Ch 15: The Chromosomal Basis of Inheritance

I. Relating Mendelism To ChromosomesA. In 1900, cytologists began to see parallels

between the behavior of genes and chromosomes. 1. They came up with the Chromosome

Theory of Inheritance:a. Genes are found on specific loci

(locations) on chromosomes. b. Chromosomes undergo segregation

and independent assortment.

B. Scientist Thomas Morgan, an embryologist,was the first to associate a specific gene with a specific chromosome. 1. He studied Drosophila melanogaster, a

fruit fly species that eats fungi on fruit.2. Fruit flies have a generation time of 2

weeks.3. They have 3 pairs of autosomes and 1

pair of sex chromosomes (XX = females,XY = males).

4. When he studied these fruit flies, theyfound one male fly with white eyes, instead of the usual red eyes.

a. The normal is called the “wild type”and the white eye type would be called an mutant phenotype.

5. Morgan crossed a wild type female with a white-eyed male. What was the phenotype of the F1?F1 = Red eyesa. Red eyes is dominant to white eyes.

b. Crosses between the F1 generation created a 3:1 ratio in the F2 generation. c. 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.

d. Genotypes of femaleswith red eyes = XRXR

XRXr

e. Genotypes of maleswith red eyes =

f. Genotypes of males with white eyes =

XRY

XrY

C. Linked genes tend to be inherited togetherbecause they are located on the same chromosome.

1. 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.

2. Morgan studied two linked genes forwing 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?):

a. 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).

b. When Morgan breed true-breeding wild-type flies with true-breeding mutant flies, he produced heterozygouswild-type flies = b+b, vg+vg

P generation b+b+, vg+vg+ X bb, vgvg (wild-type) (mutant)

F1 generation b+b, vg+vg X bb, vgvg (TESTCROSS) (wild-type) (mutant)

b+b, vg+vg b+b, vgvg bb, vg+vg bb, vgvg

b+vg+ b+vg bvg+ bvg

bvg

F2 generation 1:1:1:1

c. However, Morgan did not observe a 1:1:1:1 ratio. He observed mostlygray and normal individuals and mostlyblack and vestigial individuals.

d. 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.

D. Two forms of Genetic Recombination:1. 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% Parental Types

50% Recombinants

This 50% frequency of recombination is observed for any two genes that are located on different chromosomes.

2. Crossing Over: a. Most of the offspring will have parental

phenotypes. b. About 17% of Morgan’s flies were

recombinants. c. Crossing over takes place in Prophase I

of meiosis.

E. Geneticists can use recombination data to map a chromosome’s genetic loci (location).1. Alfred Sturtevant used crossing over of

linked genes to develop a chromosome map or linkage map.

a. 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.

c. Sturtevant mapped the relativeposition of three fruit fly genes, body color (b), wing size (vg), and eye color (cn).

d. Distance between genes, the recombination frequency = map units.

e. One map unit is also called a centimorgan. One map unit is equal to 1% recombination frequency.

2. 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 theywere on separate chromosomes.

II. Sex Chromosomes:A. The chromosomal basis for sex varies

with organism.1. Humans: X, Y

Males = XY, Females = XX2. Grasshoppers, crickets, roaches: X, 0

Males = X, Females = XX3. Birds: Z, W

Males = ZZ, Females = ZW4. Bees and Ants: Haploid, Diploid

Males = Haploid (unfertilized eggs; no fathers)

Females = Diploid

B. In humans, anatomical differences betweenmales and females show up at 2 months after fertilization.

1. The SRY gene on the Y chromosomecauses the generic embryonic gonadsto become testes. 2. No SRY gene or inactivation of the gene

forms ovaries.C. Sex-linked genes:

1. Sex-linked Human Disorders: a. Duchenne muscular dystrophy

affects one in 3,500 males born in the United States.

Duchenne muscular dystrophy is caused by an absence of a proteincalled dystrophin, a gene found on the X chromosome. Absence of this protein causes weakening of muscles.

b. Hemophilia: due to one or two absences of blood clotting protein. These proteins help stop bleeding.

D. X Inactivation in Female Mammals:1. Only 1 of the 2 X chromosomes are

activated in cells of females.

2. During female development, one of theX 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).

3. Mary Lyon showed that the X chromosome that becomes a Barr body is randomly selected.

4. She also showed that in a female, some cells have the maternal X activated whileother cells may have the paternal X activated.

a. After a Barr body forms, all of its mitotic descendent cells will have the same inactive X chromosome. b. Females have a mosaic pattern of

activated and inactivated X chromosomes. Example: Tortoise Shell Cats

III. Errors and Exceptions in ChromosomalInheritance

A. Alterations of chromosome number orstructure cause some genetic disorders.

1. Alterations in chromosome number:a. Nondisjunction: Occurs when a pair

of homologous chromosome fail toseparate in meiosis I or sister chromatids fail to separate duringmeiosis 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)-Tetraploid (4n):

A diploid zygote fails to divideafter DNA is replicated.

Commonin plants

b. 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.

B. Some human disorders due to chromosomalalterations:(Most alterations in human embryos are

so disastrous that the embryo will be spontaneously aborted long before birth.)

1. Down Syndrome: Otherwise known as“Trisomy 21” = 3 copies of chromosome #21.

Down Syndrome affects 1 in 700 birthsin the U.S. There is a higher frequencyof Down Syndrome amongst older mothers. Effects: characteristic facial features,short stature, heart defects, varying degrees of mental retardation, some are sterile.

2. Klinefelters Syndrome: Extra X in amale (XXY). Occurs 1 in 2000 births.Effects: Have male organs but sterile.

3. Trisomy X: (XXX), occurs 1 in 1000 births, produces normal females.

4. Turner’s Syndrome (Monosomy X):occurs 1 in 5000 births, produces immature females.

5. Cri du Chat: due to a deletion in chromosome #5. Effects: Mental retardation, small headwith unusual facial features, cries like acat meowing, and fatal.

C. The phenotypic effects of certain genes candepend on which parent passed along thegenes. 1. Two disorders are caused by the same

chromosomal alteration: the partial deletion of chromosome #15.

a. If the partial deletion is passed downfrom the mother, it causes AngelmanSyndrome.

Effects: spontaneous laughter, jerky movements, and other motor and mental symptoms

b. If the partial deletion of chromosome #15 is inherited from the father, itcauses Prader-Willi Syndrome.

Effects: mental retardation, obesity, short stature, and unusually small hands and feet.

2. These phenotypic effects are due to genomic imprinting.

a. The process of genomic imprinting isstill being studied. We do know that methylation of DNA (-CH3 groups added to pieces of DNA) can either turn on or turn off genes.

D. Cytoplasmic DNA: 1. Mitochondrial DNA: passed down from

mother. mDNA codes for the proteins found in the mitochondrial membrane(Ex. ATP Synthase).