Sex linkage Flashcards
chromosomes
A structure composed of DNA and associated proteins that in total contain the genome of an organism.
Autosomes
equally represented in both sexes and are the majority of our chromosomes – 22 of our chromosomes are autosomes
Sex chromosomes:
differently represented in the sexes (last chromosomes, 23 is sex chromosome)
homogametic
the sex of a species in which the sex chromosomes are identical
Females
XX in mammals and some insects such as Drosophila
heterogametic
Males
XY in mammals and some insects
Thomas Hunt Morgan
helped support this finding in his studies in Drosophila melanogaster (Common fruit fly) ~1910’s
Flies were captured from the environment and phenotypes observed under a microscope
Wildtype
phenotype that is most common in a population, denoted with “+”(usually corresponding to dominant trait, talking about most common not whether its dominant or recessive)
Mutants
phenotype different from wildtype, commonly denoted with “m” - not always recessive
Carriers
carries both the dominant and recessive alleles for a trait (ie heterozygous)
Why did Morgan use flies?
Easy to study due to short life cycles (12-14 days)
Could raise 100s of generations in short amount of time
Autosomal inheritance
Hereditary transmission of genes carried on autosomes.
not sex linked
means that males and females are equally likely to inherit the gene
Mendel looked at autosomal inheritance
Sex-linked inheritance
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X-linked inheritance
inheritance of genes on the X chromosome
What are the indicators of Sex-linked inheritance
Males and females have different phenotypic ratios in the F1 and F2
Reciprocal crosses give different results
Female wildtype x male mutant ≠ female mutant x male wildtype
“Criss-cross” inheritance
Eg a recessive trait is passed from recessive father to carrier daughter to recessive grandson
“Criss-cross” inheritance
Eg a recessive trait is passed from recessive father to carrier daughter to recessive grandson
What did Morgan first do with the flies?
Did a single mendilian cross for eye color
Morgan’s wife found a mutant male fly with white eyes
Wildtype is brick red eyes
Mutant white-eyed male was crossed with wild-type red-eyed female
1273 F1 flies, all red eyes
Suggests red is dominant over white
F1’s were then crossed to produce F2 generation
2459 red-eyed females
1011 red-eyes males
782 white-eyed males
Morgan had expected a 3:1 ratio of red eyes to
NOTE: No white-eyed females!
What was the second cross that Morgan do and what did it suggest?
When Morgan performed a reciprocal cross of his first cross (ie red-eyed male with a white eyed female), it resulted int:
Red-eyed females in F1
White-eyed males in F1
The F2 showed equal proportions of red-eyed and white-eyed males and females
The results of this cross were completely different from the first cross he did (previous slide)
Confirmed eye-color was inherited differently from other traits
Inherited differently in sexes
Hemizygous
have only one x chromosome so whatever allele is present on that x chromosome that’s what trait they will express
Need only 1 copy of the allele for the phenotype to be expressed
Inherited from the mother
Do sex linked traits follow mendel rules
Yes if we look at the traits as a whole excluding the sex
ex- morgans flies in F2 just looking at eye colour have 3:1 ratio
What did morgan’s first cross using sex chromosomes show?
Single X chromosome in the male carries a recessive allele w
X^w
Females carry 2 X chromosomes, each carrying an allele for eye colour
X^w+ X^w+
In the F2, we see that males are 50% red eye and 50% white eye
What did Morgans reciprocal cross using sex chromosomes using sex chromsomes show?
now supports Morgan’s idea of X-linked inheritance
The results in the F1 and F2 from this cross and the one on the previous slide are different
If you need to, go back to Mendel’s peas and look at the results of reciprocal crosses there
Notice there that the results are the same in the F1 and F2
Note in this reciprocal cross, we get a phenotypic ratio in the F2 as 1:1 red to white
Why were some of the results the flies produced in the reciprocal cross were they expected why did this occur?
Some of the flies did produce unexpected results in the reciprocal cross
White-eyed females
Red-eye males
Occurred ~1 in every 2000 F1 flies
Due to nondisjunction (failed chromosome separation)
Produces viable XXY(females in flies) and X0(males in flies)(0 means missing a chromosome) flies
XXX and YO flies are lethal
X-linked recessive
transmission of recessive allele on the X chromosome
X-linked dominant
transmission of the dominant alleles on the X chromosome
Autosomal recessive
transmission of recessive alleles on autosomes (ie aa)
Autosome dominant
transmission of dominant alleles on autosomes (ie AA, Aa)
Autosomal inheritance follows the typical Mendelian genetics patterns we’ve been discussing thus far
What are some examples of X-linked recessive transmission in humans?
Colour-blindness (OMIM 303800)
Hemophilia A (OMIM 306700)
Muscular dystrophy (OMIM 310200 (Duchenne type), OMIM 300376 (Becker type))
Phenotype is expressed in homozygous recessive females (X^aX^a) and hemizygous males (X^aY)
Same pattern as the white-eyes in Drosophila
If we had wildtype mother (X+X+ ) and hemizygous affected father (X^cbY)
All daughters will be carriers (X+Xcb)
All sons would be wildtype (X+Y)
If we had a carrier mother (X+Xcb) and wildtype father (X+Y)
Half of the daughters will be carriers (X+Xcb) and half wildtype (X+X+)
Half of the sons will be affected (XcbY) and half wildtype (X+Y)
Can we have a colour-blind female? How?
Father in colour blind mother is a carrier
What are some examples of X-linked Dominant transmission in humans?
Rett syndrome (OMIM 312750)- only affects females
Amelogenesis imperfecta (abnormal tooth enamel development, OMIM 301200)
Phenotypes expressed in homozygous dominant females (XDXD) and hemizygous males (XDY)
What are some things to note in X-linked Inheritance?
- Recessive and dominant refer to X-linked inheritance in females
Males are hemizygous so will also express the trait based on what is present on the X-chromosome
- Probability of transmission of X-linked traits to offspring is not the same for each sex
Females follow typical patterns seen in autosomal inheritance
Males always transmit their X chromosome to female offspring and Y to male offspring
- Females get one X chromosome each from Mom and Dad while males get their X chromosome (and therefore X-linked alleles) from Mom and Y chromosome from Dad
Y-linked genes are solely within male lineages (patrilineal)
Generally linked to male fertility and development and other male-specific traits
y- linked transmission
Occur on the Y chromosome
Smaller chromosome compared to X chromosome
Fewer genes (<50), not involved in sex determination
Females never carry Y chromosomes, so traits are solely male
Traits are “holandric”, or wholly male
Recessive and dominance not applicable
What is an example of a y linked trait?
Hairy ears (HE) we had a wildtype mother (X+X+) and affected male (X+Y HE)
Daughters will all be wildtype (not hairy ears, X+X+)
Sons will all have hairy ears (X+Y HE), ie all affected
complete penetrance
Most geneticists assume that the variation we see in phenotypes is due to differences in genotypes
Only valid if the genotype always produces the same phenotype
Mendels peas
Sex-limited traits
Alleles are carried in both sexes, but the phenotype only shows up in one sex
Behave like autosomal traits but only in one sex
Ex. Breast development and ability to produce milk in female mammals
FYI -> Males can get breast cancer too (but less than 1% of all breast cancers)! They just don’t have developed breast tissues
Sex-influenced traits
Inheritance patterns vary between the sexes even if genotype is the same
Usually influenced by hormone levels
Often autosomal
What are some examples of sex-limited traits
Breast and milk production in female mammals
Horn development in male sheep, cows, and other hoofed animals
Certain behaviors in sexually dimorphic animals
Eg courtship behaviors in cranes differ between males and females
Eg male canaries sing different songs as a result in increased male hormones at certain times of the year
What is an example of sex influenced traits?
chin beards in goats
Bearding is inherited as an autosomal trait determined by two alleles, B1 and B2
Homozygous states in both sexes are the same: B1B1 = beardless (recessive)
B2B2 = bearded (dominant)
The heterozygote state varies among the sexes though
Males B1B2 = bearded
Females B1B2 = beardless
Overall, a 3:1 ratio present for the trait for each sex
Another common example in humans: male patterned baldness
Sex determination
Encompasses the genetic and biological processes that produce male and female characteristics in a species
Chromosomal sex
the presence of sex chromosomes associated with male and female in a species
Determined at fertilisation based on contribution from parental gamete
Phenotypic sex
the internal and external morphology found in each sex
Determined by gene expression and development of sexual characteristics
Aneuploidy
In humans, the X and Y chromosomes are homologues
Small region in the Y chromosome acts as a homologue to the X chromosome
Results in sperm with either 1X or 1Y
What happens in Drosophila, nondisjunction?
, nondisjunction in meiosis 1 within females results in:
1 gamete with no X chromosomes (0)
1 gamete with extra X chromosome (XX)
When mated with a normal male gamete, can result in
XXX (lethal), XXY (non -lethal) females
X0 (non -lethal), YO (lethal) males
How is sex determined in Drosophilia?
The number of X chromosomes in relation to the number of haploid autosomal chromosomes determines sex
X:A or X:autosome ratio
XY, XYY, X0 = males, Y0 males are never observed (lethal)
XX, XXY = females, XXX females rarely observed (lethal)
How is sex determined in mammals?
Mammals also have 2 sex chromosomes: X and Y Sex determination depends on the presence of the Y
chromosome
Y chromosome has a SRY (sex determining region on
Y) gene that is expressed Males = XY, XXY, XYY
Females = XX, X0, XXX
What does the expression of the SRY do?
also elicits a cascade of events that lead to the development of male internal and external structures
As embryos, we contain undifferentiated gonadal tissues
Wolffian ducts will develop into male sexual and
reproductive structures
In males SRY expression initiates development
Müllerian ducts will develop into female sexual and reproductive structures
In females, the absence of SRY allows development
How is sex determined in birds, some reptiles, certain fish, moths,
butterflies?
Z/W system
Females = ZW (heterogametic and
hemizygous)
Males = ZZ (homogametic)
Reciprocal crosses exhibit different results compared to normal Mendelian reciprocal crosses
Z-linked genes behave like X-linked genes in mammals
W-linked genes are matrilineal (only occurs in females)
How is sex determined in Roundworms, C. elegans, nematodes
X:A ratio determines sex
XX = hermaphrodite, usually self-fertilises
Occasional nondisjunction results in X0(male) individuals
These individuals act as males producing “super-sperm” that can outcompete
the sperm from hermaphrodites Increases genetic variation!
When the mate with an XX, offspring are 1XX: 1X0 (hermaphrodite: males)
How is sex determined in monotremes?
Multiple sets of sex chromosomes can be involved
Monotremes (echidnas and platypus)-> 5 pairs of sex chromosomes!
What other ways can sex be determined?
environmental factors
Temperature determines sex in sea turtles (<28C = males, >32C = females, 28-32C =
both males and females)
Some fish change sex throughout their life depending on the temperature of the water! Ex. Clown fish……yes, like in Finding Nemo
How is sex determined in plants
90% of species are bisexual (“hermaphroditic”) Remaining 10% of species may have
Separate male and female plants
Hermaphrodites and males
Hermaphrodites and females (“inconstant males”)
