The Chromosomal Basis of Inheritance
The
Chromosomal
Basis of
Inheritance
Factors and Genes
Mendel’s model of inheritance was based on the
idea of “factors” that were independently assorted
and segregated into gametes
We now know that these factors are actually genes
that are located on chromosomes
Concept 15.1 Mendelian inheritance has its
physical basis in the behaviour of
chromosomes
• Mendel’s laws of segregation and independent assortment can be accounted for by the
behaviour of chromosomes during meiosis
• Morgan’s experiments with Drosophila provided
the first evidence that specific genes are
associated with specific chromosomes
Genes and Chromosomes Even after chromosomes were visualized and
observed through microscopy, there were no
indications that Mendel’s factors were related to
the chromosomes
It was only in the early 1900s when scientists noticed
similarities between meiosis and Mendel’s model
Chromosome Theory of Inheritance Genes have specific loci (positions) along
chromosomes
Chromosomes undergo segregation and
independent assortment
Thomas Hunt Morgan
Morgan conducted experiments with Drosophila
melanogaster (fruits flies)
Drosophila have many offspring and only 4 pairs of
chromosomes
He mated many pairs of Drosophila until a mutant
trait (different from the common wild type) was
found in a male fly that had white eyes as opposed
to the common red eyes.
This fly was then mated to red eyed females
P generation
female (wild-type eyes) X
male (mutant eyes)
F1 generation
All offspring display wild-type
phenotype
F2 generation
Offspring exhibit a 3:1 ratio of
wild-type to mutant eyes
Results similar to Mendel`s
data with pea plants.
However, only males
displayed the white eyed
phenotype.
Possibility of correlation with
sex chromosome.
Concept 15.2 Sex-linked genes exhibit unique
patterns of inheritance
• Sex chromosomes determine sex
• Sex-linked genes on sex chromosomes have a
unique pattern of inheritance
The Chromosomal Basis of Sex
The X-Y System
Human sex chromosomes are known as the X-
chromosome and Y-chromosome
The Y-chromosome is the smaller chromosome, with
the shorter arms containing regions that are
homologous to regions of the X-chromosome.
XX develops as a female, XY develops as a male
Therefore, the male gamete determines the sex of
the offspring with a fifty-fifty chance (X to for a
female and Y for a male)
Inheritance of Sex-Linked Genes
A sex-linked gene is a gene on a sex chromosome
Although a gene appears on a sex chromosome, it is not necessarily related to sex determination or sex characteristics
If a recessive sex-linked trait is on the X-chromosome, a female must be homozygous recessive for the trait to display the phenotype.
Alternatively, a male only has one copy of the X-chromosome. Therefore there is no possibility to be heterozygous for that gene.
Examples: colour blindness, hemophilia
X Inactivation in Female Mammals
While females have two X-chromosomes, they do
not make double of the proteins coded for by the
X-chromosomes
In each cell, one of the X-chromosomes will
become inactivated through DNA methylation
This inactive X-chromosome condenses into a Barr
body that remains near the nuclear envelope
The X-chromosome that is inactivated in each cell is
random, meaning that each female has a mosaic
of two cell types
Concept 15.3Linked genes tend to be inherited
together because they are located
near each other on the same
chromosome
• Mendel’s law of independent assortment dictates that genes segregate independently of each other. However, genes that are closer together do not always sort independently and are considered linked
• Recombination frequencies between genes can be used to map genes on a chromosome
Chromosomes and Linked Genes
Each chromosome has hundreds or thousands of
genes
Linked genes are genes located on the same
chromosome that tend to be inherited together
These linked genes cause deviations from the
expected outcome of Mendel’s law of
independent assortment.
How Linkage Affects Inheritance In Mendel’s experiments, he observed that some
offspring have combinations of traits that do not
match either parent in the P generation
We now know this is due to crossing over of
chromosomes during meiosis
Gametes from green-wrinkled homozygousrecessive parent (yyrr)
Gametes from yellow-roundheterozygous parent (YyRr)
Parental-type offspring
Recombinantoffspring
YyRr yyrr Yyrr yyRr
YR yr Yr yR
yr
How Linkage Affects Inheritance
In Morgan’s experiments, inheritance of certain
traits were found to deviate from Mendel’s law of
independent assortment.
For the characteristics of body colour and wing
type, there are two observable phenotype.
The wild-type phenotypes are grey bodies and
normal-sized wings. The mutant phenotypes are
black bodies and small vestigial wings.
Morgan found a higher proportion of parental
phenotypes than would be expected from
independent assortment
Cross between two flies that are true-breeding for
two different genes that follow a two-allele
complete dominance mode of inheritance.
Recombination
The offspring that show new combinations of the
parental traits are known as recombinant offspring
If 50% of all offspring are recombinants, then there is
a 50% frequency of recombination.
Based on his results, Morgan proposed that some
process must occasionally break the physical
connection between genes on the same
chromosome
This process is the mechanism of crossing over which
occurs between homologous chromosomes
Linked Genes
The further apart genes are, the more likely they are
to assort independently and recombine in new
combinations during crossover.
If the frequency of recombinants is less than 50%,
the genes are likely linked (closer together on the
same chromosome).
Mapping the Distance Between
Genes Using Recombination Data
After the discovery of linked genes and recombination due to crossing over, one of Morgan’s students came up with a method for constructing a genetic map.
Genetic map - An ordered list of the genetic loci along a particular chromosome.
Linkage map – A genetic map based on recombination frequencies
His prediction: The farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency
Concept 15.4 Alterations of chromosome number
or structure cause some genetic
disorders
• Nondisjunction during meiosis can result in abnormal chromosome number
• Chromosomal breakage can result in abnormal
chromosome structure
• Chromosomal abnormalities can lead to various disorders
Abnormal Chromosome Number
Nondisjunction – Error in segregation of homologous
chromosomes or sister chromatids during meiosis
Abnormal Chromosome Number
Aneuploidy – Zygote produced with an abnormal
number of chromosomes
Monosomic – missing chromosome in an aneuploid
zygote
Trisomic – extra chromosome in an aneuploidy
zygote
Polyploidy – extra chromosome set in an aneuploid
zygote
Alterations of Chromosome
Structure
Deletion – removes a chromosomal segment
Duplication – repeats a chromosomal segment
Inversion – reverses a chromosomal segment
Alterations of Chromosome
Structure
Translocation – moves a chromosomal segment
from one chromosome to a nonhomologous
chromosome
Reciprocal translocation
– segments are exchanged Nonreciprocal translocation
– one direction, no exchange
Human Disorders Due to
Chromosomal Alterations
Down Syndrome (Trisomy 21)
Resulting from an extra copy of chromosome 21
(usually fro nondisjunction during meiosis I)
Frequency increases with age of the mother
Symptoms include characteristic facial features and
stature, heart defects and mental delay
Prone to other diseases
Human Disorders Due to
Chromosomal Alterations
Aneuploidy of Sex Chromosomes
Nondisjunction of sex chromosomes causing either an extra sex chromosome or a missing sex chromosome
Klinefelter syndrome (genotype XXY)
Male sex organs (though sterile) but may show female secondary sex characteristics
Males with an extra Y (XYY)
No real distinguishing features
Females with trisomy X (genotype XXX)
No real distinguishing features
Turner syndrome (genotype X0)
Phenotypically female but sterile. Need hormone therapy to development secondary sex characteristics
Human Disorders Due to
Chromosomal Alterations
Disorders caused by structurally altered
chromosomes (deletions, translocations, etc.)
Cri du chat
Results from a specific deletion in chromosome 5
Symptoms include mental retardation, unusual facial
features, a cat-like cry
Early death
Chronic myelogenous leukemia
Reciprocal translocation (between chromosome 22 and
9) during mitosis of cells that are precursors to white blood
cells
Concept 15.5 Some inheritance patterns are
exceptions to the standard
chromosome theory
• Genomic imprinting can inhibit certain alleles during gamete production, which then leads to relationships between allele expression and parental lineage (maternal or paternal)
• Extranuclear genes are usually passed on maternally through the cytoplasm and its contents
Genomic Imprinting Occurs during the formation of gametes and results
in the silencing of one allele of certain genes
Occurs differently in males and females so that
either the alleles are imprinted in the egg, or in the
sperm
Therefore, only one allele is expressed for each
imprinted gene, either male or female
Genomic Imprinting Imprinted alleles usually have alterations such as a
methylation of cytosine nucleotides
However, methylation can also result in activation
of expression of an allele
Affects only a small number
of genes
Not the same as sex-linked
alleles
Inheritance of Organelle Genes
Extranuclear genes exist outside the nucleus
They can be found in organelles such as
mitochondria or chloroplasts
Theses organelles are obtained from the egg and
therefore the offspring will inherit
these specific extranuclear genes
from the maternal source
Extranuclear gene disorders
Mitochondrial myopathy
Weakness, intolerance of exercise, muscle
deterioration
Leber’s hereditary optic neuropathy
Sudden blindness in 20s or 30s
Possible connection to may other disorders