Topic 2 Chromosomal inheritance Flashcards
(41 cards)
Thomas Hunt Morgan’s theory:
Chromosomal theory of inheritance: Genes are located on chromosomes
Morgan’s experiments with fruit flies provided evidence that genes (Mendel’s heritable factors) are located on chromosomes
wild type
normal phenotype
characteristics make fruit flies a convenient
organism for genetic studies: (3)
– They breed at a high rate
– A generation can be bred every 2 weeks
– They have only 4 pairs of chromosomes
X and Y chromosomes are different =>
nonhomologous; Only the ends of the Y chromosome have regions that are homologous with the X chromosome (X and Y chromosomes have about 18 genes in common)
Sex-linked gene -
gene located on either sex chromosome
Why are sex-linked recessive disorders are much more common in males than in females?
For a recessive sex-linked trait to be expressed
– A female needs 2 copies of the allele
– A male needs only 1 copy of the allele
Example of Y chromosome gene
SRY gene on the Y chromosome codes for the development of testes: absence in females => gonads are developed into ovaries
abnormalities => XY males develop as females (Swyer syndrome), XX as males (XX male syndrome)
X-linked disorders, examples: (3)
- haemophilia (progressive bleeding is prolonged in the affected individual following an injury; due to clotting factor VIII deficiency)
- colour blindness
- Duchenne muscular dystrophy (progressive weakening of the muscles and loss of coordination; affected individuals rarely pass the age of 20)
Swyer syndrome (XY gonadal dysgenesis):
reason: Mutations in SRY gene (=> inactivation) => XY females with gonadal dysgenesis
46XY individuals that are normally male will have female characteristics
XX male syndrome:
reason: translocation (abnormal exchange of chromosomal fragments between non-homologous chromosomes) of part of the Y chromosome containing the SRY gene to the X chromosome => X chromosome carries the normally male SRY gene => females have male characteristics
translocation vs genetic recombination:
- Translocation: exchange of chromosomal fragments between non-homologous chromosomes (abnormal)
- Genetic recombination: exchange of chromosomal fragments between homologous chromosomes (normal)
Barr body
1 of the 2 X chromosomes in each somatic cell is randomly inactivated during embryonic development in mammalian females => inactive X chromosome turns into Barr body, lies inside of the nuclear envelope;
=> If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character
In the ovaries, the Barr body chromosomes are reactivated in the cells that give rise to eggs, so every female gamete has an active X
Mosaicism
Some somatic cells will express the phenotype of one X-linked gene and some cells of the other
in humans: X-linked mutation that prevents the development of sweat glands (hypohidrotic ectodermal dysplasia): woman who is heterozygous has patches of normal skin and patches of skin lacking sweat glands
How Linkage Affects Inheritance: linked genes -
Genes located near each other on the same chromosome that tend to be inherited together => Mendel’s law of independent assortment does not apply to linked genes
If the genes were completely linked, the offspring genotypes would be:
- 50% GgWw
- 50% ggww
If the genes are incompletely linked:
genetic recombination can sometimes break the physical connection between genes on the same chromosome => unlinks (separates) genes
- genetic recombination: crossing over of non-sister chromatids of homologous chromosomes during Meiosis I
Chromosomal abnormalities group: (2)
- Chromosome number
- Chromosome structure
Abnormal Chromosome Number:
Aneuploidy - presence of abnormal chromosome number;
reason: fertilization of gametes in which nondisjunction occurred => offspring with this condition have an abnormal number of a particular chromosome => abnormal karyotype
Nondisjunction -
abnormal separation of homologous chromosome pairs during meiosis I OR sister chromatids during meiosis II
Nondisjunction of homologous chromosomes in meiosis I => one gamete receives both homologous chromosomes from a pair, and another gamete receives none (0 chromosomes)
Nondisjunction of sister chromatids in meiosis II: non-separation of sister chromatids during meiosis II => half the gametes have 1 more or 1 less chromosome at the end of meiosis II, the other half are normal
Aneuploidy vs Polyploidy
Aneuploidy - condition in which the number of chromosomes in a cell is not an exact multiple of the monoploid (haploid) number of a particular species
Polyploidy - condition in which an organism has more than 2 complete sets of chromosomes
– Triploidy (3n) is three sets of chromosomes
– Tetraploidy (4n) is four sets of chromosomes
- Polyploids are more normal in appearance than aneuploids
4 types of changes in chromosome structure:
– Deletion: removal of a chromosomal segment
– Duplication: repetition of a segment
– Inversion: reversal of a segment within a
chromosome
– Translocation: exchange of segments between nonhomologous chromosomes (movement of a segment from one chromosome to another)
Human Disorders Due to Chromosomal Alterations
Some types of aneuploidy appear to upset the genetic balance less than others, resulting in individuals surviving to birth and beyond
surviving individuals have syndrome - set of symptoms characteristic of the type of aneuploidy (birth defects, intellectual disability (mental retardation) and shortened life expectancy)
ex:
Trisomy 21 (Down syndrome)
Trisomy 18 (Edwards syndrome)
Trisomy 13 (Patau syndrome)
Down Syndrome
Trisomy 21
Klinefelter syndrome
XXY individuals
reason: non-disjunction of sex chromosomes
result: males with an extra X chromosome => males with some female characteristics and developmental abnormalities (e.g. gynecomastia)
