The Chromosomal Basis of inheritance Flashcards
Chromosomal theory of inheritance:
- Stating that genes are located at specific positions (loci) on chromosomes
- that the behavior of chromosomes during meiosis accounts for Mendel’s laws of segregation and independent assortment
Law of segregation
the two alleles for each gene separate during gamete formation
Law of independent assortment
- Pairs of alleles assort independently of other pairs during gamete formation;
- applies when genes for two characters are located on different pairs of homologous chromosomes or when they are far enough apart on the same chromosome to behave as though they are on different chromosomes
When Morgan mated a white-eyed male fly with a red-eyed female he came to the startling conclusion that the trait for eye colour was located on the chromosome that determines sex. Show this cross. (parental generation: A red-eyed female was crossed with a white-eyed male.)
What unusual result of Morgan’s experiments suggested that the eye-color trait is located on the X chromosome?
the white-eye trait showed up only in males
What is the SRY gene?
refers to the Sex determining Region of Y
sex-linked gene
- a gene located on either sex chromosome.
- Most sex-linked genes are on the X chromosome and show distinctive patterns of inheritance; there are very few genes on the Y chromosome
In humans how have sex-linked genes been historically modified?
the sex-linked traits are actually X-linked genes
Name three human sex-linked disorders.
- Duchenne muscular dystrophy
- Hemophilia
- color blindness
Duchenne muscular dystrophy
A human genetic disease caused by a sex-linked recessive allele; characterized by progressive weakening and a loss of muscle tissue
Hemophelia
A human genetic disease caused by a sex-linked recessive allele resulting in the absence of one or more blood-clotting proteins; characterized by excessive bleeding following injury
Color blindness
A mild disorder almost always inherited as an X-linked allele
Barr body
a dense object lying along the inside of the nuclear envelope in cells of female mammals representing a highly condensed/inactivated X chromosome
X inactivation maintains the proper gene dosage. How is the X chromosome inactivated?
The selection of which X chromosome will form the Barr body occurs randomly and independently in each embryonic cell present at the time of X inactivation.
Linked genes
genes located close enough together on a chromosome that they tend to be inherited together
Do linked genes sort independently?
No, they are transmitted as a unit.
- Morgan’s experiments illustrated that some mechanism (later discovered to be “crossing over”) occasionally breaks the linkage between specific alleles of genes on the same chromosome.
Genes that do not independently assort…?
- two genes linked on the same chromosome:
- transmitted as a unit and will not sort independently
What occurs during meiosis between homologous chromosomes causing the linked genes to become unlinked?
crossing over
genetic recombination
the process during which linked genes become unlinked
What do geneticists call the offspring that show these new combinations after crossing over and experimenting with genetic recombination?
they call the offspring that show these new combinations recombinant types
When does crossing over occur in meiosis?
prophase I
Linkage map
a genetic map based on the frequencies between markers during crossing over of homologous chromosomes
Map unit
unit of measurement of the distance between genes (equivalent to a 1% recombination frequency)
Nondisjunction
an error in meiosis or mitosis in which members of a pair of homologous chromosomes or a pair of sister chromatids fail to separate properly from each other
What are the two ways nondisjunction can occur?
- in meiosis 1, where homologous chromosomes do not separate or,
- meiosis 2, where sister chromatids do not separate
Aneuploidy
- Fertilisation of gametes where nondisjunction occurred.
- Offspring have an abnormal number of a particular chromosome.
Monosomy
- A diploid cell that has only one copy of a particular chromosome instead of the normal two.
- Turner syndrome is a human monosomy; the female has only one X chromosome
Trisomy
- A diploid cell that has three copies of a particular chromosome instead of the normal two.
- Trisomy 21 is trisomic for chromosome 21
Polyploidy
- an organism that possesses more than two complete chromosome sets. It is the result of an accident of cell division
- examples include:
1. Triploidy (3n)
2. Tetraploidy (4n)
3. Hexaploidy (6n)
What causes Down syndrome?
- usually the result of an extra chromosome 21 so that each cell has a total of 47 chromosomes.
- Because the cells are trisomic for chromosome 21 Down syndrome is often called trisomy 21
What are the four characteristics of Down syndrome?
- Characteristic facial features
- Short stature
- Correctable heart defects
- Developmental delays
Example and sex of XXY
- Klinefelter syndrome
- Male; sterile
- male sex organs - but abnormally small/ sterile testes; some breast enlargement and other female characteristics
Example and sex of XXX
- Trisomy X (Triple X syndrome)
- Female, fertile
- no unusual physical features other than being slightly taller than average; at risk for learning disabilities
Example and sex of XO
- Turner syndrome
- Female; sterile due to lack of maturation in sex organs; secondary sex characteristics developed with estrogen replacement; normal intelligence
Example and sex of XYY
- Jacobs syndrome
- Male, Normal sexual development; taller than average stature
What are the ways chromosome structure can be altered?
- deletion
- duplication
- inversion
- translocation
Deletion
- loss of a segment
Duplication
- repeated segment
Inversion
- reverses orientation of a segment within a chromosome
Translocation
- a segment moves from one chromosome to another
Explain genomic imprinting.
- Occurs during gamete formation
- Results in the silencing of a particular allele of certain genes.
- These genes are imprinted differently in spern and eggs, so zygote expresses only one allele of an imprinted gene: inherited from either the female or male parent.
- The imprints are transmitted to all body cells during development.
What group of genes do you inherit from your mother only?
mitochondrial DNA
Besides mitochondrial DNA what other organelle has its own genes? (known as extranuclear genes)
chloroplasts or mitochondria
