Biology Dept
Kenyon College
Sex Determination 
Fall Section Spring Section 1 Spring Section 2
Sex determination
What is the function of sex?  Why have so many animal and plant species evolved the complex mechanisms of sexual recombination?

Natural selection favors diverse combinations of traits, because when the environment changes,  there is greater chance that some individuals will survive.

How do animals and plants develop two different sexual types (sexes)? Different species do it differently:

  • Male and female organs develop on the same individual.  (Garden pea plant; invertebrate worms)
  • Juvenile is born female, later develops into male; or vice versa.  (Some fish and other vertebrates)
  • Diploid (female) versus haploid, from unfertilized eggs (male)  (Ants, bees, other social insect colonies)
  • Sex chromosomes--X, Y (mammals) or W, Z (birds; moths).  One member of pair (male Y, or female W) is largely degenerate, having lost most of its genes through evolution.  (Why?)
You're already familiar with the complement of X and Y chromosomes in mammals (and Drosophila!) Read about how sex chromosomes determine gender in birds.

X-linked inheritance

Species that show X, Y sex determination can have two different mechanisms of addressing gene dosage:

  • Random inactivation of one X or the other, in early embryonic cells.  (Humans)
  • Half down-regulation of gene expression from both X chromosomes.  (Drosophila)
In either case, traits encoded by genes on the X chromosome will show X-linked inheritance.  A female carrying two recessive X-linked alleles, when crossed with a wild-type male will produce criss-cross inheritance.  This is because the Y chromosomes from the male all behave as null alleles (Why?).  So the recessive allele from the female parent is always expressed in the male offspring.  But her female offspring will receive one wild-type allele from the father.
The other, normally paired chromosomes are called autosomes.

For  X-linkage, you need to know the results of these crosses:

          A   A              a
        X   X   with  X    Y   ---->  offspring?

          a   a              A
        X   X   with  X    Y   ---->  offspring?

          A   a              a
        X   X   with  X    Y   ---->  offspring?

          A   a             A
        X   X   with  X    Y   ---->  offspring?

You also need to FIGURE OUT THE PARENTS of a given combination of offspring.

X-linked traits

X-linked traits are particularly common because they only need one recessive allele present for the phenotype to be expressed in the male.  Some examples:

  • Colorblindness. About a third of all men  are partly  color-blind.  Defective alleles are so common because (a) their effect is non-lethal, and (b) the genes for red and green photoreceptors are extremely similar and lie close together on the X chromosome, where they can recombine (cross over) with each other by mistake.  Sometimes a color-blind person can see a different "hybrid" color that no one else can!
  • Duchenne Muscular Dystrophy
One of the interesting features of Fragile X syndrome is the role of imprinting by methylated genes. (See next week.)

Pedigree analysishere.
Autosomal Recessive.  Trait appears rarely, only when two parents by chance carry the hidden allele.
Autosomal Dominant. Trait appears in every generation, in about half of descendants (assuming a heterozygous carrier.)
X-linked Recessive.   Mother passes on to half  of sons; half of daughters carry it.  Father never passes on trait.
  X-linked Dominant.  Father passes trait to all daughters; no sons.  Mother passes on to half of children.

Problems: More Pedigrees, here   SOLUTIONS

To understand "dominant" and "recessive":
  • A Dominant gene makes MUCH MORE than enough protein to cause a trait.So only one is needed; perhaps only in some cells.
  • An Incompletely Dominant gene makes BARELY ENOUGH protein for the trait.  So TWO COPIES are needed for the FULL trait.
  • A Recessive gene makes no protein; inactive or partly active protein; or not enough protein for the trait.

Partial Penetrance and Variable Expressivity

In real pedigrees of real people, inheritance of any trait (dominant or recessive) is often confounded by partial penetrance or by variable expressivity of a trait.

  • Penetrance is the percentage of individuals with a genotype who actually show the trait.  If only 80% of people with the genotype actually develop the trait, then you could pass on a trait without showing it -- even if the trait is "dominant"!
  • Expressivity  is the degree of the trait.  For example a genetic defect causing mental retardation (such as Fragile X) can result in individuals with a very wide range of intellect; and you cannot predict the degree of expression.