MENDELIAN GENETICS

What is genetics?

The study of how traits are inherited or how genetic information is passed from one generation to the next.

It also explains biological variation

Gregor Mendel

• 1850’s Grew up in a farm wanting to garden • Austrian monk ( Flunked out of college twice ) but became a mathematician • Experimented with garden pea plants • Using pea plants looked at seven different characters ( height of plants, seed color , texture, flower color ) and found evidence of how parents transmit genes to offspring • Mendel’s statistical analysis provided a model for predicting what the next generation would be like

What was the prevalent believe about inheritance before Mendel?

• People believed in “spontaneous generation” and in the “blending of characters” • Blending theory – Problem: • Would expect variation to disappear • Variation in traits persists Ex: Yellow and green parakeets should have all blue babies. This is not what you observe.

The gene theory

• An alternative idea is the “gene” idea. Parents pass on discrete individual heritable units: genes

• Experimental genetics began in an abbey garden – Modern genetics • Began with Gregor Mendel’s quantitative experiments with pea plants Petal

Figure 9.2 A

Stamen Carpel

Figure 9.2 B

The Garden Pea Plant

• Mendel chose to work with the pea plant because he could control which plant mated with which. Pea plants are • Self-pollinating • True breeding (different alleles not normally introduced) • Can be experimentally cross-pollinated

– Mendel crossed pea plants that differed in certain characteristics • •

And traced traits from generation to generation Mendel started his experiments with plants that were “true breeding”.

1

Removed stamens from purple flower

Parents (P)

Carpel White Purple

2

Transferred pollen from stamens of white flower to carpel of purple flower Stamens

3

Pollinated carpel matured into pod

4

Planted seeds from pod

Offspring (F 1 ) Figure 9.2 C

– Mendel hypothesized that there are alternative forms of genes • The units that determine heritable traits

Flower color

Purple White

Flower position

Axial

Seed color

Yellow

Seed shape

Round

Pod shape

Inflated Terminal Green Wrinkled Constricted

Pod color

Green Yellow

Figure 9.2 D Stem length

Tall Dwarf

•

Mendel’s Principles of Genetics

Mendel refuted the “blending theory” of heredity and provided an explanation of how inheritance works without knowing anything about chromosomes or genes . 1. He figured that traits must be coded for by some kind of inheritable particle which he called “factors” and now we call “genes”. 2. He said that those

genes were transmitted as independent entities from one generation to the next.

Mendel’s insight continued… 3 . He figured that there must be different versions of these “genes” ( we call them now “alleles”)and that

every individual has two genes for each trait. (

Or we can say that:

For each characteristic an organism inherits two alleles, one from each parent) He identified one as dominant, the other as recessive.

4. He figured that the two alleles a parent has are separated into different cells when gametes (sex cells) are formed. This actually happens during metaphase of meiosisI about meiosis in those days). ( no one knew This is known as the

Law of Segregation What are alleles?

Different versions of the same gene

Mendel’s Theory of Segregation

• An individual inherits a unit of information (allele) about a trait from each parent • During gamete formation, the alleles segregate from each other

– Mendel’s law of segregation • Predicts that allele pairs separate from each other during the production of gametes

P plants Genetic makeup (alleles)

PP pp

Gametes All

p

All

P

F 1 plants

(hybrids) All P

p

1 2

P

1 2

p

Gametes

P

Sperm

p

F 2 plants

Phenotypic ratio 3 purple : 1 white Genotypic ratio 1

PP

: 2

Pp

: 1

pp

Eggs

P p PP Pp Pp pp

Figure 9.3 B

•Mendel’s law of segregation describes the inheritance of a single characteristic – From his experimental data • Mendel deduced that an organism has two genes (alleles) for each inherited characteristic

P generation

(true-breeding parents) Purple flowers  White flowers

F 1 generation

All plants have purple flowers Fertilization among F (F 1  F 1 ) 1 plants

F 2 generation

3 4 of plants have purple flowers 1 4 of plants have white flowers

Figure 9.3 A

What is a dominant trait?

The trait that shows, the allele that

is

fully expressed

What is a recessive trait?

The alleles that is masked, the gene is there but it doesn’t show

What is the phenotype?

The observable traits

What is the genotype?

The genetic make up

– If the two alleles of an inherited pair differ • Then one determines the organism’s appearance and is called the

dominant allele ( use capital letters)

– The other allele • Has no noticeable effect on the organism’s appearance and is called the

recessive allele

Vocabulary

• When you mate two contrasting true breeding plants you get a Hybrid.

• The true breeding parents are called the “P” (parent) generation • The hybrid offspring of the P generation are called the F1 generation • When two F1 individuals self pollinate you get the F2 generation

F

1

Results of One Monohybrid Cross

F

2

Results of Monohybrid Cross

Mendel’s Monohybrid Cross Results

F 2 plants showed dominant-to recessive ratio that averaged 3:1

5,474 round 1,850 wrinkled 6,022 yellow 2,001 green 882 inflated 299 wrinkled 428 green 152 yellow 705 purple 224 white 651 long stem 207 at tip 787 tall 277 dwarf

Punnett Square of a Monohybrid Cross

Male gametes

A a

Female gametes

A a AA Aa Aa aa

Dominant phenotype can arise 3 ways, recessive only one

A Test cross

• In a pea plant with purple flowers the genotype is not obvious. Could be homozygous or heterozygous • Why do a test cross?

It allows us to determine the genotype of an organism with a dominant phenotype but unknown genotype

Test Cross

• You cross an individual that shows the dominant phenotype with an individual with recessive phenotype ( one who is homozygous recessive for that trait) • Examining offspring allows you to determine the genotype of the dominant individual

Punnett Squares of Test Crosses

Homozygous recessive Homozygous recessive

a a a a A Aa Aa A Aa Aa a aa aa

Two phenotypes

A Aa Aa

All dominant phenotype

Geneticists use the testcross to determine unknown genotypes – The offspring of a testcross, a mating between an individual of unknown genotype and a homozygous recessive individual • Can reveal the unknown’s genotype

Figure 9.6

Testcross: Genotypes Gametes Offspring



b B_ BB

Two possibilities for the black dog: or

Bb B B b

Bb

All black

bb b

Bb bb

1 black : 1 chocolate

Homologous chromosomes bear the two alleles for each characteristic – Alternative forms of a gene • Reside at the same locus on homologous chromosomes

Dominant

allele

Gene loci

a P B P a

Genotype:

PP

Homozygous

for the dominant allele

Figure 9.4

aa

Homozygous

for the recessive allele

b Bb

Heterozygous Recessive

allele

Web sites to check

• • • http://gslc.genetics.utah.edu/units/basics/tou r/inheritance.swf

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

http://library.thinkquest.org/20465/games.ht

ml

Mendel’s two Laws

•

1. Law of segregation

The two alleles for a trait segregate during gamete formation and

only one allele for a trait is carried in a gamete

. The gametes combine at random • ( In other words : A cell contains two copies of a particular gene, they separate when a gamete is made).

2. Law of Independent Assortment

Alleles from one trait behave independently from alleles for another trait. Traits are inherited independently from one another

Independent Assortment

• Mendel concluded that the two “units” for the first trait were to be assorted into gametes independently of the two “units” for the other trait • Members of each pair of homologous chromosomes are sorted into gametes at random during meiosis

• The law of independent assortment is revealed by tracking two characteristics at once – By looking at two characteristics at once • Mendel tried to determine how two characteristics were inherited

– Mendel’s law of independent assortment • States that alleles of a pair segregate independently of other allele pairs during gamete formation

Figure 9.5 A P generation Hypothesis: Dependent assortment

RRYY rryy

F 1 generation

Gametes

RY ry RrYy

F 2 generation

Eggs 1 2

RY

1 2

ry

Sperm 1 2

RY

1 2

ry

Actual results contradict hypothesis

Hypothesis: Independent assortment

RRYY rryy

Gametes

RY



ry RrYy

1 4

RY RRYY

1 4 Eggs 1 4

ry Ry

1 4

RY

1 4

ry

Sperm 1 4

RY

1 4

ry RrYY RRYy RrYY rrYY RrYy RRYy RrYy RRyy RrYy rrYy Rryy

1 4

ry RrYy rrYy Rryy

Actual results support hypothesis

rryy

9 16 3 16 3 16 1 16 Yellow round Green round Yellow wrinkled Green wrinkled

– An example of independent assortment Blind Blind

Phenotypes Genotypes

Black coat, normal vision

B_N_

Mating of heterozygotes

(black, normal vision)

Phenotypic ratio of offspring 9 black coat, normal vision Figure 9.5 B

Black coat, blind (PRA)

B_nn

Chocolate coat, normal vision

bbN_

Chocolate coat, blind (PRA)

bbnn BbNn



BbNn

3 black coat, blind (PRA) 3 chocolate coat, normal vision 1 chocolate coat, blind (PRA)

A Dihybrid Cross - F

1

Results

TRUE BREEDING PARENTS: GAMETES

:

purple flowers, tall

AABB

x

aabb AB AB AaBb ab ab

white flowers, dwarf

F

1 HYBRID OFFSPRING: All purple-flowered, tall

16 Allele Combinations in

F

2 1/4

AB

1/4

Ab

1/4

aB

1/4

ab

1/4

AB

1/4

Ab

1/4

aB

1/4

ab

1/16 1/16 1/16 1/16

AABB AABb AaBB AaBb

1/16 1/16 1/16 1/16

AABb AAbb AaBb Aabb

1/16

AaBB

1/16

AaBb

1/16 1/16

AaBb aaBB

1/16 1/16

Aabb aaBb

1/16

aaBb

1/16

aabb

Phenotypic Ratios in

F

2

AaBb AaBb

X Four Phenotypes: – Tall, purple-flowered – Tall, white-flowered (3/16) (9/16) – Dwarf, purple-flowered (3/16) – Dwarf, white-flowered (1/16)

Explanation of Mendel’s Dihybrid Results

If the two traits are coded for by genes on separate chromosomes, sixteen gamete combinations are possible

1/4

AB

1/4

Ab

1/4

aB

1/4

ab

1/4

AB

1/4

Ab

1/4

aB

1/4

ab

1/16 1/16 1/16 1/16

AABB AABb AaBB AaBb

1/16 1/16 1/16 1/16

AABb AAbb AaBb Aabb

1/16

AaBB

1/16

AaBb

1/16 1/16

AaBb aaBB

1/16 1/16

Aabb aaBb

1/16

aaBb

1/16

aabb

• Mendel’s laws reflect the rules of probability – Inheritance follows the rules of probability

– The rule of multiplication • Calculates the probability of two independent events – The rule of addition • Calculates the probability of an event that can occur in alternate ways

Figure 9.7

F 1 genotypes

Bb

male Formation of sperm

Bb

female Formation of eggs

F 2 genotypes

1 2

B

1 2

b

1 2

B

1 2

b B B

1 4

b B

1 4

B

1 4

b b

1 4

b

Genetic traits in humans can be tracked through family pedigrees – The inheritance of many human traits • Follows Mendel’s laws

Dominant Traits Recessive Traits

Freckles No freckles Widow’s peak Straight hairline Free earlobe Attached earlobe

Figure 9.8 A

– Family pedigrees • Can be used to determine individual genotypes

Dd

Joshua Lambert

Dd

Abigail Linnell

D ?

Abigail Lambert

D ?

John Eddy

dd

Jonathan Lambert

Dd

Elizabeth Eddy

D ?

Hepzibah Daggett

Figure 9.8 B

Dd Dd dd Dd Dd Dd dd

Female Male Deaf Hearing

•

Recessive Disorders

– Most human genetic disorders are recessive

Parents

Normal

Dd

 Sperm

D

Normal

Dd d DD

Normal

Dd

Normal (carrier)

D

Offspring

Eggs

d

Figure 9.9 A

Dd

Normal (carrier)

dd

Deaf

VARIATIONS ON MENDEL’S LAWS

The relationship of genotype to phenotype is rarely simple – Mendel’s principles are valid for all sexually reproducing species • But genotype often does not dictate phenotype in the simple way his laws describe

Genetics is not as simple as Gregor Mendel concluded, (one gene, one trait).

We know now that there is a range of dominance and that genes can work together and interact.

Incomplete dominance:

When the F1 generation have an appearance in between the phenotypes of the parents.

Ex

: pink snapdragons offspring of red and white ones.

Another way to say it is In incomplete dominance Heterozygote phenotype is somewhere between that of two homozygotes

Flower Color in Snapdragons: Incomplete Dominance

Red-flowered plant X White-flowered plant (homozygote) (homozygote) Pink-flowered

F

1

plants (heterozygotes)

Incomplete dominance in snapdragon color

Flower Color in Snapdragons: Incomplete Dominance

• Red flowers - two alleles allow them to make a red pigment • White flowers - two mutant alleles; can’t make red pigment • Pink flowers have one normal and one mutant allele; make a smaller amount of red pigment

Flower Color in Snapdragons: Incomplete Dominance

Pink-flowered plant X Pink-flowered plant (heterozygote) (heterozygote) White-, pink-, and red-flowered plants in a 1:2:1 ratio

Incomplete dominance in carnations

Co-Dominance or multiple alleles

:

• Codominance – Non-identical alleles specify two phenotypes that are both expressed in heterozygotes • • Having more than 2 alleles for a given trait and both alleles show in the phenotype. No single one is dominant over the other.

Example

: ABO blood types

Genetics of ABO Blood Types: Three Alleles

• Gene that controls ABO type codes for enzyme that dictates structure of a glycolipid on blood cells • Two alleles (

I

A

when paired and

I

B

) are codominant • Third allele (

i

) is recessive to others

ABO blood types

– The ABO blood type in humans • Involves three alleles of a single gene – The alleles for A and B blood types are codominant • And both are expressed in the phenotype

Figure 9.13

Blood Group (Phenotype) Genotypes Antibodies Present in Blood Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left O A B AB O

ii

Anti-A Anti-B A B

I A I A

or

I A i I B I B

or

I B i

Anti-B Anti-A AB

I A I B

—

Multiple alleles for the ABO blood groups

More exceptions to the dominant/recessive rule

Pleiotropy: One genes having many effects

. Only one gene affects an organism in many ways.

Ex

: sickle cell anemia and cystic fibrosis

Pleiotropy

• Alleles at a single locus may have effects on two or more traits • Classic example is the effects of the mutant allele at the beta-globin locus that gives rise to sickle-cell anemia

• A single gene may affect many phenotypic characteristics – In pleiotropy • A single gene may affect phenotype in many ways Individual homozygous for sickle-cell allele Sickle-cell (abnormal) hemoglobin Abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped Sickle cells Breakdown of red blood cells Clumping of cells and clogging of small blood vessels Accumulation of sickled cells in spleen Physical weakness Anemia Heart failure Pain and fever Brain damage Damage to other organs Spleen damage Impaired mental function Paralysis Pneumonia and other infections Rheumatism Kidney failure

Figure 9.14

Genetics of Sickle-Cell Anemia

• • Two alleles 1)

Hb

A

Encodes normal beta hemoglobin chain 2)

Hb

S

Mutant allele encodes defective chain

Hb

S

homozygotes produce only the defective hemoglobin; suffer from sickle cell anemia

Pleiotropic effects of the sickle-cell allele in a homozygote

Epistasis:

• Interaction between the products of gene pairs Interaction between two genes in which

one of the genes modifies the expression of the other.

Ex

: fur /hair color in mammals and albinism

Albinism

• Phenotype results when pathway for

melanin production

is completely

blocked

• Genotype - Homozygous recessive at the gene locus that codes for tyrosinase, an enzyme in the melanin-synthesizing pathway

Genetics of Coat Color in Labrador Retrievers

• Two genes involved -

One gene

influences melanin

production

• Two alleles -

B

(black) is dominant over

b

(brown) -

Other gene

influences melanin

deposition

• Two alleles -

E

promotes pigment deposition and is dominant over

e

Allele Combinations and Coat Color

• Black coat - Must have at least one dominant allele at both loci –

BBEE, BbEe, BBEe,

or

BbEE

• Brown coat -

bbEE, bbEe

• Yellow coat -

Bbee, BbEE, bbee

An example of epistasis

Human Variation

• Some human traits occur as a few discrete types – Attached or detached earlobes – Many genetic disorders • Other traits show continuous variation – Height – Weight – Eye color

More modifications to Mendel’s rule

Polygenic Inheritance:

In this case many genes have an additive effect. The characteristic or trait is the result of the combined effect of several genes.

Ex

: human skin color, height. Controlled by more than one pair of genes

Continuous Variation

• Polygenic inheritance results in a

continuous range

of small differences in a given trait among individuals • The greater the number of genes that affect a trait, the more continuous the variation in versions of that trait

A simplified model for polygenic inheritance of skin color

Environmental effects:

The degree to which an allele is expressed depends on the environment

Ex:

Siamese cat fur color ( enzyme for melanin production inhibited by heat), hydrangea flowers ( depends on acidity of soil), height (nutrition)

Temperature Effects on Phenotype

• Himalayan rabbits are Homozygous for an allele that specifies a heat-sensitive version of an enzyme in melanin-producing pathway • Melanin is produced in cooler areas of body

Environmental Effects on Plant Phenotype

•

Hydrangea macrophylla

• Action of gene responsible for floral color is influenced by soil acidity • Flower color ranges from pink to blue

The effect of environment of phenotype

Web sites to check

• • • http://gslc.genetics.utah.edu/units/basics/tou r/inheritance.swf

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

http://library.thinkquest.org/20465/games.ht

ml

Thomas Hunt Morgan (1910) and Sex Linked Inheritance Morgan’s Experimental Evidence:

Scientific Inquiry

• The first solid evidence associating a specific gene with a a specific chromosome came from Thomas Hunt Morgan • Morgan’s experiments with fruit flies (Columbia University, 1910) provided convincing evidence that chromosomes are the location of Mendel’s heritable factors. He provided confirmation of the correctness of the chromosomal theory of inheritance.

– Morgan’s experiments • Demonstrated the role of crossing over in inheritance

Experiment

Gray body, long wings (wild type)

GgLI

Female  Black body, vestigial wings

ggll

Male Gray long Offspring Black vestigial Gray vestigial Black long 965 Parental phenotypes Recombination frequency = 944 206 185 Recombinant phenotypes 391 recombinants 2,300 total offspring = 0.17 or 17%

Explanation

GgLI

(female)

G L g l g l g l ggll

(male)

G L g l G l g L g l G L g l

Eggs

g l g l

Offspring

G g l l

Sperm

g L g l

Figure 9.20 C

– Thomas Hunt Morgan • Performed some of the early studies of crossing over using the fruit fly

Drosophila melanogaster

Figure 9.20 B

– In Drosophila • White eye color is a sex-linked trait

Figure 9.23 A

SEX LINKED INHERITANCE •

CHROMOSOMES

• Humans have 22 pairs of AUTOSOMES and one pair of SEX CHROMOSOMES : total=23 prs • Thomas Morgan discovered

SEX LINKED INHERITANCE

studying Drosophila (fruit fly) • In fruit flies red eyes is the wild type and white eyes is a mutant. He noticed the connection between gender and certain traits. Only the male flies had mutant white eyes.

SEX LINKED TRAITS

ARE THOSE CARRIED BY THE X CHROMOSOME • Red-Green color blindness Inability to see those colors. Red and green look all the same ,like gray • Hemophilia Blood clotting disorder. The clotting factor VIII is not made, individual can bleed to death.

• Muscular dystrophy X linked recessive, gradual and progressive destruction of skeletal muscles .

• Faulty teeth enamel Extremely rare, X linked Dominant

•Sex-linked genes exhibit a unique pattern of inheritance – All genes on the sex chromosomes • Are said to be sex-linked – In many organisms • The X chromosome carries many genes unrelated to sex

• new technologies can provide insight into one’s genetic legacy – New technologies • Can provide insight for reproductive decisions

•

Identifying Carriers

– For an increasing number of genetic disorders • Tests are available that can distinguish carriers of genetic disorders

•

Newborn Screening

– Some genetic disorders can be detected at birth • By simple tests that are now routinely performed in most hospitals in the United States

•

Fetal Testing

–

Amniocentesis and chorionic villus sampling

(CVS) • Allow doctors to remove fetal cells that can be tested for genetic abnormalities

Chorionic villus sampling (CVS) Amniocentesis

Ultrasound monitor Needle inserted through abdomen to extract amniotic fluid Ultrasound monitor Suction tube inserted through cervix to extract tissue from chorionic villi Fetus Placenta Fetus Placenta Chorionic villi Uterus Cervix Cervix Uterus Amniotic fluid Fetal cells Centrifugation Fetal cells Several weeks Biochemical tests Several hours Karyotyping

Figure 9.10 A

•

Ethical Considerations

– New technologies such as fetal imaging and testing • Raise new ethical questions

Mutations

• Mutations are permanent changes in DNA Causes?

Errors in DNA replication that can be spontaneous. Also caused by high energy radiation (X rays, gamma rays),toxic chemicals in the environment ( pesticides,asbestos, tar) and viruses.

MUTATION:

A PERMANENT CHANGE IN THE DNA. When it happens in the gametes it is inheritable. Some mutations are lethal but most are harmless. Mutations are very important because it creates

DIVERSITY

•

WHAT CAUSES MUTATIONS

?

• Most mutations are spontaneous, changes in DNA caused by errors in replication ( the DNA is copied incorrectly during cell division). The cell has mechanism to find and correct mistakes but those that get through get passed along.

• Some mutations can cause genetic disorders.

• Some environmental factors can cause molecular changes in DNA.

• X rays, toxic chemicals (insecticides, fertilizers, dry cleaning fluids, tar), some viruses, high energy radiation.

• Many inherited disorders in humans are controlled by a single gene – Some autosomal disorders in humans

Table 9.9

DISORDERS RESULTING FROM AUTOSOMAL RECESSIVE INHERITANCE

•

These are conditions in which the gene that is defective is recessive

.

• It is only expressed when the child receives both recessive genes for the disorder (

one from each parent

) • • • • • • • • If a person is heterozygous, that is it has one dominant regular gene and one recessive abnormal gene for the condition, he will be a CARRIER but not have the disorder. The dominant allele will mask the expression of the abnormal condition.

EXAMPLES: ALBINISM: SICKLE CELL ANEMIA: CYSTIC FIBROSIS: TAY- SACHS DISEASE; PHENYLKETONURIA; GALACTOSEMIA:

DISORDERS RESULTING FROM RECESSIVE INHERITANCE

Many not life threatening traits are inherited this way.

widows peak, and attached earlobes.

• ALBINISM: No pigmentation in skin This is

also

an example of “EPISTASIS”(one pair of genes modifies the expression of another) • SICKLE CELL ANEMIA: This is

also

example of “PLEIOTROPY” an Red blood cells curved shape. Decreased oxygen to brain and muscles (offers resistance to Malaria)

DISORDERS RESULTING FROM RECESSIVE INHERITANCE

• CYSTIC FIBROSIS: Excessive mucus secretions.Impaired lung function, lung infections. Protein channel that transport chloride across cell membrane does not function. Protects against cholera.

This is

also

an example of “PLEIOTROPY” • TAY –SACHS DISEASE: Nervous system degeneration in infants. Enzyme fails to breakdown lipids which accumulate in nerve cells and kills the cells. Progressive degeneration starting with the brain cells.

DISORDERS RESULTING FROM RECESSIVE INHERITANCE

• GALACTOSEMIA: Produces brain, liver, eye damage.

Enzyme that breaks down lactose is lacking

. It accumulates to toxic levels. Death in infancy • PHENYLKETONURIA: Results in mental retardation

Disorders resulting from Autosomal Dominant Inheritance

Dominant genes

: Many are harmless for example:freckles, dimples, cleft chin, free earlobe, short big toe, tongue rollers, left thumb on top, curly hair and dark hair • Dominant traits appear in each generation since the allele shows in the heterozygous individual.

•

Dominant Disorders

– Some human genetic disorders are dominant

Figure 9.9 B

Disorders resulting from Dominant Inheritance

•

Acondroplasia or dwarfism:

• A condition where the bone does not grow properly and can’t make proper cartilage. Person is less than 4 feet with short arms and legs but a regular size trunk.

Cholesterolemia:

High cholesterol levels in the blood causing arteries to clog and high incidence of early heart attacks.

•

Marfan Syndrome:

Abnormal connective tissue

•

Disorders resulting from Autosomal Dominant Inheritance Huntington’s Disorder:

Progressive degeneration of nervous system and muscle control. Affects motor and mental abilities and it is irreversible. Late onset, usually late 30’s. Usually the person already had children.

•

Progeria:

Premature accelerated aging. Usually dead by 18. Genes that bring about growth and development are abnormal.

•

Polydactily:

Extra toes and fingers

Karyotype

• A karyotype is a visual display of an individual’s chromosomes. A man made picture of a person’s 23 pairs of chromosomes. ( the photo is taken during metaphase when the sister chromatids are lined up together) • It is useful in sex determination and diagnosis of certain conditions.

INHERITED DISORDERS DUE TO CHROMOSOMES CHANGES

• Chromosome changes can cause a lot of genetic disorders as well as a lot of variety •

WHEN AND HOW CAN A CHROMOSOME CHANGE?

• Mistakes in replication. During the S phase of the cell cycle segments of a chromosome could be

deleted, duplicated, inverted or moved

each gamete of the new individual.

to a new location. Also during Metaphase I (meiosis) there can be improper separation after duplication. This can change the total number of chromosomes in

• If during meiosis the paired chromatids fail to separate correctly this is called

NON DISJUNCTION

•

ANEUPLOIDY

chromosomes.

means an abnormal number of • When an individual ends up with the wrong number of chromosomes most of the time it is miscarried ( spontaneous abortion). • The wrong number of somatic chromosomes are almost always lethal. Ex: trisomy 21(three chrom. 21):

Down Syndrome

• You can live with the wrong number of sex pair chromosomes.

CHANGES IN THE NUMBER OF SEX CHROMOSOMES

• • •

X Turner syndrome

characteristics.

One X instead of a pair. This happens because of non disjuction of sperm. Most are aborted spontaneously. If they live, she is very short, infertily and with reduced sex

XXY Klinefelter syndrome

injections help.

One in 500 live male births. Taller than average, infertile, some low intelligence, some normal. Testosterone

XYY

“

super male”

about 1 in 1000. taller, mildly retarded but normal phenotype.

SEX CHROMOSOMES AND SEX-

species

LINKED GENES

• Chromosomes determine sex in many – In mammals, a male has one X chromosome and one Y chromosome • And a female has two X chromosomes (male) 44 + XY Parents’ diploid cells (female) 44 + XX 22 + X 22 + Y Sperm 22 + X Egg 44 + XX Offspring (diploid) 44 + XY

Figure 9.22 A

– Other systems of sex determination exist in other animals and plants 22 + XX 22 + X

Figure 9.22 B

76 + ZW 76 + ZZ

Figure 9.22 C

32 16

Figure 9.22 D

– The Y chromosome • Has genes for the development of testes – The absence of a Y chromosome • Allows ovaries to develop

Comb Shape in Poultry

Alleles at two loci (

R

and

P

) interact • Walnut comb -

RRPP, RRPp, RrPP, RrPp

• Rose comb -

RRpp, Rrpp

• Pea comb -

rrPP, rrPp

• Single comb -

rrpp

Campodactyly: Unexpected Phenotypes

• Effect of allele varies: – Bent fingers on both hands – Bent fingers on one hand – No effect • Many factors affect gene expression