Download Lecture#18 - Sex chromosomes and sex linkage Concepts: In many

Lecture 18; 2007
Biology 207; Section B2; Good
Lecture#18 - Sex chromosomes and sex linkage
Readings: Griffiths et al (2004)
8th Edition: pp 29-54
7th Edition: Ch. pp 37-40; 51; 73-76; 146-147; 681-684
Assigned Problems:
8th Ch. 2; 25-27, 29-33, 51-55: Ch. 3: 28
7th Ch. 2: 33-39, 41-49, 52-54; Ch. 3: 30
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 during meiosis.
3. Chromosomal sex determination gives a non-Mendelian pattern of inheritance.
In many animals sex is determined by sex chromosomes
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 due to, a pair of "sex" chromosomes. Note:
1- Not all species use chromosomes to determine sex. In some species sex is
determined by environmental conditions (eg. temperatures; sex-ratio in the population)
2- Some species are parthenogenic - females lay fertile eggs - no males required
Sex chromosomes are usually 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. 2-22 (8th) 2-13 (7th)
Morphologically dissimilar -- size, structure
Genetically dissimilar -- carry different genes
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-pg. 54 (8th) 682(7th), which is responsible for
the person developing as a male. Note: This is the same as SRY gene in Fig. 2-22
If mutant XY -> develops as a female; although she has both X and Y chromosomes.
X and Y chromosomes act as a homologous pair in meiosis
Although morphologically and genetically dissimilar
- the X and Y act as a homologous chromosome pair in meiosis
- they do share some regions of sequence (gene) similarity.
- region of similarity --> aids in pairing of chromosomes during meiosis. ->
Pseudoautosomal region
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Lecture 18; 2007
Biology 207; Section B2; Good
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They segregate in meiosis. The 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 X or Y > XX (Female) or XY Male 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.
Y-chromosome + Y-linked genes
Genes located on the Y-chromosome exhibit Y-linkage. Father to son transmission only.
eg. - TDF --> sex determination - - hairy ear rim --> Fig 2-39 (8th) 2-33 (7th)
X-chromosome + X-linked genes
Genes and 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
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.
Genetic test for sex-linkage vs. autosomal location
Example of white gene locus in Drosophila. T.H. Morgan in 1910 correlated the
hereditary behavior of a 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: see Fig 2-24 (8th) 2-15 (7th)
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)
Non-X/Y Sex Chromosomes
Alternative systems:
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Lecture 18; 2007
Biology 207; Section B2; Good
A) ZZ ZW System
ZZ-males (homogametic)
ZW-females (heterogametic)
- found in birds and moths Fig 3-6 (8th) 3-11 (7th)
- reminder here X and Y sex linkage pattern is backwards
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?
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)
C) Environmental Factors
- Rearing temperature - alligator
- Sex ratio, etc – tropical reef fish
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Lecture 18; 2007
Biology 207; Section B2; Good
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