Biology

Patterns of Inheritance

             

Key Terms & Scientists

Genetics Traits Blending hypothesis Mendel Self-fertilization (true breeding) Hybrid Cross pollination Monohybrid/Dihybrid Parental generation First/second filial (generation) Genes/alleles Dominant/recessive Homozygous & heterozygous Law of Segregation & Law of Independent Assortment              Probability Punnett Square Testcross Complete & incomplete dominance Codominance Intermediate inheritance Polygenic traits Multiple alleles Pleiotropy Chromosomal Theory of Inheritance Linked & sex-linked genes Sex Chromosomes Autosomes

Inheritance



Genetics

heredity

.

is the scientific study of

  A

trait

is a characteristic that is passed from parent to offspring

(ex. Eye color).

The

blending hypothesis

was once believed to be the way traits were inherited from generation to generation. Think mixing paints. This

is the idea that each generation is a mix (or blend) of both

parents genes (traits).

not account for the appearance

of unexpected traits.

This does



Traits are passed to offspring through chromosomes

.

Genes DON’T Mix!

http://www.google.com/imgres



Gregor Mendel, an Austrian Monk, (1860’s) studied the pea plant.

 He

knew nothing of molecular biology

(or chromosomes).  He

did NOT support the blending hypothesis, and in fact, disproved it through his studies .



He is the father of genetics

.

http://mendel.imp.ac.at/mendeljsp/images/mendel3.jpg

Mendel used the pea plant for 3 reasons:

1. The

structure of the pea flowers allowed:



self fertilization

(

which means the plant can breed with itself

, a process called

pure breeding

) 

OR he could cross pollinate the flowers and produce a

hybrid

is

an organism that receives different forms of a genetic trait from each parent, or 2 sets of DNA: 1 from each parent

).

(this

2.

The rapid reproduction cycle: the pea plant reproduces about every 90 days.

http://www.google.com/imgres

3.

The presence of distinctive traits allowed Mendel to observe his results easily

. He studied 7 traits (we will look at 5). Traits in the pea plant have only 2 forms

(there is NO intermediate or in between form; it is either/or): Purple (P) vs. white (p)= flower color Yellow (Y) vs. green (y)= pea color Round (R ) vs. wrinkled (r )= pea shape Green (G) vs. yellow (g)= pod color Tall (T) vs. short (t)= height

http://www.google.com/imgres

Mendel’s Observations:



When Mendel worked with the pea plants he used 2 different groups of purebred plants , looking at 1

trait at a time.

 For example, he used 1

group of purebred purple flower pea

plants & 1 group of

purebred white flower pea plants .

http://www.google.com/imgres

http://www.google.com/imgres  (

He crossed these 2 groups with each other

cross pollinated

them)

and

P

.

called them the

parental generation, or

◦ This is a

monohybrid cross

(crossing 1 trait).



All of the offspring had purple flowers

. ◦

This generation did not show up as a blend of parents (no mix b/c they

are not less purple). But,

where did the white flower trait go?



He called this generation of offspring

First Filial or F1 generation

offspring

).

(

the filial refers to



The offspring is a hybrid of the parents .



He allowed the F1 generation to self fertilize . He called this generation the

second filial, or F2 generation

.



The F2 offspring revealed 3 out of 4 had purple flowers

and 1

out of 4 had white

flowers.

Again, no blending resulted.

Also, the white flower trait had NOT disappeared.

 Mendel performed this experiment with all 7 traits and received the same results:

the offspring is not a mixture of the parents; the original traits do not disappear.

 In his work, all F1 revealed 1 characteristic: this characteristic is dominant. All F2 generations were in a 3:1 ratio (3 dominant: 1 recessive).

F1 generation

http://wps.prenhall.com/wps/media/objects/487/498795/CDA10_1.jpg

F2 generation



Genes

trait.

are sections of a chromosome that code for a

◦

Most organisms have (1 from each parent).

2 copies for every gene and chromosome

  An

allele

is a distinct form of a gene. ◦

If an organism has 2 different alleles for 1 trait, only 1 allele is expressed or visible (usually).

http://www.google.com/imgres

 The

dominant allele

that is fully expressed when 2 different

alleles are present.

is a form of a gene

◦ ◦

This is represented with a capital letter (and is written 1st). Ex. Purple= P

 The

recessive allele

that is not expressed when paired with a dominant allele is a form of a gene (it takes 2 recessives

to be expressed). ◦ ◦

This is represented by a lower case letter & is written 2nd

.

Ex. White= p



The Chromosome Theory of Heredity

(developed by Walter Sutton)

chromosomes .

states that the material of inheritance is carried by the genes in the

 A

genotype

is the genetic makeup of an organism. Ex: GG, Gg, gg or BB, Bb, or bb

 A

phenotype

is the physical expression of the genotype or the outward expression of that trait.

Ex: yellow peas.

http://www.google.com/imgres



Homozygous

is having 2 of the same alleles (2 identical alleles). Ex: GG or gg



Heterozygous

is having 2 different alleles. Ex: Gg

Mendel’s Laws:

These are the Rules of inheritance:

1. The Law of Segregation:

Gene pairs separate when gametes form.

This means: genes (alleles) are on chromosomes; chromosomes separate during meiosis; gametes form during meiosis; therefore, genes separate when gametes form.

2. The Law of Independent Assortment:

When looking at each other.

2 traits at the same time , it is seen that traits are inherited independently from Gene pairs segregate into gametes randomly and independently of each other.

Genetics & Predictions:

 In genetics we use

mathematical

probability (P

◦ P= ½ or 50% ). If you flipped a coin what are the chances of it landing on heads?

 If you flipped a coin 10X what would you expect the chances of it landing on heads?

◦ About 5 times or 50% or ½ or 1:1 (ratio)  In science, we generally use the ratio.

  A

punnett square

is used to organize & predict genetic information.

Let’s use Mendel’s purebred purple flowers & purebred white flowers: PP X pp

Always show the cross Set up square

Genotype= 4Pp Phenotype= All Purple

Always use ratios!

Use WHOLE #s (no fractions)!

 Let’s cross the F1 generation.

Pp X Pp   Genotype= 1PP: 2Pp: 1pp Phenotype= 3 purple: 1 white  Now you have some practice problems!



What happens if we have a purple flower but we don’t know if it is heterozygous or homozygous? How would we figure out what it is?

 We would perform a phenotype)

testcross

genotype of the unknown.

.

This is a cross between a recessive organism

(in this case a white flower because we know the genotype) (the organism that is showing the dominant

in an attempt to discover the with an organism that has an unknown genotype



If the offspring result in a recessive organism then the unknown parent must be heterozygous.

Variations in Inheritance:



Complete dominance

is what Mendel saw. One trait is completely dominant (expressed) over another .

Either/or; dominant or recessive

. Purple flowers or white flowers.

Intermediate Inheritance:



Not all genes are cut and dry;

just 2 distinct forms in nature.

one allele is not always clearly dominant over another & there are not always



Intermediate inheritance

offspring has its own trait (different than either parent) is when the heterozygous

. This is not seen in pea plants. This includes codominance & incomplete dominance.



Incomplete dominance

is when there is a heterozygote BUT neither the dominant or recessive allele is completely expressed.

Look at snapdragons.



A red snapdragon (RR) is crossed with a white snapdragon (rr).

◦

As you would expect, the F1 generation is Rr BUT they are not Red, they are PINK!



This almost looks like the blending hypothesis, right? But it is not. Why??

http://www.nkellogg.com/codominance.gif

http://fig.cox.miami.edu/~cmallery/150/m endel/c14x9incomplete-dominance2.jpg

Allow the F1 generation to self fertilize.

Rr 2Rr: 1rr X Rr The genotypic results are 1RR: The phenotypic results are

1 red: 2 pink: 1 white



The original traits are NOT lost; therefore this is NOT the blending hypothesis .



An example of incomplete dominance in humans is

hypercholesterolemia

blood ).

( having too much cholesterol in the



Codominance

alleles for 1 trait and 2 different dominant alleles are together is seen when there are more than 2 but neither dominant alleles overpower the other.

◦ This is seen in human blood types.



There are 4 blood types in humans: type A, type B, type AB, and type O. These are phenotypes!



Alleles for blood types in humans are represented with the letter I.

◦

I A

represents A, I

B

represents B, and i represents O.



Codominance is human blood types is phenotypically represented by type AB and genotypically represented by I

A I B

.

http://www.biologycorner.com/resources/bloodtype_chart.gif

http://science.uniserve.edu.au/mirror/biolproject/mende lian_genetics/problem_sets/monohybrid_Cross/graphic s/12T.gif



Polygenic traits

are when traits are affected by

more than 1 gene.

◦

Eye color, hair color & skin color are examples of polygenic traits.



Multiple alleles

are when there are more than 2 alleles per trait.

◦

Again human blood types are examples .



Pleiotropy

is when 1 gene affects more than 1 trait

. An example of this is

sickle cell anemia

or

sickle cell disease

This affects the shape of red blood cells (RBCs)

. . 

RBCs are normally round .

◦

In sickle cell anemia, they are crescent-moon shaped (sickle shaped).

◦

This blocks normal blood flow through blood vessels causing circulatory system damage, weakness, anemia, brain damage & other organ damage.

Chromosomal Theory of Inheritance



Specific genes are located on specific chromosomes, or have

loci

http://www.anselm.edu/homepage/jpitocch/genbio/locus.JPG



Genetic Linkage

Genetic linkage

(or

linked genes

)

genes that are located on the same chromosome.

◦ Generally, these genes will be

inherited together .

 The

closer these genes are on a chromosome, the higher the chances are that they will be inherited together.



Thomas Morgan worked with fruit flies (

Drosophila melanogaster

) and discovered linked genes .



Sex-Linked Traits

Sex chromosomes

determine the sex of the organism. In humans, XX is female; XY is male .



Autosomes

are non-sex chromosomes .



Sex-linked traits

chromosomes. There are more genes on the X than the Y.

are genes that are located on the X or Y

Sex-linked Traits in Humans:



Colorblindness

found on the X chromosome.

◦

This is when someone cannot see red or green . is recessive and

◦

More males suffer from this than females.



Hemophilia

linked also. is recessive and X-

◦

This causes excessive bleeding and no normal blood clotting.

◦

More males suffer from this than females.

http://healthresources.caremark.com/Imagebank/Articles_images/Hemophilia_02.gif

FYI: Environmental Effects:



External & internal environmental conditions can affect genetic expression

.

Some examples: 

Environmental temperature affects the Himalayan rabbit’s fur coat & the western white butterfly’s wing coloration for flight.



Soil acidity affects the color of hydrangeas (acidic=blue; neutral=pink)



Japanese Goby fish changes sex in response to social environment

Nature vs Nuture:

 Study of identical twins that were separated at birth & brought up differently revealed that there are genetic links between individuals. The results of the studies revealed that these twins had similar likes, dislikes, opinions, etc.