GENETICS(Study of heredity)

Pedigree Charts Reveal Mode of Inheritance.

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II

21 1 2 3 4

1 2 3

1 2 3

4 5

I

2 3 4 5 6 7

1 2 3 4 5

1 8

21

2 3 4 5 6

1 2 3 4 5

1

a. Achondroplasia (autosomal dominant)

Female

Male

Normal Carrier Affected

II

III III

II

III

II

b. Albinism (autosomal recessive) c. Red-green color blindness (X-linked recessive)

a: © Rick Wilking/Reuters/Corbis; b: © Reuters/STRINGER Brazil; c: © BSIP/Photo Researchers

Useful websitehttp://

www.explorescience.com/index.cfm?method=cResource.dspView&ResourceID

=449

NOTES• Offspring resemble parents because genes

(alleles) pass from parents to offspring.

• Individuals have two genes (alleles) for every trait in each body cell.

• Homozygous individuals have identical alleles for a trait.

• Heterozygous individuals have two different alleles for a trait.

GENETIC TERMS• Alleles - different molecular forms of a gene• Loci - locations of alleles on homologous chromosomes• Genotype - genes present in an individual• Phenotype - an individual’s observable traits

GENES • Symbols represent genes (alleles).• Upper case letters represent dominant genes.•Lower case letters represent recessive genes.•Dominant genes are always expressed.• Recessive genes are expressed in the absence of dominant genes. •Gametes have one gene for each trait

NOTES• Monohybrid inheritance - inheritance of a

single trait e.g. height

• Dihybrid inheritance - inheritance of two traits e.g. color and texture

• Punnett squares - used to determine possible combinations of genes in offspring

• Testcross - cross individual of unknown genotype with recessive individual & observe offspring

Using a Punnett Square

Gametes

pP

P

p

Gametes

• Place possible gametes along two opposite sides of the square

• Record genotypes of potential offspring in appropriate cells of the square

• Note the frequency (probability) of offspring with each genotype

ppPp

PpPP

Example of a Punnett Square

Testcrosses: detect heterozygotes

Monohybrid Inheritance

Y

y

Y

Mal

e g

amet

es

y

YY

Female gametes

Yellow seeds (Yy): 100% chanceYy

Yy

Yy

Yy

yy

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Monohybrid inheritance of seed color of garden pea plants

Note: All the offspring produced by homozygous parents are genetically identical

R Y

R Y

r y r y

Mal

e g

amet

es

F1 generation

rr yy

Female gametes

RR YY

Yy

YyRrRr Yy

Rr Yy Rr

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Dihybrid Inheritance

Inheritance of seed color and texture of garden pea plants

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Y Y

R Y R y r Y r y

R Y

R y

r Y

r y

9331

F2 generation

Female gametes

Mal

e g

amet

es

Phenotypic ratio 9:3:3:1

Smooth, yellowSmooth, greenWrinkled, yellowWrinkled, green

RR

RR

Rr

Rr

Rr Yy

YyRrYYRrYyRR

Yy RR yy Rr Yy yyRr

YY Rr Yy rr YY rr Yy

Yy Rr yy rr Yy rr yy

YyRr

Dihybrid InheritanceInheritance of seed color and texture of garden pea plants

SPECIAL GENES

• Codominant genes - both alleles for a trait are equally dominant and are expressed in the phenotype

• Incompletely dominant genes - cause heterozygotes to be intermediate in features to homozygous dominant and recessive individuals

Incomplete dominance in flower color of the Japanese four o’clock

HUMAN GENES & BLOOD

• Multiple alleles A, B, and O determine blood type.

• Genes A & B are codominant.

• Gene O is recessive to genes A & B.

• Rh factor of blood is determined by a dominant and a recessive gene.

Multiple alleles controlling the ABO blood groups

Possible alleles from female

IA or orIB i

IA

IB

i

or

or

Po

ssib

le a

llel

es f

rom

mal

e

A AB B OBlood types

IAIA IAIB IAi

IAIB IBIB IBi

IAi IBi ii

SPECIAL GENES

• X-linked genes - carried only on X chromosomes - recessive - rare - usually expressed in males and not in females

• Epistatic genes - cause epistasis - interaction occurs between genes & one of the genes modifies the phenotypic expression of the other gene

• Pleiotropic gene - has more than one effect on the phenotype e.g. sickle-cell gene

• There are two genes that contribute to kernel color

– B Production of pigment

– A Deposition of pigment

• Either gene can block the other’s expression

– To produce pigment a plant must possess at least one functional copy of each gene

Epistasis

DEFINITIONS• Sex linked trait - trait determined by a

gene on the sex chromosome e.g. hemophilia

• Karyotype - an individuals particular array of chromosomes

• Nondisjunction - failure of chromosomes to separate correctly during meiosis I or II

• Aneuploidy - having an abnormal number of chromosomes

Nondisjunction of the X chromosome

DEFINITIONS• Mutations - sudden change in

genetic material

• Amniocentesis - procedure that permits prenatal diagnosis of many genetic disorders

Genetic Screening

Fig. 8.34

• Amniocentesis– Usually performed in the

fourth month of pregnancy

• Ultrasound– Used to locate the fetus

during amniocentesis– Used to examine the fetus for

signs of major abnormalities

Fig. 8.35

HUMAN HEREDITARY DISORDERS

• Sickle-cell Anemia - recessive disorder - hemoglobin is defective - red blood cells are sickle-shaped

• Galactosemia- recessive disorder –galactose rises to abnormally high levels in blood – eyes, liver and brain may become damaged

• Huntington’s Disorder - caused by a dominant allele – nervous system deteriorates

Sickle-Cell Anemia: Recessive Trait

Fig. 8.30

Normal red blood cell Sickled red blood cell

Smooth shape allows for easy passage through capillaries

Irregular shape causes blockage

of capillaries

Sickle-Cell Anemia: Recessive Trait

• Sickle-cell anemia is an autosomal recessive trait in which the protein hemoglobin is defective. – The affected individuals cannot properly transport oxygen to

their tissues.

HUMAN HEREDITARY DISORDERS

• Red-green color blindness – X-linked recessive disorder

• Hemophilia –X-linked recessive disorder - blood clots slowly or fails to clot

• Down syndrome – individuals have 3 chromosome 21 instead of 2 – mental retardation and physical defects occur

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XH

XH Xh

YFa

the

r: h

eal

thy

Phenotype of Offspring: Healthy daughter, noncarrier (XHXH): 25% chanceHealthy daughter, carrier (XHXh): 25% chanceHealthy son (XHY): 25% chanceAffected son (XhY): 25% chance

Mother: heterozygous

Female gametes

Healthydaughter(carrier)

Ma

le g

am

ete

s

Son withhemophilia

Healthyson

Healthydaughter

XHXHXHXh

XHY XhY

Inheritance of Hemophilia A.

Down Syndrome

• Caused by trisomy 21

Down Syndrome

• Frequency is about 1 in 750 children

• Much more common among children of older women

1 in 1,500 if mother is under 30

1 in 16 if mother is over 45

GREGOR MENDEL

Gregor Mendel

• Monk and Scientist • Regarded as the father

of genetics• Studied heredity of

garden pea plants • Discovered the first

laws of heredity • Laid the foundation

for modern genetics

Mendel’s Experimental Design

Fig. 8.3 How Mendel conducted his experiments

Mendel’s Laws

• Law of Segregation of Characteristics

• Law of Independent Assortment

Law of Segregation of Characteristics

• Mendel's First Law

• States that of a pair of characteristics (e.g. blue and brown eye color) only one can be represented in a gamete

• Means that for any pair of characteristics there is only one gene in a gamete even though there are two genes in ordinary body cells

Law of Independent Assortment

• Mendel's Second Law

• States that for any two characteristics the genes are inherited independently

Solving the Heredity Puzzle: Key Players

• Assignment:

Refer to the website below then write a few descriptive sentences about each scientist involved with solving the heredity puzzle.

• http://www.nobelprize.org/educational/medicine/dna_double_helix/readmore.html