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Transcript of Heredity
Walter Sutton proposed a chromosomal theory of inheritanceHereditary traits are carried on chromosomes
Thomas Hunt Morgan
1910Drosphila Melanogasterthe fruit fly
Chromosomes condense and spindles begin to form. DNA has been replicated.Crossing over occurs
Homologous chromosomes align along the metaphase plate (in the center) Kinetochoremicrotubules attach to each end of a chromosome
Kinetochore microtubules begin to shorten and homologous chromosomes are pulledapart.They move to opposite ends. Sister chromatids DO NOT separate (opposite ofmitosis)
Separated chromosomes cluster together at each end of the cell and the nuclearenvelope reforms. Cytokensis occurs and forms two daughter cells with half the numberof chromosomes as the original cell. Chromatids are NOT identical because crossingover occurred in Prophase 1
Prophase 2A new spindle apparatus forms and the nuclear envelope breaks down
Metaphase 2Formation of spindle apparatus is complete.
Chromosomes align along the metaphaseplate. New
kinetochore microtubles form at opposite poles of each cell.
Anaphase 2Microtubles shorten and the centromeres split. Sister chromatids are pulled to opposite ends.
Telophase 2Nuclear membranes reform around the clusters of chromosomes.After cytokenisis 4 haploid cells result.No two cells are identical due to crossing over.
-Austrian monk who mated peas to produce offspring and recorded the phenotype to determine inheritance-He discovered many statistical laws of heredity-Learned that a monohybrid cross (cross that involves a single character in heterozygous parents) results in a phenotype ratio of 3:1-dihybrid cross (crossing of 2 different hybrid characters) results in a phenotype ratio of 9:3:3:1
every organism carries a pair of alleles for each trait and the members of the pair separate during the formation of gametes
Law of Segregation
members of each pair of factors are distributed independently
Law of Independent Assortment
-something Mendel’s experiment did not account for-incomplete dominence- neither allele dominates the other. Ex. Red snapdragon and whitesnapdragon make a pink snapdragon-hypercholesterolemia-codominence- both alleles express themselves fully in a heterozygous organism. Ex.uman blood groups
-polygenic traits: traits that are affected by more than one gene. Ex: eye color-multiple alleles: human blood group-epistasis: when the expression on one gene affects the expression of another gene. Ex: thecoat color of mice. The gene for color also affects the gene that controls the deposition ofpigmentation of the fur-pleiotropy: a single gene has multiple effects on an organism. Ex: sickle cell anemia
Other forms of Inheritance
White Mutant Male Fly
The testcross revealed that white-eyed females are viable. Therefore eye color is linked to the X chromosome and absent from the Y chromosome.
presented first clear evidence that the genes determining Mendelian traits do indeed reside on the chromosomes
Sex Chromosomes and Sex Determination
The structure and number of sex chromosomes vary in differenct species
In humans the Y chromosome generally determines maleness
Some Human genetic disorders display sex linkage
Red-green colorblindness is more common in males because the gene affected is carried on the x chromosome.
Hemophilia is a disease caused by an X-linked recessive allele. Meaning women who are heterozygous for the allele are asymptomatic carriers and men who receive an X chromosome with the recessive allele exhibit the disease.
The number of copies an individual has of the X chromosome determines the sex of the individual.
inactivation of one of the X chromosomes in a female during early embryonic development
prevents doubling of any sex linked gene
the inactivated X chromosome appears as a Barr body attached to a nuclear membrane
can lead to genetic mosaics
their individual cells may express different alleles depending on which chromosome is inactivated.
ex: Calico Cata female that has a patchy distribution of dark fur, orange fur, and white fur
Sex Linkage and the Chromosomal Theory of Inheritance
Selected Human Genetic Disorders
genetic disorders can be due to altered proteins
ex: sickle cell anemia
sickle cell anemia is caused by a defect in the oxygen carrier, hemoglobin, that leads to impaired oxygen delivery to tissues. The defective hemoglobin molecules stick to one another, forming stiff, rod like structures.
nondisjunction: the failure of homologues or sister chromatids to separate properly during meiosis
Nondisjunction of Autosomes
Monosomics=humans who have lost even one copy of an autosome
Trisomics=humans who have gained an extra autosome
ex: Down Syndrome
Down Syndrome=when a particular small portion of chromosome 21 is present in three copies instead of two
Nondisjunction of Sex Chromosomes
individuals that gain or lose a sex chromosome do not generally experiance the severe developmental abnormalities caused by similar changes in autosomes
genomic imprinting=the phenotype caused by a specific allele is exhibited when the allele comes from one parent, but not from another.
genetic counseling=the process of identifying parents at risk for having children with genetic defects and of assessing the genetic state of early embryos.
amniocentesis= a procedure that permits the prenatal diagnosis of many genetic disorders
chorionic villi sampling=the doctor removes cells from the chorion, a membranous part of the placenta that nourishes the fetus