Ch 15: The Chromosomal Basis of Inheritance

1. Ch 15: The Chromosomal Basis of Inheritance

2. 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.

4. 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).

5. When he studied these fruit flies, they • found one male fly with white eyes, • instead of the usual red eyes.

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

7. 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.

8. Genotypes of females • with red eyes = XRXR XRXr • Genotypes of males • with red eyes = • Genotypes of males • with white eyes = XRY XrY

9. 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?):

10. 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)

11. 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.

12. 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.

14. 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

15. 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.

16. 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.

17. Sturtevant mapped the relative • position of three fruit fly genes, body • color (b), wing size (vg), and eye • color (cn).

18. 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.

20. 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

22. 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.

23. 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.

24. 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.

25. 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

26. 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.

28. 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.

29. 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.

30. 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.

31. 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.

32. 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.

33. 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.

34. 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

35. 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.

36. Cytoplasmic DNA: • 1. Mitochondrial DNA: passed down from mother. mDNA codes for the proteins found in the mitochondrial membrane (Ex. ATP Synthase).