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chromosomal basis of inheritance

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Page 1: chromosomal basis of inheritance
Page 2: chromosomal basis of inheritance

CHROMOSOMAL BASIS OF

INHERITANCE

Page 3: chromosomal basis of inheritance

B.s biochem 3rd semester

Presented To:

Sana AkramRoll.no.2

BAHAUDDIN ZAKARIYA UNIVERSITY MULTAN

Presented By:

Sir Ibrahim

Page 4: chromosomal basis of inheritance

Genes and Chromosomes

Genes Are located on chromosomes

Can be visualized using certain techniques

Page 5: chromosomal basis of inheritance

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

Page 6: chromosomal basis of inheritance

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.

Page 7: chromosomal basis of inheritance

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

Page 8: chromosomal basis of inheritance

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).

Page 9: chromosomal basis of inheritance

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

Page 10: chromosomal basis of inheritance

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

Page 11: chromosomal basis of inheritance

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.

Page 12: chromosomal basis of inheritance

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

Page 13: chromosomal basis of inheritance

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

Page 14: chromosomal basis of inheritance

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%

Page 15: chromosomal basis of inheritance

Unlinked genes

B bY y

Located on different chromosomes assort independently:

BY

By

bY

by

25%

25% 25% 25%

Page 16: chromosomal basis of inheritance

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

Page 17: chromosomal basis of inheritance

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

Page 18: chromosomal basis of inheritance

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

Page 19: chromosomal basis of inheritance

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

Page 20: chromosomal basis of inheritance

Human Sex linked Disorders

Some recessive alleles found on the X chromosome in humans cause certain types of disorders Color blindness

Page 21: chromosomal basis of inheritance

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

Page 22: chromosomal basis of inheritance

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

Page 23: chromosomal basis of inheritance

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

Page 24: chromosomal basis of inheritance

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.

Page 25: chromosomal basis of inheritance

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.

Page 26: chromosomal basis of inheritance

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.

Page 27: chromosomal basis of inheritance

TranslocationA chromosomal fragment joins a non homologous chromosome.

Some translocations are reciprocal, others are not.

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