Chapter 15 The Chromosomal Basis of Inheritance. Timeline 1866- Mendel's Paper 1875- Mitosis worked...
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Transcript of Chapter 15 The Chromosomal Basis of Inheritance. Timeline 1866- Mendel's Paper 1875- Mitosis worked...
Chapter 15 The Chromosomal Basis of Inheritance
Timeline
• 1866- Mendel's Paper
• 1875- Mitosis worked out
• 1890's- Meiosis worked out
• 1902- Sutton, Boveri et. al. connect chromosomes to Meiosis.
• Parents are two true-breeding pea plants
• Parent 1 Yellow, round Seeds (YYRR)
• Parent 2 Green, wrinkled seeds (yyrr)
These 2 genes are on different chromosomes.
Draw meiosis to determine the resulting gametes of parent 1.
How do the resulting gametes connect to Punnett squares?
• F1: YyRr x YyRr
• What are the predicted phenotypic ratios of the offspring?
• ¾ yellow ¾ round
• ¼ green ¼ wrinkled
¼ (green) x ¼ (wrinkled) = 1/16 green, wrinkled
9:3:3:1 phenotypic ratio
First Experimental Evidence to connect Mendelism to the chromosome
• Thomas Morgan (1910)
• Chose to use fruit flies as a test organism in genetics.
• Allowed the first tracing of traits to specific chromosomes.
Fruit Fly
• Drosophila melanogaster
• Early test organism for genetic studies.
Reasons• Small • Cheap to house and feed• Short generation time• Many offspring
• 3 pairs of Autosomes• 1 pair of sex chromosomes
Examples
• Wild type is most common, NOT dominant or recessive• Recessive mutation:
• w = white eyes• w+ = red eyes
• Dominant Mutation• Cy = Curly wings• Cy+ = Normal wings
Morgan Observed:
• A male fly with a mutation for white eyes.
Morgan crossed
• The white eye male with a normal red eye female.
• Male ww x Female w+w+
The F1 offspring:
• All had red eyes.
• This suggests that white eyes is a _________?
• Recessive.
• F1= w+w
• What is the predicted phenotypic ratio for the F2 generation?
F1 X F1 = F2
• Expected F2 ratio - 3:1 of red:white
• He got this ratio, however, all of the white eyed flies were MALE.
• Therefore, the eye color trait appeared to be linked to sex.
Morgan discovered:
• Sex linked traits.
• Genetic traits whose gene are located on the sex chromosome
Fruit Fly Chromosomes
• Female Male• XX XY
• Presence of Y chromosome determines the sex
• Just like in humans!
Morgan Discovered
• There are many genes, but only a few chromosomes.
• Therefore, each chromosome must carry a number of genes together as a “package”.
Sex-Linked Problem• A man with hemophilia (a recessive, sex-
linked, x-chromosome condition) has a daughter of normal phenotype. She marries a man who is normal for the trait.
• A. What is the probability that a daughter of this mating will be a hemophiliac?
• B. That a son will be a hemophiliac? • C. If the couple has four sons, what is the probability that
all four will be born with hemophilia?
• Original Man - XhY • Daughter - must get the dad’s X chromosome XHXh
(normal phenotype, so she’s a carrier)• Daughter’s husband XHY (normal phenotype)
• A. daughter must get XH from the dad. 0% (50% carrier, 50% homo dom.)
• B. son must get Y from dad. 50% chance to be hemophiliac
• C. ½ x ½ x ½ x ½ = 1/16
Linked Genes• Traits that are located on the same
chromosome.• Result:
• Failure of Mendel's Law of Independent Assortment.
• Ratios are different from the expected
Example:• Body Color - gray dominant
• b+ - Gray • b - black
• Wing Type - normal dominant• vg+ - normal • vg – vestigial (short)
Example
b+b vg+vg X bb vgvg
Predict the phenotypic ratio of the offspring
Show at board
b+b x bb vg+vg x vgvg
½ gray ½ black ½ normal ½ vestigial
-----------------------------------------------------¼ gray normal, ¼ gray vestigial,
¼ black normal, ¼ black vestigial
1:1:1:1 phenotypic ratio
Conclusion• Most offspring had the parental phenotype.
Both genes are on the same chromosome.
• What do you expect to happen if they’re on the same chromosome? (draw chromosomes)
• bbvgvg parent can only pass on b vg
• b+b vg+vg can pass on b+ vg+ or b vg
b+b vg+vg - Chromosomes (linked genes)
Crossing-Over
• Breaks up linkages and creates new ones.
• Recombinant offspring formed that doesn't match the parental types.
• Higher recombinant frequency = genes further apart on chromosome
If Genes are Linked:
• Independent Assortment of traits fails.
• Linkage may be “strong” or “weak”.
• Strong Linkage means that 2 alleles are often inherited together.
• Degree of strength related to how close the traits are on the chromosome.
Genetic Maps• Constructed from crossing-over
frequencies.
• 1 map unit = 1% recombination frequency.
• Can use recombination rates to ‘map’ chromosomes.
• Comment - only good for genes that are within 50 map units of each other. Why?
• Over 50% gives the same phenotypic ratios as genes on separate chromosomes
Genetic Maps
• Have been constructed for many traits in fruit flies, humans and other organisms.
Sex Linkage in Biology• Several systems are known:1. Mammals – XY and XX2. Diploid insects – X and XX3. Birds – ZZ and ZW4. Social insects – haploid and diploid
Chromosomal Basis of Sex in Humans
• X chromosome - medium sized chromosome with a large number of traits.
• Y chromosome - much smaller chromosome with only a few traits.
Human Chromosome Sex
• Males - XYFemales - XX
• Comment - The X and Y chromosomes are a homologous pair, but only for a small region at one tip.
SRY• Sex-determining Region
Y chromosome gene.• If present - male • If absent - female• SRY codes for a cell receptor.
Sex Linkage
• Inheritance of traits on the sex chromosomes.
• X- Linkage (common)
• Y- Linkage (very rare if exists at all)
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
• Color blindness
• Duchenne's Muscular Dystrophy
• Hemophilia (types a and b)
• Immune system defects
Samples of X-linked patterns:
X-linked Patterns• Trait is usually passed from a
carrier mother to 1/2 of sons.
• Affected father has no affected children, 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?
Can Females be color-blind?
• Yes, if their mother was a carrier and their father is affected.
Y-linkage• Hairy ear pinnae.
• Comment - new techniques have found a number of Y-linked markers 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
Sex Influenced Traits
• Traits whose expression differs because of the hormones of the sex.
• These are NOT on the sex chromosomes.
• Ex. – beards, mammary gland development, baldness
Baldness• Testosterone – the trait act as a
dominant.• No testosterone – the trait act as a
recessive.• Males – have gene = bald• Females – must be homozygous to
have thin hair.
Barr Body
• Inactive X chromosome observed in the nucleus.
• Way of determining genetic sex without doing a karyotype.
Lyon Hypothesis• Which X inactivated is random.• Inactivation happens early in
embryo development by adding CH3 groups to the DNA.
• Result - body cells are a mosaic of X types.
Examples• Calico Cats.
• Human examples are known such as a sweat gland disorder.
Calico Cats
• XB = black fur
• XO = orange fur
• Calico is heterozygous, XB XO.
Question?
• Why don’t you find many calico males?
• They must be XB XOY and are sterile.
Chromosomal 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.
Nondisjunction
• When chromosomes fail to separate during meiosis
• Result – cells have too many or too few chromosomes which is known as aneuploidy
Meiosis I vs Meiosis II
• Meiosis I – all 4 cells are abnormal
• Meiosis II – only 2 cells are abnormal
Aneuploidy
• Caused by nondisjunction, the failure of a pair of chromosomes to separate during meiosis.
Types
• Monosomy: 2N - 1
• Trisomy: 2N + 1
Turner Syndrome
• 2N - 1 or 45 chromosomesGenotype: 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 need for ovary development.
Homework
• Read Chapter 15 (Hillis – 8)
• Genetics Lab Report – today
• No class Feb. 4 and 5
• Chapter 15 – Thurs. 2/7
Other Sex Chromosome changes
• Kleinfelter Syndrome
• Meta female
• Supermale
Kleinfelter Syndrome• 2N + 1
• Genotype: XXY
• Phenotype: male, but sexual development may be poor. Often taller than average, mental development fine, usually sterile.
Meta female
• 2N + 1 or 2N + 2
• Genotype: XXX or XXXX
• Phenotype: female, but sexual development poor. Mental impairment common.
Super male
• 2N + 1 or 2N + 2
• Genotype: XYY or XYYY
• Phenotype: male, usually normal, fertile.
Trisomy events
• Trisomy 21: Down's Syndrome
• Trisomy 13: Patau Syndrome
• Both have various physical and mental changes.
Question?
• Why is trisomy more common than monosomy?
• Fetus can survive an extra copy of a chromosome, but being hemizygous is usually fatal.
Question?
• Why is trisomy 21 more common in older mothers?
• Maternal age increases risk of nondisjunction.
Polyploid
• Triploid= 3N
• Tetraploid= 4N
• Usually fatal in animals.
Question?
• In plants, even # polyploids are often fertile, why odd # polyploids are sterile. Why?
• Odd number of chromosomes can’t be split during meiosis to make spores.
Structure Alterations
• Deletions
• Duplications
• Inversions
• Translocations
Translocations
Result
• Loss of genetic information.
• 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.
• Good survival, but low birth weight and slow gain.
• Severe mental impairment.
• Small sized heads common.
Cri Du Chat Syndrome
Philadelphia Chromosome
• 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.
Example:
• Prader-Willi Syndrome and Angelman Syndrome
• Both lack a small gene region from chromosome 15.• Male imprint: Prader-Willi
Female imprint: Angelman
Cause:
• Imprints are "erased" in gamete producing cells and re-coded by the body according to its sex.
• Gametes are methylated to code as “male “ or “female”.
Result
• Phenotypes don't follow Mendelian Inheritance patterns because the sex of the parent does matter.
Extranuclear Inheritance
• Inheritance of genes not located on the nuclear DNA.
• DNA in organelles.• Mitochondria
• Chloroplasts
Result
• Mendelian inheritance patterns fail.
• Maternal Inheritance of traits where the trait is passed directly through the egg to the offspring.
Chloroplasts
• Gives non-green areas in leaves, called variegation.
• Several different types known.
• Very common in ornamental plants.
Variegation in African Violets
Variegated Examples
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.
Comment
• Cells can have a mixture of normal and abnormal organelles.
• Result - degree of expression of the maternal inherited trait can vary widely.
Summary
• Know about linkage and crossing-over.
• Sex chromosomes and their pattern of inheritance.
Summary
• Be able to work genetics problems for this chapter.