Lecture XII: Chromosomal Basis of Inheritance & Differential gene expression Flashcards
Sex linked inheritance
Humans have 2 different sex chromosomes, X and Y
Males have one of each
Females have 2 X chromosomes
Sex of a child is determined by which sex chromosome is present in the sperm cell.
Sex determination (XY system, XO, ZW, Haplo-diploid System)
X-Y system. In mammals, sex of offspring determined by whether the sperm cell has an X or a Y chromosome.
X-O system. Some insects. There is only 1 type of sex chromosome. Sex of offspring determined by the presence or absence of the sex chromosome in the sperm cell.
Z-W system. Birds, some fish and insects. Females have different sex chromosomes instead of males. The sex chromosome in the egg, not the sperm, determines the sex of the offspring (either Z or W).
Haplo-diploid system. Bees and ants. No sex chromosomes. Females develop from fertilized eggs (2n) and males from unfertilized eggs (n).
Human Sex Linked Traits
Any trait that is determined by a gene on one of the sex chromosomes is considered sex linked.
X-linked if on the X chromosome
Y-linked if on the Y chromosome
Define sex chromosomes
Sex chromosomes contain genes that:
determine sex of the individual
control development of the secondary
sexual characteristics
BUT also carry genes for other
characteristics
Sex chromosomes are not truly homologous, but they do pair up during meiosis.
Small region where they are homologous
X-linked traits in humans
For genes that are only present on the X chromosome:
Dominance of traits can only apply to females since they have 2 copies (alleles).
Males only have one X chromosome, therefore the allele they have will always be expressed.
Because of this, males will express recessive deleterious X-linked traits more often than females.
Example:
Females XX
Possible Genotypes Possible Phenotypes
XN XN normal vision
XN Xn normal vision but
carrier for the trait
Xn Xn colour blind
Males XY
Possible Genotypes Possible Phenotypes
XN Y normal vision
XnY colour blind
X-inactivation in female Mammals
Females have 2 copies of the X chromosome in every cell, but a strange thing happens early on in development.
One of the copies of the X chromosome in each cell becomes inactivated around the 100 cell stage in embryonic development and condenses into a compact object called a Barr Body. This is called X-inactivation.
Once X inactivation occurs within a cell, all descendants of that cell will have the same inactive X chromosome.
X inactivation is random (as long as chromosomes are normal) so about 50% of cells will have one allele and 50% the other allele for genes.
X-Inactivation and X-linked disorders
If half of a female’s X chromosomes are inactivated what happens when a female is heterozygous (a carrier) for an X-linked disorder?
The alleles for most disorders create a dysfunctional protein or do not produce the protein at all. However, because half of the female’s cells have a healthy normal allele they will compensate and make enough of the required protein.
Chromosomal Disorders
Caused by having the wrong number of chromosomes or having damaged chromosomes.
Due to Physical or chemical disturbance causing mutation or errors during meiosis.
Types of chromosomal abnormalities
Aneuploidy (wrong number of chromosomes)
Polyploidy (more than 2 complete sets of chromosomes)
Alterations to chromosome structure
Aneuploidy
Having fewer or more than the normal diploid number of a chromosome.
Typically caused by an error during meiosis called a Nondisjunction
Monosomic Aneuploidy
Individual is born missing a chromosome (2n – 1)
Occurs when one of the gametes that created the individual was missing a chromosome usually from nondisjunction in meiosis.
Trisomic Aneuploidy
Individual born with 1 extra chromosome (2n + 1)
Occurs when one of the gametes that created the individual had an extra chromosome usually from nondisjunction in meiosis.
e.g. Down Syndrome (trisomy of chromosome 21)
Non-disjunction
When homologous chromosomes or sister chromatids do not separate properly during anaphase I or II of meiosis respectively.
Aneuploidy of sex chromosomes in Males/Females
Male:
OY- Not viable
XYY- Male sexual development. Taller than usual.
XXY - Klinefelter syndrome
Born with male sex organs but testes small and individual is often sterile.
At puberty may develop secondary female sex characteristics.
Female
XXX- Triple X. Healthy.
XO - Turner syndrome
Only known viable monosomy in humans.
Female sexual development occurs, but internal organs are underdeveloped resulting in sterility.
Polyploidy
Organism with more than 2 complete sets of chromosomes in all somatic cells (3n or more)
Caused by fertilization of an abnormal diploid egg. This is rare in animals but common in plants
Alterations to chromosome structure
4 possible kinds of alterations:
Deletion = Chromosome fragment lost. (missing some genes or incomplete gene sequences)
Duplication = Extra chromosome fragment added. (2 copies of some genes)
Inversion = DNA segment is put in reverse (backwards code)
Translocation = DNA segment from non-homologous chromosomes gets traded. (wrong genes on chromosomes)
Differential gene expression
Genes can be activated or inactivated.
Different cell types have a different selection of active genes.
Leads to different types of proteins being made, different cell structure and function.
The genes that are active in a cell dictate the cell’s structure and function by coding for specific proteins to be made.
