Lecture 19/20 Sex Chromosomes and Linked genes

Question 1

Beyond Mendel

Answer 1

Some traits do not show the inheritance
patterns predicted by Mendel
– E.g. color blindness in humans and white eyes
in fruit flies
* There are some examples in which the
expected ratios and pedigree patterns are
modified by principles Mendel knew nothing
about

Question 2

Human Sex Chromosomes

Answer 2

In humans and some other animals, the sex
chromosomes determine the sex of the individual
* X chromosome is larger than the Y chromosome
* X chromosome: 150 Mb; about 1000 genes
* Y chromosome: 50 Mb; about 50 genes
* These sex chromosomes
differ quite a bit and only have
a small sequence homology
(shown in red)
* The SRY gene on the Y chromosome
codes for the development of testes

Question 3

X-linked Genes

Answer 3

Thomas Hunt Morgan (1910) used fruit flies
(Drosophila melanogaster) for his genetic
experiments
* Significance: first experimental evidence that genes are
found on chromosomes
* Genes on the X-chromosome are called X-linked
genes
* In his work, he found a white-eyed mutant male fly
and used this fly to cross with wild-type (red-eyed)
female

Question 4

X-linked Genes – Morgan’s Crosses

Answer 4

Cross between red-eyed female and
white-eyed male resulted in all red
eyes in F1 as would be expected
with recessive trait
* Crosses between F1
progeny resulted in a
surprise:
* Only males had white eyes,
so white-eyed phenotype
was associated with sex of
fly!
* All females –> red eyed
* Males–> 1:1 red:white eyes

Question 5

Crisscross Inheritance Pattern in X-linkage

Answer 5

X-linked recessive alleles are
expressed in males because males
have only one X-chromosome
* Homozygous female
crossed with male
with recessive
mutation
* Result: all offspring
(males and females)
are red-eyed
* Why?

Question 6

Crisscross Inheritance Pattern in X-linkage

Answer 6

If cross heterozygous females
(from previous cross) with wild-
type (normal) males….
* Result: all females have red
eyes; males have 1:1 ratio
of red:white eyes
* This is crisscross
inheritance:
* X chromosome of male
can only pass to
females of next generation
then transmitted back to males
in generation after that

Question 7

x-linked Recessive Mutation in Humans

Answer 7

Features of X-linked inheritance include:
1. Affected individuals almost always males
2. Affected males have unaffected sons
3. A female whose father is affected can have
affected sons

Question 8

Y-linked Genes

Answer 8

The SRY gene on the Y chromosome encodes a protein that is the
trigger for male development
* Y-linked genes are those that are present in a unique region of the Y
chromosome, where they cannot cross over with the X
chromosome
* These genes are transmitted from father to son to grandson
* Other than maleness itself
and some types of impaired
fertility, no physical traits
are known that follow a
strict Y-linked pattern of
inheritance. This
emphasizes the extremely
low density of functional
genes in the Y chromosome

Question 9

Genetic Linkage (Linked Genes)

Answer 9

each chromosome has hundreds or
thousands of genes
* Genes located close together on the same
chromosome that tend to be inherited
together are called linked genes
– Linked genes do not assort independently
* Linked genes can occur on autosomes or on
sex chromosomes
– Do not confuse this with “x-linked gene

Question 10

Genetic Linkage

Answer 10

morgan’s student, Alfred Sturtevant
discovered genetic linkage – looked at two
recessive mutations on the X-chromosome
Genetic Linkage
14
– White eyes (w - )
* non-mutant is w + ( red
eyes)
– Crossveinless (cv - )
* non-mutant is cv +
( crossveined)

Question 11

Genetic Linkage

Answer 11

Sturtevant crossed female,
homozygous for both non-
mutant alleles with male
having both mutant alleles
* Result in F1 generation:
– Females are red-eyed and
crossveined (heterozygous for
both alleles)
– Males are red-eyed and
crossveined

Question 12

Genetic Linkage

Answer 12

turtevant then crossed the
* F1 generation flies to get the
F2 generation
* If the genes were not linked
what ratio would you
expect?
* Results in F2 generation:
– 4 types of progeny but in
different ratio than expected if
genes not linked
– Non-recombinants
– Recombinants

Question 13

Recombinant and Nonrecombinant Alleles

Answer 13

crossing over without recombination:
* When crossing over occurs outside the interval between
genes, there is no recombination between the alleles of
the genes.
* Nonrecombinant chromosomes have the same
configuration of alleles as one of the parental
chromosomes

Question 14

Recombinant and Nonrecombinant Alleles

Answer 14

Crossing over with recombination:
* When crossing over occurs within the interval between
genes, recombination occurs
* Crossing over results in two recombinant and two
nonrecombinant chromosomes
* Because only one sister chromatid from each homologous
chromosome is included in any crossover

Question 15

Frequency of Recombination

Answer 15

The frequency of recombination - measure of the genetic
distance between linked genes

– When two genes are on separate chromosomes, the expected ratio
is 1:1:1:1 for the nonrecombinant and recombinant gametes
* Whether crossing over occurs or not, depends on the physical
distance between the genes
– For two genes found on the same chromosome, if they are located
very far from each other–> crossing over is likely to occur
– If the two genes are found very close together–> crossing over is
less likely to occur

Question 16

Frequency of Recombination

Answer 16

BIOL1010 and BIOL1011
* The frequency of recombination between any two
genes ranges from:
– 0% (when crossing over never takes place) to
– 50% (when the genes are so far apart that crossing over
between the genes always takes place)
* Linked genes have a recombination frequency between 0% and
50%
* Why is the upper limit 50% and not 100%?
Frequency of Recombination
20–>non-linked genes produce
1:1 parental to non-parental.
Therefore, the recombination frequency
would be:
non-parental/total = 1/(1+1) = 50%