Transcript No Slide Title

Genes and Inheritance III
Genes and sex
• The examples we have seen so far were genes on
autosomes, so it didn’t matter which parent was
mother or father
• Many organisms are “monoecious” - an individual
can produce both male and female gametes
• Others (including humans, birds, fruit-flies) are
“dioecious” so individuals are either male or female
• But the way in which chromosomes determine sex is
different in different species
Sex determination
• Bees - unfertilised eggs (haploid) produce males
(drones) and fertilised eggs (diploid) produce
females (workers, queen); same in ants and many
other invertebrates
• Grasshoppers - all diploid, males have only one X
chromosome, females have 2 X’s
• Humans, fruit flies - sperms (haploid) contain an X
or a Y chromosome, females and males are XX and
XY
• Birds, butterflies - males are ZZ, and females are
WZ
The sex life of bees
The sex life of bees
• Here is the story
• To be female, bee has to be heterozygous for CSD
gene
• CSD has 19 different alleles so diploid bee is likely
to be heterozygous
• Rare homozygous diploid bees develop as sterile
males and are killed by workers
• Intensive selection by bee-keepers can reduce
genetic diversity and increase chances of CSD
homozygosity (which is bad for the bees)
Functions of sex chromosomes
• In humans (and other mammals) there is a gene SRY on
the Y, and DAX1 on the X
• Dax1 protein prevents the development of testis, allows
development of ovary, whereas Sry protein overcomes
Dax1 and allows testis to develop - primary sexual
characteristics
• XO individuals are female, XXY male
• Secondary sexual characteristics (body type, voice, body
hair) are controlled by hormones, not by X and Y directly
Functions of sex chromosomes
•
•
•
•
In fruit flies it’s a bit different
XO is a sterile male, XXY a fertile female
It is the ratio of X to Y that determines sex
There is no gene on the Y corresponding to
the mammal’s SRY
Sex-linked inheritance
• There are very few genes on the Y, but lots on the
X, and their inheritance shows sex-linkage in
pedigrees
• White eye colour in fruit flies is sex linked (Dr
Thomas Morgan again)
• The outcome of a cross depends on which parental
genotype is from the father and which from the
mother
• Figure 10.23 in textbook
Sex-linkage in humans
• There are thousands of genes on the human
X, some cause diseases like muscular
dystrophy and haemophilia that mostly
affect males
• This can be seen from inheritance in
pedigrees - for example, colour blindness
• Figure 10.24
Haemophilia in the Royal Family
• Haemophilia is a hereditary disorder caused
by defective gene on X-chromosome for a
blood clotting factor
• Queen Victoria was a carrier – her family
was linked to other European royals
• It did not get passed on to the present
Royals…….
Non-nuclear inheritance
• Everything we have seen so far, concerns genes on
chromosomes in the nucleus
• Mitochondria and plastids are organelles that have
their own little chromosome
• They may have evolved from bacteria that once
colonised a primitive eukaryotic cell
• They are found in eggs (but not sperm) so they
show maternal inheritance - passed from mothers
to all children
Mitochondrial (maternal) inheritance