Lecture 15 (Errors in Meiosis and X-inactivation)

Question 1

Down syndrome

Answer 1

Trisomy 21 (3 copies of chromosome 21) 
1/750 live births

Question 2

Aneuploidy

Answer 2

Abnormal number of a particular chromosome

Many aneuploids in humans are fatal

Question 3

Statistics for Down syndrome

Answer 3

Mothers over 45 have 40% of all cases of down syndrome

95% of down syndrome babies have two maternal chromosome 21

Question 4

Nondisjunction

Answer 4

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

Question 5

When non-disjunction occurs in mitosis (in terms of a plant)

Answer 5

The rest of the plant made from that cell is a polyploid.

Question 6

When non-disjunction occurs in meiosis (in terms of a plant)

Answer 6

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.

Question 7

Nondisjunction and Down syndrome

Answer 7

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.

Question 8

Klinefelter syndrome

Answer 8

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

Question 9

Turner syndrome

Answer 9

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

Question 10

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?

Answer 10

N+1
N+1
N-1
N-1

Question 11

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?

Answer 11

N+1
N-1
N
N

Question 12

Polyploidy

Answer 12

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.

Question 13

Autopolyploid

Answer 13

Made from only one species.

Question 14

Allopolyploid

Answer 14

Come from more than one species.

Question 15

Tetraploid

Answer 15

4n, containing four homologous sets of chromosomes.

Question 16

Triploid

Answer 16

3n, containing three homologous sets of chromosomes.

Question 17

Offspring with polyploid karyotypes may be…

Answer 17

Viable and self-fertile

Question 18

Deletion

Answer 18

A deletion removes a chromosomal segment

Question 19

Inversion

Answer 19

An inversion reverses a segment within a chromosome

Question 20

Duplication

Answer 20

A duplication repeats a segment

Question 21

Translocation

Answer 21

A translocation moves a segment from one chromosome to a non-homologous chromosome

Question 22

Reciprocal translocation

Answer 22

Reciprocal translocation is a chromosome abnormality caused by exchange of parts between non-homologous chromosomes. Two detached fragments of two different chromosomes are switched.

Question 23

Insertion

Answer 23

The addition of one or more nucleotide base pairs into a DNA sequence

Question 24

Lejeune syndrome (Cri du chat)

Answer 24

Deletion of tip of short arm on chromosome 5. Individuals affected are severely intellectually disables and usually die in childhood.

Question 25

Results of meiosis in a human inversion heterozygote

Answer 25

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

Question 26

Familial Down syndrome

Answer 26

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

Question 27

Individuals with familial Down syndrome have which of the following karyotypes?

Answer 27

46 chromosomes, translocation of 21 (these individuals have the right number of chromosomes but have a translocation on chromosome 21)

Question 28

Lyon hypothesis

Answer 28

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

Question 29

X inactivation (lyonisation)

Answer 29

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)

Question 30

Barr body

Answer 30

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

Question 31

Barr body numbers in 
XX
XY
XXY
XO
XYY
XXX

Answer 31

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.

Question 32

Mosaicism

Answer 32

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

Question 33

Calico cats

Answer 33

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.

Question 34

Sex linked traits

Answer 34

Traits where the genes that code for this trait are on the X chromosome

Question 35

Many genetic disease map to the …

Answer 35

X chromosome

Most do not show a mosaic effect in carrier females because the gene products can move around the body

Question 36

An XXY human will

Answer 36

Have a Barr body in each nucleus

Question 37

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…

Answer 37

X-inactivation in a heterozygote

A somatic mutation

Damage to one eye