Download Basic Principles of Heredity

Basic Principles of Heredity
Packet #18
Mendel
Vocabulary Word Introduction
• Heredity
▫ Transmission of genetic information from parent
to offspring
• Genetics
▫ The science of heredity
 Studies both genetic similarities and genetic
variation
Vocabulary II
• Genes
– Located on the chromosome
– Composed of DNA
• Locus
– The location of a gene on the chromosome
• Allele
– Different form, of a particular gene, that is located
at a specific locus on a specific chromosome
• Allele is used when investigation two or more forms
of a particular gene
Allele
Mendel’s Laws
• When Mendel carried out his research, the
processes of mitosis and meiosis had not yet
been discovered.
• Principle of Segregation
– During meiosis, the alleles for each locus, separate
from each other
– When haploid gametes are formed, each contain
only one allele for each locus
– Segregation of alleles is a direct result of
homologous chromosomes separating during
meiosis
Mendel’s Laws
• Principle of Independent Assortment
– The random distribution of alleles, of different loci,
into gametes
– Results in recombination
• The presence of new gene combinations not present in
the parental (P) generation.
– Independent assortment occurs because there are two
ways in which two pairs of homologous chromosomes
can be arranged at metaphase I of meiosis.
• The orientation of homologous chromosomes on the
metaphase plate determines the way chromosomes are
distributed into haploid cells.
Mendel’s Laws
Mendel’s Laws
Mendel’s Law
Law of Independent Assortment
Vocabulary III
• Dominant Allele
▫ May mask the expression of the other allele
known as the recessive allele
 There must be two alleles present
• Recessive Allele
▫ May only be expressed when paired with another
recessive allele
Homozygous vs. Hetereozygous
• Homozygous Dominant
▫ Two identical alleles that are in a dominant state
• Homozygous Recessive
▫ Two identical alleles that are in a recessive state
• Hetereozygous
▫ Two different alleles
 One dominant
 One recessive
Genotype vs. Phenotype
• Genotype
▫ Composition of a specific region of DNA, in an
individuals genome, that varies within a
population
▫ The allele composition found within a cell
 Allows the expression of the phenotype
• Phenotype
▫ The physical effect of a particular genotype.
Genotype vs. Phenotype
Punnett Square
• Punnett Square
▫ A diagram used
in the study of
inheritance
▫ Shows the result
of random
fertilization in
genetic crosses.
Solving Genetics Problems
Test/Monohybrid/Dihybrid Cross
• Monohybrid Cross
– A cross, between parents (P generation), involving ONE allele
• Test Cross
– A cross between individuals of an unknown genotype and a
homozygous recessive individual
• Still involving ONE allele
• Dihybrid Cross
– A cross, between parents (P generation), involving TWO alleles.
• The first generation of offspring
– F1 generation
• First filial
• The second generation of offspring
– F2 generation
• Second filial
Punnett Square
• Example #1
▫ Sex determination
• Sex is determined by sex
chromosomes
▫ X&Y
• The Y chromosome determines
male sex in most species of
mammals
▫ The Y chromosome contains
the SRY gene
 Sex reversal on Y gene
Punnett Square
• Example #2
▫ Monohybrid cross
Punnett Square
• Example #3
▫ Test Cross
Punnett Square
• Example #4
▫ Dihybrid cross
Blood Groups
Multiple Alleles
• Three, or more alleles, can potentially occupy a
particular locus.
▫ A diploid individual any two of the three alleles
▫ A haploid individual, or gamete, has only one
Blood Groups II
Rh Factor
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• Determines whether someone has positive or
negative blood
• A protein antigen that is on the surface of blood
cells and if that antigen is present, the individual
is positive
– A+; B+; O+; AB+
• If the antigen is not present, then the individual
is negative
– A-; B-; O-; AB-
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Rh Factor II
• If an RH-negative mother is exposed to blood
from an Rh-positive fetus, the mother’s blood
will produce antibodies that will attack the blood
of the fetus--potentially killing the unborn child.
• This is why, blood types should be determined
before having children
• If, the male and female are negative, and
positive, the mother must receive medication to
prevent her immune system from attacking the
child.
Punnett Square
• Example #5
▫ Blood Type Cross
 We WILL NOT be doing Punnett Squares involving
the Rhesus factor.
Incomplete Dominance
• Occurs when hybrids have an appearance
between the phenotypes of the parental varieties.
▫ The hetereozygote is intermediate in phenotype
▫ Example
 The color between red and white
 Pink
Incomplete Dominance
Incomplete Dominance
Punnett Square
• Example
▫ Incomplete Dominance
Codominance
• Situation in which the phenotypes of both alleles
are exhibited in a heterozygote
▫ Hetereozygote simultaneously expresses the
phenotypes of both parents.
• Example
▫ Red Flower crossed with a White Flower
 The child will display flowers with red and white
spots
 Both alleles are exhibited
Punnett Square
• Example #
▫ Codominance
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Epistasis
• Epistatis occurs when one gene alters the
expression of another gene
▫ The genes are independent of each other
Epistasis
Linkage
• Each chromosome behaves genetically as if it
consisted of genes arranged in a linear order
• Linkage is the tendency for a group of genes, on the
same chromosome, to be inherited together via
crossing over
• Therefore, groups of genes on the same
chromosome are linked genes.
– Independent assortment does not apply if two loci are
linked close together on the same pair of homologous
chromosomes.
• Normally, they are passed on together.
– However, recombination of linked genes can result from
crossing-over during Prophase I of Meiosis I
Linked vs. Unlinked
• Recombination of unlinked genes = Independent
Assortment of chromosomes
• Recombination of Linked genes = Crossing Over
Linkage II
• Measuring the frequency of
recombination between linked
genes may provide an
opportunity to construct a
linkage map of a chromosome.
Distinguishing Between Independent
Assortment and Linkage(Linked Genes)
• Perform a two-point test cross
▫ One individual must be hetereozygous for the
linked genes
▫ One individual must be homozygous recessive for
the both characteristics
• Linkage is recognized when there is an excess of
parental type offspring (majority) and a
deficiency of recombinant type offspring are
produced in the two-point cross.
Two Point Cross
• Parent #1
▫ BbVv
 Grey with normal wings
• Parent #2
▫ bbvv
 Black with vestigial wings
Linked Genes
Two-Point Cross
BV
bv
Bv
bV
• Calculations
– Parental Genotypes
bv
BbVv
bbvv
Expec 575
ted
Resul
ts
575
Actua 965
l
Resul
ts
944
Bbvv
575
206
bbVv
575
185
• 965 (42%) +944 (41%) =
1909
• 1909/2300 = 83%
– Recombinant Genotypes
• 206 (9%)+185 (8%) = 391
• 391/2300 = 17%
– If independent assortment
was to occur, the percentages
would be 25% a piece.
– The recombinants arose
because of crossing over
Gene Mapping
• By measuring the
frequency of
recombination between
linked genes, it is
possible to construct a
linkage map of a
chromosome
▫ This is how scientists were
able to develop a detailed
genetic map of Neurospora
(fungus), fruit fly, the
mouse, yeast and many
plants that are particularly
important as crops
Sex-Linked Genetics
• Sex is determined by sex chromosomes
– X and Y
• XX = female
• XY = male
• The X chromosome contains many important
genes that are unrelated to sex determination
– These genes are required for both males and
females
• A male receives ALL of his X-linked genes from his
mother while a female receives her X-linked genes
from both parents.
Sex-Linked Genetics
Female Mammals
• Display Dosage Compensation
– In females, only one of the two chromosomes is
expressed in each cell
– Equalizes the expression of x-linked genes for both
genders.
• The other allele is inactive
• Seen as a dark-staining Barr body at the edge of the
interphase nucleus.
– A random event that occurs in each somatic cells
• A female that is hetereozygous expresses one of the alleles
in about half her cells and the other allele in the other half
Dosage Compensation II
• Mice and cats have several alleles that code for
coat color on the x-chromosome.
▫ Females that are hetereozygous for such genes
may show patches of one color in the middle areas
of the other color.
 Variegation
 Not always visible in other circumstances
 May require special techniques
Dosage Compensation
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X-Linked Recessive Disorder
• Males will show this trait if they have the recessive
allele on the X chromosome
– Considered as hemizygous for the trait
• Females will show this trait if they have the
recessive allele on both X chromosomes
– Homozygous recessive
• Hemophilia
– Inability to have clotting of blood
– xh
• Color blindness
– xc
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X-Linked Dominant Disorder
• Baldness
▫ XBXb
 This female will not go bald due to lack of
testosterone
▫ XBXB
 This individual will start to lose her hair in the future
Pleiotrophy
• The ability of one gene to have several effects on
different characteristics.
▫ Normally, can be traced to a single cause
 Defective enzyme
Disorders caused by some form of alteration (mutation) on an
autosome
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Autosomal Disorders
Huntington Disease
• Caused by a rare autosomal dominant allele that affects
the nervous system
▫ Gene found at one end of chromosome #4
• No symptoms appear until 30’s and 40’s
• Symptoms
▫ Uncontrollable muscle spasms
 Degeneration of the nervous system
▫ Personality changes
• Ultimately fatal 10-20 years after onset of symptoms
• No effective treatment has been found
• Problem with symptoms appearing in the 30’s and 40’s
▫ These individuals have children of their own before the disease
develops
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Autosomal Disorders
Sickle Cell Anemia
• Caused by a change in polypeptides found in hemoglobin
– Hemoglobin is the protein that carries oxygen in red blood
cells
– The recessive allele causes the change in the polypeptide
chain
• Individuals that are hetereozygous display co-dominance
– Both alleles are expressed
– Individuals are partially resistant to malaria
• Caused by Plasmodium, a protist (protozoan), carried by the
Anepheles mosquito
• Mild Symptoms
–
–
–
–
Fatigue (feeling tired)
Paleness
Jaundice (Yellowing of the skin and eyes)
Shortness of breath
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Sickle Cell Anemia
Autosomal Disorders
Phenylketonuria (PKU)
• Autosomal recessive disorder
• Lack enzyme that converts amino acid
phenylalanine to another amino acid
▫ Tyrosine
• The excess phenyalanine is converted to toxic
phenylketones
▫ Damages the developing nervous system
• Can be screened for early in life and lifestyle
changes made to prevent severe symptoms
that result in mental retardation
Autosomal Disorders
Cystic Fibrosis
• Autosomal recessive disorder
• Gene responsible for the disorder codes for a
protein that transports chloride ions across
cell membranes
• Defective protein, found in the epithelial cells
lining the passageways of lungs, intestines,
pancreas, liver, sweat glands ad reproductive
organs result in the production of a thick
mucus
• Leads to tissue damage
• What are some treatments available?
Autosomal Disorders
Tay-Sachs Disease
• Autosomal recessive disorder
• Caused by abnormal lipid metabolism in the
brain
• Results in blindness and severe mental
retardation
• Symptoms begin in the first year and
normally result in death before the age of 5
years.
• Lack of enzyme results in the inability to
break down a lipid in the brain
• Lipids build in the lysosomes
• Lysosomes swell and burst causing the nerve
cells to malfunction
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Ploidy
• Degree of repetition of the basic number of
chromosomes
• Diploidy
▫ Chromosomes repeat 2X
 Remember, in humans, you have one copy of a
chromosome from the maternal father and one from
the maternal mother
Euploidy
• “True” ploidy
▫ Having 2 copies of each chromosome
Polyploidy
• Definition
▫ The presence of multiple sets of chromosomes
• Common in plants but rare in animals
• Normally lethal in humans
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Aneuploidy
• Either missing, or having, extra copies of certain
chromosomes.
• Trisomy
▫ Indicates the individual has an extra chromosome
• Monosomy
▫ Indicates that one member of a pair of
chromosomes is missing
Non-Disjunction
• Causes trisomy or monosomy
• Causes
▫ Homologous pairs fail to
separate
 During Anaphase I of
Meiosis I
▫ Sister chromatids fail to
separate
 During Anaphase II of
Meiosis II
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Sex Chromosome Aneuploidy
Turner Syndrome
• 2n - 1
▫ 45 XO karyotype
 44 autosomes + 1 X chromosome
 There is the absence of a sex chromosome
▫ No Barr bodies
• Female in appearance but their female sex organs do not
develop at puberty and they are sterile
▫ Ovaries degenerate in late embryonic life
• Short in stature
• Shows normal intelligence but some cognitive functions
are defective
• There are no Barr bodies
▫ Due to the lack of the other X chromosome
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Sex Chromosome Aneuploidy
Klinefelter Syndrome
• 2n + 1
▫ 47 XXY karyotype
 44 autosomes + 3 sex chromosomes
 There is an extra X chromosome
▫ One Barr body per cell
• Male in appearance and they too are sterile
▫ Male with slowly degenerating testes
• Female type pubic hair pattern
• May have breast development
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Turner Syndrome vs. Klinefelter
Syndrome
Klienfelter Syndrome
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Sex Chromosome Aneuploidy
XYY karyotype
•
•
•
•
•
•
Males that are usually fertile
Some are unusually tall with heavy acne
Others may have some mental disabilities
Predisposition to be more violent in behavior
Gametes never YY or XY--meiosis is normal
After age of 35, extra Y chromosome often
degenerates and is not passed onto offspring
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Sex Chromosome Aneuploidy
XXX karyotype
• Fertile females
• May be some mental disabilities
▫ Rare
• Eggs will produce only X after meiosis--not XX
Autosomal Aneuploidy
Down Syndrome
Trisomy 21
• Caused by an extra copy of chromosome #21
▫ There are three copies of chromosome #21 in their
somatic cells
•
•
•
•
•
0.15 percent of all live births
Growth failure and mental retardation
Big toes widely spaced
Congenital heart disease
Mean life expectancy is about 17 years and only
8 % survive past age 40
Trisomy 21
Autosomal Aneuploidy
Patau Syndrone
Trisomy 13
• Multiple defects
• Death is typical by the age of 3
Autosomal Aneuploidy
Edward’s Syndrone
Trisomy 18
•
•
•
•
Ear deformities
Heart defects
Spasticity and other damage
Death is typical by the age of 1
▫ Some may survive longer
Abnormalities in Chromosome
Structure
Disorders
• The changes in the shape of the chromosome
may be due to either of the following
▫ Translocation
▫ Deletions
▫ Fragile sites
Translocation
• A chromosome
fragment breaking off
and attaching to a nonhomologous
chromosome
▫ Reciporcal translocation
 Two non-homologous pairs
exchange genetic
information
• Can result in deletion
and/or duplication of
genes
Translocation Down Syndrome
• 4% of Down Syndrome cases
• Individuals actually have 46 chromosomes
• One of copies of chromosome #14 has combined
with chromosome #21
▫ The large arm of chromosome #21 has been
translocated to the large arm of another chromosome-usually chromosome #14
Deletion
• The loss of part of a chromosome
• The abnormal chromosome is known as a
deletion
• Sometimes chromosomes break and fail to rejoin
Cri du Chat Syndrome
• Part of the short arm of chromosome #5 is
deleted
▫ Breakage point varies from case to case
• Infants normally have a small head with altered
features
▫ Moon face
• Infants have a distinctive cry that sounds like a
cat mewing
• Infants normally survive childhood
• Exhibit severe mental retardation
Fragile Sites
• Weak points at specific locations in chromatids
• Appears to be a place where part of a chromatid
appears to be attached to the rest of the
chromosome by a thin thread of DNA
▫ Have been identified on the X chromosome and
certain autosomes
Fragile X Syndrome
• Fragile site occurs near the tip of the X
chromosome
▫ Where nucleotide triplet CGG is repeated many more
times than normal
• Most common cause of mental retardation
Genetic Screening & Genetic
Counseling
• Genetic Screening
▫ Identifies individuals who might carry a serious
genetic disease
 Screening of newborns is the first step in preventative medicine
• Genetic Counseling
▫ Provide couples, concerned about the risk of
abnormality in their children, medical and genetic
information
Screening
Pedigrees
• Definition
▫ A family tree that shows the transmission of genetic
traits within a family over several generations.
• Pedigree Analysis
▫ Useful in detecting autosomal dominant mutations,
autosomal recessive mutations, X linked recessive
mutations and defects due to genomic imprinting
 Genomic Imprinting
 Expressions of a gene based on its parental origin
Pedigree Analysis
Pedigree Analysis
Homework
•
•
•
•
•
•
Bioinformatics
Proteomics
Aminocentesis
Chronic villus sampling (CVS)
Preimplantation genetic diagnosis (PGD)
Know how to discuss (argue for/against)
▫ Genetic discrimination
▫ The Human Genome Project