Lecture 15 (Errors in Meiosis and X-inactivation) Flashcards
Down syndrome
Trisomy 21 (3 copies of chromosome 21) 1/750 live births
Aneuploidy
Abnormal number of a particular chromosome
Many aneuploids in humans are fatal
Statistics for Down syndrome
Mothers over 45 have 40% of all cases of down syndrome
95% of down syndrome babies have two maternal chromosome 21
Nondisjunction
Non-disjunction is an error in cell division in which members of a pair of homologous chromosomes (in meiosis I) or a pair of sister chromatids (in meiosis II or mitosis) fail to seperate properly from each other. This leads to chromosomes not being distributed equally in the daughter cells
Affects the proportion of chromosomes in meiotic products
Creates aneuploidy
When non-disjunction occurs in mitosis (in terms of a plant)
The rest of the plant made from that cell is a polyploid.
When non-disjunction occurs in meiosis (in terms of a plant)
The resulting 2n gamete may either fuse with another 2n gamete to create a 4n polyploidy plant. Or it could fuse with a normal haploid gamete to make a 3n plant. A 3n plant would usually be sterile, however sometimes it can have a chromosome doubling event to make a fertile 6n plant.
Nondisjunction and Down syndrome
Down syndrome is usually caused by an error in cell division called “nondisjunction.” Nondisjunction results in an embryo with three copies of chromosome 21 instead of the usual two. Prior to or at conception, a pair of 21st chromosomes in either the sperm or the egg fails to separate.
Klinefelter syndrome
A genetic condition in which a male is born with an extra copy of the X chromosome (XXY)
Phenotypic features - tall, male, womanly physique, lack of chest hair, breast development
Turner syndrome
Turner syndrome, a condition that affects only females, results when one of the X chromosomes is missing or partially missing (XO). Turner syndrome can cause a variety of medical and developmental problems, including short height, failure of the ovaries to develop, short neck fold in skin and heart defects.
Interesting to note that there is a difference in social abilities depending on whether the X chromosome comes from the mother or father
A pair of chromosomes fails to disjoin at anaphase of meiosis 1, what will be the likely chromosome numbers (N) of their four resulting gametes?
N+1
N+1
N-1
N-1
A pair of chromosomes fails to disjoin at anaphase of meiosis 2, what will be the likely chromosome numbers (N) of their four resulting gametes?
N+1
N-1
N
N
Polyploidy
Having more than 2 haploid sets of chromosomes (being more than diploid.)
Possession of multiple sets of chromosomes (whole set of chromosomes is duplicated)
Occurs in plants and some other animals
Many polyploids are infertile, depending on the number of chromosome sets they have inherited. If the individual has inherited an odd number of chromosome sets (3n, 5n, etc), they are usually infertile.
Autopolyploid
Made from only one species.
Allopolyploid
Come from more than one species.
Tetraploid
4n, containing four homologous sets of chromosomes.
Triploid
3n, containing three homologous sets of chromosomes.
Offspring with polyploid karyotypes may be…
Viable and self-fertile
Deletion
A deletion removes a chromosomal segment
Inversion
An inversion reverses a segment within a chromosome
Duplication
A duplication repeats a segment
Translocation
A translocation moves a segment from one chromosome to a non-homologous chromosome
Reciprocal translocation
Reciprocal translocation is a chromosome abnormality caused by exchange of parts between non-homologous chromosomes. Two detached fragments of two different chromosomes are switched.
Insertion
The addition of one or more nucleotide base pairs into a DNA sequence
Lejeune syndrome (Cri du chat)
Deletion of tip of short arm on chromosome 5. Individuals affected are severely intellectually disables and usually die in childhood.
Results of meiosis in a human inversion heterozygote
Inversion heterozygote - A diploid organism in which one member of a pair of homologous chromosomes has an inverted gene sequence and the other has the normal gene sequence.
Usually fine with transcribing the gene as all the information is still there. A problem sometimes arises when the chromosomes synapse in meiosis because the synapse by recognising piece that are homologous to each other all the way along, if one of those chromosomes have fiipped around then they can’t pair up properly, they pair up in a way that gives the chromosome big loops and as a consequence this can lead to deletions and insertions
Familial Down syndrome
Translocation Down syndrome happens when an extra copy of chromosome 21 is attached to another chromosome. This is then present in all cells of the body. Chromosomes contain all of the genetic information that tells our body how to grow and function. Like trisomy 21, mosaic Down syndrome is not inherited.
Process - A type of chromosome rearrangement, also called a Robertsonian translocation, in which there is fusion of an entire long arm of one acrocentric chromosome (A chromosome in which the centromere is located quite near one end of the chromosome) with a similarly intact long arm of another acrocentric chromosome. Down syndrome is caused by an extra acrocentric chromosome (chromosome21).
This is not spontaneous
Just as likely to have come from mother or father
Individuals with familial Down syndrome have which of the following karyotypes?
46 chromosomes, translocation of 21 (these individuals have the right number of chromosomes but have a translocation on chromosome 21)
Lyon hypothesis
A hypothesis explaining why the phenotypic effect of the X chromosome is the same in the mammalian female which has two X chromosomes as it is in the male which has only one X chromosome: one of each two somatic X chromosomes in mammalian females is selected at random and inactivated early in embryonic development
X inactivation (lyonisation)
X chromosome inactivation prevents females from having twice as many gene products as males, who only have a single copy of the X chromosome.
Random event that occurs very early on in development (random which X chromosome condenses, mum or dad’s)
Barr body
Women shut down one of their two X chromosomes in a process called X-inactivation. In X-inactivation, an X chromosome is compacted, to make a small, dense structure called a Barr body.
A Barr body is the inactive X chromosome in a female somatic cell, rendered inactive in a process called lyonization
Barr body numbers in XX XY XXY XO XYY XXX
XX - 1 XY - no barr bodies XXY - 1 XO - no barr bodies XYY - no barr bodies XXX - 2
In humans with more than one X chromosome, the number of Barr bodies visible at interphase is always one fewer than the total number of X chromosomes.
Mosaicism
In genetics, a mosaic or mosaicism refers to the presence of two (or more) populations of cells, each with different genotypes in an individual
Females are a mosaic of different kinds of cells, cells have one or the other X inactivated so effectively their bodies are therefore a mosaic of different kinds of cells
Calico cats
In cats, the fur pigmentation gene is X-linked, and depending on which copy of the X chromosome each cell chooses to leave active, either an orange or black coat colour results. X inactivation only occurs in cells with multiple X chromosomes, which explains why almost all calico cats are female.
Sex linked traits
Traits where the genes that code for this trait are on the X chromosome
Many genetic disease map to the …
X chromosome
Most do not show a mosaic effect in carrier females because the gene products can move around the body
An XXY human will
Have a Barr body in each nucleus
A female is colour blind in one eye only. Her father is colour blind in both eyes. The causes of the females condition could be…
X-inactivation in a heterozygote
A somatic mutation
Damage to one eye
