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CHROMOSOMAL BASIS OF
INHERITANCE
B.s biochem 3rd semester
Presented To:
Sana AkramRoll.no.2
BAHAUDDIN ZAKARIYA UNIVERSITY MULTAN
Presented By:
Sir Ibrahim
Genes and Chromosomes
Genes Are located on chromosomes
Can be visualized using certain techniques
INTRODUCTION
In 1875 process of Mitosis and in 1890 Meiosis was worked out
Independently, Karl Correns, Erich von Tschermak, and Hugo de Vries all found that Mendel had explained the same results 35 years before
But there was still resistance
Around 1900, cytologists and geneticists began to see parallels between the behavior of chromosomes and the behavior of Mendel’s factors Chromosomes and genes are
both present in pairs in diploid cells
Homologous chromosomes separate and alleles segregate during meiosis
Fertilization restores the paired condition for both chromosomes and genes.
Chromosomal Theory of InheritanceIn 1902, Walter Sutton, Theodor Boveri, and others noted these parallels and a chromosome theory of inheritance began to take form
The chromosome theory of inheritance states that
Mendelian genes have specific loci on chromosomesChromosomes undergo segregation and independent assortment
Evidence that Genes Associated to Chromosomes Thomas Hunt Morgan was the first to associate a
specific gene with a specific chromosome He provided convincing evidence that
chromosomes are the location of Mendel’s heritable factors
Experimental animal, Drosophila melanogaster, a fruit fly species that eats fungi on fruit.• Fruit flies prolific breeders generation time of
two weeks.• Fruit flies three pairs of autosomes and a pair of
sex chromosomes (XX in females, XY in males).
Thomas Hunt Morgan was the first to associate a specific gene with a specific chromosome
He provided convincing evidence that chromosomes are the location of Mendel’s heritable factors
Evidence that Genes Associated to Chromosomes
In one experiment Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type)• The F1 generation all had red eyes• The F2 generation showed the 3:1 red:white
eye ratio The white-eyed trait appeared only in males. All the females and half the males had red eyes
Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair
Morgan concluded that a fly’s eye color was linked to its sex
Morgan concluded that a fly’s eye color was linked to its sex
Females (XX) may have two red-eyed alleles and have red eyes or may be heterozygous and have red eyes.
Males (XY) have only a single allele and will be red eyed if they have a red-eyed allele or white-eyed if they have a white-eyed allele.
Morgan’s Discovery
That transmission of the X chromosome in fruit flies correlates with inheritance of the eye-color trait Was the first solid evidence indicating that a
specific gene is associated with a specific chromosome
Linkage of Genes Affects Inheritance
Each chromosome Has hundreds or thousands of genes
Genes located on the same chromosome, linked genes,
tend to be inherited together because the chromosome is passed along as a unit
Morgan did other experiments with fruit flies To see how linkage affects the inheritance of
two different characters
Linked genes Located on the same chromosome often do NOT assort independently:
Parental type
BN
B bN n
Bn
Parental type
bN
bn
Recombinant types
47% 5% 5% 43%
Unlinked genes
B bY y
Located on different chromosomes assort independently:
BY
By
bY
by
25%
25% 25% 25%
Genetic Recombination and Linkage
Recombinant offspring are those that show new combinations of the parental traits
Morgan discovered that genes can be linked but due to the appearance of recombinant phenotypes, the linkage appeared incomplete
Morgan proposed that some process must occasionally break the physical connection between genes on the same chromosome
Crossing over of homologous chromosomes was the mechanism
Recombinant Types
Produced when a crossover occurs between the 2 genes being studied
AA
B
a
b B
A a
b
b
Recombinant types Non recombinant types
B ab
aA
B
The Chromosomal Basis of Sex
The Chromosomal basis for determining sex is rather simple
The X-Y System Female homogameticXX
Male heterogametic XYSex of offspring depends on whether the sperm has an X
e.g human
The Z-W System
The X-O SystemIn insects Only one type of chromosomeFemales XX, Males XOSex of off spring sperm has X or Oe.g
In birds, some fishes and some insectsFemales ZW, Males ZZSex chromosome is in the ovum e.g birds , fishes ,insect etc
Human Sex linked Disorders
Some recessive alleles found on the X chromosome in humans cause certain types of disorders Color blindness
Hemophilia Sex linked recessive trait Absence of one or more clotting factors in the blood Gene that controls formation of clotting factors are
recessive and present on the X chromosome Individuals bleed excessively when injured Approx. 1/10,000 males are affected
Female Hemophiliacmust have this trait on both the XX Male only one X chromosome from mother, will be a
hemophiliac if mother has this trait
Alteration of Chromosome Number and Genetic Disorders
Sex-linked traits are not the only notable deviation from the inheritance patterns observed by Mendel
Also, gene mutations are not the only kind of changes to the genome that can affect phenotype
Physical and chemical disturbances, errors in meiosis damage chromosomes and alter #s
Large-scale chromosomal alterations lead to spontaneous abortions or cause a variety of developmental disorders
Trisomy 21 Results from an error during meiosis Affects 1/700 children born in the USDowns Syndrome Extra chromosome on # 21 This affects phenotype Facial features
Broad round faceFlattened noseSmall irregular teeth
Stature usually short Heart defects Susceptible to infection, leukemia, Alzheimer’s Life span shorter Varying degrees of mental retardation
AneuploidyKlinefelter’s syndrome, an XXY male, occurs once in every 2000 live births.
These individuals have male sex organs, but are sterile. There may be feminine characteristics, but their intelligence is
normal Males with an extra Y chromosome (XYY) tend to somewhat taller
than average
Monosomy X or Turner’s syndrome (X0), occurs once in every 5000 births, produces phenotypic, but immature females.
Alterations of Chromosome Structure
Breakage of a chromosome can lead to four types of changes in chromosome structure
DeletionA deletion occurs when a chromosome fragment lacking a centromere is lost during cell division.
This chromosome will be missing certain genes.
DuplicationA duplication occurs when a fragment becomes attached as an extra segment to a sister chromatid
InversionOccurs when a chromosomal fragment reattaches to the original chromosome but in the reverse orientation.
TranslocationA chromosomal fragment joins a non homologous chromosome.
Some translocations are reciprocal, others are not.