Please enable JavaScript.
Coggle requires JavaScript to display documents.
Ch. 14 Mendel and the Gene Idea, CH 15 Chromosomal Basis of Inheritance -…
Ch. 14 Mendel and the Gene Idea
Mendel and His Pea Plants
Mendel discovered basic principles of heredity by breeding garden peas in carefully planned experiments.
~Mendel chose peas because they were available in different varieties.
~He chose to track only characters with TWO distinct alternative forms.
Advantage to using peas:
-short generation time
-large number of offspring
-could cross-pollinate or self-pollinate
Terms to Know
Mendelian genetics
- basics of genetics.
character
- hair color, eye color (GENE)
trait
- variants of character- BROWN hair, BLUE eyes (ALLELE)
True-breeding
- plants that produce offspring of the same variety when they self-pollinate. AA/aa.
Homozygote/homozygous
- organism has two identical alleles for a gene.
-true-breeders
homozygous dominant
- PP
homozygous recessive
- pp
Heterozygote/heterozygous
- organism has two different alleles for a gene.
-NOT true-breeder
Pp
Phenotype
- physical appearance
-purple/white
Genotype
- genetic make-up
PP/Pp/pp
Mendel's Process
~Mendel mated two contrasting true-breeding varieties.
Purple flowering pea plants- PP
White flowering pea plants- pp
hybridization
- mating two contrasting true-breeding varieties.
P generation
- true-breeding parents.
F1 generation
- HYBRID offspring of P generation.
F2
- offspring of F1 self-pollination or cross-pollination.
heritable factor
- what we now call a GENE.
dominant allele
- traits that is always expressed if present.
~capital letter
recessive trait
- both parents must pass on the recessive trait for it to be expressed.
~lower case
Mendel's Model
~When Mendel crossed contrasting true-breeding white and purple flowering pea plants- ALL of F1 hybrids were purple.
-Refuted the "blending" hypothesis.
~Mendel then crossed the F1 hybrids
F2 generation had a ratio of 3 purple:1 white.
Mendel called the purple flower color the DOMINANT trait and the white flower color the RECESSIVE trait.
-discovered white flowers were not destroyed or lost
Mendel's model was developed to explain the 3:1 inheritance pattern in the F2 offspring.
FOUR CONCEPTS:
1st: Alternative versions of genes account for the variations in characters.
-each gene is located at a specific
locus
on a specific chromosome.
-alternative versions of a gene are called
alleles
.
2nd: an organism inherits two alleles for each character; one from each parent.
-two alleles at specific locus may be true-breeders or hybrids.
3rd: If two alleles at locus differ- the
dominant allele
determines the
phenotype
.
-the
recessive allele
has no noticeable effect on phenotype.
Law of Segregation
Mendel's 4th concept
Two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes.
-an egg or a sperm only gets one of the two alleles that are present.
anaphase 1 and anaphase 2 of meiosis
Punnett square
- model that shows the possible combinations of sperm and egg.
~Derived by following a single character
Monohybrids
- F1 offspring were heterozygous for ONE CHARACTER.
Monohybrid cross
- cross between monohybrids (heterozygous for one character)
Law of Independent Assortment
Each pair of alleles segregate independently of any other pair of alleles during gamete formation.
~applies to genes on different nonhomologous chromosomes or those far apart on same chromosome.
~genes located near each other on the same chromosome tend to inherit together.
metaphase 1 of meiosis
~Derived following two characters at the same time.
Dihybrids
- F1 generation is heterozygous for TWO characters.
Dihybrid cross
- cross between F2 hybrids that determines whether TWO characters are transmitted to offspring together or independently.
Dihybrid Punnett Square
- use FOIL method to calculate possible gametes produced from each parent.
Ex. YyRr - ONE PARENT
YxR= YR, Yxr= Yr, yxR= yR, yxr= yr
Testcross
Dominant phenotypes can be homozygous dominant or heterozygous.
-Can determine genotype by
testcross
- breeding the mystery individual with a homozygous recessive.
-If offspring display recessive phenotype- mystery parent is heterozygous.
Probability Laws and Mendelian Inheritance
Mendels laws are a reflection of probability, such as tossing a coin.
-the outcome of one toss has NO IMPACT on the outcome of the next toss.
-alleles of one gene segregate into gametes independently of another gene's alleles.
-Can apply rules of probability to predict the outcome of
multicharacter crosses
- two or more independent monohybrid crosses occurring simultaneously.
-calculate the chances for each character separately and then multiply individua probabilities.
Multiplication and Addition Rules: Monohybrid Crosses
Multiplication rule
- the probability that two or more independent events will occur together is the product of their individual probabilities.
Ex. Segregation in a heterozygous plant: each gamete has a 1/2 chance of carrying dominant allele and a 1/2 chance of carrying the recessive allele.
Addition rule
- the probability that any one of two or more mutually exclusive events will occur is calculated by adding together their individual probabilities.
Extending Mendelian Genetics
~Many heritable characters are not determined by only one gene with two alleles, BUT basic principles of segregation and independent assortment apply to complex patterns of inheritance.
~inheritance of characters by a SINGLE gene can deviate from Mendelian patterns when:
-alleles are not completely dominant
-a gene has more than two alleles
-a gene produces multiple phenotypes
Degree of Dominance
Complete dominance
- phenotypes of the heterozygote and dominant homozygote are identical
~Mendelian genetics
PP- purple, Pp- purple
Incomplete dominance
- BLENDING of two of more traits.
~Phenotype of F1 is somewhere between phenotypes of parents.
PP-purple, pp- white, Pp- pink
Codominance
- two or more alleles are dominant simultaneously.
~affect the phenotype separately.
AB bloodtype
Incomplete Dominance
Dominance and Phenotype
Tay-Sachs disease
-a fatal inherited disorder where a dysfunctional enzyme causes an accumulation of lipids in the brain.
-organismal level- allele is recessive
-biochemical level- phenotype (enzyme activity level) is incompletely dominant
-molecular level- alleles are codominant
~lysosomes are dysfunctional and cannot break down lipids in the brain.
Dominant alleles are not more common than recessive alleles.
Polydactyl
-born with extra finger or toes caused by a dominant allele
-1 in 400 babies are born polydactyl.
Multiple Alleles
~most genes exist in populations in more than two allelic forms.
Ex. ABO blood groups in humans.
~four phenotypes that are determined by three alleles.
Type A(IAIA or IAi): Antigen A with B antibodies
Type B(IBIB or IBi): Antigen B with A antibodies
Type AB(IAIB): Antigens A & B with NO antibodies
-AB+ is universal recipient
Type O (ii): NO Antigens with AB antibodies
-O- is universal donor
RH factor (Rhesus factor)
+(dominant) or - (recessive)
A+ or A-
B+ or B-
AB+ or AB-
O+ or O-
Pleiotropy
gene that has multiple phenotypic effects.
most genes have this property.
pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases (cystic fibrosis and sickle-cell)
Mendelian Genetics for Two or More Genes
Epistasis
expression of a gene at one locus alters that phenotypic expression of a gene at a second locus.
-One gene effects the phenotype of another (down the line) due to interaction of their gene products.
Ex. Labrador retrievers' coat color depends on two genes
-One gene determines the pigment color (B-black and b-brown)
-The other gene determines whether the pigment will be deposited in the hair (E-color and e-no color)
-epistatic interactions produce a variety or ratios, all of which are modified versions of 9:3:3:1
Polygenic Inheritance
multiple genes independently affect a single trait
-additive effect of two or more genes on a single phenotype.
Ex. Height
-Over 180 genes affect height
Ex. Skin pigmentation
Quantitative characters
- those that vary in the population along a continuum.
-indicates polygenic inheritance.
Environmental Impact on Phenotype
Multifactorial
- traits that depend on multiple genes combined with environmental influences.
-phenotype for a character depends on environment as well as genotype.
Ex. Hydrangeas
Study of Human Traits
-Geneticists analyze the results of human matings that have already occurred.
Pedigree
- family tree that describes the inheritance of a trait across generations.
-used to make predictions about future offspring.
Disorders
Recessively Inherited Disorders
-Recessively inherited disorders show up only in homozygous recessive alleles.
Carriers
- heterozygous individuals who carry the recessive allele but are phenotypically normal.
-Mom and Dad can be carriers of autosomal gene traits.
-Only Mom can be a carrier for sex-linked genes.
Ex. Albinism is a recessive condition.
-Consanguineous matings (close relatives) increase chance that both parents of child carry the same recessive allele.
Cystic Fibrosis
-allele results in defective or absent chloride transport channels that lead to a buildup of chloride ions outside the cell.
~most common lethal genetic disease in US (1 out of every 2,500)
-if untreated- death by 5 yrs old.
-if treated- antibiotics and physical therapy can prolong life to survive into their 40's.
Symptoms: mucus buildup in internal organs and abnormal absorption in small intestines.
Sickle-Cell Disease
-substitution of one amino acid in the hemoglobin protein in RBC's.
~affects 1 out of 400 African-Americans (1 out of 10 have trait)
~homozygous individuals- all hemoglobin is abnormal
~heterozygous are usually healthy but may have some symptoms, BUT less susceptible to malaria
Symptoms: weakness, pain, organ damage, paralysis.
Dominantly Inherited Disorders
-Dominantly inherited disorders are cause by dominant alleles.
~lethal diseases are rare and arise by mutation.
Ex. Achondroplasia- form of dwarfism caused by rare dominant allele.
Huntington's Disease
-degenerative disease of the nervous system.
~no obvious phenotypic effects until 35-40 yrs old.
~once deterioration begins, condition is irreversible and fatal.
~testing can detect the presence of the Huntington's allele in the genome.
Multifactorial Disorders
-Heart disease, cancer, alcoholism, mental illnesses have both genetic and environmental components.
~Lifestyle has a tremendous effect on phenotype.
Genetic Testing/Counseling
-Fetal and newborn testing can reveal genetic disorders.
-Genetic counselors can determine the risk a couple will have a child with a disease.
~if both the parents had a sibling with recessively inherited illness that means both their parents were carriers.
-Tests are available that identify carriers to help define the odds of passing the trait down.
Fetal Testing
amniocentesis
- amniotic fluid is removed and tested for certain genetic disorders.
chorionic villus sampling (CVS)
- sample of the placenta is removed and tested.
-Ultrasounds assess fetal health visually
Newborn Screening
-Some genetic disorders are detected at birth by simple tests.
Ex. Phenylketonuria (PKU)- recessively inherited disorder.
CH 15 Chromosomal Basis of Inheritance
Morgan and the Fruit Flies
Mende's work was theoretical until mitosis and meiosis were figured out.
Afterward, biologists began to develop the
chromosome theory of inheritance
- theory that Mendelian genes have specific loci (locations) along chromosomes.
First evidence of this theory came from the work of Thomas Hunt Morgan.
Morgan chose to study a common species of fruit fly,
Drosophila melanogaster
because:
-They produce many offspring
-Can breed a generation every two weeks
-They only have four pairs of chromosomes
Morgan bred these fruit flies over many months to get a variant fruit fly.
Finally, one of the fruit fly offspring had white eyes instead of red.
Wild type
- normal phenotypes that are common in the population.
-red eyed flies
Mutant phenotypes
- the alternative trait.
-the whit eyes flies
Morgan's Scientific Inquiry
Morgan then mated the mutant (white eyed) male fly with the wild type (red eyed) females:
F1 generation had red eyes
F2 generation had a 3:1 ratio of red to white eyes, BUT only MALES had white eyes.
Morgan reasoned that the white-eyed mutant alleles must be located on the X chromosome.
Linked Genes
Linked genes
- genes close together on the same chromosome tend to be inherited together.
Morgan continued his study with fruit flies to see how linkage affects inheritance.
Crosses flied that differed in traits of body color and wing size.
First cross generated F1 dihybrid flies
Second was a testcross- F1 dihybrids and double mutant (recessive)
Many of the offspring resembled the P generation flies (parental phenotypes).
-Reasoned that they must be on the same chromosome and do not assort independently.
Nonparental phenotypes were also produced.
-suggested that two traits could be separated sometimes.
genetic recombination
Genetic Recombination
Genetic Recombination
- the production of offspring with combinations of traits differing from either parent.
Parental types
- offspring with a phenotype matching one of the parental (P generation) phenotypes.
Recombinant types (recombinants)
- offspring with a nonparental phenotypes (new combinations of traits).
Recombination of unlinked Genes: Independent Assortment of Chromosomes
Recombination of Linked Genes: Crossing Over
Morgan observed although some genes are linked, nonparental allele combinations are still produced.
Proposed some process must occasionally break the physical connection between genes on the same chromosome.
crossing over
- mechanism to explain recombination of linked genes.
Occurs during Prophase 1
Increases genetic diversity
Recombination of chromosomes bring alleles together in new combinations for genetic variation.
Random fertilization further increases the variant combinations.
Raw material for natural selection to work.
Chromosomal Basis of Sex
Humans (mammals) have two types of sex chromosomes:
X chromosome
- larger
Y chromosome
- smaller
XX
- female
XY
- male
Short segments of the Y chromosomes are homologous with the X chromosome.
SRY (sex-determining region on the Y)
- gene of the Y chromosomes responsible for development of the testes, muscle mass, and deeper voice.
-Only on the Y chromosome.
Other systems of sex determination:
Z-W system- chickens
X-O system- grasshoppers
Haplo-diploid system- bees
Sex-Linked Genes
Sex-linked genes
- gene that is located on EITHER sex chromosome.
Y-linked
- genes on the Y chromosome.
-Only 78 genes coding for ~25 proteins on human Y chromosome.
-contain genes mainly related to sex determination, such as SRY.
X-linked genes
- genes on the X chromosome.
-human X chromosome has ~1,100 genes.
-contain genes unrelated to sex.
X-linked Inheritance
X-linked genes follow specific pattern of inheritance.
For recessive X-linked trait to be expressed:
-female needs two copies of the recessive alleles (
homozygous recessive
)
-male only needs on copy of the allele passed from the mother.
hemizygous
- term for males having one allele instead of two.
Because males only need on copy of the allele, X-linked recessive disorders are much more common in males.
Disorders caused by recessive alleles on the X chromosome
:
-Colorblindness
-Duchenne muscular dystrophy
-Hemophilia
X Inactivation in Females
Female mammals inherit two X chromosomes.
One of the two in EACH CELL is RANDOMLY inactivated during embryonic development.
Barr body
- formed when the inactive X chromosome condenses.
Women are considered a mosaic of the characters from the X chromosomes, because some cells inactivate one X chromosome and other cells inactivate the other X chromosome.
Inactivation involves modification of the DNA and histones.
-Part of the chromosome itself contains several genes involved in the inactivation process.
XIST (X-inactive specific transcript)
- gene that becomes active on the chromosome being inactivated.
Alfred Sturtevant and Gene Mapping
Sturtevant was one of Morgan's students that constructed a way to map genes.
Genetic map
- ordered list of the genetic loci along a particular chromosome.
Predicted that the farther apart two or more genes are on the same chromosome, higher the likelihood of crossing over to occur, which increases recombination frequency.
linkage map
- genetic map of a chromosome based on recombination frequencies.
map units
- measurement of distance between genes.
1 map unit= 1% recombination frequency.
Genes that are far apart on the same chromosome can have a recombination frequency near 50%.
These genes are technically linked, but genetically unlinked-behave as if on different chromosomes.
Sturtevant used recombination frequencies to make linkage maps of the fruit fly genes.
Discovered
linkage groups
- genes clustered into four groups of linked genes.
The linkage maps further proved genes were located on chromosomes.
Alterations of Chromosomes
Large alterations of chromosomes in humans often leads to miscarriages or causes a variety of developmental disorders.
Alterations include:
-Alterations in chromosome number
-Alterations in chromosome structure
-Plants tolerate these genetic changes better.
Abnormal Chromosome Number
Nondisjunction
Nondisjunction
- pairs of homologous chromosomes do not separate normally during meiosis.
Law of Segregation gone wrong
-One gamete receives two of the same type of chromosome, and the other gamete receives no copy.
Anaphase I or anaphase II
Aneuploidy
- results from the fertilization of the gamete in which nondisjunction occurred, resulting in the offspring having an abnormal number of a particular chromosome.
Monosomic
- zygote has only one copy of a particular chromosome. (missing one chromosome)
Trisomic
- zygote has three copies of a particular chromosome. (and extra chromosome)
Polyploidy
- an organism has more than two COMPLETE sets of chromosomes.
Triploid (3n)
- three sets
**Tetraploid (4n)- four sets
-common in plants-more normal appearance than aneuploids.
Aneuploidy Disorders
Down Syndrome (Trisomy 21)
- aneuploid condition that results from three copies of chromosome 21.
~affects 1 out of every 830 children in U.S.
~frequency increases with age of the mother
Aneuploidy of Sex Chromosomes
Klinefelter syndrome
- extra chromosome in a male- XXY
~affects 1 in 1,000 males
~no syndrome
~XXX females occur in 1 out of 1,000-at risk for learning disabilities.
Turner Syndrome
- monosomy X in females produces an XO female
~female is sterile, but only viable monosomy in humans
Alterations of Chromosome Structure
Breakage of a chromosome can lead to four types of changes in structure:
Deletion
- removes a chromosomal fragment- LOST
Duplication
- fragment attaches as extra segment-REPEAT
Inversion
- fragment reattaches INVERTED
Translocation
- moves segment from one chromosome to another.
Disorders from Structurally Altered Chromosomes
Cri du chat ("cry of the cat")
- results from a
deletion
in chromosome 5.
~children born with this syndrome are severely intellectually disabled and have a catlike cry.
~usually die in infancy or early childhood
Chronic myelogenous leukemia (CML)
- caused by translocations of chromosomes.
Genomic Imprinting
An exception to Mendelian genetics involves whether the genes are located in the nucleus or outside the nucleus
extranuclear genes (cytoplasmic genes)
- found in organelles in the cytoplasm.
Mitochondrial DNA
~inherited maternally because the zygote's cytoplasm comes from the egg.
Some defects in mitochondrial genes prevent cells from making enough ATP and results in diseases that affect the muscular and nervous system
~Mitochondrial myopathy and Leber's hereditary optic neuropathy
It is possible to avoid passing along mitochondrial disorders by transferring the chromosomes from the affected egg to an egg of a healthy donor.
Genomic imprinting
- variation of phenotype is dependent on whether allele was inherited from male or female parent.
~silences certain genes depending on which parent passes them on.
~most imprinted genes are on autosomes.
~most imprinted genes are critical for embryonic development (body temperature regulation, sleep, some metabolic functions).
Mouse gene for Igf2 (insulin-like growth factor 2) was one of first imprinted genes to be identified.
~only the paternal allele of this gene is expressed.
