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BIOLOGY 207 - Dr.Locke
Lecture#16 Sex chromosomes and sex linkage
Required readings and problems:
Reading: Open Genetics, Chapter 3
Problems: Chapter 3
Optional
Griffiths (2008) 9th Ed. Readings: pp 61-65
Problems: 9th Ed. Ch. 2; 46-49, 52,53, 62-65
Campbell (2008) 8th Ed. Readings: Concept 15.2
Concepts:
How do sex chromosomes behave in meiosis and heredity?
1. In many species the sex of an individual is determined by the sex chromosomes.
2. Morphologically and genetically dissimilar sex chromosomes act as a homologous
pair in meiosis.
3. Chromosomal sex determination gives a non-Mendelian pattern of inheritance.
Biol207 Dr. Locke section
Lecture#17
Fall'11
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In many animals sex is determined by sex chromosomes (genetic)
Plants: Most have both male (stamens) and female (pistil) reproductive organs (called
a hermaphrodite) and therefore we do not need to consider the determination of sex
in plants
Animals: Often, sex determination is associated with, and due to, a pair of "sex"
chromosomes (genetically determined)
Note:
1- Not all species use chromosomes to determine sex. In some species sex is
environmental determined:
1a- Growth Temperature – e.g. Alligators (Tuatara -> effects of global warming?)
Ferguson & Joanen. 1982 Nature 296: 850 - 853; doi:10.1038/296850a0
Sex is fully determined at the time of hatching. Temperatures of 30°C produce all females
(nests constructed on levees). Temperatures of 34°C yielding all males (wet marsh nests).
The natural sex ratio at hatching is five females to 1 male.
1b- Social organization (sex-ratio in a population)
e.g. Most Reef fish – change sex during their lifetime.
2- Some species are parthenogenic - females lay fertile eggs - no males required
e.g. – walking stick insects, some fish/lizards (sharks in captivity).
Biol207 Dr. Locke section
Lecture#17
Fall'11
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Sex chromosomes - morphologically & genetically dissimilar
In many animals (most mammals - including humans and model systems) the males
have one pair of chromosomes that appear morphologically different
--> heteromorphic pair - referred to as X-chromosome Y-chromosome
Human Female has (X X) + 22 autosome pairs
Human Male has (Y X) + 22 autosome pairs
Autosomes = non-sex chromosomes
Sex chromosomes Fig
Morphologically dissimilar -- size, structure
Genetically dissimilar -- carry different genes (although some loci are allelic)
Humans:
X-chromosome contains most of the genes associated with the sex-chromosomes.
Y-chromosome contains relatively few genes - most of which are on one arm.
There is one important Y-chromosome gene, called:
TDF = Testis Determining Factor on the Y = SRY,
which is responsible for the person developing as a male.
If mutant?? -> develops as a female; although she has both X and Y chromosomes.
Biol207 Dr. Locke section
Lecture#17
Fall'11
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X and Y chromosomes act as a homologous pair in meiosis
Although morphologically and genetically dissimilar:
- X and Y act as a homologous chromosome pair in meiosis
- share some regions of sequence (gene) similarity.
- aids in pairing of chromosomes during meiosis.
-> Pseudo-autosomal region
X and Y segregate in meiosis.
X/Y meiocytes yield 4 gametes, each with one of the two chromosomes
Therefore, in XX XY species:
X Y heterogametic sex (different gametes) -> X Y
Female
gametes
X
X
Male gametes Y XY XY Male progeny
X XX XX Female progeny
Progeny: 2 XX and 2 XY --> 2 : 2 ratio of hetero- and homogametic progeny again
Apparent Non-Mendelian Inheritance of gene on sex chromosomes - due to genetic
dissimilarities.
Biol207 Dr. Locke section
Lecture#17
Fall'11
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Y-chromosome -> Y-linked genes
Genes / alleles located on the Y-chromosome exhibit Y-linkage
Father to son transmission only.
eg. - TDF -> sex determination - hairy ear rim --> Fig
X-chromosome -> X-linked genes
Genes / alleles on the X-chromosome show X-linkage.
1) Fathers X --> always goes to daughters only
Y --> always goes to sons only
2) Mothers X --> equally to both daughters and sons
Biol207 Dr. Locke section
Lecture#17
Fall'11
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Simple Genetic test for sex-linkage vs. autosomal location:
(remember test cross)
Reciprocal Crosses
Autosomal genes: the exact same pattern of inheritance is seen in reciprocal
crosses.
Sex linked genes: (on the X-chromosome) produce reciprocal crosses do not result
in the same pattern.
Example: white gene locus in Drosophila
T.H. Morgan in 1910 correlated the hereditary behavior of the white gene with that of
the X-chromosome.
Supported the idea that genes were on chromosomes
- Sutton & Boveri (1903) -> Chromosome theory of heredity
Dominant wild type allele (w +) -> has red colour eyes.
Recessive mutant allele (w -) -> has white eyes.
The red colour is due to the presence of red pigments, which are absent in the white
mutant.
Morgan observed an inheritance pattern consistent with the white gene locus being on
the X-chromosome.
The reciprocal crosses resulted in different outcomes:
Biol207 Dr. Locke section
Lecture#17
Fall'11
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Outcomes: Reciprocal crosses are not equivalent
1) First cross: red female X white male
F1 -->all red females and males
F2 --> get 3:1 red:white but all females= red,
males= 1/2 red 1/2 white, (all white flies are male)
2) Second cross: white female X red male
F1 --> red females and white males
F2 --> get 2:2 red:white (1/2 white 1/2 red female) (1/2 red 1/2 white males)
3) Autosomal locus
AA (x) aa
Reciprocal = same results
A
a
Aa (x) Aa
A
a
A
AA Aa
a
Aa aa
Genotype 1 AA : 2 Aa : 1 aa Phenotype 3 A : 1 a Both male & female are 3:1
Conclusion: use reciprocal crosses to test for sex-linkage of a
locus (just two test crosses!)
Biol207 Dr. Locke section
Lecture#17
Fall'11
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Non-X/Y Sex Chromosomes
Alternative sex chromosome systems:
A) ZZ / ZW System
W X Y Z
ZZ-males (homogametic)
ZW-females (heterogametic)
- found in birds and moths
- here Z and W sex linkage pattern is backwards of the X and Y sex linkage pattern
1) Z in males and females 2) W-female only
B) X/O system
XX-female
X/O male where O is an absence of a chromosome
- found frequently in insects
Question:
But how can X/O be male if, as in humans, sex determining gene (eg. TDF) is on the Y
chromosome?
Biol207 Dr. Locke section
Lecture#17
Fall'11
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Sex determination mechanisms:
A) Single Gene specified
Y-chromosome gene (eg. TDF) induces gonadal cells to differentiate into testis.
Male hormones then induce the body to develop into a male.
B) Chromosome ratios
- specifically X : Autosome ratio
2N individuals =
(Autosome set x 2) + (XX) = female 2A:2X
(Autosome set x 2) + (X -) = male 2A:1X
This system is found in most insects including Drosophila (which has XX XY sex
chromosomes)
X/O ->
C) Environmental Factors
- Rearing temperature - alligator
- Sex ratio, etc – tropical reef fish
D) Haploid/Diploid system
- social insects
Biol207 Dr. Locke section
Lecture#17
Fall'11
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