Transcript genetics review

Family resemblance - Habsburgs of Europe
Haploid
sperm
Maternal
Paternal
homologue homologue
Maximilian I,
Rudolf II, Holy
Konstanze, queen
Holy Roman Emperor,Roman Emperor, of Poland, 1604
1530
~1590
Ferdinand II,
Holy Roman
Emperor, ~1620
Family resemblance has long been noted and commented on
Inheritance – a historical perspective
•
Until the middle ages people thought bizarre composite
animals could result from breeding widely different species
•
The giraffe (Giraffa camelopardalis) was once thought to
have resulted from a camel breeding with a leopard
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Also, species were thought to have been maintained
without significant change since the time of their creation
Constancy of species - Variation and heredity occur within the
boundaries of a species
Inheritance – a historical perspective
•
Constancy of species is false
•
1760 - Josef Koelreuter conducted
hybridization experiments with
different strains of tobacco plants
and obtained fertile offspring
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Well known animal hybrids include,
ligers (lion x tiger), mule (donkey x
horse)
•
Hybridization may produce fertile
or sterile offspring depending on
genetic compatibility
Inheritance – a historical perspective
•
Inheritance was thought to occur via pangenesis in which
particles (pangenes) traveled from different parts of the
body to sperm and egg
•
People also believed that acquired traits or characteristics
throughout life were inherited
•
During the 19th century the idea of blending was popular;
traits from parents were thought to be blended together
Blending
white horse
black horse
x
gray horse
Inheritance – a historical perspective
Historical ideas about inheritance
• Constancy of species
• Blending
• Acquired traits
• Pangenesis
•
All shown to be false during the mid 1800’s
So, how does inheritance actually work?
How does inheritance actually work?
Gregor Mendel’s experiments
• Mendel conducted the first
quantitative studies of
inheritance
•
Pure breeding adult peas of
different flower colors were
crossed
•
Flower color is a trait - an
observable characteristic
Mendel was an Austrian monk
How does inheritance actually work?
Gregor Mendel’s experiments
• Crossing experiment:
Purple X White
Purple
•
All offspring produced had
purple flowers
Why were peas used?
Mendel used a monohybrid cross
What is a monohybrid cross?
• Monohybrid cross - A cross between two individuals
involving to observe inheritance of one trait
Hybridization terminology
• P generation – True breeding parents
•
F1 generation – First generation offspring; resulting
from a cross between pure breeding individuals
(parents)
•
F2 generation – Second generation offspring;
resulting from a cross between F1 plants
Results of Mendel’s monohybrid cross
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F1 generation peas had purple
flowers
•
F2 generation peas consisted
of mostly purple flowers (3/4)
and small number of white
flowers (1/4)
•
Why?
F1
F2
So, what is the mechanism of inheritance?
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Genes code for traits, each
gene has two different
forms called alleles
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Law of segregation – Alleles
separate during meiosis;
sperm and egg possess
one allele for every gene
•
Genotype – Combination of
alleles one has
Mendel’s experiments showed that purple
flower color is dominant over white
So, what is the mechanism of inheritance?
•
Genotype – Combination of
alleles one has
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Two forms of alleles –
DOMINANT and recessive
alleles
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Two alleles = two possible
phenotypes
•
Phenotype – Outward
appearance expressed by a
gene
•
Dominant vs. recessive…
what’s the difference?
Mendel’s experiments showed that purple
flower color is dominant over white
Punnett square
•
Letters represent alleles,
typically the first letter of a
word that defines a trait
•
Capital P for purple (dominant
trait) , lowercase p for white
(recessive trait)
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Genotypes are either
homozygous or heterozygous
Punnett square
Punnett square
•
Punnett square – A tool
developed by Reginald Punnett
used to predict the number and
variety of genetic combinations
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Homozygous – Having the same
two alleles for one gene (either
both dominant or both recessive)
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Heterozygous – Having two
different alleles for one gene (one
dominant allele, and one
recessive allele)
Reginald Punnett
Dihybrid crossing
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Mendel did not know if 2+ traits
were inherited together or
separately
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If inherited together phenotypic
ratio of the F2 generation would
be 3:1 (dependent assortment)
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In other words, were dominant
alleles inherited together and
were recessive alleles inherited
together?
Dihybrid crossing
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•
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Dihybrid cross – Breeding
individuals having for to
observed inheritance of two
different traits
For example, seed color
(yellow, green) and seed
texture (round, wrinkled)
Independent assortment –
each pair of alleles
segregates independently
of the other pairs of alleles
during meiosis
P
F1
F2
Both segregation and independent assortment are explained
by the distribution of chromosomes during meiosis
Other inheritance patterns
•
Mendel’s experiments
illustrate complete
dominance offspring
always resembled one of
the two parents
•
Dominant allele had the
same phenotypic effect
whether present in one or
two copies
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Incomplete dominance heterozygous individuals
have an intermediate
phenotype
Crossing red and white snap dragons
produces pink snap dragons
Blending inheritance vs. Particulate inheritance
F1
F2
Offspring remain pink
WRONG
Return of parental phenotypes
RIGHT
Examples of inherited traits in humans
•
Many human traits are controlled by
alleles which seem to be inherited
according to Mendellian inheritance laws
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Some of the most obvious traits include
straight hairline vs. widow’s peak,
straight thumb or hitchhiker’s thumb
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Widow’s peak
Are you dominant or recessive for such
traits?
Hitchhiker’s thumb
Some genes have more than one allele
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Blood phenotypes are
controlled by a
combination of two of
three different alleles
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Three blood type alleles:
i, IA, IB
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The alleles for blood
types A and B are
codominant
Blood clumping results when certain
antibodies bind to specific antigens
Blood groups and blood types
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Erythrocytes (red blood cell) have cell-surface proteins called
antigens
•
Blood types are based upon the types of antigens one has
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Defensive chemicals called antibodies are produced by your body to
protect itself from foreign cells or organisms
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Antibodies circulate through the body in the fluidic blood plasma
Type A
Type B
Type AB
Type O
Sex linkage
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•
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Some traits are controlled by
alleles found on sex
chromosomes
These traits are commonly
referred to as either X-linked or
Y-linked
Use a punnett square to figure
out genotypic and phenotypic
proportions
Each allele of each sex
chromosome written as a
superscript
Mother
A
X
a
Father
•
X
Y
A
A
X
a
A
a
X X X X
A
A
X Y X Y
Punnett square for sex
determination
A = dominant allele
a = recessive allele
Sex-linked disorders and pedigrees
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Sex-linked disorders are
caused by genes located on
sex chromosomes (usually X
chromosome)
•
Sex-linked disorders and other
traits may be traced through a
pedigree
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Sex-linked disorders include
red-green color blindness,
hemophilia, and Duchenne
muscular distrophy
•
Are males or females mostly
affected by such disorders?
Pedigree shows inheritance of
traits among family members:
Circle = female, Square = male
Pedigree for normal hearing and
deafness
Autosomal disorders
Recessive disorders
Dominant disorders
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Cause: recessive alleles
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Cause: dominant alleles
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Albinism, Cystic fibrosis,
Sickle-cell anemia,
Tay-Sachs disease
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Dwarfism (Achondroplasia),
Alzheimer’s disease,
Huntington’s disease
•
More common than dominant
disorders
•
Less common than
recessive disorders
Autosomal disorders are more prevalent in certain
geographic regions or cultures
Human chromosomes
Let’s look at this again
• There are 46 chromosomes
(23 homologous pairs) in each
somatic cell
• 22 pairs of autosomes
• 1 pair of sex chromosomes
XX = Female, XY = Male
Normal human karyotype
• Karyotype - chromosomes are
arranged according to shape and
size
Nondisjunction and chromosomal disorders
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Nondisjunction – failure of
chromosomes to separate and
segregate into daughter cells
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Nondisjunction may occur during
meiosis 1 or meiosis 2
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Abnormal number of chromosomes
may result (such as a trisomy
disorder like Down’s syndrome)
Nondisjunction can
cause more significant
problems causing the
fetus to die
Autosomal nondisjunction-related disorders
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Down syndrome (Trisomy-21)
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Patau syndrome (Trisomy-13)
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Edward syndrome (Trisomy-18)
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Again, individuals with these disorders have an
extra chromosome
Nondisjunction and chromosomal disorders
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Nondisjunction may also affect sex chromosomes
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Sex chromosome disorders include Klinefelter syndrome
(XXY, or also XXYY, XXXY) in males, and Turner syndrome
(X) in females
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These disorders cause poor genital development, and various
physical abnormalities such as breast development in men
(Klinefelter syndrome), poor breast development in women
(Turner Syndrome)
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Other abnormalities may occur which produce normal male or
normal female offspring (XYY, or XXX)
Synopsis of monohybrid crosses
C
C
X
c
X
Y
C
X
c
C
X X
X X
C
C
X Y
c
X Y
What type of monohybrid cross is this an example of?
Synopsis of monohybrid crosses
A
i
i
B
I
I
A
B
I i
I i
A
B
I i
I i
What type of monohybrid cross is this an example of?
Synopsis of monohybrid crosses
E
e
E
EE
Ee
E
EE
Ee
What type of monohybrid cross is this an example of?
Synopsis of monohybrid crosses
X
X
XX
XXX
XXX
Y
XY
XY
What type of monohybrid cross is this an example of?