Genetics: Sex Determination and Sex Linkage
==Mechanisms of Sex Determination==
- a) Environmental Sex Determination, where external cues initiate development into one sex or another.
- i) Genes still encode traits, but the initial switch is determined by an external cue.
- ii) Cues may be temperature, environmental chemicals, social environment, etc.
- iii) Some species that use this include many reptiles (temperature cues) and clownfish (social cues).
- b) Genetic (Chromosomal) Sex Determination is initiated by the presence of a gene OR an allele.
- i) Sex Determination by differentiated sex chromosomes
- (1) Dedicated homologous pair of sex chromosomes, one or both of which will determine sex. Other chromosomes are autosomes.
- (a) Differentiation means that the chromosomes, although arising from a homologous pair, now have different size, shape, banding pattern, and gene content.
- (b) Sex chromosomes are differentiated in appearance due to a lack of crossing over, which allows mutations to accumulate separately on the two chromosome types.
- (c) These chromosomes are still called homologous because they pair during meiosis and because there may be an area where crossing over occurs between the two types, called the pseudo-autosomal region (at the ends of chromosomes)
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- (2) XY systems, where females are XX and males are XY
- (a) Males, the heterogametic sex, determines the offspring’s sex by passing either the X chromosome or the Y chromosome to gametes during meiosis. Females are the
- homogametic sex.
- (b) The Y chromosome is smaller and has fewer genes than the X chromosome.
- (i) Many of the genes on the X are essential for development, but not the Y.
- (c) Not all XY systems determine sex via the same mechanism.
- (i) mammals: sex is determined by the presence of the SRY gene (on Y chromosome).
- (ii) Drosophila: sex is determined by the ratio of Xs to pairs of autosomes.
- (iii) The difference between these can be understood by comparing the phenotypes of mammals and Drosophila with atypical sex chromosome genotypes (XO, XXY, etc).
- XXY = male in mammals, XXY= female in Drosophila
- (d) Because the X has many genes that are necessary for development, problems arise when gene expression of these necessary genes is unequal in male cells (1 X) and female cells (2 Xs).
Dosage compensation: process to equalize the level of gene expression of X linked genes between individuals that have two copies of the X and those who have one copy.
Solutions include:
- (i) Hypertranscription: Double expression of one X in males so that both sexes make 2 “doses”.
- (ii) Hypotranscription: Reducing expression level of each X to ½ so that total expression equals 1 “dose”.
- (iii) X-inactivation: Turning off all but one X by X inactivation so that 1 “dose” is expressed.
- 1. An inactivated X is a Barr body, visible in stained cells at all stages of the cell cycle.
- 2. In XX females, that means one X is inactivated. In individuals that are XXX or XXY, all but one X will be inactivated.
- 3. Identity of the inactivated X(s) is not constant, which may produce a mosaic phenotype if a female is heterozygous for an X-linked gene.
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- (3) Haplo-diploid sex chromosomes (XO) where females have two Xs and males have 1 X only (no Y)
- Females = XX
- Males = XO
- One sex chromosome is haploid and one is diploid
- Ex: grasshoppers
- Male still determines sex
- (4) ZW systems
- females are ZW and males are ZZ
- (a) Females, the heterogametic sex, determine offspring sex.
- (b) Z and W are differentiated, where W is the smaller chromosome.
- (c) Sex-linked traits in ZW species show similar patterns of inheritance to sex-linked traits in XY species.
- ii) Genic Sex Determination – where the genotype of an individual at one locus determines sex AND the chromosome carrying that locus is not differentiated.
- iii) Haplo-Diploid Genomes Sex Determination
(1) One sex, males, are all haploid, and the other, females, are all diploid
Female offspring are produced when a female fertilizes her eggs. Male offspring are produced when the female lays eggs that are unfertilized.
Queen (2n) --meiosis→ eggs = n … with fertilization = 2n female, without fertilization = n male
(2) Genetic variation in male offspring arises by crossing over an independent assortment, but not fertilization.
- In females, has independent assortment, crossing over, and contribution of father
3) Inheritance of sex-linked traits – traits that are affected by the identity of the parent
- This term includes genes on a sex chromosome (X, Y, Z, or W) and mitochondrial genes.
a) Reciprocal Crosses – switching the identity of the parent with the recessive phenotype will not affect the inheritance pattern of autosomal traits but will affect the pattern of sex-linked traits.
- i) For example, if you start with a white eyed male and red eyed female in the P generation of one cross, the reciprocal cross would start with a white eyed female and red eyed male
b) Properties of X-linked Traits
- i) Alleles may be dominant or recessive
- Recessive alleles will always be shown in males.
- Recessive
- ii) Traits show sex specific patterns of inheritance -
- males do not pass their 1 X to their sons.
- iii) often skips a generation
- v) male passes allele to all daughters as carrier → then will pass trait to half of sons
- vi) appears more in males than females
- Dominant
- i) does not skip a generation
- ii) affected fathers pass one allele to all daughters but not sons
- iii) affected mothers pass allele to ½ daughter and ½ sons
- v) both males and females affected → tend to be female more often
c) Properties of Y-linked Traits –
- Y chromosomes are exclusive to males, alleles cannot be recessive
- Y-linked phenotype is inherited only via the paternal lineage (father to all son).
- Does not skip generations
- No dominant or recessive
d) Properties of Mitochondrial Traits
- mitochondria are carried by all individuals but inherited from the mother only
- Males cannot pass mitochondrial traits to their offspring.
- All progeny of mother inherit trait
- Ends at son → cannot further pass to offspring
