CHAPTER 15. THE CHROMOSOMAL BASIS OF INHERITANCE. I. Concept 15.1: Behavior of Chromosomes. A. Background 1. Genetics 1860’s— Mendel proposed that discrete inherited factors segregate and assort independently during gamete formation 2. Cytology
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THE CHROMOSOMAL BASIS OF INHERITANCE
1860’s—Mendelproposed that discrete inherited factors segregate and assort independently during gamete formation
1875—Cytologists worked out process of mitosis
1890’s—Cytologists worked out process of meiosis
1900—Correns, von Tschermak, and de Vries independently discovered Mendel’s work
1902—2 areas converge as Walter Sutton, Theodor Boveri, and others noticed parallels between the behavior of Mendel’s factors and the behavior of chromosomes:
5. Chromosomal Theory of Inheritance is based on these observations. According to this theory:
Mendelian factors (genes) are located on chromosomes.
Chromosomes segregate and assort independently during meiosis
1. Walter Flemming(1882)
2. August Weismann (1887)
4. Walter Sutton (1902)
1. Reasons for using Drosophila
Ex: w+-red eyes
-alternatives to wild type
-due to mutations in wild type gene
Ex: w-white eyes
a.Eye color is linked to sex.
b. The gene for eye color is located on the X chromosome.
had red eyes
3. Morgan’s finding of the correlation between a particular trait and an individual’s sex provided support for the chromosome theory of inheritance
3. The SRY gene on the Y chromosome codes for the development of testes
7. Sex-linked recessive disorders are much more common in males than in females
8. If a sex-linked trait is due to a recessive allele, a female will express the trait only if she is homozygous.
9. A heterozygous female is a carrier.
10. Males only need 1 allele of a sex-linked trait to show the trait.
11. Males are hemizygous (only one copy of a gene is present in a diploid organism)
N= normal color vision
a. Color Blindness
-can’t distinguish certain colors
-red-green most common
b. Duchenne’s Muscular Dystrophy
-atrophy of muscle
-blood fails to clot because of lack of a clotting factor
1. Female mammals have only one fully functional X chromosome in diploid cells.
2. Proposed by Mary F. Lyon and known as the Lyon Hypothesis
3. Each of the embryonic cells inactivates one of the two X chromosomes.
4. Inactive X contracts into densely staining object called a Barr Body.
1. Sex-limited Traits
2. Sex-influenced Traits
both—length of index finger as compared to ring finger
-dominant in ♂ shorter index finger
-dominant in ♀ longer index finger
1. Production of offspring with new combinations of traits different from those combinations found in the parents (crossing over)
2. Results from the events of meiosis and random fertilization
B. Recombination of Unlinked Genes: Independent Assortment
one of the parental phenotypes.
Gametes from yellow-round
heterozygous parent (YyRr)
Gametes from green-
recessive parent (yyrr)
Recombinant Types or Recombinants- offspring that have new
combinations of phenotypes.
(yellow, round) (green, wrinkled)
F1 ¼ YyRr ¼ yyrrParental types 50%
¼ yyRr ¼ YyrrRecombinant types 50%
b—black body vg—vestigial wings
b+--gray body vg+--wild type wings
b+ b vg+ vg x b b vg vg (testcross)
(gray, normal wings) (black, vestigial wings)
offspring if the genes were located on different chromosomes?
What if they were located on the same chromosome and parental
alleles are always inherited together?
total # offspring
--results in new allelic combination
1. Recombination frequencies are used to construct chromosome maps [show locations of genes on a particular chromosome (linear)]
2. Sturtevant constructed chromosome maps for Drosophila using recombination frequencies
3. 1 map unit = 1% recombination frequency (now called centimorgans)
b vg 17%
cn b 9%
cn vg 9.5%
a. Establish the distance between the genes with the highest RF
b. Determine RF between third gene and first.
c. Consider the two possible placements of the third gene.
--correct sequence b-cn-vg
5. If linked genes are so far apart on a chromosome that the RF is 50%, they are indistinguishable from unlinked genes that assort independently.
6. Maps from XO data give relative positions of linked genes
7. Cytological mapping pinpoints actual location of genes and real distance between them.
A. Alterations of Chromosomal Number
1. Breakage of a chromosome can lead to four types of changes in chromosome structure:
a. Deletion removes a chromosomal segment
c. An inversion occurs if the fragment reattaches to the original chromosome in reverse order.
Inversion reverses a segment within a chromosome
d.A translocation occurs if a chromosomal fragment joins to a nonhomologous chromosome
1. Alterations of chromosome number and structure are associated with some serious disorders
2. Some types of aneuploidy appear to upset the genetic balance less than others, resulting in individuals surviving to birth and beyond
3. These surviving individuals have a set of symptoms, or syndrome, characteristic of the type of aneuploidy
a. Down’s Syndrome
b. Patau Syndrome
c. Edward’s Syndrome
a. Klinefelter’s Syndrome--Genotype usually XXY
b. Extra Y--XYY
c. Trisomy X (metafemales)--XXX
d. Turner’s Syndrome (monosomy X)--XO
6. Examples of Deletions:
a. Cri du chat Syndrome--Deletion on chromosome 5
7. Examples of Translocations:
a. Chronic Myelogenous Leukemia (CML)--Portion of chromosome 22 switched with a fragment of chromosome 9
b. Type of Down’s Syndrome--Translocation from chromosome 21 to chromosome 15
A. Genomic Imprinting
4. It appears that imprinting is the result of the methylation (addition of –CH3) of DNA
5. Genomic imprinting is thought to affect only a small fraction of mammalian genes
6. Most imprinted genes are critical for embryonic development
5. Some defects in mitochondrial genes prevent cells from making enough ATP and result in diseases that affect the muscular and nervous systems
6. Cytoplasmic genes described in plants by Karl Correns (1909)