Chapter 15: The Chromosomal Basis of Inheritance
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Transcript of Chapter 15: The Chromosomal Basis of Inheritance
Chapter 15:
The Chromosomal
Basis of Inheritance
Essential Knowledge
3.a.4 – The inheritance pattern of many traits cannot be explained by simple Mendelian genetics (15.1, 15.2, 15.3, 15.5).
3.c.1 – Changes in genotype can result in changes in phenotype (15.4).
Sutton (1902)
Developed the “Chromosome Theory of Inheritance”1) Mendelian factors or alleles are located on chromosomes2) Chromosomes segregate and show independent assortment
Morgan
Embryologist at Columbia University Chose to use fruit flies as a test
organism in genetics Allowed the first tracing of traits to
specific chromosomes
Fruit Fly Drosophila melanogasterFeeds on fungus growing on fruitEarly test organism for genetic
studies
Reasons he chose fruit fly
Small Cheap to house and feedShort generation time
New generation every 2 weeks100s of offspring producedFew chromosomes
4 pairs (8 total) 3 pairs autosomes, 1 pair sex
Genetic Symbols
Mendel: use of uppercase or lowercase letters T = tall t = short
Morgan: symbol from the mutant phenotype + = wild phenotype (natural
pheno) No symbol = mutant phenotype
(any pheno different from wild)
Examples
Recessive mutation: w = white eyes w+ = red eyes
Dominant mutation: Cy = Curly wings Cy+ = Normal wings
Letters come from 1st mutant trait observed
Morgan Observed:
A male fly with a mutation for white eyes
Then, he crossed the white eye male with normal red eye female
All had red eyes Same as Mendel’s F1
This suggests that white eyes is a genetic recessive
F1 X F1 = F2
Morgan expected the F2 to have a 3:1 ratio of red:white
He got this ratio However, all of the white eyed flies
were MALE Most red eyed flies were FEMALE
Therefore, the eye color trait appeared to be linked to sex
Morgan discovered:
Sex-linked traits Genetic traits whose expression are
dependent on the sex of the individual
Genes on sex chromosomes exhibited unique patterns
Eye color gene located
on X chromo (with
no corresponding
gene on Y)
Morgan Discovered
There are many genes, but only a few chromosomes
Therefore, each chromosome must carry a number of genes together as a “package” There was a correlation between a particular trait and an individual’s sex
Linked Genes
Traits that are located on the same chromosome (that tend to be inherited together)
Result:Failure of (deviation from) Mendel's Law of Independent Assortment.
Ratios mimic monohybrid crosses.
Body Color and Wing type
Body color and wing type Wild: gray and normal (dom +) Mutant: Black and vestigal (rec)
This is why we use “b” for body color alleles and “vg” for wing alleles
Symbols: Body color - b+: gray; b: black Wings - vg+: normal; vg: vestigal
Example
b+b vg+vg X bb vgvg#1: b+b = gray; vg+vg = normal#2: bb = black; vgvg = vestigal
(b+ linked to vg+)(b linked to vg)
If unlinked: 1:1:1:1 ratio. If linked: ratio will be altered
Crossing-Over Occurs during Pro I
of meiosis Breaks up linkages
and creates new ones
Recombinant offspring formed that doesn't match the parental types
If Genes are Linked:
Independent Assortment of traits fails
Linkage may be “strong” or “weak”
Linkage Strength
Degree of strength related to how close the traits are on the chromosome Weak - farther apart Strong - closer together
Usually located closer to centromere
Genetic Maps
Constructed from crossing-over frequencies
1 map unit = 1% recombination frequency
Have been constructed for many traits in fruit flies, humans and other organism.
Sex Linkage in Biology
Several systems are known:1. Mammals – XX and XY2. Diploid insects – X and XX3. Birds – ZZ and ZW4. Haploid-Diploid
Sex Linkage in Biology
1. Mammals Determined by whether sperm has X or Y
2. Diploid insects Only X chromosomes present
3. Birds Egg determines sex
4. Haploid-Diploid Females develop from fert egg Males develop from unfert egg
Chromosomal Basis of Sex in Humans
Sex determination ALWAYS 50-50
X chromosome - medium sized chromosome with a large number of traits
Y chromosome - much smaller chromosome with only a few traits
Human Chromosome Sex
Eggs – only contain XSperm – either X or Y
Males - XYFemales - XX
Comment - The X and Y chromosomes are a homologous pair, but only for a small region at one tip
Sex Linkage
Inheritance of traits on the sex chromosomes NOT TO BE CONFUSED WITH sex-linked
traits!!!!! X Linkage - common; Y- rare Dads: only to daughters (b/c dads
ONLY give X chromo to daughters)
Moms: to either sex
Males Hemizygous - 1 copy of X
chromosome Show ALL X traits (dominant or
recessive) More likely to show X recessive
gene problems than females
X-linked Disorders and PatternsDisorders on X-chromo:
Color blindness Duchenne's Muscular Dystrophy Hemophilia (types a and b)
Patterns Trait is usually passed from a carrier
mother to 1 of 2 sons Affected father has no affected sons, but
passes the trait on to all daughters (who will be carriers for the trait)
Comment
Watch how questions with sex linkage are phrased: Chance of children? Chance of males? Chance of females?
You MUST practice genetics problems w/ these traits: Hemophilia, Muscular dystrophy and colorblindness (they all work the same!)
Can Females be color-blind?
Yes!!! ONLY if their mother was a carrier and their father is affected
How? Mother contributes X (with affected allele) and dad contributes all he can to make a daughter – affected X
Are you color blind?
25
45
29
56
6 8
Y-linkage Hairy ear pinnae Comment - new techniques have
found a number of Y-linked factors that can be shown to run in the males of a family
Ex: Jewish priests
Sex Limited Traits
Traits that are only expressed in one sex
Ex: prostate development, gonad specialization, fallopian tube development
Sex Influenced Traits
Traits whose expression differs because of the hormones of the sex
Ex: beards, mammary gland development, baldness
Baldness: Testosterone – makes the trait act as a
dominant No testosterone – makes the trait act as a
recessive Males – have gene = bald Females – must be homozygous to have thin
hair (rare)
X chromosome inactivation In every somatic cell (in
females), one X chromosome is inactivated Humans: differs/random Kangaroos: always paternal X that
is inactivated Called Barr bodies
Barr Body
Inactive X chromosome observed in the nucleus▪ Becomes inactive during embryonic development
Way of determining genetic sex (without doing a karyotype)
Barr body description
Compact body which lies close to nuclear envelope
Most genes on this X are NOT expressed
Inside developed ovaries, these are reactivated (so that each ova will get an active X)
Lyon Hypothesis
Which X inactivated is random Inactivation happens early in
embryo development by adding CH3 groups to the DNA Changes DNA nucleotide
Result - body cells are a mosaic/combo of X types Some have active X from mom, others
active X from dad
Examples Calico Cats Human examples are known
(sweat gland disorder)
Question?
Why don’t you find many calico males? They must be XB XOY and are always sterile
Why? They MUST have an extra X chromo (to have an inactive X - you must have TWO!)
Chromosomal Alterations
Two types of alterations: Changes in number Changes in structure
Number Alterations
Aneuploidy - too many or too few chromosomes, but not a whole “set” change
Polyploidy - changes in whole “sets” of chromosomes
Aneuploidy
Caused by nondisjunction the failure of a pair of chromosomes to
separate during meiosis Result: too many or too few
chromosomes in a gamete Nondisjunction in Meiosis I produces
4 abnormal gametes. Nondisjunction in Meiosis II produces
2 normal and 2 abnormal gametes.
Types of Aneuoploidy
Monosomy: 2N – 1 (very rare) Mono = one (missing copy)
Trisomy: 2N + 1 (more common) Tri = three (extra copy)
Normal: 2N
Turner Syndrome Monosomy 2N - 1 or 45 chromosomes
Genotype: X_ or X0 Phenotype: female, but very poor
secondary sexual development. Characteristics:
Short stature. Extra skin on neck. Broad chest. Usually sterile Normal mental development except for some
spatial problems.
Question
Why are Turner Individuals usually sterile? Odd chromosome number Two X chromosomes needed for ovary
development.
Other Sex Chromosome changes Kleinfelter Syndrome Meta female Supermale
Kleinfelter SyndromeTrisomy2N + 1 (2N + 2, 2N + 3)Genotype: XXY (XXXY, XXXXY)Phenotype: male, sexual
development may be poor/slow Often taller than average, mental development fine (in XXY), usually sterile
More X = more mental problems
George Washington
May have been a Kleinfelter Syndrome individual.
Much taller than average Produced no children/sterile
individual
Meta female
Trisomy 2N + 1 or 2N + 2 Genotype: XXX or XXXX Phenotype: female, but sexual
development poor. Mental impairment common.
XYY Syndrome OR Super male Trisomy 2N + 1 or 2N + 2 Genotype: XYY or XYYY Phenotype: male, usually normal
development, fertile w/ normal sex organ development
Trisomy events
Trisomy 21: Down Syndrome Trisomy 13: Patau Syndrome Both have various physical and
mental changes
Down’s
Down’s Syndrome
Increases with maternal age (especially above 35) How? An embryo’s ovaries are
halted in meiosis I (during egg development) When ovulation occurs, the eggs resume
meiosis and nondisjunction occurs then This is why it is often seen more in older
women Mental retardation Heart defects Characteristic facial features
Patau
Question?
Why is trisomy more common than monosomy? Fetus can survive an extra copy of a
chromosome, but being hemizygous for somatic cell is usually fatal
Why is trisomy 21 more common in older mothers? Maternal age increases risk of
nondisjunction
Polyploid
Triploid= 3N Tetraploid= 4N Usually fatal in animals Cells receive AN ENTIRE EXTRA
COPY of all homologous chromosomes (including sex chromo)
Question? In plants, even # polyploids are
often fertile, while odd # polyploids are sterile.
Why? Odd number of chromosomes can’t be split during meiosis to make spores.
Chromosome Structure Alterations
Deletions: loss of genetic info Duplications: extra copies of
genetic info Inversions and translocations:
Position effects: a gene's expression is influenced by its location to other genes
Cri Du Chat Syndrome
Part of p arm of #5 missing Deletion chromosomal abnormality
Good survival rate Severe mental retardation Small sized heads common Malformed larynx w/ vocal/speech
problems
Cri du chat
Fragile X
Part of X chromo is missing Deletion
Sterile Mental retardation Oversized testes (if male); ovaries
(if female) “Double jointedness”
Philadelphia Chromosome Caused by translocation An abnormal chromosome produced
by an exchange of portions of chromosomes 9 and 22
Causes chronic myeloid leukemia
Parental Imprinting of Genes Gene expression and inheritance depends
on which parent passed on the gene Usually caused by different methylations of
the DNA CAUSE:
Imprints are "erased" in gamete producing cells and re-coded by the body according to its sex
RESULT: Phenotypes don't follow Mendelian Inheritance
patterns because the sex of the parent does matter
Example:
Prader-Willi Syndrome and Angelman SyndromeBoth lack a small gene region from chromosome 15
Male gene contribution missing: Prader-WilliFemale: Angelman
Why have parental imprinting?
Method that cells might use to detect that TWO different sets of chromosomes are in the zygote
Extranuclear Inheritance
Inheritance of genes not located on the nuclear DNA
Where does it come from? DNA in organelles (Mitochondria and
chloroplasts) Result:
Mendelian inheritance patterns fail. Maternal Inheritance of traits where the
trait is passed directly through the egg to the offspring
Mitochondria Myoclonic Epilepsy Ragged Red-fiber Disease Leber’s Optic Neuropathy All are associated with ATP
generation problems and affect organs with high ATP demands Muscle, brain
Chloroplasts
Gives non-green areas in leaves Called variegation
Several different types known Very common in ornamental plants
Examples
Summary Recognize the relationships between Mendelian
inheritance patterns and chromosomes. Identify linked genes and their effect on
inheritance patterns. Recognize the chromosomal basis of
recombination in unlinked and linked genes. Recognize how crossover data is used to
construct a genetic map. Identify the chromosomal basis of sex in
humans. Recognize examples of sex-linked disorders in
humans.
Summary Continued Identify X-inactivation and its effect in
females. Recognize sources and examples of
chromosomal alterations in humans. Identify examples of abnormalities in sex
chromosome number in humans. Recognize the basis and effects of parental
imprinting of genes in human inheritance patterns.
Recognize the basis and effect of extranuclear inheritance on genetic inheritance patterns.