Transcript non mendelian genetics_1 (Ms. Shivani Bhagwat)

Non- Mendelian inheritance
Ms. Shivani Bhagwat
Lecturer,
School of Biotechnology
DAVV
• Mendelian inheritance patterns
– Involve genes directly influencing traits
– Obey Mendel’s laws
• Law of segregation
• Law of independent assortment
– Include
• Dominant / recessive relationships
• Gene interactions
• Phenotype-influencing roles of sex and environment
– Most genes of eukaryotes follow a Mendelian
inheritance pattern
• Many genes do not follow a Mendelian
inheritance pattern
– e.g., Closely linked genes do not follow Mendel’s law
of independent assortment
– non-Mendelian inheritance patterns are:
• Maternal effect
• Epigenetic inheritance
• Extranuclear inheritance
MATERNAL EFFECT
• Maternal effect
– Inheritance pattern for certain nuclear genes
– Genotype of mother directly determines phenotype
of offspring
• Genotype of father and offspring are irrelevant
– Explained by the accumulation of gene products
mother provides to developing eggs
MATERNAL EFFECT
A. E. Boycott (1920s)
• First to study an example of maternal effect
• Involved morphological features of water snail
– Limnea peregra
– Shell and internal organs can be either right- or lefthanded
• Dextral or sinistral, respectively
• Determined by cleavage pattern
of egg after fertilization
– Dextral orientation is more
common and dominant
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MATERNAL EFFECT
A. E. Boycott (1920s)
• Began with two different true-breeding strains
– One dextral, one sinistral
• Dextral ♀ x sinistral ♂  dextral offspring
• Reciprocal cross  sinistral offspring
• Contradict a Mendelian pattern of inheritance
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Reciprocal cross is a breeding experiment designed to test the role of parental
sex on a given inheritance pattern.
All parent organisms must be true breeding to properly carry out such an
experiment.
In one cross, a male expressing the trait of interest will be crossed with a female
not expressing the trait. In the other, a female expressing the trait of interest will
be crossed with a male not expressing the trait
MATERNAL EFFECT
A. E. Boycott (1920s); Alfred Sturtevant (1923)
• Sturtevant proposed that Boycott’s results could be explained
by a maternal effect gene
– Conclusions drawn from F2
and F3 generations
– Dextral (D) is dominant to
sinistral (d)
– Phenotype of offspring is
determined by genotype of
mother
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EPIGENETIC INHERITANCE
• Epigenetic inheritance
– Modification occurs to a nuclear gene or
chromosome
– Occur during spermatogenesis, oogenesis, and
early stages of embryogenesis
– Gene expression is altered
• May be fixed during an individual’s lifetime
– Expression is not permanently changed over
multiple generations
• DNA sequence is not altered
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EPIGENETIC INHERITANCE
• Two types of epigenetic inheritance:
– Dosage compensation
• Offsets differences in the number of sex chromosomes
• One sex chromosome is altered
– Genomic imprinting
• Occurs during gamete formation
• Involves a single gene or chromosome
• Governs whether offspring express maternally- or
paternally-derived gene
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DOSAGE COMPENSATION
• Males and females of many species have
different numbers of certain sex chromosomes
– e.g., X chromosomes
– The level of expression
of many genes on sex
chromosomes is similar
in both sexes
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DOSAGE COMPENSATION
• Apricot eye color in Drosophila
– Conferred by an X-linked gene
– Homozygous females resemble hemizygous males
– Females heterozygous for the apricot allele and a
deletion have paler eye color
– Two copies of the allele in a female produce a
phenotype similar to one copy in a male
– The difference in gene dosage is being
compensated at the level of gene expression
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DOSAGE COMPENSATION
• Dosage compensation does not occur for all
eye color alleles in Drosophila
– e.g., Eosin eye color
• Conferred by an X-linked gene
• Homozygous eosin females have
darker eye color than hemizygous
eosin males
– Dark eosin and light eosin
• Two copies of the allele in a female produce a
phenotype different than one copy in a male
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DOSAGE COMPENSATION
• Most X-linked genes show dosage compensation
• Some X-linked genes do not
• Reasons for the difference are not understood
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DOSAGE COMPENSATION
• Mechanisms of dosage compensation
– Mammals
• One X chromosome is inactivated in females
– “X inactivation”
– Paternally derived in marsupial mammals
– Paternal or random, depending on species of placental
mammal
– Drosophila melanogaster
• Twofold increase in expression of genes on the X
chromosome of males
– The nematode Caenorhabditis elegans
• 50% reduction in expression of X-linked genes in XX
individuals
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DOSAGE COMPENSATION
Murray Barr and Ewart Bertram (1949)
• Identified a highly condensed structure in interphase nuclei of
somatic cells of female cats
– This structure was absent in male cats
– “Barr body”
– Later identified as a highly
condensed X chromosome
– A normal human female has only one Barr body per
somatic cell, while a normal human male has none
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DOSAGE COMPENSATION
Mary Lyon (1961)
• Aware of Barr and Bertram cytological evidence
• Also aware of mammalian mutations producing a variegated
coat color pattern
– e.g., Calico cats are heterozygous
for X-linked alleles determining
coat color
• Possess randomly distributed patches of black and
orange
• Lyon hypothesis: A single X chromosome was inactivated in
the cells of females
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Calico cats are almost always female because the X chromosome determines the
color of the cat and female cats, much like all female mammals, have two X
chromosomes, whereas male mammals, including common male cats, have one
X and one Y chromosome.
Since the Y chromosome does not have any color genes, there is no chance he
could have both orange and non-orange together.
One main exception to this is when, in rare cases, a male has XXY chromosomes
(see Klinefelter's syndrome), in which case the male could have tortoiseshell or
calico markings.
Male calico or tortoiseshell cats are sterile due to the abnormality of carrying two
X chromosomes.
The orange mutant gene is found only on the X, or female, chromosome. The
female cat, therefore, can have the orange mutant gene on one X chromosome
and the genes for a black or white coat on the other, and those can affect or
modify the orange mutant gene.
If that is the case, those several genes will be expressed in a blotchy coat of the
tortoiseshell or calico kind.
DOSAGE COMPENSATION
• Genetic control of X inactivation
– Human cells (and those of other mammals)
possess the ability to count their X chromosomes
– Only one is allowed to remain active
•
•
•
•
•
XX females  1 Barr body
XY males  0 Barr bodies
XO females  0 Barr bodies
(Turner syndrome)
XXX females  2 Barr bodies (Triple X syndrome)
XXY males  1 Barr body
(Kleinfelter syndrome)
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DOSAGE COMPENSATION
• Genetic control of X inactivation
– Not entirely understood
– X-inactivation center (Xic) is involved
• Short region of the X chromosome
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Genomic Imprinting
• Genomic imprinting involves the physical
marking of a segment of DNA
– Mark is retained and recognized throughout the
life of the organism inheriting the marked DNA
– Resulting phenotypes display non-Mendelian
inheritance patterns
– Offspring expresses one allele, not both
– “Monoallelic expression”
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Extranuclear inheritance
Extranuclear inheritance (also known as cytoplasmic inheritance) is a form of nonMendelian inheritance first discovered by Carl Correns in 1908. While working with
Mirabilis jalapa. Correns observed that leaf color was dependent only on the genotype of
the maternal parent. Based on these data, he determined that the trait was transmitted
through a character present in the cytoplasm of the ovule.
Gene conversion
Gene conversion is a reparation process in DNA recombination, by which a piece of DNA
sequence information is transferred from one DNA helix (which remains unchanged) to
another DNA helix, whose sequence is altered. This may occur as a mismatch repair
between the strands of DNA which are derived from different parents. Thus the mismatch
repair can convert one allele into the other. This phenomenon can be detected through the
offspring non-Mendelian ratios, and is frequently observed, e.g., in fungal crosses
Infectious heredity
Infectious particles such as viruses may infect host cells and continue to reside in the
cytoplasm of these cells. If the presence of these particles results in an altered phenotype,
then this phenotype may be subsequently transmitted to progeny.Because this phenotype is
dependent only on the presence of the invader in the host cell’s cytoplasm, inheritance will
be determined only by the infected status of the maternal parent. This will result in a
uniparental transmission of the trait, just as in extranuclear inheritance.
Genomic imprinting
Genomic imprinting is a genetic phenomenon by which certain genes are expressed in a
parent-of-origin-specific manner. Imprinted alleles are silenced such that the genes are either
expressed only from the non-imprinted allele inherited from the mother (e.g. H19 or
CDKN1C), or in other instances from the non-imprinted allele inherited from the father (e.g.
IGF-2).
Forms of genomic imprinting have been demonstrated in insects, mammals and flowering
plants.
Effects can include
A single gene
A part of a chromosome
An entire chromosome
All the chromosomes from one parent
Genomic imprinting can also be correlated with the process of X inactivation
In some species, imprinting determines which X chromosome will be inactivated
e.g., The paternal X chromosome is always inactivated in marsupials
e.g., The paternal X chromosome is inactivated in extra embryonic tissue
(e.g., the placenta) of placental mammals
X inactivation is random in the placental embryo itself.
Involves the physical marking of DNA. Also involves differentially
methylated regions (DMRs) located near imprinted genes
Maternal or paternal copy is methylated, not both
Mosaicism
Individuals who possess cells with genetic differences from the other cells in their body are
termed mosaics. These differences can result from mutations that occur in different tissues
and at different periods of development.
Mosaicism also results from a phenomenon known as X-inactivation. All female mammals
have two X chromosomes. To prevent lethal gene dosage problems, one of these
chromosomes is inactivated following fertilization. This process occurs randomly for all of
the cells in the organism’s body. Because a given female’s two X chromosomes will almost
certainly differ in their specific pattern of alleles, this will result in differing cell phenotypes
depending on which chromosome is silenced. Calico cats, which are almost all female,
demonstrate one of the most commonly observed manifestations of this process.
Trinucleotide repeat disorders
These diseases are all caused by the expansion of microsatellite tandem repeats consisting
of a stretch of three nucleotides. In normal individuals, the number of repeated units is
relatively low. With each successive generation, there is a chance that the number of
repeats will expand. As this occurs, progeny can progress to permutation and ultimately
affected status. Individuals with a number of repeats that falls in the permutation range
have a good chance of having affected children.
Uniparental disomy
Uniparental disomy (UPD) occurs when a person receives two copies of a chromosome, or
part of a chromosome, from one parent and no copies from the other parent.
1. UPD could involve isodisomy (meiosis II error)
2. heterodisomy (meiosis I error)
Uniparental inheritance of imprinted genes can also result in phenotypical anomalies.
Though few imprinted genes have been identified, uniparental inheritance of an imprinted
gene can result in the loss of gene function which can lead to delayed development,
mental retardation, or other medical problems.
The most well-known conditions include Prader-Willi syndrome and Angelman syndrome.
Both of these disorders can be caused by UPD or other errors in imprinting involving genes
on the long arm of chromosome 15.
Other conditions, such as Beckwith-Wiedemann syndrome, are associated with
abnormalities of imprinted genes on the short arm of chromosome 11.
Chromosome 14 is also known to cause particular symptoms such as skeletal
abnormalities, mental retardation and joint contractures among others.
Multifactorial (Complex) Inheritance
Expression of many traits may involve multiple genes.
Many such traits (eg, height) are distributed along a bell-shaped curve (normal
distribution). Normally, each gene adds to or subtracts from the trait independently of
other genes. In this distribution, few people are at the extremes and many are in the
middle because people are unlikely to inherit multiple factors acting in the same direction.
Environmental factors also add to or subtract from the final result.
Many relatively common congenital anomalies and familial disorders result from
multifactorial inheritance. In an affected person, the disorder represents the sum of
genetic and environmental influences.
Risk of the occurrence of such a trait is much higher in 1st-degree relatives (siblings,
parents, or children who share, on average, 50% of the affected person's genes) than in
more distant relatives, who are likely to have inherited only a few high-liability genes.
Genetically determined predisposing factors, including a family history and specific
biochemical pathways often identified by molecular markers (eg, high cholesterol), can
sometimes identify people who are at risk and are likely to benefit from preventive
measures.
Multigenic, multifactorial traits seldom produce clear patterns of inheritance; however,
these traits tend to occur more often among certain ethnic and geographic groups or among
one sex or the other.
Exceptions to Simple Inheritance
Polygenic Traits(stature, body shape, hair and skin color)
Intermediate Expression(AA,Aa,aa)
Codominance (A,B,AB blood gps)
Multiple-allele Series (ABO blood grouping)
Modifying and Regulator Genes: Modifying genes alter how certain genes are expressed in
the phenotype. For instance, there is a dominant cataract gene which will produce varying
degrees of vision impairment depending on the presence of a specific allele for a companion
modifying gene. However, cataracts also can be promoted by diabetes and common
environmental factors such as excessive ultraviolet radiation and alcoholism.
Regulator genes can either initiate or block the expression of other genes. They control the
production of a variety of chemicals in plants and animals. For instance, the time of
production of specific proteins that will be new structural parts of our bodies can be
controlled by such regulator genes.
they control the maturation and aging processes. Regulator genes that are involved in
subdividing an embryo into what will become the major body parts of an individual are also
referred to as control, homeotic , homeobox , or Hox genes.
They are responsible for setting generalized cells on the path to become a head, torso, arms,
legs, etc.
Incomplete Penetrance
Some genes are incompletely penetrant. That is to say, their effect does not normally occur
unless certain environmental factors are present.
For example, you may inherit the genes that are responsible for type 2 diabetes but never
get the disease unless you become greatly overweight, persistently stressed psychologically,
or do not get enough sleep on a regular basis.
Stuttering Alleles
It is now known that some genetically inherited diseases have more severe symptoms each
succeeding generation due to segments of the defective genes being doubled in their
transmission to children. These are referred to as stuttering alleles or unstable alleles.
Examples of this phenomenon are Huntington's disease, fragile-X syndrome, and the
myotonic form of muscular dystrophy.
Mendel believed that all units of inheritance are passed on to offspring unchanged.
Unstable alleles are an important exception to this rule.
Behavioural genetics
The field of study that examines the role of genetics in animal (including human) behaviour.
Often associated with the "nature versus nurture" debate, behavioural genetics is highly
interdisciplinary, involving contributions from biology, genetics, ethology, psychology, and
statistics. Behavioural geneticists study the inheritance of behavioural traits.
In humans, this information is often gathered through the use of the twin study or
adoption study. In animal studies, breeding, transgenesis, and gene knockout techniques
are common; psychiatric genetics.
Sir Francis Galton (1822-1911) was the first scientist to study heredity and human
behaviour systematically.
What indications are there that behaviour has a biological basis?
Behavior often is species specific
Behaviors often breed true
Behaviors change in response to alterations in biological structures or processes
In humans, some behaviors run in families
Behavior has an evolutionary history that persists across related species
 Genes are evolutionary glue, binding all of life in a single history that dates back some
3.5 billion years. Conserved behaviors are part of that history.
Twin study
Twin studies help disentangle the relative importance of environmental and genetic
influences on individual traits and behaviors.
Twins are a valuable source for observation due to their genotypes and family
environments tending to be similar. More specifically, monozygotic (MZ) or "identical"
twins, share nearly 100% of their genetic polymorphisms, which means that most variation
in pairs' traits (measured height, susceptibility to boredom, intelligence, depression, etc.) is
due to their unique experiences.
Dizygotic (DZ) or "fraternal" twins share only about 50% of their polymorphisms. DZ twins
are helpful to study because they tend to share many aspects of their environment (e.g.,
parenting style, education, wealth, culture, community) by virtue of being born in the same
time and place.
The basic logic of the twin study can be understood with very little mathematics beyond an
understanding of correlation and the concept of variance.
Quantitative genetics
Quantitative genetics is the study of continuously measured traits (such as height or weight)
and their mechanisms. It can be an extension of simple Mendelian inheritance in that the
combined effects of one or more genes and the environments in which they are expressed
give rise to continuous distributions of phenotypic values.
Basic principles
The phenotypic value (P) of an individual is the combined effect of the genotypic value (G)
and the environmental deviation (E):
P=G+E
The genotypic value is the combined effect of all the genetic effects, including nuclear
genes, mitochondrial genes and interactions between the genes.
Resemblance between relatives
Central in estimating the variances for the various components is the principle of
relatedness. A child has a father and a mother. Consequently, the child and father share 50%
of their alleles, as do the child and the mother. However, the mother and father normally do
not share alleles as a result of shared ancestors.
The principle of relationship (R) is central to understanding the resemblances within
families and can be useful when calculating inbreeding.
Relationship has two definitions that can be applied:
1. The probable portion of genes that are same for two individuals due to common
ancestry exceeding that of the base population.
2. Additive/numerator relationship: the relationship coefficient (Rxy¬) = twice the
probability of two genes at loci in different individuals being identical by descent.
Rxy values can range from 0 to 2.
Relationship can be calculated in several ways;
a. from the known relationships of the individual
b. from bracket pedigrees
c. from pedigree path diagrams.
Bracket pedigree
The number of common
alleles is halved with each
generation
Path pedigreediagram
Qualitative vs. Quantitative Traits
The classical Mendelian traits encountered have been qualitative in nature; i.e., traits that
are easily classified into distinct phenotypic categories.
These discrete phenotypes are under the genetic control of only one or a very few genes
with little or no environmental modification to obscure the gene effects.
In contrast, the variability exhibited by many traits fails to fit into discrete phenotypic
classes (discontinuous variability), but instead forms a spectrum of phenotypes that blend
imperceptively (continuous variability).
Economically important traits such as body weight gains in livestock, mature plant
heights, egg or milk production, and yield of grain per acre are quantitative or complex
traits with continuous variability.
 The basic differences between qualitative and quantitative traits involve the number of
genes contributing to the phenotypic variability and the degree to which the phenotype
can be modified by environmental factors.
Quantitative traits are governed by many genes (perhaps 10–100 or more), each
contributing a small amount to the phenotype are their individual effects cannot be
detected by Mendelian methods.
For this reason, quantitative traits are also referred to as polygenic traits. Stretches of DNA
with closely linked genes responsible for phenotypes associated with quantitative traits are
called quantitative trait loci or QTLs.
Each gene usually has effects on more than one trait. The idea that each character is
controlled by a single gene (the one-gene-one-trait hypothesis) has often been falsely
attributed to Mendel. But even he recognized that a single factor (or gene) might have
manifold effects on more than one trait.
For example, he observed that purple flowers are correlated with brown seeds and a dark
spot on the axils of leaves; similarly, white flowers are correlated with light-colored seeds
and no axillary spots on the leaves. In Drosophila, many loci (e.g., genes named dumpy, cut,
vestigial, apterous) contribute to a complex character such as wing development. Each of
these genes also has pleiotropic effects on other traits. For example, the gene for vestigial
wings also effects the halteres (balancers), bristles, egg production in females, and longevity.
Some genes encode products such as enzymes that participate in multistep biochemical
pathways or proteins that regulate the activity of one or more other genes in metabolic,
regulatory, or developmental pathways. Because of the complex interactions within these
pathways, a gene product acting at any one step might have phenotypic effects on other
characters. For a given gene, some of its pleiotropic effects may be relatively strong for
certain traits, whereas its effects on other traits may be so weak that they are difficult or
impossible to identify by Mendelian techniques. It is the totality of these pleiotropic effects
of numerous loci that constitutes the genetic base of a quantitative character.
Genetic Testing
Non-disjunction
Nondisjunction ("not coming apart") is the failure of chromosome pairs to separate
properly during meiosis stage 1 or stage 2.
This could arise from a failure of homologous chromosomes to separate in meiosis
I, or the failure of sister chromatids to separate during meiosis II or mitosis.
The result of this error is a cell with an imbalance of chromosomes. Such a cell is
said to be aneuploid.
Loss of a single chromosome (2n-1), in which the daughter cell(s) with the defect
will have one chromosome missing from one of its pairs, is referred to as a
monosomy.
Gaining a single chromosome, in which the daughter cell(s) with the defect will have
one chromosome in addition to its pairs is referred to as a trisomy (2n+1).
Once in a Blue Moon
One of the best-known facts of genetics is that a cross between a horse and a
donkey produces a mule.
Actually, it’s a cross between a female horse and a male donkey that produces the
mule; the reciprocal cross, between a male horse and a female donkey, produces a
hinny, which has smaller ears and a bushy tail, like a horse . Both mules and
hinnies are sterile because horses and donkeys are different species with different
numbers of chromosomes: a horse has 64 chromosomes, whereas a donkey has
only 62. There are also considerable differences in the sizes and shapes of the
chromosomes that horses and donkeys have in common. A mule inherits 32
chromosomes from its horse mother and 31 chromosomes from its donkey father,
giving the mule a chromosome number of 63.
The maternal and paternal chromosomes of a mule are not homologous, and so
they do not pair and separate properly in meiosis; consequently, a mule’s gametes
are abnormal and the animal is sterile.
Karyotype
Preparation and staining techniques help to distinguish among chromosomes of
similar size and shape.
1. chromosomes may be treated with enzymes that partly digest them; staining
with a special dye called Giemsa reveals G bands, which distinguish areas of
DNA that are rich in adenine–thymine base pairs.
2. Q bands are revealed by staining chromosomes with quinacrine mustard and
viewing the chromosomes under UV light.
3. C bands which are regions of DNA occupied by centromeric heterochromatin.
4. R band, which are rich in guanine–cytosine base pairs.
Chromosome abnormality
A chromosome anomaly, abnormality or aberration reflects an atypical number of
chromosomes or a structural abnormality in one or more chromosomes.
A Karyotype refers to a full set of chromosomes from an individual which can be
compared to a "normal" Karyotype for the species via genetic testing.
Chromosome anomalies usually occur when there is an error in cell division following
meiosis or mitosis. There are many types of chromosome anomalies.
They can be organized into two basic groups:
a. numerical
b. structural anomalies.
Numerical Disorders
This is called Aneuploidy (an abnormal number of chromosomes), and occurs when
an individual is missing either a chromosome from a pair (monosomy) or has more
than two chromosomes of a pair (Trisomy, Tetrasomy, etc.).
In humans an example of a condition caused by a numerical anomaly is Down
Syndrome, also known as Trisomy 21 (an individual with Down Syndrome has three
copies of chromosome 21, rather than two).
Turner Syndrome is an example of a monosomy where the individual is born with
only one sex chromosome, an XO.
Structural abnormalities
When the chromosome's structure is altered, this can take several forms:
Deletions: A portion of the chromosome is missing or deleted.
Duplications: A portion of the chromosome is duplicated, resulting in extra genetic
material.
Translocations: When a portion of one chromosome is transferred to another
chromosome.
In a reciprocal translocation, segments from two different chromosomes have
been exchanged.
Inversions: A portion of the chromosome has broken off, turned upside down and
reattached, therefore the genetic material is inverted.
Insertions: A portion of one chromosome has been removed from its normal place
and inserted into another chromosome.
2n--2
2n--1
2n+1
2n+2
Y chromosome
“Functional wasteland,” “Nonrecombining desert” and “Gene-poor chromosome” are
some of the different definitions given to the Y chromosome in the last decade.
The Y chromosome is one of the two sex-determining chromosomes in most
mammals, including humans. In mammals, it contains the gene SRY((Sexdetermining region Y), which triggers testis development if present. The human Y
chromosome is composed of about 60 million base pairs.
DNA in the Y chromosome is passed from father to son, and Y-DNA analysis may
thus be used in genealogy research. With a 30% difference between humans and
chimpanzees, the Y chromosome is one of the fastest evolving parts of the human
genome
Before Y chromosome
Many ectothermic vertebrates have no sex chromosomes. If they have different
sexes, sex is determined environmentally rather than genetically. For some of them,
especially reptiles, sex depends on the incubation temperature; others are
hermaphroditic (meaning they contain both male and female gametes in the same
individual).
Origin
The X and Y chromosomes are thought to have evolved from a pair of identical
chromosomes, termed autosomes, when an ancestral mammal developed an allelic
variation, a so-called 'sex locus' – simply possessing this allele caused the
organism to be male.
The chromosome with this allele became the Y chromosome, while the other
member of the pair became the X chromosome. Over time, genes which were
beneficial for males and harmful to (or had no effect on) females either developed on
the Y chromosome, or were acquired through the process of translocation.
Recombination inhibition
Recombination between the X and Y chromosomes proved harmful—it resulted in
males without necessary genes formerly found on the Y chromosome, and females
with unnecessary or even harmful genes previously only found on the Y
chromosome. As a result, genes beneficial to males accumulated near the sexdetermining genes, and recombination in this region was suppressed in order to
preserve this male specific region.
Over time, the Y chromosome changed in such a way as to inhibit the areas around
the sex determining genes from recombining at all with the X chromosome. As a
result of this process 95% of the human Y chromosome is unable to recombine.
Shrinking theory
The human Y chromosome has lost 1,393 of its 1,438 original genes over the
course of its existence. With a rate of genetic loss of 4.6 genes per million years,
the Y chromosome may potentially lose complete function within the next 10 million
years.
High mutation rate
The human Y chromosome is particularly exposed to high mutation rates due to the
environment in which it is housed. The Y chromosome is passed exclusively
through sperm, which undergo multiple cell divisions during gametogenesis. Each
cellular division provides further opportunity to accumulate base pair mutations.
Additionally, sperms are stored in the highly oxidative environment of the testis,
which encourages further mutation.
These two conditions combined, put the Y chromosome at a risk of mutation 4.8
times greater than the rest of the genome.
Human Y chromosome
In humans, the Y chromosome spans about 58 million base pairs (the building
blocks of DNA) and represents approximately 2% of the total DNA in a male cell.
The human Y chromosome contains 86 genes, which code for only 23 distinct
proteins. Traits that are inherited via the Y chromosome are called holandric traits.
The bulk of the Y chromosome which does not recombine is called the "NRY" or
non-recombining region of the Y chromosome. It is the SNPs in this region which
are used for tracing direct paternal ancestral lines.