Download GENES AND CHROMOSOMES

Chromosomes and Inheritance
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Gene for eye color located on X chromosome
A trait determined by a gene on the sex
chromosome is said to be sex-linked.
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These experiments were the first to demonstrate
that genes were carried on chromosomes
The segregation of white eyes corresponded to
segregation of X chromosome
CALVIN KNEW WHAT FLY
CHROMOSOMES LOOKED LIKE
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He knew that Drosophilla had 4 pairs of
chromosomes numbered 1-4
Chromosomes 2, 3, & 4, are the same in both
genders
Chromosome pair # 1 looked different in males
and females
Males had an X and a Y chromosome
Females had 2 X chromosomes
Calvin reasoned that the X and Y
chromosomes determined gender
CALVIN WAS RIGHT
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Males get their X chromosome only from their
mother and their Y chromosome from their
father
If the gene for white eyes is on the X
chromosome it explains the fact that males
carry only one copy of the gene that they got
from their mother
Calvin correlated an unusual pattern of
inheritance, X linked, with an unusual pattern
of chromosome segregation, XY for males and
XX for females
GENDER DETERMINATION IN FRUIT
FLIES
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Gender in flies is determined by the number of X
chromosomes
XX
Female
X
Male
Calvin realized these white eye daughters carried
2 X chromosomes, ww, from their mother and a Y
from their father
They were wwY but still female and had white
eyes
He would see these flies about 1/1000 offspring
GENDER DETERMINATION IN FRUIT
FLIES
The males had only 1 X chromosome from
their father that was W
 The red eye males were actually W  They had no Y chromosome but they were
still males
 How was this strange distribution of
chromosomes happening?
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NONDISJUNCTION
Calvin realized that such flies could arise if
2 X chromosomes had failed to segregate
from each other during the first meiotic
division in the mother
 When the female is making eggs both of her
X chromosomes go into 1 egg
 So she will produce an egg with 2 X
chromosomes
 And an egg with no X chromosome
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GENES ON CHROMOSOMES
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Gender in flies is determined by the number of X
chromosomes
XX
Female
X
Male
The Y doesn’t make any difference
It was this finding that the chromosomes an
individual carried could determine something as
important as gender that convinced the last of the
skeptics that chromosomes were the units of
heredity
Calvin’s paper on non disjunction was the first
paper published in the Journal of Genetics
Non disjunction provided the proof that genes
reside on chromosomes
NONDISJUNCTION
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Nondisjunction of X chromosome in humans like
flies leads to XXY individual
This individual will be male because in humans Y
chromosome determines maleness
In Drosophila would be female because has two X
chromosomes
In humans severe non disjunction causes death in
utero
That is why the study of non disjunction is so
important it proved that genes are carried on
chromosomes and it causes such havoc in
organisms
Mendelian Inheritance
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Mendel’s Second Law of Heredity
 (Law of Independent Assortment)
 Genes that are located on different
chromosomes assort independently of one
another.
 The presence of an allele of one trait does not
preclude the presence or absence of any other
allele of any other trait
HUMAN GENETICS
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Humans have about 25,000 genes
If 2 genes are located very close together on the same
chromosome they may not segregate independently
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2 genes that are very close together on a chromosome may
only rarely be separated by a crossover event
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The father apart 2 genes are the more chance there is they
will be separated by a crossover event and behave as
independent genes