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Sex Linked Inheritance
T.H. Morgan provided a deeper
understanding of gender and
inheritance (1866-1945)
Chromosomal Theory
• Chromosomes carry genes, the units of
heredity
• Paired chromosomes segregate during
meiosis. Each gamete has only one of each
of the paired alleles
Fruit flies were used by Morgan
and were ideal for genetic
experiments
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Three Reasons:
1. reproduces rapidly
2. small size
3. males easily distinguished from females
Normal or Wild-type Fruit flies
have red eyes
Differences between the Sexes
• Males are smaller
• males have a rounded abdomen with a darkcoloured posterior segment
• larger females have a pointed abdomen with
a pattern of dark bands
Males have rounded abdomen,
females have pointed abdomen
Morgan’s Discovery
• Morgan found a white eyed male fruit fly
among many red eyed fruit flies.
• He did an experiment
• He mated the white eyed male with a red
eyed female
• Result: all F1 had red eyes
Morgan’s Interpretation
• All the members of the F1 had red eyes
which was expected according to normal
Mendelian genetics
• Morgan, however, decided to mate two
hybrids from the F1 generation
• the F2 had 3/4 red and 1/4 white eyes, also
seemed normal, but only males had white
eyes
• Morgan wondered Why only males?
Recall: Sex chromosomes are
not identical in shape
Morgan’s Hypothesis
• He hypothesized that the gene for white
eyes was on the X chromosome because it
was so much larger than the Y
• Therefore : Females have three genotypes:
XRXR, XRXr, XrXr
• Males have only two genotypes:
XrY or XRY
Morgan’s First Cross with White
eyed Male and Red eyed Female
Morgan’s First Cross or F1
generation
Morgan’s Second Cross or
Experiment
• All the flies resulting from his first cross
were red eyed so he decided to cross the F1
• A red eyed male and a red eyed female who
is heterozygous for red eyes
• XRY x XRXr
F2 Generation
F2 Generation
Examining the F2 Generation
• Males only inherit the trait for eye colour
from the mother
• The father supplies the Y chromosome
which makes the offspring male
• F2 males are either XRY or XrY, half have
white eyes
• F2 females are red eyed but some are XrXR
Sex Linked Traits in Humans
• Hemophilia and Color Blindness are sex
linked trait in humans
• The genes are located on the sex
chromosomes. A recessive trait located on
the X chromosome is more likely to express
itself in males than in females since males
need only one copy of the recessive gene.
• See Case Study on page 169
Review
• What sex chromosomes do females have?
Males?
Answer
• Female XX
• Male XY
Review
• On what chromosome is the sex linked trait
usually located?
Answer
• The sex-linked trait is usually recessive and
is located on the X chromosome.
Review
• A recessive sex-linked allele (h) located on
the X chromosome increases blood-clotting
time, causing hemophilia. Explain how
hemophilic offspring can be born to two
normal parents.
Answer
• Two normal parents but the mother is a
carrier for the trait.
• XHXh x XHY