Transcript 5-Sex linked - Science-with

Do Now Problem
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Would it be possible for a mother with
blood type A and a father with a blood
type B to have a baby with blood type
O? blood type A?
Support your answers with a punnett
square (for each one) and probability.
Marked out of 8 (1 mark for yes/no for each scenario, 1
mark for each probability and 2 marks for each punnett)
Answer
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Scenario 1
Yes! If both parents were
heterozygous for the
dominant traits (IAi and IBi
respectively) it would be
possible.
IA
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Scenario 2
Yes! Mother would
have to be either IAIA
or IAi and the father
would HAVE to be IBi.
i
IA
i
IB
IAIB IBi
IB
IAIB
IBi
i
IAi
i
IAi
ii
ii
Sex-linked
Chapter 19.1
Pages 628-631
Chromosomal Theory
Walter Sutton and Theodor Boveri (1902)
 observed chromosomes came in pairs and segregated
during meiosis.
• chromosomes formed new pairs when the egg and
sperm united.
• this supported Mendel’s observations on inheritance
and his “factors” became alleles of a gene.
Chromosomal theory
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humans have 44 autosomal chromosomes and 2 sex
chromosomes.
• humans have thousands of different traits.
• Sutton hypothesized that each chromosome carries
multiple genes
• genes that are located on the same chromosome are
said to be linked genes.
Chromosomal Theory
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the Chromosomal theory of inheritance:
• chromosomes carry genes, the units of heredity
• paired chromosomes segregate during meiosis. Each
sex cell or gamete has half the number of
chromosomes found in the somatic cells. This explains
why each gamete has one of each of the paired alleles.
Morgan’s Experiment
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studied the principles of inheritance using Drosophila
melanogaster, fruit flies
fruit flies a great animals to study because:
• they reproduce rapidly (in 10 to 15 days)
• offspring can mate shortly after leaving the egg
• females produce over 100 eggs
• they are small and easy to take care of.
• males can be easily distinguished from females.
• males have smaller-rounded abdomen, females
have a pointed abdomen.
Morgan’s Experiment
Morgan’s Experiment
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Morgan observed a white-eyed phenotype and after a
number of test crosses figured out it only occurred in
males.
• a sex-linked trait is one that is determined by genes
located on the sex chromosomes
Morgan first crossed a white eyed male with a red eyed
female (red eyed being dominant)
• all members of the F1 generation had red eyes
Morgan then crossed to members of the F1 generation
• he observed ¾ red eyes and ¼ whites eyes.
Morgan’s Experiment
• he noticed all the females had red eyes and the white
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eyed phenotype only appeared in the males.
because the sex chromosomes in males are not
homologous they contain different genes.
Morgan concluded that the Y chromosome does not carry
the gene to determine eye colour.
• we now know the gene for eye colour in fruit flies is on
the X chromosome.
Morgan’s Experiment
Punnett Squares for Sex Linked Inheritance
F1 Generation
♂
Xr
Y
♀ XR
XR
XRXr
XRXr
XRY
XRY
• 4/4 red eyed fruit flies
F2 Generation
♂
XR
Y
♀ XR
Xr
XRXR
XRXr
XRY
XrY
• 3/4 red eyed fruit flies
(2 female and 1 male)
•¼ white eyed, 1 male
Morgan’s Experiment
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in humans it is estimated that
• the X chromosome carries between 100 and 200 genes
• the Y chromosome carries less than 100 genes
disorders that require two recessive alleles, one on each X
chromosome only need to be present once in males.
• this is why some sex linked disorders occur more frequently in
males.
• examples: colour blindness, hemophilia, near-sightedness
(myopia), night-blindness.
• recessive lethal X-linked disorders also occurs more frequently
in males.
• example: infantile spinal muscular atrophy
Dr.Murray Barr
(Uni. Of Western Ontario
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recognized dark spots in some somatic cells of female mammals
• the spot turned out to be sex chromatin.
• it comes about when one of the X chromosomes becomes
inactivated randomly.
• this spot is now called a Barr Body
• this means not all female somatic cells are identical, some
have two active X chromosomes and some have one.
• homozygous recessive X disorders can appear in some
cells that are heterozygous but have lost the dominant
allele.
Homework
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Workbook “6.Sex linked”