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.
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