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Ch 15: The Chromosomal Basis of Inheritance. I. Relating Mendelism To Chromosomes. In 1900, cytologists began to see parallels between the behavior of genes and chromosomes. They came up with the Chromosome Theory of Inheritance:. Genes are found on specific loci
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I. Relating Mendelism To Chromosomes • In 1900, cytologists began to see parallels • between the behavior of genes and • chromosomes. • They came up with the Chromosome • Theory of Inheritance: • Genes are found on specific loci • (locations) on chromosomes. • Chromosomes undergo segregation • and independent assortment.
Scientist Thomas Morgan, an embryologist, • was the first to associate a specific gene • with a specific chromosome. • He studied Drosophila melanogaster, a • fruit fly species that eats fungi on fruit. • Fruit flies have a generation time of 2 • weeks. • They have 3 pairs of autosomes and 1 • pair of sex chromosomes (XX = females, • XY = males).
When he studied these fruit flies, they • found one male fly with white eyes, • instead of the usual red eyes.
The normal is called the “wild type” • and the white eye type would be • called an mutant phenotype.
Morgan crossed a wild type female with a • white-eyed male. • What was the phenotype of the F1? F1 = Red eyes a. Red eyes is dominant to white eyes. • Crosses between the F1 generation • created a 3:1 ratio in the F2 generation. • He also found that the white-eyed • flies were only males. • Morgan concluded that the eye color gene was linked to sex or found on the sex chromosomes.
Genotypes of females • with red eyes = XRXR XRXr • Genotypes of males • with red eyes = • Genotypes of males • with white eyes = XRY XrY
Linked genes tend to be inherited together • because they are located on the same • chromosome. • Each chromosome has hundreds or • thousands of genes. These genes are • passed along as a unit because the • chromosome is passed along as a unit. • These genes are called linked genes. • Morgan studied two linked genes for • wing shape and body color (if these • two genes are linked, and if you did a • dihybrid cross, would you get the • 9:3:3:1 ratio?):
The wild-type body color is gray (b+) • and the mutant black (b). • The wild-type wing size is normal (vg+) • and the mutant has vestigial wings (vg). • When Morgan breed true-breeding • wild-type flies with true-breeding • mutant flies, he produced heterozygous • wild-type flies = b+b, vg+vg P generation b+b+, vg+vg+X bb, vgvg (wild-type) (mutant) F1 generationb+b, vg+vg X bb, vgvg (TESTCROSS) (wild-type) (mutant)
b+vg+ b+vg bvg+ bvg bvg 1:1:1:1 F2 generation • However, Morgan did not observe a • 1:1:1:1 ratio. He observed mostly • gray and normal individuals and mostly • black and vestigial individuals.
Very few were gray and vestigial • and a few black and normal. • He attributed these few individuals with variation as a result of crossing over.
Two forms ofGenetic Recombination: • Independent Assortment: • Example = P generation YYRR X yyrr (yellow, round) (green, wrinkled) F1 generation YyRr X yyrr (TESTCROSS) (yellow, round) (green, wrinkled) F2 generation 1YyRr: 1Yyrr: 1yyRr: 1yyrr 50% Recombinants 50% Parental Types
This 50% frequency of recombination is observed for any two genes that are located on different chromosomes. • Crossing Over: • Most of the offspring will have parental • phenotypes. • About 17% of Morgan’s flies were • recombinants. • Crossing over takes place in Prophase I • of meiosis.
Geneticists can use recombination data to • map a chromosome’s genetic loci (location). • Alfred Sturtevant used crossing over of • linked genes to develop a chromosome • map or linkage map. • A linkage map is an ordered list of the • genetic loci along a chromosome. b. The farther apart two genes are, the higher the probability that a crossover will occur between them higher recombination frequency.
Sturtevant mapped the relative • position of three fruit fly genes, body • color (b), wing size (vg), and eye • color (cn).
Distance between genes, the • recombination frequency = map units. • One map unit is also called a • centimorgan. One map unit is equal to • 1% recombination frequency. • Some genes on a chromosome are so • far apart that a crossover between them • is virtually certain. Example: seed color and flower color are far enough apart that linkage is not observed. These two genes act as if they were on separate chromosomes.
Sex Chromosomes: • A. The chromosomal basis for sex varies • with organism. • Humans: X, Y • Males = XY, Females = XX • Grasshoppers, crickets, roaches: X, 0 • Males = X, Females = XX • Birds: Z, W • Males = ZZ, Females = ZW • Bees and Ants: Haploid, Diploid • Males = Haploid (unfertilized eggs; no • fathers) • Females = Diploid
In humans, anatomical differences between • males and females show up at 2 months • after fertilization. • The SRY gene on the Y chromosome • causes the generic embryonic gonads • to become testes. • No SRY gene or inactivation of the gene • forms ovaries. • Sex-linked genes: • Sex-linked Human Disorders: • Duchenne muscular dystrophy • affects one in 3,500 males born in the • United States.
Duchenne muscular dystrophy is caused by an absence of a protein called dystrophin, a gene found on the X chromosome. Absence of this protein causes weakening of muscles. • Hemophilia: due to one or two • absences of blood clotting protein. • These proteins help stop bleeding. • X Inactivation in Female Mammals: • Only 1 of the 2 X chromosomes are • activated in cells of females.
During female development, one of the • X chromosomes condenses into a • compact object called a Barr body, which • inactivates all the genes it. (The Barr body is reactivated in certain ovarian cells that become ova – eggs). • Mary Lyon showed that the X • chromosome that becomes a Barr body • is randomly selected. • She also showed that in a female, some • cells have the maternal X activated while • other cells may have the paternal X • activated.
After a Barr body forms, all of its • mitotic descendent cells will have the • same inactive X chromosome. • Females have a mosaic pattern of • activated and inactivated X • chromosomes. • Example: Tortoise Shell Cats
Errors and Exceptions in Chromosomal • Inheritance • Alterations of chromosome number or • structure cause some genetic disorders. • Alterations in chromosome number: • Nondisjunction: Occurs when a pair • of homologous chromosome fail to • separate in meiosis I or sister • chromatids fail to separate during • meiosis II. • Causes gametes to either receive too few or too many chromosomes. • Aneuploidy is when an individual have an abnormal number of chromosomes.
A type of aneuploidy where an individual has 3 copies of a chromosome is called trisomic. (2n + 1) • A type of aneuploidy where an individual has only 1 copy of a chromosome is called monosomic. (2n – 1) • A polyploid organism has more than two sets of chromosomes: -Triploid (3n): 1 gamete (n) + 1 gamete (2n) Common in plants -Tetraploid (4n): A diploid zygote fails to divide after DNA is replicated.
Alterations of Chromosome Structure: • Deletion: When a portion of a chromosome is lost during cell division. This would result in the loss of genes. • Duplication: During meiosis, a fragment of a sister chromatid can attach to the other sister, causing the recipient sister to have extra copies of genes.
Inversion: When a chromosome fragment is reattached to its original chromosome, but reattaches in reverse. • Translocation: When a chromosome attaches to a non-homologous chromosome.
Some human disorders due to chromosomal • alterations: • (Most alterations in human embryos are • so disastrous that the embryo will be • spontaneously aborted long before birth.) • Down Syndrome: Otherwise known as • “Trisomy 21” = 3 copies of chromosome • #21.
Down Syndrome affects 1 in 700 births in the U.S. There is a higher frequency of Down Syndrome amongst older mothers. Effects: characteristic facial features, short stature, heart defects, varying degrees of mental retardation, some are sterile. • Klinefelters Syndrome: Extra X in a • male (XXY). Occurs 1 in 2000 births. Effects: Have male organs but sterile.
Trisomy X: (XXX), occurs 1 in 1000 • births, produces normal females. • Turner’s Syndrome (Monosomy X): • occurs 1 in 5000 births, produces • immature females. • Cri du Chat: due to a deletion in • chromosome #5. Effects: Mental retardation, small head with unusual facial features, cries like a cat meowing, and fatal.
The phenotypic effects of certain genes can • depend on which parent passed along the • genes. • Two disorders are caused by the same • chromosomal alteration: the partial • deletion of chromosome #15. • If the partial deletion is passed down • from the mother, it causes Angelman • Syndrome. Effects: spontaneous laughter, jerky movements, and other motor and mental symptoms
If the partial deletion of chromosome • #15 is inherited from the father, it • causes Prader-Willi Syndrome. Effects: mental retardation, obesity, short stature, and unusually small hands and feet. • These phenotypic effects are due to • genomic imprinting. • The process of genomic imprinting is • still being studied. We do know that • methylation of DNA (-CH3 groups • added to pieces of DNA) can either • turn on or turn off genes.
Cytoplasmic DNA: • 1. Mitochondrial DNA: passed down from mother. mDNA codes for the proteins found in the mitochondrial membrane (Ex. ATP Synthase).