What is the
sex? Why have so many animal and plant species evolved the
mechanisms of sexual recombination?
diverse combinations of traits, because when the environment
there is greater chance that some individuals will survive.
plants develop two different sexual types (sexes)? Different
species do it differently:
You're already familiar with the complement of X and Y
chromosomes in mammals (and Drosophila!) Read
sex chromosomes determine gender in birds.
Male and female organs
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
from unfertilized eggs (male) (Ants, bees, other social
Sex chromosomes--X, Y
W, Z (birds; moths). One member of pair (male Y, or female W)
is largely degenerate, having lost most of its genes through
that show X,
Y sex determination can have two different mechanisms of addressing gene
either case, traits encoded by genes on the X chromosome will show
inheritance. A female carrying two recessive X-linked alleles,
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
male offspring. But her female offspring will receive one
allele from the father.
inactivation of one X or the other, in early embryonic cells.
down-regulation of gene expression from both X chromosomes. (Drosophila)
paired chromosomes are called autosomes.
X-linkage, you need to know the results of these crosses:
X X with X
X X with X
X X with X
X X with X
also need to FIGURE OUT THE PARENTS of a given combination of offspring.
particularly common because they only need one recessive allele present
for the phenotype to be expressed in the male. Some examples:
One of the
features of Fragile X syndrome is the role of imprinting
by methylated genes. (See next week.)
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
and lie close together on the X chromosome, where they can recombine
over) with each other by mistake. Sometimes a color-blind person
can see a different "hybrid" color that no one else can!
Trait appears rarely, only when two parents by chance carry the hidden
Trait appears in every generation, in about half of
a heterozygous carrier.)
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
no sons. Mother passes on to half of children.
Pedigrees, here SOLUTIONS
understand "dominant" and "recessive":
Dominant gene makes MUCH MORE than enough protein to cause a trait.So
one is needed; perhaps only in some cells.
Incompletely Dominant gene makes BARELY ENOUGH protein for the
So TWO COPIES are needed for the FULL trait.
Recessive gene makes no protein; inactive or partly active protein; or
not enough protein for the trait.
Penetrance and Variable Expressivity
real people, inheritance of any trait (dominant or recessive) is often
confounded by partial penetrance or by variable
of a trait.
percentage of individuals with a genotype who actually show the
If only 80% of people with the genotype actually develop the trait,
you could pass on a trait without showing it -- even if the trait is
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