Basic Principles of HeredityPacket #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 LawLaw 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 ProblemsTest/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• 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-
Saturday, April 22, 2023
24
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.
Saturday, April 22, 2023
25
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
Epistasis•Epistatis occurs when one gene alters the
expression of another gene▫The genes are independent of each other
Saturday, April 22, 2023
34
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 CrossBV bv Bv bV
bv BbVv bbvv Bbvv bbVv
Expected Results
575 575 575 575
Actual Results
965 944 206 185
• Calculations– Parental Genotypes
• 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
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
Saturday, April 22, 2023
50
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
Saturday, April 22, 2023
51
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
Autosomal DisordersHuntington 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
Saturday, April 22, 2023
54
Autosomal DisordersSickle 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
Saturday, April 22, 2023
55
Sickle Cell Anemia
Saturday, April 22, 2023
56
Autosomal DisordersPhenylketonuria (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 DisordersCystic 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 DisordersTay-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
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
Saturday, April 22, 2023
61
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
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
Saturday, April 22, 2023
64
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
Saturday, April 22, 2023
66
Sex Chromosome AneuploidyTurner 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
Saturday, April 22, 2023
67
Saturday, April 22, 2023
68
Saturday, April 22, 2023
69
Saturday, April 22, 2023
70
Sex Chromosome AneuploidyKlinefelter 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
Saturday, April 22, 2023
71
Saturday, April 22, 2023
72
Turner Syndrome vs. Klinefelter Syndrome
Saturday, April 22, 2023
73
Klienfelter Syndrome
Sex Chromosome AneuploidyXYY 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
Saturday, April 22, 2023
75
Sex Chromosome AneuploidyXXX karyotype•Fertile females•May be some mental disabilities
▫Rare•Eggs will produce only X after meiosis--
not XX
Saturday, April 22, 2023
76
Autosomal AneuploidyDown SyndromeTrisomy 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 AneuploidyPatau SyndroneTrisomy 13•Multiple defects•Death is typical by the age of 3
Autosomal AneuploidyEdward’s SyndroneTrisomy 18•Ear deformities•Heart defects•Spasticity and other damage•Death is typical by the age of 1
▫Some may survive longer
Abnormalities in Chromosome StructureDisorders•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 non-homologous 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
Top Related