outcome of self-fertilization of a true-breeding pea plant with purple flowers will always be pea plants with purple flowers (because there is only one genetic variety of that gene present in the plant’s genome).

  Or if we cross-fertilize two true-breeding pea plants with purple flowers, the result will always be 100 % plants with purple flowers.


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  True-breeding plants


Self-fertilization Cross fertilization

Offspring

X

  BUT a true breeding plant for flower type may not be true breeding for another trait such as plant height, or seed color,… !


  The offspring of two different varieties are hybrids.

  The cross-fertilization is a hybridization, or genetic cross.

 Hybrid offspring from a P generation cross are the F1 generation.   In this example, we have two parental true breeders : one

producing purple flowers, the other producing white flowers.

  In this example, the F1 generation is also 100% purple flowered.

Removal of stamens

Carpel

White

Stamens Transfer of pollen Purple

Carpel matures into pea pod

Seeds from pod planted

Offspring (F1)

Parents (P)

2

3

1

4

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 A cross of F1 plants or self-fertilization among F1 plants produces a new generation of plants referred to as the F2 generation.


9.3 Mendel’s law of segregation describes the inheritance of a single character

 A cross between two individuals differing in a single character is a monohybrid cross.

 So, crossing plants and looking only for the differences in flower color, where the flower colors have only two possibilities, refers to a mono-hybrid cross

 Mendel performed several monohybrid crosses between pea plants and initially only looked at each trait as individual events.


Character Traits

Dominant Recessive

Flower color

Purple White

Flower position

Axial Terminal

Seed color Yellow Green

Seed shape

Round Wrinkled


•  The following two slides shows the traits he was crossing for

•  Each of them nicely showing up as discrete characters (thus there were no in-between states)

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Figure 9.2D_2

Character Dominant Recessive

Traits

Pod shape Inflated Constricted

Pod color Green Yellow

Stem length

Tall Dwarf


 Mendel performed a monohybrid cross between a plant with purple flowers and a plant with white flowers.

 Doing hunderds of crosses, planting the resulting seeds and observing the flowers of over a thousand plants he observed that

–  The F1 generation produced all plants with purple flowers.

–  A cross of F1 plants with each other produced an F2 generation with ¾ purple and ¼ white flowers.


The Experiment P generation (true-breeding parents)

F1 generation

F2 generation

of plants have purple flowers

of plants have white flowers

Purple flowers

White flowers

All plants have purple flowers

Fertilization among F1 plants (F1 × F1)

×

3 4

1 4

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  Thus the keen eye of Mendel was the he observed that none of the F1 generation plants were light purpled AND the offspring of the all-purple F1 generation did not produce any light purple flowers either.

  This refuted the old standing hypothesis (the blending hypothesis) that assumed traits become blended when individuals reproduce and “mix” their gametes ( a mix of purple and white would have generated light purpled flowers, but it didn’t ).



 Mendel observed the same patterns of inheritance for six other pea plant characters.


Trait Dominant Recessive Ratio

Stem length 787 277 2.84

Seed color 6022 2001 3.00

Pod color 428 152 2.82

Seed shape 5474 1850 2.96

Pod shape 882 299 2.95

  All characteristics showed up as either/or (no mixing).

  All ratios between dominant and recessive trait in the F2 generation approached 3:1


 Mendel needed to explain why

–  white color seemed to disappear in the F1 generation and

–  white color reappeared in one quarter of the F2 offspring.

 Mendel developed four hypotheses, described below using modern terminology.

 Keep in mind that in Mendel’s time, nothing was known about DNA and genes. He used terms as “heritable factors”, that retain their individuality from generation to generation


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1. Those heritable factors can exist in alternative versions that account for variations in inherited characters. These days we call these alternative versions of genes alleles

2. For each characteristic, an organism inherits two alleles, one from each parent. The alleles can be the same or different.

–  A homozygous genotype has two identical alleles.

–  A heterozygous genotype has two different alleles.



3.  If the alleles of an inherited pair differ, then one determines the organism’s appearance and is called the dominant allele. The other has no noticeable effect on the organism’s appearance and is called the recessive allele.

–  The phenotype is the appearance or expression of a trait.

–  The genotype is the genetic makeup of a trait.

–  The same phenotype may be determined by more than one genotype.



4.  A sperm or egg carries only one allele for each inherited character because allele pairs separate (segregate) from each other during the production of gametes. This statement is called the law of segregation.


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  Thus Mendel viewed cells as having 2 heritable factors for each trait, and they segregate independently into the gametes


A A

A a

Dominant trait (allele)

Recessive trait (allele)

A A A a

Homozygous cell Heterozygous cell

Possible gametes Possible gametes


  In terms of what we have seen with respect to chromosomes and meiosis, the homologous chromosomes carry the alleles. Meiosis would look like this for a heterozygous cell .


A a A A

a a A a

A

a

Meiosis I Meiosis I Meiosis II

A

a

Segregation of the alleles into gametes


 Often, to simplify things, the first letter of a dominant trait is used to designate that specific trait.

  For example :

–  Purple flowers are dominant, white flowers are recessive. Thus the letter P is used to designate the flower color trait

–  Tall plants are dominant over small plants. The letter T is used to designate the plant length trait.


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  To differentiate between the dominant allele and recessive allele of a trait , a capital letter is used for the dominant allele, a small letter for the recessive allele

  With respect to flower color, since purple is dominant

  P is for the dominant allele, p for the recessive allele ( so even though recessive color is white, we do not use the letter w).

  With respect to length, since tall is dominant

  T is for the dominant allele, t for the recessive allele ( so even though recessive length is small, we do not use the letter s here).



 Remembering now that a diploid cell has pairs of homologous chromosomes, thus each trait is represented by 2 alleles

  In simple terminology, we write these alleles within the cells out as follows


Trait Homozygous Dominant

Heterozygous Homozygous Recessive

Flower Color PP Pp pp

Plant length TT Tt tt

Pod color GG Gg gg

Seed shape RR Rr rr


  Looking back at Mendel’s experiment, he initially crossed two true-breeders: one purple flowered pea plant with a white flowered pea plant.

  Thus these are the P generation plants. True-breeders indicating that they are homozygous for their trait.

  The purple homozygous flowered plants are thus PP, the white homozygous flowered plants are pp.

 What possible gametes can each of them produce ?


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 Since they are homozygous, they only have one kind of allele (present twice), and thus can only produce one kind of gamete with respect to this trait.


PP

pp

P generation gametes

100%

100%

P

p


  Crossing these two parent plants provides us the F1 generation. Only one possible genotype can result from this cross: all offspring will be Pp , 100% heterozygous


PP

pp

P generation gametes

100%

100%

P

p Genotype of F1 : Pp

Phenotype : purple flower

Pp

F1 generation: 100% Pp


  If we now cross the F1 generation with each other, we create the F2 generation.

  So we first have to find out what possible gametes the F1 generation can produce ? Possible gametes are P and p.


F1 generation crossing

Possible gametes

Pp

Possibilities for F2 generation ?

P

Pp P

p

p

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PP


  The possibilities for the F2 generation are the possible combination of gametes between the 2 individuals being crossed.


F1 generation crossing

Possible gametes

Pp

Possibilities for F2 generation ?

P

Pp P

p

p

Pp

Pp

pp


 Mendel’s hypotheses also explain the 3:1 ratio in the F2 generation.

–  The F1 hybrids all have a Pp genotype and purple phenotype

–  Their gametes are a 50:50 % of P and p

–  The F2 generation ends up having the genotypes 1 PP, 2 Pp and 1 pp (1:2:1 ratio, see previous slide)

–  The phenotypes are 3 purple to 1 white flower (3:1 ratio) since every genotype with at least one P allele will display the purple flower trait


9.3 Mendel’s law of segregation and Punnet Squares

  Punnett squares make it easier to show the four possible combinations of alleles that could occur when these gametes combine.

  They also show the % possibilities of these outcomes, very similar as what one would expect when flipping coins.

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  Question : what are the possible outcomes when flipping a coin ?

  Question : does the outcome of flipping of a coin influence the outcome flipping the coin again ?

  The answer to the first question is of course heads or tails ( only two possibilities)

  The answer to the second question is NO. These are called independent events.

  Similar, flipping two coins are independent events as well. The outcome of flipping one coin has no effect on flipping a second coin.


– What are the probabilities of “heads” vs “tails” ?

– We call the outcome usually 50% or half and half.

–  If the two possible events have to add up to 1, the outcome of each would be ½ (= 0.5) ( heads would be ½ and tails would be ½; together ½ + ½ = 1)

–  The other way of asking, what is the % chance of each. In this case, since the sum of the two events has to add up to 100%, each event has a 50% chance of occurring (50 % + 50% = 100%)


  So what are the possible outcomes of flipping two coins in a row ? Heads = H, tails = t

  HH

  Ht

  tH

  tt

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H

t H

t

Possible outcome

HH Ht

Ht tt

  HH

  HT

  TH

  TT

  We can put the possible outcome of each flip into a Punnet square and that visualizes the 4 possibilities easier.


H

t H

t

Possible outcome

HH 25%

  This also provides us with the probability of each event.

Ht 25%

Ht 25%

tt 25%

  The outcome of two independent events, is multiplying their individual possibility.

  H = ½, t= ½, thus for example the outcome HH = (1/2)x(1/2)=1/4 (or 25%)

  And since there are are 4 squares, each one has a 25 % possibility

1/2 1/2

1/2

1/2


H

t H

t

Possible outcome

HH 25%

  This also provides us with the probability of each event.

Ht 25%

Ht 25%

tt 25%

  The outcome of all events in this case :

  HH 25 %

  HT 25+25= 50%

  tt 25 %

1/2 1/2

1/2

1/2

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  Punnett squares are used to show the four possible combinations of alleles that could occur when gametes combine during fertilization.

Pp

Pp

P

p P

p

gametes

PP Pp

Pp pp

  The squares show the combinations between the gametes.

Male

Female

Figure 9.3B_s3 The Explanation

P generation

F1 generation (hybrids)

F2 generation

Genetic makeup (alleles) Purple flowers White flowers

Gametes P All p

pp PP

P

P

P

p

p

p

PP Pp

Pp pp

Eggs from F1 plant

Gametes

Fertilization

All Pp Alleles

segregate

Phenotypic ratio 3 purple : 1 white

Genotypic ratio 1 PP : 2 Pp : 1 pp

Sperm from F1 plant

2 1

2 1

All

F2 generation P

P

p

p

PP Pp

Pp pp

Eggs from F1 plant

Phenotypic ratio 3 purple : 1 white

Genotypic ratio 1 PP : 2 Pp : 1 pp

Sperm from F1 plant


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Practice Examples

  Assume in rabbits, white coat is recessive, brown is dominant.

  How would you write out the following genotypes ?

  Homozygous white :

  Homozygous dominant :

  Heterozygous :

  What color would the heterozygous rabbit have ?

Practice Examples

  Using a Punnet Square, what are the possible genotypes and phenotypes when crossing a homozygous brown bunny with a white bunny ?

  Using a Punnet Square, what are the possible genotypes and phenotypes when crossing a homozygous brown bunny with a heterozygous brown bunny ?

Practice Examples

  Straight hair is dominant in humans and curly hair is recessive .

  How would you write out the following genotypes ?

  A person with straight hair :

  A person with curly hair :

Patterns of Inheritancefaculty.collin.edu/cdoumen/1408/1408_Ch8_9/1408_Ch9A.pdf · 2015-11-22 ·...

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