Chapter 11 – Patterns of Chromosomal Inheritance
Chapter 11 – Patterns of Chromosomal Inheritance
Inheritance of Chromosomes
How many chromosomes did our parents gametes contain when we were conceived?
23, 22 autosomes, 1 sex chromosome
Autosomes are identical in both male & female offspring
For the sex chromosomes, females possess 2 X chromosomes so they are genetically identical (homologous)
Males on the other hand receive 1 X & 1 Y for their sex chromosome pair & even though they align themselves as a homologous pair during metaphase I, they contain vastly different genes & are not true homologues of one another
Chapter 11 – Patterns of Chromosomal Inheritance
Defects in Inheritance of Chromosomes
Non-disjunction
Failure of homologous chromosomes to separate during meiosis
Where does chromosome separation occur during meiosis?
Anaphase I or Anaphase II
So what would the resulting gametes look like?
Chapter 11 – Patterns of Chromosomal Inheritance
Chapter 11 – Patterns of Chromosomal Inheritance
Chapter 11 – Patterns of Chromosomal Inheritance
Defects in Inheritance of Chromosomes
Aberrations in chromosome # are usually non-viable.
Down’s syndrome involves an extra chromosome #21 , & it is one of the smaller chromosomes
This form of trisomy (three copies of a chromosome, monosomy = only one copy of a chromosome) involves one of the smaller chromosome, indicating that having just a little extra genetic material is not good!
The other trisomic conditions (with the exception of the sex chromosomes) which are viable births also involve relatively small chromosomes.
All other trisomic or monosomic conditions usually results in spontaneous abortions
Chapter 11 – Patterns of Chromosomal Inheritance
Chapter 11 – Patterns of Chromosomal Inheritance
Chapter 11 – Patterns of Chromosomal Inheritance
Chapter 11 – Patterns of Chromosomal Inheritance
Chromosomal mutations
Permanent genetic changes in chromosome structure
Inversions
A segment of a chromosome is turned around 180 degrees ( backward ). This type of mutation will become more evident when we look at how a gene will encode a protein
Translocation
Movement of a chromosome segment from one chromosome to another nonhomologous chromosome
Deletion
The loss of a segment of a chromosome
Duplication
The presence of a chromosomal segment more than once in the same chromosome, due primarily to uneven crossing over , which would lead to a deletion in the other homologue
Chapter 11 – Patterns of Chromosomal Inheritance
Chapter 11 – Patterns of Chromosomal Inheritance
Inheritance of sex linked traits
All is not equal when it comes to inheriting traits located on the sex chromosomes!
Why is this SO!
Because males inherit one of each sex chromosomes, while females inherit the same sex chromosome
So what! Why should this matter?
Let’s look at the Punnett square!
Chapter 11 – Patterns of Chromosomal Inheritance
Chapter 11 – Patterns of Chromosomal Inheritance
Inheritance of sex linked traits
As we can see, the female will inherit 2 alleles for each trait located on the X chromosome, while the male will inherit only 1 allele for each trait located on the X
Why is this such a big deal?
Let’s look at the transmission of color blindness again!
Chapter 11 – Patterns of Chromosomal Inheritance
Inheritance of sex linked traits
Let’s look at the transmission of color blindness again!
X B = normal vision, X b = color blind
Chapter 11 – Patterns of Chromosomal Inheritance
Inheritance of sex linked traits
Male children do not have a heterozygous option for traits located on the X, so they either have the genetic disorder or don’t have it. They are hemizygous with respect to traits on the X chromosome (possess only one copy of an allele)
Females can possess the heterozygote condition & are deemed carriers for the genetic disorder
Since male children only receive their X from their mother (they get the Y from Dad by default), male children are at the mercy of their mother’s X chromosomes
Brings up some unique problems!
Chapter 11 – Patterns of Chromosomal Inheritance
Inheritance of sex linked traits
Brings up some unique problems!
Chapter 11 – Patterns of Chromosomal Inheritance
Chapter 11 – Patterns of Chromosomal Inheritance
Chapter 11 – Patterns of Chromosomal Inheritance
Chapter 11 – Patterns of Chromosomal Inheritance
Linked genes
All the alleles on one chromosome form a linkage group because they tend to be inherited together
Hence if we were to take 2 traits: eye color & hair line & place them on the same chromosome, the traits would be transmitted together
Chapter 11 – Patterns of Chromosomal Inheritance
Linked genes
Example:
Brown eye = B, Blue eye = b
Widow’s peak = W, Straight hair line = w
Bob is BbWw & Sue is BbWw
Chapter 11 – Patterns of Chromosomal Inheritance
Chapter 11 – Patterns of Chromosomal Inheritance
PRACTICE QUESTIONS
1.
What is non-disjunction? During what stage of meiosis does it occur?
2.
Define trisomy? Define monosomy?
3.
What is an inversion? Translocation? Deletion? Duplication?
4.
Define hemizygous.
5.
Sue is a carrier for color blindness & she marries Bob who has normal vision. What percentage of their offspring will be color blind?
6.
Sue conceives with Tom instead, who is color blind. What percentage of their offspring will be color blind?
7.
Why are linked genes transmitted together?
8.
Practice genetic problems on page 503 & 504!
