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GENETICS(Study of heredity)
Pedigree Charts Reveal Mode of Inheritance.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Dihybrid Inheritance
Inheritance of seed color and texture of garden pea plants
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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