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Sex inheritance. In humans, fruit flies, XX = female; XY = male. Inheritance of sex is just like any other trait, except it involve inheritance of an entire chromosome. Because there are genes on sex chromosomes, inheritance of certain traits can be sex-linked. Sex Linkage and Determination.

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Sex inheritance
Sex inheritance

  • In humans, fruit flies, XX = female; XY = male.

  • Inheritance of sex is just like any other trait, except it involve inheritance of an entire chromosome.

Because there are genes on sex chromosomes, inheritance of certain traits can be sex-linked.

Sex linkage and determination
Sex Linkage and Determination

  • Early 1900s, Thomas Hunt Morgan was doing classical genetics on fruit flies, looking for mutants and checking out the patterns of inheritance.

  • He studied the white eye phenotype and discovered something odd…

What morgan saw
What Morgan saw

Reciprocal cross produced a different result:

Inheritance of eye color depended on sex of the fly.

Morgan and the f2 generation
Morgan and the F2 generation

When the all red eyed F1 heterozygotes were crossed, close to a 3:1 ratio was observed, but the traits were not evenly divided between the sexes.


  • The traits of sex and eye color did not assort independently (as the traits in peas did).

  • The traits are linked.

  • The gene is NOT PRESENT on the Y chromosome.

R = red eye

R = white eye

XX = female

XY = male


  • With regard to X linked traits, males have only one allele, not two. They are said to be hemizygous.

  • Morgan’s work led to the understanding that genes are located on chromosome’s because inheritance of certain traits corresponded to inheritance of a visibly different chromosome.

  • Inheritance of X-linked traits results in typical crisscross inheritance: mother to son.

Crisscross inheritance
Crisscross inheritance

Carrier mother passes allele to son who expresses it, passes allele to daughter who carries it, etc.

Hemophilia & color blindness: examples in humans.

Sex determination
Sex determination

  • Different organisms have different chromosomal mechanisms for determining sex.

    • XX/XO: typically, the male has one copy.

      • Nematodes, e.g. C. elegans

    • XX/XY: as in humans, fruit flies, XX = female; XY = male.

  • Heterogametic sex is the one that produces a mixture of gametes. Usually the male but:

    • Female can be heterogametic in some species

    • Designation is ZZ/ZW where female is ZW

More on sex determination
More on sex determination

  • Temperature affects sex determination in many reptile species

    • Females result from low, high, or extremes of temperature.

    • Hypothesis:


  • Sex determination studies began in late 1800’s

  • Work in humans started around 1912, but didn’t get it right until 1956.

  • Keys to understanding sex determination in humans:

    • Improved karyotype methods

    • Study of aneuploidy of sex chromosomes

    • Aneuploidy is the wrong number of a particular chromosome.

    • Aneuploidy results from non-disjunction

Abnormalities in chromosome number result from non disjunction
Abnormalities in chromosome number result from non-disjunction

Homologues fail to separate during Meiosis I.

Abnormalities in chromosome number result from non disjunction 2
Abnormalities in chromosome number result from non-disjunction-2

Sister chromatids fail to separate during Meiosis II.

Evidence for xx xy
Evidence for XX/XY non-disjunction-2

  • 47, XXY Klinefelter syndrome

    • male in appearance, but some feminization; sterile.

    • slow to learn, but not retarded.

    • XXXY etc. similar, but more severe symptoms

  • 45, XO Turner syndrome

    • Monosomy, the only one occurring in humans

    • female, sterile, short webbed neck, broad chest, short.

    • majority aren’t born

  • If XXY is still male and XO is female

    • Y must be determinant of maleness

About the y
About the Y non-disjunction-2

  • Y chromosome has been shrinking.

    • Now missing many of genes that X has.

  • Two regions: PAR and MSY

  • PAR= pseudoautosomal region

    • Regions near p telomere and q telomere are homologous to X chromosome. Crossing over can occur there during meiosis. Because of this, genes in this location do not behave as sex-linked traits, thus said to be pseudoautosomal because they behave like genes on autosomes rather than sex chromosomes.

Structure of y
Structure of Y non-disjunction-2

Human Genome project has revealed much about the Y chromosome.

Male specific region y msy
Male specific region Y (MSY) non-disjunction-2

  • X-transposed region

    • 99% identical to X chromosome region, but only 2 genes; the rest are not expressed.

  • X-degenerative region

    • Contains DNA related to X chromosome regions

    • Several functional genes and pseudogenes

    • Contains SRY that codes for testis-determining factor, necessary for maleness during development.

  • Ampliconic region

    • Highly similar or repeated genes, some related to male development and fertility.

Evidence for sry
Evidence for SRY non-disjunction-2

  • SRY contains gene for testis determining factor

  • Crossing over in meiosis

    • Males with two X chromosomes; SRY found on one

    • Females with X and Y; SRY is missing from Y

  • Transgenic mice

    • Remove SRY from Y chromosome

    • Mice are XY but are female

    • Reciprocal experiment also done

Article about y chromosome
Article about Y chromosome non-disjunction-2


  • Copy and paste

Gene dosage
Gene dosage non-disjunction-2

  • It matters how many copies of genes there are.

    • Snapdragons: heterozygous flowers are pink.

    • Multiple histone genes.

    • Too many of some genes is deleterious.

      • 3 copies of chromosome 21 = Down Syndrome

  • What about sex chromosomes? XX vs. XY

    • Y chromosomes are missing most of genes X has.

    • So, if 1 set of genes on the X is good for males, is two sets (2 X chromosomes) bad for females?

Dosage compensation barr ohno and lyon
Dosage compensation: Barr, Ohno, and Lyon non-disjunction-2

  • Barr noticed that in the nucleus of females, but not males, a darkly staining body is visible.

  • Ohno hypothesized that this was an inactivated X chromosome in females so that there would only be 1 functional copy of genes, as in males.

  • Inactivated X is called a Barr body.

  • Individuals with incorrect numbers of sex chromosomes have appropriate number of Barr bodies.

    • E.g. XXX females have 2 Barr bodies

Lyon hypothesis
Lyon Hypothesis non-disjunction-2

  • X chromosome inactivation takes place early in development.

  • In placental mammals, it can be either X chromosome.

    • All the descendents of that cell have the same X chromosome inactivated.

    • Results in a mosaic, patches of tissue with different lineages. Seen with X-linked traits.

      • Human females: anhidrotic epidermal dysplasia, no sweat glands; female has patches of skin w/o sweat glands, cells descended from a cell in which the X chromosome with the normal gene was inactivated.

      • G6PD alleles; Patches of color blindness

Sex inheritance

Descent of cells: non-disjunction-2

How mosaics are made.

Events during development.

Two homologous chromosomes, blue & red.

Black indicates inactivation = Barr body

Formation of barr bodies 2
Formation of Barr bodies-2 non-disjunction-2

Classic example: the calico cat.

One X chromosome codes for orange fur, the other for black. Cat shows characteristic mosaic patterns caused by one or the other X chromosome being inactivated.

White fur results from the effect of another gene.

Molecular basis of barr body formation
Molecular basis of Barr body formation non-disjunction-2

  • Xic is a region on the X near the centromere.

  • Xic region includes a region called Xist (X inactivation specific transcript)

    • This area is transcribed, but RNA isn’t used to make a protein; it binds to the DNA of the rest of the X chromosome.

    • This promotes molecular changes that inactivate the chromosome including extensive methylation (except for XIC) and condensation of DNA (into smaller space).

  • In the OTHER X chromosome, Xic region is methylated so it will NOT be active.

Active and inactive regions
Active and inactive regions non-disjunction-2

Red: active genes.

Black: inactive

Xic is responsible for this process; if moved to an autosome, that chromosome will be inactivated.