Genetics

Genetics

• Genetics is the science of heredity• Genetics explains how genes bring about

characteristics in living organisms and how those characteristics are transmitted from parents to offspring

• Genetics is at the center of all biology because gene activity underlies all biological processes!

Genetics

• Remember, genes are discrete units of genetic (hereditary) information consisting of a specific nucleotide sequence in DNA

Experimental Genetics

• The modern science of genetics began in the 1860’s when Gregor Mendel, an Austrian monk studied the principles of genetics by breeding garden peas– Available in a wide variety of

shapes and colors– Cheap and abundant– Short generation times with

large amounts of offspring

Experimental Genetics

• Mendel studied 7 characters (heritable features) each with its own distinctive trait (variant of that character)

• He created true-breeding lines; lines of peas that were homologous for each trait– A true breeding line had only the genes that

coded for that trait, both chromosomes had the same version of the gene

– For example, a true breeding purple pea plant had only ‘purple’ genes, not white

Flower color White

Axial

Purple

Flower position Terminal

YellowSeed color Green

RoundSeed shape Wrinkled

InflatedPod shape Constricted

GreenPod color Yellow

TallStem length Dwarf

Experimental Genetics

• Mendel wanted to see what happened when he crossed true-breeding lines for one trait with true-breeding lines for another trait

• His results led to the establishment of several principles:– Mendel’s Law of Dominance– Mendel’s Law of Segregation– Mendel’s Law of Independent Assortment

• What would happen if you cross a purple flower with a white flower?

• Mendel’s results indicated that 3 of the 4 flowers produced had purple flowers, while 1 had white

• How?

Mendel’s Law of Dominance

• The white and purple flowers of the pea plants are two versions of a gene for flower color

• Alternative versions of a gene are called alleles

• Mendel’s law of dominance states that when an organism has 2 different alleles for any given character, 1 allele will dominate

P plants

1–21–2

Genotypic ratio1 PP : 2 Pp : 1 pp

Phenotypic ratio3 purple : 1 white

F1 plants(hybrids)

Gametes

Genetic makeup (alleles)

All

All Pp

Sperm

PP

p

ppPp

Pp

P

pP

pP

P

p

PP pp

All

Gametes

F2 plants

Mendel’s Law of Dominance

• For each character, an organism inherits 2 alleles, 1 from each parent

• These alleles may be the same or different• An organism that has 2 identical alleles for a

gene is said to be homozygous• An organism that has 2 different alleles for a

gene is said to be heterozygous

Mendel’s Law of Dominance

• If the 2 alleles of an inherited pair differ, then one allele will determine the organism’s appearance over the other, and is called the dominant allele

• The other allele has no noticeable effect on the organism’s appearance and is called the recessive allele– We use upper and lower case letters to describe

the dominant and recessive alleles, respectively

Mendel’s Law of Segregation

• A sperm or egg carries only 1 allele for each inherited character

• This is because allele pairs segregate (separate) during gamete formation (meiosis!)

• When sperm and egg unite during fertilization, they each contribute their own allele, restoring the paired ‘condition’ to the offspring

Mendel’s Law of Independent Assortment

• The alleles of a gene pair separate from one another independently of the other alleles of another gene pair during segregation (meiosis)

• The origin of any particular allele will be randomly selected from paternal or maternal chromosomes via the process of crossing-over (why, for example, a cat’s color is independent of its tail length)

Mendel’s Law of Independent Assortment

• For example, Aa will segregate from Bb, or in other words, the color of the flower is independent from the inheritance of the height of the plant

1–4

1–4

1–4

1–4

1–41–4

1–4

1–4

9––16

3––16

3––16

1––16

Sperm

Gametes

RRYY rryy

RrYy

ryRY

RRYY

rryy

RrYy

ry

RY

RrYy

RrYy

RrYy

rrYYRrYY

RRYyRrYY

RRYy

rrYy

rrYy

Rryy

Rryy

RRyy

rY

Ry

ry

YellowroundGreenround

Greenwrinkled

Yellowwrinkled

RY rY Ry

• In this example, yellow and green are 2 traits for the color character (indicated by Y and y, respectively) and round and wrinkled are 2 traits of another character (indicated by R and r, respectively)

Homozygousfor the

dominant allele

Homozygousfor the

recessive alleleHeterozygous

Genotype:

P Ba

P

PP

a

aa

b

Bb

Genetics terminology

• The complete genetic make-up of an organism is called its genotype

• The physical expression of the genotype is its phenotype

Phenotypes can reveal genotypes

• Chocolate labs are labrador retrievers that are homozygous recessive for coat color

• Black labs have at least 1 copy of the dominant allele; but their genotype can be Bb or BB

B_ bb

Phenotypes can reveal genotypes

• How can you determine your dog’s genotype (without a blood test)?

• You can testcross your dog; mating your dog with a homozygous recessive dog (bb; a chocolate lab)

• If the black lab was BB, all of its offspring will be black (Bb)

• If the black lab was Bb, half would be black (Bb) and half would be brown (bb)

B_

or

Two possibilities for the black dog:

Testcross:

Genotypes

Gametes

Offspring 1 black : 1 chocolateAll black

Bb

bb

BB

Bb bb

B

b

Bb

b

bB

Geneticists use the testcross to determine unknown genotypes

• Mendel used testcrosses to verify that he had true-breeding lines of pea plants

• Mendel performed his experiments nearly 100 years before the discovery of DNA!

• The testcross continues to be an important tool of geneticists for determining genotypes

Mendel’s laws reflect the rules of probability

• Mendel’s strong background in mathematics (and physics and chemistry…) served him well in his studies of inheritance

• He knew he needed large sample sizes• The laws of inheritance reflect the probability

of an event occurring– The probability of having a girl: 1 in 2– The probability of rolling a 5 on a dice: 1 in 6– The probability of drawing a queen from a deck of

cards: 4 in 52 (1 in 13)

Probability

• An event that is certain to occur has a probability of 1

• An event that is certain not to occur has a probability of 0

• When you flip a coin, the probability of getting heads (or tails) is 1 in 2 every time you toss the coin; independent of previous tosses!

1–2

1–2

1–2

1–2

1–41–4

1–41–4

Formation of eggs

Bb female

F2 genotypes

Formation of sperm

Bb male

B

BB B B

B

b

b

bbbb

Segregation and fertilization as chance events

Extra credit opportunity!!!

• Try it at home…• Toss a coin 100 times and record the

outcomes; your answer should be close to ½ for heads and ½ for tails (if you are using a fair coin…)

• Submit your answers (and perhaps some photographic/video proof) for extra credit!!!

Genetic traits may be tracked

• Individuals exhibiting a recessive trait would be homozygous recessive (carry 2 copies of the recessive allele)

• Individuals exhibiting a dominant trait, however, could be homozygous dominant (carry 2 copies of the dominant allele) or be heterozygous (carry 1 copy of the dominant and 1 copy of the recessive allele)

Freckles

Widow’s peak

Free earlobe

No freckles

Straight hairline

Attached earlobe

Dominant Traits Recessive Traits

F_ ff

W_ ww

E_ ee

Genotype Genotype

Genetic disorders

• Genetic disorders may be inherited as a recessive or dominant trait

• Most human genetic disorders are recessive; most people who have recessive disorders are born to normal parents who are both heterozygous for the allele controlling the disorder

• In this way, the parents are carriers of the recessive allele, but are phenotypically normal

ParentsNormalDd

Offspring

Sperm

Eggs

ddDeafd

DdNormal(carrier)

DDNormalD

D d

DdNormal(carrier)

NormalDd

Offspring produced by parents who are carriers for a recessive trait

• Does this mean that deaf parents always have deaf children?

It is said that everything should be tried once, except square-dancing and

inbreeding….

• It is relatively unlikely for 2 carriers of a rare, harmful allele will meet and mate

• However, the probability increases greatly if close relatives marry and have children

• A mating of close relatives, called inbreeding, is more likely to produce offspring homozygous for recessive traits

Genetic disorders

• Let’s take a non-human example…• Dog breeds that have been inbred for

appearance frequently exhibit serious genetic disorders, such as weak hip joints, eye problems, etc.

• Endangered species frequently suffer from inbreeding (reduced numbers increase chances of close matings)

A case study: The Florida Panther

• The Florida panther population once numbered in the 30’s in the 1990’s

• Close matings resulted in reduced sperm counts, heart defects, and low survival rates among kittens

• Introduction of the Texas Panther in recent years has yielded hybrids with a higher survival rate (controversial!)

www.bigcatrescue.org/catswild/florida_panther.htm

Genetic disorders

• Why are most genetic disorders recessive?• Dominant alleles that cause lethal diseases are

much less common than lethal recessives• This is because the dominant allele cannot be

carried by heterozygotes without it affecting them (and subsequently kill the embryo)

• In contrast, recessive alleles are continually carried from generation to generation by healthy (unaffected) heterozygous carriers

Genetic disorders

• Most dominant genetic disorders can be eliminated when it causes the death of an individual before he/she has a chance to mate (and pass along his/her alleles)

• A lethal dominant allele, however, can escape elimination when it does not cause death until a relatively advanced age– Huntington’s Disease – degenerative disease of

the nervous system does not appear until 35-40 years of age (50% chance of inheriting it)

Three’s a crowd…

• Mendel was fortunate in that he chose characters for which there were only 2 alleles

• Many genes, however, have more than 2 alleles in the population

• More often than not, the inheritance patterns of a particular trait are more complex

www.flickr.com/photos/27887160@N02/2601298345/

Incomplete dominance

• In some allele combinations, dominance does not exist

• Instead, 2 traits are blended together to form a 3rd trait

• In snapdragons (a plant, not a cool dragon, unfortunately) when a red plant is crossed with a white plant, some offspring are red, some are white and some are pink!

P generation

1–2

1–2

1–2

1–2

1–2

1–2

F1 generation

F2 generation

RedRR

Gametes

Gametes

Eggs

Sperm

RR rR

Rr rr

R

r

R r

R r

PinkRr

R r

Whiterr

Incomplete dominance

• In this case, heterozygous individuals exhibit a third phenotype, pink.

• The resulting pink flowers are Rr and can produce red, white or pink offspring of their own

• In the case of incomplete dominance, the phenotype does reveal the genotype for all traits!

Multiple alleles

• Multiple alleles exist for most genes• For example, the ABO blood group in humans

involves 3 alleles of a single gene: A, B, and O• An individual can have type A, B, O, or AB

blood• The A and B alleles are co-dominant; both

alleles are expressed in heterozygous individuals

BloodGroup(Phenotype) Genotypes

O

A

OO

AO or AA

Red Blood Cells

Carbohydrate A

BBO or BB

Carbohydrate B

AB AB

Safe a life, give blood

• Blood group AB can receive blood from any blood type, but can only donate to AB

• Blood group A can receive blood from only A or O, but can donate to A or AB

• Blood group B can receive blood from only B or O, but can donate to B or AB

• Blood group O can receive blood only from O, but can donate to A, B, O or AB!

Blood type complications

• In addition to blood ‘type’, our red blood cells may (or may not) have a protein known as the Rh factor

• Individuals without this factor have a “negative” blood type, while those with this factor have a “positive” blood type

• Problems can occur when an Rh- mother carries an Rh+ child, especially for children conceived after the birth of an Rh+ child

Multiple alleles

• No matter how many alleles for a given gene are in a population, a diploid individual will only have 2 alleles, one on each homologous chromosome

Homozygousfor the

dominant allele

Homozygousfor the

recessive alleleHeterozygous

Genotype:

P Ba

P

PP

a

aa

b

Bb

The chromosome basis of inheritance

• Mendel established his principles (laws) of inheritance long before mitosis and meiosis were understood, and longer still before chromosomes were ‘discovered’

• The chromosome theory of inheritance states that genes occupy specific loci, or positions, on chromosomes and it is the chromosomes that undergo segregation and independent assortment during meiosis

F1 generation R

Metaphase Iof meiosis(alternative

arrangements)

r

Y

y

Rr

Y y

R r

Y y

All round yellow seeds(RrYy)

Anaphase Iof meiosis

Metaphase IIof meiosis

R

y

r

Y

r

y

R

Y

R r

Y y

Rr

Y y

1–4

R

y

Ry

R

y

r

Y

1–4 rY

r

Y

1–4 ry

r

y

1–4RY

R

Y

R

YGametes

Fertilization among the F1 plants

:39 :3 :1F2 generation

r

y

Genes on the same chromosome tend to be inherited together

• The number of genes in a given cell is far greater than the number of chromosomes

• Each chromosome contains hundreds or thousands of genes

• Genes located close together on the same chromosome tend to be inherited together and are called linked genes

• Linked genes do not follow Mendel’s law of independent assortment

Purple longPurple roundRed longRed round

Experiment

Purple flower

PpLl Long pollenPpLl

Prediction(9:3:3:1)

ObservedoffspringPhenotypes

284212155

215717124

Explanation: linked genes

Parentaldiploid cellPpLl

Mostgametes

Meiosis

PL

pl

PL

PL pl

pl

Fertilization

Sperm

Mostoffspring Eggs

3 purple long : 1 red roundNot accounted for: purple round and red long

PL PL

PL

PL

plPL

pl

plpl

pl

Sex chromosomes and sex-linked genes

• The X-Y system of sex chromosomes is only 1 of several sex-determining systems

• Insects have an X-O system; females have 2 X chromosomes, while males have only 1 (XO)

• Some organisms lack sex chromosomes altogether and sex is instead determined by chromosome number

• Other organisms have temperature-dependent sex determination!

Sex chromosomes and sex-linked genes

• In addition to bearing genes that determine sex, sex chromosomes contain genes for characters unrelated to gender

• A gene located on a sex chromosome is called a sex-linked gene

• Sex-linked disorders typically affect human males, since they have only one copy of each chromosome

Sex-linked disorders

• Hemophilia is a sex-linked recessive trait• Hemophiliacs lack 1 or more proteins required

for blood clotting, and bleed excessively when injured as a result

• Hemophilia is a caused by a recessive allele on the X chromosome

• Woman can be carriers, but rarely suffer from the condition……Why?